Mechanical Design of Tank Structure

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Evaluating Mechanical Design of Tank Structures

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Vessels File Reference: MEX20303

For additional information on this subject, contact J.H. Thomas on 875-2230

Engineering Encyclopedia

Vessels Evaluating Mechanical Design of Tank Structures

MODULE COMPONENT

PAGE

INTRODUCTION ..................................................................................................................... 1 DESIGN FACTORS THAT ARE CONSIDERED IN THE MECHANICAL DESIGN OF STORAGE TANKS .................................................. 2 Metal Temperature .................................................................................................................... 2 Pressure ..................................................................................................................................... 3 Specific Gravity of Stored Liquid ............................................................................................. 3 Corrosion Allowance ................................................................................................................. 5 Other Loads ............................................................................................................................... 7 Settlement ................................................................................................................................ 10 DETERMINING WHETHER CONTRACTOR-SPECIFIED SHELL THICKNESSES ARE CORRECT ............................................................................ 16 API-650 Requirements ............................................................................................................ 16 One-Foot Method .................................................................................................................... 19 Variable-Design-Point Method ............................................................................................... 25 Hydrostatic Test Case.........................................................................30 Design Case........................................................................................ 35 DETERMINING WHETHER CONTRACTOR-SPECIFIED WIND GIRDER REQUIREMENTS FOR OPEN-TOP TANKS ARE CORRECT .......................................... 39 Pertinent Sections of API-650 ................................................................................................. 41 General Wind Girder Requirements ........................................................................................ 42 Top Wind Girder Design Calculations .................................................................................... 43 Intermediate Wind Girder Design Calculations ....................................................................... 48 DETERMINING WHETHER CONTRACTOR-SPECIFIED DETAILS FOR OPENING DESIGN ARE ACCEPTABLE ................................................................... 54 General .................................................................................................................................... 55 Reinforcement and Welding .................................................................................................... 55 Thermal Stress Relief .............................................................................................................. 56 Manholes, Nozzles, and Flush-Type Cleanout Fittings ........................................................... 57

Saudi Aramco DeskTop Standards



DETERMINING WHETHER CONTRACTOR-SPECIFIED DESIGN DETAILS FOR TANK ROOFS ARE ACCEPTABLE .......................................... 63 Cone Roofs .............................................................................................................................. 63 Supported Cone Roof ................................................................................................. 63 Self-Supporting Cone Roof ........................................................................................ 65 Self-Supporting Dome Roof .................................................................................................... 65 Internal Floating Roof ............................................................................................................. 66 External Floating Roofs ........................................................................................................... 66 Single-Deck Floating Roof ........................................................................................ 67 Double-Deck Floating Roof ....................................................................................... 68 Special Considerations for External Floating Roofs .................................................. 68 Saudi Aramco and API Design Requirements ........................................................................ 69 32-SAMSS-005 Requirements .................................................................................. 71 API-650 Requirements ............................................................................................... 71 Sizing Inlet Diffusers ............................................................................................................... 76 DETERMINING WHETHER CONTRACTOR-SPECIFIED DESIGN DETAILS FOR TANK BOTTOMS ARE ACCEPTABLE .................................... 78 Minimum Thickness ................................................................................................................ 78 Cone Up or Down ................................................................................................................... 78 Annular Ring ........................................................................................................................... 78 Water Withdrawal.................................................................................................................... 81 Saudi Aramco and API Design Requirements ........................................................................ 81 SUMMARY ............................................................................................................................ 82 WORK AID 1:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED SHELL THICKNESSES ARE CORRECT ........................... 83

Work Aid 1A:

Procedures (One-Foot Method) and Additional Information for Calculating the Required Shell Thickness for Atmospheric Storage Tanks ...................................................... 83

Work Aid 1B:

Procedure (Variable-Design-Point Method) and Additional Information for Calculating the Required Shell Thickness for Atmospheric Storage Tanks ............................................................ 89




Calculation of Bottom Shell Course Thickness ...................................................... 89 Calculation of the Second Shell Course Thickness ................................................ 90 Calculation of Third and Higher Shell Course Thicknesses ................................... 92 Conclusion ................................................................................................................ 94 WORK AID 2:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED WIND GIRDER REQUIREMENTS FOR OPEN-TOP API-650 TANKS ARE CORRECT ............................................................ 95

Top Wind Girder Evaluation ................................................................................... 95 Intermediate Wind Girder Evaluation .................................................................... 105 WORK AID 3:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED DETAILS FOR OPENING DESIGN ARE ACCEPTABLE ......................................................................................... 111

Shell Manholes ...................................................................................................... 112 Shell Nozzle and Flange ........................................................................................ 112 Flush-Type Cleanout Fittings ................................................................................ 117 Flush-Type Shell Connections ............................................................................... 117 Roof Manholes....................................................................................................... 119 Roof Nozzles ......................................................................................................... 119 WORK AID 4:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED DESIGN DETAILS FOR TANK ROOFS ARE ACCEPTABLE ......................................................................................... 120

General................................................................................................................... 120 Frangible Fixed Roof ............................................................................................. 120 General Fixed Roof Tanks ..................................................................................... 123 Supported Cone Roof Tank ................................................................................... 124 Self-Supporting Cone Roof ................................................................................... 125 Self-Supporting Dome Roof .................................................................................. 126 External Floating Roof ........................................................................................... 127 Internal Floating Roof ............................................................................................ 130 WORK AID 5:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED DESIGN DETAILS FOR TANK BOTTOMS ARE ACCEPTABLE ......................................................................................... 131

GLOSSARY .......................................................................................................................... 138




DESIGN Factors THAT ARE Considered in the Mechanical Design of Storage Tanks This section discusses the primary factors that are considered in the mechanical design of storage tanks. These factors are as follows: •

Metal temperature

•

Pressure

•

Specific gravity of the stored liquid

•

Corrosion allowance

•

Other loads

•

Settlement

Specifically, this section discusses the effect that each of the above-listed design factors could have on tank reliability if the design factor is not properly considered in the mechanical design of the storage tank.

Metal Temperature The metal temperature of storage tank components is determined by the operating requirements of the stored liquid, and by the ambient temperature at the tank location. The operating requirements and operating conditions of the stored liquid are determined by process engineers. The mechanical design of storage tank components must consider both the highest and the lowest temperatures to which the tank can be exposed. As discussed in MEX 203.02, the maximum operating temperature is the highest temperature that must be considered in the tank design, and the design metal temperature is the lowest temperature that must be considered in the tank design. Both of these temperatures are specified on either Saudi Aramco Drawing 2696 or the Storage Tank Data Sheet (API-650 Appendix L). The maximum operating temperature determines the allowable stress that is used for the mechanical design of storage tank components. The allowable stress of each specific material is constant for all temperatures up to 93°C (200°F); however, the allowable stress of each material decreases for temperatures that are above 93°C (200°F). API-650 Appendix M contains additional design criteria that must be followed for tanks that have maximum operating temperatures over 93°C (200°F). The tank could experience a permanent deformation or a ductile fracture if the design requirements of API-650 Appendix M are not followed. As discussed in MEX 203.02, the design metal temperature affects the fracture toughness characteristics of the material and has a significant influence on tank material selection. If the design metal temperature that is specified is higher than it needs to be, based on the specific tank application, the tank may experience a brittle fracture. The design metal temperature is based on ambient conditions for most storage tanks. As discussed in MEX 203.02, SAES-D-100 specifies the appropriate design metal temperatures for Saudi Aramco storage tanks.


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Pressure The internal pressure at which a storage tank will operate determines which API standard is to be used for the mechanical design of the tank and its associated components. API-650, Welded Steel Tanks for Oil Storage, is the design standard for tanks that operate at internal pressures approximating atmospheric pressure. API-650 may also be used for tanks that operate at internal pressures up to 17 kPa (ga; 2.5 psig); however, additional design requirements that are contained in API-650 Appendix F must be followed if the internal pressure exceeds atmospheric pressure. This course will only consider API-650 tanks that operate at atmospheric pressure. API-620, Design and Construction of Large, Welded, Low-Pressure Tanks, is the design standard that is used for tanks that operate at pressures that exceed the pressure limits of API-650 and are less than 103 kPa (ga; 15 psig). When tanks have internal design pressures over 103 kPa (ga; 15 psig), they are designed as pressure vessels in compliance with Section VIII of the ASME Code. Tanks that are designed in accordance with API620, or Section VIII of the ASME Code, are not discussed in this course. Saudi Aramco has a small number of API-620 tanks, and pressure vessels are discussed in MEX 202. The internal operating pressure is determined by process engineers based on the operating requirements of the stored liquid. The internal operating pressure is specified on either Saudi Aramco Drawing 2696 or the Storage Tank Data Sheet (API-650 Appendix L). If the operating pressure is specified incorrectly, the mechanical design of the tank will not be correct. For example, if the actual internal pressure for a particular application exceeds 17 kPa (2.5 psig) but is specified to be less than that, API-650 would be used for the mechanical design of the tank instead of API-620. The use of API-650 for the mechanical design of the tank would be incorrect in this case. Depending on the magnitude of the actual pressure, this specification error could result in shell and roof sections that are too thin or nozzles that are not adequately reinforced for the actual applied pressure. The consequences of this error could be permanent deformations or a ductile fracture of tank components.

Specific Gravity of Stored Liquid The specific gravity of the liquid that is being stored, (G), in conjunction with the depth of the liquid, determines the hydrostatic pressure of the liquid, as illustrated in Figure 1. The total hydrostatic pressure at a given elevation in a tank must be considered in determining the required thickness of the tank shell. The specific gravity of the stored liquid must be specified on either Saudi Aramco Form 2696 or the Storage Tank Data Sheet (API-650 Appendix L).


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Figure 1. Hydrostatic Pressure in a Storage Tank

Storage tanks must be designed for the specific gravity of water (1.0), because the tanks are filled with water for testing purposes after they are constructed. If the specific gravity of the liquid that is to be stored exceeds 1.0 (water), the tank must be designed for the higher specific gravity. Most Saudi Aramco storage tank applications store hydrocarbons with specific gravities that are less than 1.0. As discussed later in this module, the required tank shell thicknesses are calculated separately for the hydrotest case and the design liquid case using different allowable stresses for each case.


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A later section of this module discusses the procedures that are used to determine the required thickness of the tank shell. The required shell thickness is directly proportional to the specific gravity of the stored liquid. If the specific gravity is not correctly specified, the calculated shell thickness will be incorrect. In extreme cases, the shell can become permanently deformed if it is too thin, and a ductile fracture may occur. It may be desirable for operational reasons to change the liquid that is being stored after the tank has been in service for some period of time. In situations where the stored liquid is changed, the tank must be evaluated based on the specific gravity of the new liquid. MEX 203.08 discusses the evaluation of existing storage tanks.

Corrosion Allowance As discussed in MEX 203.02, the components of a storage tank may lose metal due to corrosion that is caused by the stored liquid. To compensate for this metal loss, a "corrosion allowance" (CA) may be added to the metal thickness that is required for strength, as illustrated in Figure 2. This "corrosion allowance" offsets the expected deterioration of the tank components while they are in service. When needed, the corrosion allowance is typically added to the calculated required thicknesses of the shell, internal components, and structural members that may be used to support a fixed roof. A corrosion allowance is typically not added to the required thicknesses of the roof itself or the bottom.


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Where: t

=

Minimum required shell thickness

CA

=

Corrosion allowance

T

=

Total required shell thickness

Figure 2. Corrosion Allowance in Tank Shell


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Corrosion will sometimes occur on the exterior of the tank shell or roof sections. However, this corrosion is normally caused by factors such as deterioration of the external paint system or poor local drainage. A corrosion allowance is not added to the metal thickness to account for external corrosion. Periodic maintenance must be performed to ensure that external corrosion does not become a problem. As discussed in MEX 203.02, SAES-D-100 specifies the corrosion allowance requirements for Saudi Aramco storage tanks. In brief, a corrosion allowance is normally not specified for storage tanks that are in crude oil service; however, a corrosion allowance may be specified by the proponent of the project for tanks that are in crude oil or hydrocarbon services based on actual experience with other tanks in similar services. Corrosion allowances are specified for the shell and internal structures of storage tanks that are in water service if there is no internal cathodic protection system. A corrosion allowance, when required, must be specified on either Saudi Aramco Drawing 2696 or the Storage Tank Data Sheet (API-650 Appendix L). MEX 203.02 contains a table that summarizes Saudi Aramco corrosion allowance requirements. If a storage tank shell corrodes during operation, and a corrosion allowance was not properly specified, the stresses in the shell will increase and may eventually exceed the allowable stress. In extreme cases, the shell can become permanently deformed (or hole through) if it becomes too thin, and a ductile fracture may occur. MEX 203.08 discusses the evaluation of existing storage tanks that have experienced corrosion during operation.

Other Loads The mechanical design of a storage tank must also consider loads other than pressure. These other loads include wind and earthquake, loads that are imposed by connected piping systems (and other attachments) on nozzles, and rainwater accumulation on external floating roofs. Figure 3 illustrates the application of wind and earthquake loads on a storage tank.


6



Where: Fw

=

Base shear force due to wind

M

=

Overturning moment due to wind or earthquake

Figure 3. Wind and Earthquake Loads


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32-SAMSS-005 does not specify any requirements with respect to other loads. SAES-D-100 specifies the following loading and analysis requirements for the design of storage tanks: •

All tanks must be designed for a wind velocity of 137 km/h (85 MPH) in accordance with ANSI A58.1, Exposure C. Tanks must also be checked for stability against overturning moments that are caused by wind pressure. A tank shell may become out-of-round if it is not adequately designed for the expected wind velocity. Excessive shell out-of-roundness could cause binding of floating roofs and/or the formation of cracks at shell welds. The overturning moment requirement is most relevant for relatively small diameter storage tanks, because the tank weight might not be sufficient to resist the wind load. Anchor bolts might be needed to keep the tank from turning over.

•

The minimum required thickness of the tank shell for the design liquid, excluding corrosion allowance, must be used to determine wind girder requirements for external floating roof tanks. One or more wind girders are used to prevent tank shell out-of-roundness that could be caused by wind loads. Wind girder requirements are discussed later in this module.

•

Seismic loads are to be considered, and the applicable seismic zone (to be found in SAES-M100) must be specified on either Saudi Aramco Drawing 2696 or the Storage Tank Data Sheet (API-650 Appendix L). A tank rupture can occur if the tank is not designed for the required seismic loads.

•

Nozzles must be designed for the static liquid load and loads that are applied by connected piping. Special design considerations are not required to account for the static liquid load as long as standard API-650 design details are used for the nozzles. Loads that are applied by piping that is connected to a tank nozzle cause additional local stresses in the nozzle neck, the adjacent shell, and the associated attachment welds. These additional stresses, if they are too high, can cause the formation of local weld cracks in the nozzle or shell. Therefore, it must be confirmed that the loads that are applied by the pipe are not excessive.

•

For nozzles with diameters greater than 150 mm (6 in.) that are located in the first shell course of tanks greater than 20 m (65 ft.) in diameter, a stress analysis of the tank shell-to-piping intersection must be performed to assure adequacy of the design. The analysis must be submitted to the Consulting Services Department for review prior to fabrication. This analysis is done to ensure that the local stresses at the nozzle-to-shell junction are not excessive.


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Settlement Settlement of the tank bottom or shell is caused by compression or movement of the soil under the tank or the tank foundation. Settlement can be the result of improper foundation design, unusual flooding or high tides, or from slowly flowing soil in marshy or swampy locations. When the foundation settles evenly around the tank periphery, the resulting settlement causes only minor problems. However, nonuniform settlement causes tank shell out-of-roundness. Excessive shell out-ofroundness can cause flat spots or buckles on the tank shell and roof of fixed roof tanks, and seal damage, binding, and/or excessive gaps between the roof and shell in floating roof tanks. Figure 4 illustrates differential shell settlement.


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Figure 4. Differential Shell Settlement

A tank bottom may also experience differential settlement of several different types, as illustrated in Figures 5 through 7. Settlement, depending on its type and severity, can overstress the bottom plates or bottom-to-shell junction, or buckle the bottom shell course. This overstress can cause the bottom plate welds to crack and leak. If the leaks are large enough, portions of the foundation may become unstable. This instability could lead to the loss of local bottom support, further increase the stress in the bottom plates, and eventually result in a significant tank failure.


10



Figure 5. Center-to-Edge Bottom Settlement


11



Figure 6. Bottom Edge Settlement


12



Figure 7. Bottom Settlement or Bulges


13



Soil samples are typically taken from the area where a tank is to be erected. This sampling is done to determine the composition of the soil, and whether there are any significant variations in soil composition over the area upon which the tank will rest. From this soil composition data, civil engineers are then able to determine the maximum amount of settlement that is expected during the life of the tank, and whether this settlement will be relatively uniform. If the civil engineers determine that the predicted settlement is excessive, a soils improvement program is undertaken in order to reduce the amount of predicted settlement, and its nonuniformity, to acceptable values. The predicted settlement values are not specified on either Saudi Aramco Drawing 2696 or the Storage Tank Data Sheet (API-650 Appendix L). The predicted settlement is not directly considered in the mechanical design of a new storage tank because it is assumed that the tank foundation has been stabilized to the extent that is necessary to keep settlement within an acceptable limit.


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DETERMINING WHETHER CONTRACTOR-SPECIFIED Shell thicknesses ARE CORRECT This section first reviews the requirements that are in API-650 for shell thicknesses of storage tanks. This section then explains the following methods that are used to calculate shell thicknesses: •

The one-foot method

•

The variable-design-point method

The discussion of each calculation method is accompanied by a sample problem that illustrates the application of the method. The normal job function of Saudi Aramco engineers is to review the drawings and calculations for storage tank shells that are provided by tank manufacturers in Contractor Design Packages. For example, the shell plate thicknesses that are specified by the contractor must be reviewed to determine if they are acceptable. The Saudi Aramco engineer is not responsible for making the shell thickness calculations himself for the initial tank design. Situations may arise during the review of Contractor Design Packages when the Saudi Aramco engineer must do check-calculations himself in order to verify the contractor’s work. The Saudi Aramco engineer must know how these calculations should be done to be able to verify that the contractor-specified thicknesses are acceptable, to confirm that the contractor’s calculations are done correctly, and to perform check-calculations himself as required.

API-650 Requirements Figure 8 shows the relevant paragraphs of API-650 that specify requirements for tank shell thickness.


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Topic

Paragraph

Shell Design - General

3.6.1

Allowable Stress

3.6.2

Calculation of Thickness by the One-Foot Method

3.6.3

Calculation of Thickness by the Variable-Design-Point Method Calculation of Thickness by Elastic Analysis

3.6.4 3.6.5

Figure 8. API-650 Tank Shell Thickness Requirements

Work Aid 1 contains the specific procedures to be used to calculate shell thicknesses using either the one-foot method or the variable-design-point method. Tank shell thicknesses will normally only be calculated by the tank manufacturer. However, the contractor may make preliminary estimates of the shell course thicknesses using the one-foot method for tanks that are no more than 61 m (200 ft.) in diameter. The sections that follow summarize general shell thickness requirements that are contained in API-650 and elaborate on several of the procedural items that are contained in Work Aid 1. API-650 requires that an elastic stress analysis be done for tanks that cannot be designed by either of the other two thickness calculation methods. Elastic stress analysis of storage tank shells is not discussed in this course. API-650 contains several general requirements that relate to shell thickness. Several of these requirements are highlighted as follows: •

The required thickness of a storage tank shell must be calculated for two separate cases:

Case 1 considers the specific gravity of the stored liquid. The shell thickness that results from this case is the “design shell thickness,” t d.


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Case 2 considers the specific gravity of the water (1.0) which will be used for the hydrostatic test of the tank. The shell thickness that results from this case is the “hydrostatic test shell thickness,” tt. •

td for a tank is computed on the following basis: (1)

The assumption that the storage tank is filled to its design capacity with a liquid having a specific gravity specified by the purchaser.

(2)

The design allowable stress (discussed in MEX 203.02).

(3)

Any corrosion allowance specified by the purchaser.

The shell thickness computed on this basis is for the normal, maximum design loading for the tank. •

tt for a tank is computed on the following basis: (1)

The assumption that the storage tank is filled to its design capacity with water.

(2)

The hydrostatic test allowable stress (discussed in MEX 203.02).

(3)

No corrosion allowance.

•

The tank shell is to be checked for stability against buckling from the design wind velocity as previously cited from SAES-D-100, (i.e., 137 km/h [85 MPH]). Intermediate wind girders, increased shell-plate thickness, or both, are to be included in the design in order to stiffen the shell, if required for stability. The most common approach is to weld circumferential wind girders to the shell. The use of wind girders will be discussed later in this module.

•

The tank manufacturer is to furnish the purchaser with a drawing with the following data for each shell course:


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Required shell thicknesses for both the design condition (including corrosion allowance) and the hydrostatic test condition.

-

Nominal thickness used.

-

Material specification.

-

Allowable stresses.

•

Isolated radial loads on the tank shell, such as those caused by heavy loads on platforms and elevated walkways between tanks, are to be distributed to avoid excessive local shell stresses.

One-Foot Method The one-foot method is based on limiting the approximate membrane stress to the allowable stress at a location that is 1 ft. above the bottom of the course being considered. The required shell thickness is then determined based on that stress. A distance of 1 ft. above the bottom of the course is assumed to be the location of maximum membrane stress. A step-by-step procedure for the one-foot method of API-650 for calculation of the required shell thickness for atmospheric storage tanks is provided in Work Aid 1A. This is the most commonly used method for the calculation of shell thicknesses because it is the simplest method. The one-foot method is not valid for tanks that are over 61 m (200 ft.) in diameter. The following briefly summarizes the overall calculation approach of the one-foot method and several additional points. Complete procedural details are contained in Work Aid 1A. Use Figure 9 as a reference.


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Figure 9. Tank Shell Courses


19



• A tank shell is constructed of individual rows of plates. Each individual plate row is called a “course” (see Figure 9), and each course is of a specified height. The total height of the tank shell is the sum of the individual course heights. • The required thickness of each individual course is calculated separately. The required thickness considers the depth of liquid (H) measured from the bottom of the course to the maximum liquid level. • The hydrostatic pressure that is imposed by the stored liquid is a maximum at the bottom course and a minimum at the top course. If the same plate material specification is used for all the courses, the required plate thickness to resist the hydrostatic pressure decreases in going from the bottom course to the top course. • Different plate material specifications may be used for different courses. For example, a higher-strength material specification may be used for the bottom course than is used for the upper courses. Use of a higher strength material for one or more lower courses minimizes the required plate thicknesses for the courses that are subjected to the highest hydrostatic pressures. • The required thickness is calculated for each course for both the design liquid and for water. The minimum acceptable course thickness based on hydrostatic pressure considerations is the higher of the two calculated thicknesses (i.e., t d or tt). • The actual course thickness that is used for each course must also be greater than minimum values that are specified in API-650, based on tank diameter. In addition, no course may be thicker than the course that is under it. •

Sample Problem 1 illustrates the application of the one-foot method.


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Sample Problem 1:

Calculation of Shell Thickness Requirements Using the One-Foot Method

Figure 10 is an excerpt from a Contractor Design Package for an atmospheric storage tank. Calculate the required shell thicknesses for this storage tank using the one-foot method.

Tank Size: Diameter - 100 ft.

Total Height - 40 ft.

Shell Courses: 5-courses, each 8 ft. high Maximum Design Liquid Storage Height: 38 ft. Liquid Specific Gravity: 0.85 Corrosion Allowance: 1/16 in. Shell Plate Material: A516 Grade 70 for bottom course A516 Grade 60 for all other courses Bottom Plate Material: Annular Plate:

A516 Grade 70

Rest of Bottom: A36 Roof Plate Material: A36 Roof Type: External Floating

Figure 10. Sample Problem 1 Data


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Solution Work Aid 1A is used to solve this problem.

•

td =

2.6D(H − 1)G + CA Sd

tt =

2.6D(H − 1) St

For the first (bottom) shell course (H = 38 ft.): For the A516, Grade 70 steel, Sd = 25 300 psi, and St = 28 500 psi (Table 3-2 of API-650).

2.6(100 )(38 − 1)(0. 85) + 0.0625 25 300 t d = 0.386 in. td =

2.6(100 )(38 − 1) 28 500 t t = 0.338 in. tt =

The design condition is the governing case for the first course; therefore, the shell must be at least 0.386 in. thick. In practice, the next larger readily available, standard plate thickness will be ordered by the vendor for each course.


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•

For the second shell course (H = 30 ft.): Note that the shell plate material has changed to A516 Grade 60 for this course and all higher courses. For A516 Grade 60, Sd = 21 300 psi and St = 24 000 psi. 2.6(100 )(30 − 1)(0.85 ) + 0.0625 21 300 t d = 0.363 in. td =

2.6(100 )(30 − 1) 24 000 t t = 0.314 in. tt =

The design condition is the governing case for the second course; therefore, the shell must be at least 0.363 in. thick. •

For the third shell course (H = 22 ft.):

2.6(100 )(22 − 1)(0.85 ) + 0.0625 21 300 t d = 0.28 in. td =

2.6(100 )(22 − 1) 24 000 t t = 0.23 in. tt =

The design condition is the governing case for the third course. Therefore, the shell must be at least 0.28 in. thick. If the calculations were continued, they would show that the minimum permissible thickness is the governing factor for the fourth course; therefore, the shell must be at least 0.25 in. thick. Since the minimum permissible thickness determined the required thickness of the fourth course, the fifth course must be at least 0.25 in. thick also.


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The required minimum shell thicknesses are summarized as follows: •

First course:

0.386 in.

•

Second course: 0.363 in.

•

Third course:

•

Fourth course:

•

Fifth course:

0.28 in. 0.25 in. 0.25 in.

If the Contractor Design Package for this tank included shell course thicknesses, you must confirm that the contractor-specified thicknesses are at least these minimum values.

Variable-Design-Point Method Recall from the previous discussion that the one-foot method cannot be used if the tank diameter exceeds 61 m (200 ft.). The variable-design-point method of API-650 is normally used to calculate the required shell thicknesses for these larger-diameter tanks. A step-by-step procedure for calculation of the required shell thickness for atmospheric storage tanks by the variable-design-point method is provided in Work Aid 1B. The paragraphs that follow describe the overall approach. The variable-design-point method calculates the required thickness of each shell course at an elevation that is closer to the actual point of maximum stress than the one-foot method assumes. The variable-design-point method may be used when the purchaser does not require use of the one-foot method, and when the following equation based on tank geometry is satisfied: SI Units

0.268 Dt ≤2 H Where:

English Units

6Dt ≤2 H

D

=

Tank diameter, m (ft.)

t

=

Bottom shell course thickness, mm (in.)

H

=

Maximum design liquid level, m (ft.)

If this inequality is not satisfied for a particular tank geometry, the shell thicknesses must be calculated by elastic stress analysis.


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The variable-design-point method is an iterative calculation procedure. The calculation is begun by picking a thickness for the shell course, and by then performing a calculation using that thickness in order to calculate the required thickness. Normally, the starting thickness for the first iteration is the thickness that is determined by the one-foot method. The calculation results in a revised required thickness for the shell course. The calculation is then repeated using the result of the first calculation as the initial estimated thickness. The procedure continues until the starting and calculated thicknesses converge. Calculations are made for both the design and hydrotest cases, as in the one-foot method. The variable-design-point method for calculation of the required tank shell thickness has the following characteristics: •

It is a more sophisticated design calculation than the one-foot method and usually results in a thinner tank shell. The reduction in shell thickness is more important for larger diameter tanks because it results in a greater reduction in the amount of material that is needed to fabricate the shell. The reduction in shell thickness then results in a greater cost saving for large diameter tanks.

•

It is much more time-consuming than the one-foot method if the calculations are done manually. However, tank suppliers will typically use a computer program for these calculations.

•

It is a valid method for calculation of tank shell thicknesses for tanks that are over 61 m (200 ft.) in diameter that satisfy the previously stated equation based on tank geometry. This method may also be used for smaller diameter tanks. However, it normally is not worthwhile to employ this method for small diameter tanks.

•

It cannot be used when the one-foot method is specified by the Purchaser. Saudi Aramco does not specify the use of one calculation method over another.

Refer to Tables K-1 through K-3 in API-650 Appendix K. These tables summarize the results of shell thickness calculations based on the variable-design-point method for a variety of tank diameters and shell heights. These tables may be used to help make an initial assessment of shell thickness results that are provided in a Contractor Design Package when the variable-design-point method is used. These tables should be used cautiously for the following reasons:


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• The tables are based on the use of just three specific allowable stresses (i.e., the results are only accurate for the material specifications that correspond to these allowable stresses). Therefore, their results are not correct for any other allowable material stresses, and must be adjusted. •

The tables summarize the results for only the hydrostatic test case calculations. Therefore, the acceptability of the thicknesses for the design case must still be verified.

•

The tables assume that all of the shell courses are fabricated using the same material specification. Therefore, the thicknesses are not correct if more than one material is used in the fabrication of the shell. It is common for the shell of a large-diameter tank to be fabricated using at least two material specifications.

•

The tables assume that each shell course is 2.4 m (96 in.) high. Therefore, the thicknesses are not correct if the courses above the first course are not of this height.

•

The tables are based on specific tank diameters and shell heights. The thicknesses must be adjusted for other tank diameters and heights.

Because of the above limitations, Tables K-1 through K-3 should at most be used to make an initial estimate of the shell thicknesses that are required based on the variable-design-point calculation method. Any final decision regarding the acceptability of contractor-specified shell thicknesses should be based on the specific tank geometry and material specifications and must also consider the calculations for the design case.


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Sample Problem 2:

Calculation of Shell Thickness Requirements Using the VariableDesign-Point Method

Figure 11 is an excerpt from a Contractor Design Package for an atmospheric storage tank. Calculate the required shell thicknesses for this tank using the variable-design-point method. Work Aid 1B is used to solve this problem. Tank Size: Diameter - 280 ft.


Shell Courses: 8-courses, each 8 ft. high Maximum Design Liquid Storage Height: 64 ft. Liquid Specific Gravity: 0.85 Corrosion Allowance: 0.05 in. Shell Plate Material: A573 Grade 70 for all courses Bottom Plate Material: Annular Plate:

A573 Grade 70


Figure 11. Sample Problem 2 Data Solution: Since the tank exceeds 61 m (200 ft.) diameter, Work Aid 1B must be used. For the A573 Grade 70 steel, Sd = 28 000 psi, St = 30 000 psi.

t pd =


2.6D(H− 1)G + CA Sd

27



t pd =

2.6(280)(64 − 1)(0.85 ) + 0.05 (28 000)

= 1. 442 in. 2.6D(H − 1) tpt = St tpt =

2.6(280)(64 − 1) (30 000)

= 1.529 in. First Shell Course:

 0. 463D HG   2.6HDG  t 1d = 1.06 − + CA H S d   Sd  

 0. 463(280) t1d = 1.06 −  (64)  = 1.423 in.

(64)(0.85 )   2.6(64 )(280)(0.85)  + 0. 05  (28 000 )   (28 000 )

 0. 463D H   2.6HD  t 1t = 1.06 −  H St   St 

 0. 463(280) t1t = 1.06 − (64)  = 1. 501 in.

(64)

  2. 6(64 )(280 ) (30 000)   (30 000) 

The required bottom shell course thickness is 1.501 in.


28



Hydrostatic Test Case The hydrostatic test case calculations will first be done for all remaining courses, then the design condition calculations will be done.

Second Shell Course:

[12 (8)]

h1 = rt1

6(280)(1.501)

= 1.912

Since this is between 1.375 and 2.625, t 2 is calculated by the equation that follows (after calculating t 2a).

  h1 t 2 = t 2a + (t 1 − t 2a ) 2.1− 1. 25 rt 1  

Use the "upper course" procedure to first calculate t 2a.

t pt2 =

2.6 (280 )[(64 − 8 ) − 1] (30 000 )

= 1. 335 in.= t u tL = 1.501 in. (from the earlier calculations).

 t t   L  L − 1  t t  u u C= 1.5   tL   1+    t   u     C=  

(1.501)  (1. 501)   −1 (1.335 )  (1. 335)    = 0. 060 1.5  (1. 501)   1+  (1. 335)  

Calculate the maximum stress point, x.


29



x1 = 0.61 rt u + 3. 84CH x1 = 0.61 [6(280 )](1. 335) + 3.84 (0. 060)(64 − 8) = 41.79 in. x 2 = 12CH

x 2 = 12 (0.060 )(56 ) = 40. 32 in. x 3 = 1.22 rt u x 3 = 1.22 6 (280 )(1. 335) = 57.78 in. x = 40.32 in.

t dx

t 2a

x  2.6D H − 12   = St

 (40. 32) 2.6(280) (56 ) −  12  = = 1. 277 in. 30 000

Since the calculated value of t2a differs significantly from the initial value assumed (1.335 in.), repeat the procedure using tu = 1.277 in.

  C=   

(1. 501)  (1.501) − 1  (1. 277)  (1.277 )    = 0.084 1.5  (1.501)   1+   (1.277 ) 

x1 = 0.61 [6(280)](1.277 ) + 3.84 (0.084 )(64 − 8 ) = 46. 24 in. x 2 = 12 (0.084 )(56 ) = 56.19 in. x 3 = 1.22 6 (280 )(1. 277) = 56.51 in. x = 46. 24 in.

t 2a

 (46.24)  2.6(280) (56 ) − 12   = = 1.265 in. 30 000

This is much closer, but do one more iteration.


30



  C=   

(1. 501)  (1. 501) − 1  (1. 265)  (1. 265)    = 0.089 1.5  (1.501)   1+   (1.265 ) 

x1 = 0.61 [6(280)](1.265 ) + 3.84 (0.089 )(64 − 8 ) = 47.18 in. x 2 = 12 (0.089 )(56 ) = 59. 57 in. x 3 = 1.22 6 (280 )(1. 265) = 56.24 in. x = 47.18 in.

t 2a

 (47.18)  2.6(280) (56 ) − 12   = = 1.263 in. 30 000

Since 1.265 in. and 1.263 in. are very close, no further interations are required. Use the value of 1.263 in. for t2a. Finish calculation of the second shell course thickness.

  12 (8) t 2 = (1.263 ) + [(1. 501) − (1.263 )] 2.1−  = 1.399 in. 1.25 6 280 1.501 ( ) ( )  


31



Third Shell Course:

t pt3 =   C=   

2.6 (280)[(64 − 16 ) − 1] = 1.141 in. (30 000 )

(1. 399)  (1. 399)   −1 (1.141)  (1.141)    = 0.106 1.5  (1.399 )  1+   (1.141)  

x1 = 0.61 [6 (280)](1.141) + 3.84 (0.106)(64 − 16 ) = 46.25 in.

x 2 = 12 (0.106)(48 ) = 61.06 in.

x 3 = 1.22 6(280 )(1.141) = 53.41 in. x = 46. 25 in.

t t3

 (46. 25 ) 2. 6(280 ) (48 ) − 12   = = 1.071 in. (30 000 )

Iterate again.   C=   

(1. 399)  (1. 399)   −1 (1. 071)  (1.071)    = 0.140 1.5  (1.399 )  1+   (1.071)  

x1 = 0.61 [6(280)](1.071) + 3.84 (0.140 )(64 − 16 ) = 51.68 in.

x 2 = 12 (0.140)(48 ) = 80.64 in.

x 3 = 1.22 6 (280 )(1. 071) = 51.75 in. x = 51. 68 in.

t t3

 (51.68)  2. 6(280 ) (48 ) − 12   = = 1.060 in. (30 000)


32



Iterate one last time.   C=   

(1. 399)  (1. 399) − 1  (1. 060)  (1.060)    = 0.146 1.5  (1.399 )  1+   (1.060)  

x1 = 0.61 [6 (280)](1.060 ) + 3.84 (0.146 )(64 − 16 ) = 52.65 in. x 2 = 12 (0.146)(48 ) = 84.10 in. x 3 = 1.22 6(280 )(1. 060) = 51.48 in. x = 51. 48 in.

t t3

 (51. 48 ) 2. 6(280 ) (48 ) − 12   = = 1. 061 in. (30 000 )

Since 1.060 in. and 1.061 in. are very close, no further iterations are required. Use 1.061 in. for the thickness of the third shell course. The minimum acceptable shell thickness for a 280 ft. diameter tank is 0.375 in. Therefore, calculation of the fourth course thickness is required. The calculations that are required for the fourth and higher courses use the same procedure as was used for the third course. Therefore, only the final values for the intermediate parameters and course thicknesses are shown in the following table:


33



Course Number

C

x1

x2

x3

x

tt

4

0.0864

36.96

41.47

47.39

36.96

0.896

5

0.126

36.46

48.50

41.96

36.46

0.703

6

0.162

32.93

46.73

35.96

32.93

0.516

7

0.237

28.94

45.5

28.77

28.77

0.330 (0.375)*

8

-

-

-

-

-

0.375*

* Minimum permitted thickness is 0.375 in. for a 280 ft. diameter tank. Proceed with the calculations for the design condition.

Design Case The design case calculations will now be done. Again, only the final iteration is shown through the third shell course, and only the final values are shown for the higher courses.

Second Shell Course: tL = 1.423 in.

Assume tu = 1.203 in. Note that the corrosion allowance is first subtracted from the values of t L and tu that are used in the following intermediate calculations, and then the corrosion allowance is added back in for the final thickness calculation.


34



  C=   

(1. 423 − 0. 05 )  (1.423 − 0.05) − 1  (1. 203 − 0. 05 )  (1.203 − 0.05)    = 0. 0902 1.5  (1. 423 − 0. 05 )  1+   (1. 203 − 0. 05 ) 

x1 = 0.61 [6(280)](1.203 − 0.05 ) + 3.84 (0. 0866)(56 ) = 46. 25 in. x 2 = 12 (0.0902 )(56 ) = 58.2 in. x 3 = 1.22 6 (280 )(1. 203 − 0. 05 ) = 53.7 in. x = 46. 25 in.

t 2a

(46.25)  (0.85) 2.6(280) 56 − 12   = 28 000 = 1.152 in.

Since (1.203 - 0.05) = 1.153 in. checks, no further iterations are needed.  h1  + CA t 2 = t 2a + (t1 − t2 a ) 2.1− 1.25 rt1   Note that the value for t1 that is used in this equation is the bottom shell course thickness that was calculated for the design case minus corrosion allowance.

  12(8 ) + 0.05 t 2 = (1.152 ) + [(1.423 − 0.05 ) − (1.152 )] 2.1− 1.25 6(280)(1.423 − 0.05 )   = 1. 312 in. Third Shell Course:

tL = 1.312 Assume tu = 1.02


35



  C=   

(1. 312 − 0. 05)  (1.312 − 0.05 ) − 1  (1.02 − 0.05)  (1. 02 − 0.05)    = 0.139 1.5  (1. 312 − 0. 05 )  1+   (1.02 − 0.05 )  

x1 = 0.61 [6 (280)](1.02 − 0. 05 ) + 3. 84(0.12 )(48 ) = 50.18 in. x 2 = 12 (0.12 )(48 ) = 79.86 in. x 3 = 1.22 6(280 )(1. 02 − 0.05 ) =~ 49.25 in. x = 49. 25 in.

t d3

 (49. 25 ) (0.85) 2. 6(280 ) (48 ) − 12   = + (0.05 ) = 1.02 in. (28 000)

Since the calculated and assumed thicknesses match, no further interations are needed. Use 1.02 in. as the thickness for the design case of the third shell course. The minimum acceptable shell thickness for a 280 ft. diameter tank is 0.375 in. Therefore, calculation of the fourth course shell thickness is required. The calculations that are required for the fourth and higher courses use the same procedure as was used for the third course. Therefore, only the final values for the intermediate parameters and course thicknesses are shown in the following table for courses four through eight: Course C

x1

x2

x3

x

td

4

0.0875

36.075

42.011

45.262

36.075

0.868

5

0.126

35.53

48.41

40.09

35.53

0.692

6

0.157

31.70

45.11

34.53

31.70

0.522

7

0.237

28.30

45.44

27.52

27.52

0.353 (0.375)*

8

-

-

-

-

-

0.375*

Number

* Minimum permitted thickness is 0.375 in. for a 280 ft. diameter tank.


36



Figure 12 summarizes the results of both sets of calculations (i.e., hydrostatic test case and design case) and the resulting minimum thickness that is required for each shell course.

SHELL COURSE

HYDROSTATIC TEST THICKNESS, in.

DESIGN THICKNESS, in.

MINIMUM REQUIRED THICKNESS, in.

1

1.501

1.423

1.501

2

1.399

1.312

1.399

3

1.061

1.020

1.061

4

0.896

0.868

0.896

5

0.703

0.692

0.703

6

0.516

0.522

0.522

7

0.375

0.375

0.375

8

0.375

0.375

0.375

Figure 12. Required Shell Thicknesses

Note from this summary that the hydrostatic test condition governs some course thicknesses, and that the design condition governs other course thicknesses. No shell course has a thickness greater than the shell course beneath it. If the Contractor Design Package for this tank included shell course thicknesses, you must confirm that the contractor-specified thicknesses are at least these minimum values.


37



DETERMINING WHETHER CONTRACTOR-SPECIFIED Wind Girder Requirements for Open-Top Tanks ARE CORRECT This section discusses the wind girder requirements for open-top tanks, and demonstrates how wind girders are designed. An open-top tank is essentially a vertical cylinder that is open at the top and closed at the bottom. As Figure 13 illustrates, this cylinder can be forced out-of-round by wind pressure that acts against it, unless adequate stiffness against deformation is provided by the shell alone or by other means. If excessive, shell out-ofroundness could prevent free vertical travel of the floating roof, or could cause the formation of cracks in shell welds.

Figure 13. Shell Out-of-Roundness Caused By Wind


38



Theoretically, there are two ways to provide adequate stiffness: •

The tank shell can be made sufficiently thick to provide all the needed stiffness, or

•

Some additional method of stiffening the shell can be provided.

In most cases, it is not economical to make the shell thick enough to provide all of the necessary stiffness. Therefore, additional stiffness to resist shell deformation is provided by welding circumferential stiffening rings around the outside of the tank. These stiffening rings are referred to as wind girders. Figure 14 shows several typical configurations for wind girders, and Figure 15 illustrates the general placement of both a top and an intermediate wind girder on a tank shell.

Figure 14. Typical Wind Girders


39



Figure 15. Wind Girder Placement on Tank Shell

Pertinent Sections of API-650 All API-650 requirements that are related to the top and intermediate wind girders are contained in Para. 3.9. Several of the specific paragraph references that are in that section and their general content are as follows: •

Para. 3.9.3

-

Restrictions on Stiffening Rings. Basic minimum size requirements.

•

Para. 3.9.4

-

Stiffening Rings As Walkways. Size and location requirements if a stiffening ring is used as a walkway.

•

Para. 3.9.5

-

Supports for Stiffening Rings. Criteria for determination of the need and the size of stiffening ring supports.


40



•

Para. 3.9.6

-

Top Wind Girder. Sizing requirements for a top wind girder.

•

Para. 3.9.7

-

Intermediate Wind Girder. Criteria for determination of the need, location, and size of intermediate wind girders.

The sections that follow discuss several of the general wind girder requirements, along with the design calculations that are required for top and intermediate wind girders. Participants are referred, for other wind girder requirements, to API-650.

General Wind Girder Requirements API-650 requires that all open-top tanks be provided with stiffening rings (i.e., wind girders), that the stiffening rings be located at or near the top of the top course, and that they preferably be located on the outside of the tank shell. Stiffening rings will typically be made of standard structural sections, or will be formed from plate sections that are welded together and then welded to the shell. The general approach to wind girder design consists of determining the following: •

The minimum required section modulus for the top wind girder.

•

Whether a second, intermediate wind girder must also be provided at some lower elevation on the shell.

•

The location of the intermediate wind girder (if one is needed).

•

The minimum required section modulus (if an intermediate wind girder is needed).

of

the

intermediate

wind

girder

Wind girder design calculations will be discussed shortly. API-650 contains additional general requirements for wind girders that cover the following topics: •

Minimum sizes are specified for angles that are used as stiffening ring components or as a top curb angle. The minimum plate thickness that may be used for built-up stiffening rings is 6.35 mm (1/4 in.). The minimum angle sizes that are specified will provide a basic level of structural rigidity to the tank. The minimum angle sizes are specified separately from the top and intermediate wind girder evaluations.


41



•

Wind girders must have drain holes to remove trapped liquid. Trapped liquid could cause accelerated corrosion of both the wind girder and the adjacent portion of the tank shell.

•

Size, location, and design details are specified for wind girders that are regularly used as walkways. These details ensure that personnel safety is considered in wind girder design.

•

Wind girder support requirements are specified. These requirements include criteria for designs where support is needed, along with maximum spacing limits between wind girder supports.

•

Design details are specified for the region of a wind girder where the tank stairway passes through the wind girder. The opening through the wind girder that is required to permit stairway access locally weakens the wind girder. Stiffening requirements are specified to reinforce this area.

Top Wind Girder Design Calculations As previously stated, API-650 requires that all open-top tanks be provided with a stiffening ring that is located at or near the top of the tank. This stiffening ring is the top wind girder. The purpose of the top wind girder is to ensure that the top section of the tank shell is stiff enough to prevent deformation which may be caused by the design wind velocity blowing across the entire tank shell. The top wind girder must be sized to have a large enough section modulus to provide adequate shell stiffening. Top wind girder design calculations consist of determining the minimum required section modulus, and then selecting a large enough wind girder section to provide this section modulus. API-650 contains an equation to calculate the minimum required top wind girder section modulus. The required top wind girder section modulus is based on the tank diameter and shell height. API-650 also contains a table which summarizes the section module of various structural shapes and attachment configurations to the tank shell. API-650 permits the inclusion of a portion of the tank shell in the wind girder section modulus. This inclusion is accounted for in the section modulus table.


42



Saudi Aramco has simplified top wind girder selection by developing several standard wind girder designs which meet API-650 requirements. These standard designs are contained on Standard Drawing AD-036211. The easiest approach to take is to use a standard API-650 or standard Saudi Aramco top wind girder detail that has at least the required section modulus. However, nonstandard wind girder details may be used in certain situations. For example: •

The tank contractor may decide to use a nonstandard detail due to cost or schedule considerations.

•

The standard details include a portion of the tank shell in determining their section modulus, as permitted by API-650. However, the shell corrosion allowance is not considered in the section modulus determination for the standard details. Saudi Aramco requires that shell corrosion allowance be considered in the wind girder calculations. Therefore, if the tank has a corrosion allowance and a standard wind girder detail is specified, the section modulus must be calculated, based on the corroded shell thickness, to confirm that the standard wind girder is still acceptable.

Work Aid 2 contains a procedure for designing or evaluating the top wind girder, in accordance with Saudi Aramco and API-650 requirements. Work Aid 2 also contains a procedure that may be used to calculate the actual wind girder section modulus, using its specified geometry.

Sample Problem 3: Calculation of Top Wind Girder Size Calculate the top wind girder size for the atmospheric storage tank that was used in Sample Problem 1. Figure 16 is an excerpt from the Contractor Design Package for this tank. The shell course thicknesses for Courses 1 through 3 are slightly higher than the minimum values that were calculated in Sample Problem 1 because these are now the as-ordered plate thicknesses. Work Aid 2 is used to solve this problem.


43



Tank Size: Diameter - 100 ft.


Shell Courses: 5-courses, each 8 ft. high Maximum Design Liquid Storage Height: 38 ft. Liquid Specific Gravity: 0.85 Corrosion Allowance: 1/16 in. Shell Plate Material/Thickness: Course

Material

Thickness, in.

1

A516 Grade 70

7/16

2

A516 Grade 60

3/8

3

A516 Grade 60

5/16

4

A516 Grade 60

1/4

5

A516 Grade 60

1/4

Bottom Plate Material: Annular Plate:

A516 Grade 70


Figure 16. Sample Problem 3 Data


44



Solution

Z T = 0.0001D2H2

 V 2  100 

 85  2 Z T = 0.0001(100) (40)  100  2

Z T = 28. 9 in.3

Detail 3 of Drawing AD-036211 with a dimension "A" of 20 in. should be considered first, based on the 100 ft. diameter of this tank. The section modulus of Detail 3 must now be checked even though it is a standard Saudi Aramco design, because the shell corrosion allowance must be considered. Work Aid 2 contains the procedure for calculating the wind girder section modulus. The following summarizes the information that is needed for making the section modulus calculation for the selected wind girder: Corroded Top Shell Course Thickness, T s

0.1875 in.

Wind Girder Thickness, Tw

0.25 in.

Wind Girder Extension, A

20 in.

Wind Girder Height, Hw

6 in.

Wind Girder Lip, Lw

2.5 in.

The results of the intermediate calculations are summarized in Figure 17:


45



Section

Area, A

x

Ax

d

I

Ad2

1

1.172

0.094

0.110

11.116

0.003

144.82

2

5.000

10.188

50.94

1.022

166.667

5.22

3

1.500

20.063

30.094

-8.853

0.008

117.56

4

0.625

18.938

11.836

-7.728

0.326

37.33

Totals

∑A

∑(Ax)

∑I

∑(Ad2)

8.297

92.979

167.003

304.93

Figure 17. Sample Problem 3 Intermediate Calculations

Determine the location of the centroid of the combined area including the wind girder itself and the portion of the shell that acts with the girder.

x = ∑(Ax)/∑A x = 92.979/8.297 = 11.21 in.

Determine the moment of inertia of the combined area. Io = ∑(Ad2) + ∑I Io = 304.93 + 167.003 = 471.933 in. 4 Determine the maximum distance to the outermost fiber of the combined area as the greater of c 1 or c2.


46



c1 = (0.1875 + 20 - 11.21) = 8.978 in. c2 = x = 11.21 in. Therefore, c = 11.21 in. Determine the section modulus of the combined area. Z = Io/c Z = 471.993/11.21 = 42.1 in.3 Since the actual section modulus of the combined area exceeds the required area modulus (i.e., 42.1 in. 3 > 28.9 in.3), the wind girder detail that was initially selected is acceptable.

Intermediate Wind Girder Design Calculations Situations exist where just a top wind girder alone will not provide enough shell stiffness for a given combination of tank height, tank diameter, and tank shell course thicknesses. Put in simple terms, the distance between the top wind girder and the tank bottom is too large, in these situations, to resist wind-induced shell deformation. Installation of an intermediate wind girder at a location between the top wind girder and the tank bottom reduces the unstiffened length of the shell, and is required in order to prevent shell deformation in these cases. Intermediate wind girder design calculations in accordance with API-650 requirements consist of the following general steps: •

Determine if an intermediate wind girder is needed, based on design wind velocity, tank diameter, and shell course thicknesses.

•

Locate the intermediate wind girder.

•

Calculate the minimum required section modulus of the intermediate wind girder and select a standard structural shape that provides this section modulus.


47



The ideal location of the intermediate wind girder is such that the portions of the tank shell between the intermediate wind girder and the top wind girder, and between the intermediate wind girder and the bottom of the tank, have approximately the same stiffnesses. It would be incorrect, however, to locate the intermediate wind girder at the mid-height between the top wind girder and the tank bottom. As we've seen from the earlier shell thickness calculations, the tank shell thickness decreases in going from the bottom to the top course. Because the lower courses are thicker than the upper courses, the lower portion of the tank shell is inherently stiffer than the upper portion of the tank shell. Therefore, if the intermediate wind girder was located at the mid-height of the shell, the upper portion of the tank shell would not be stiffened enough. The API-650 procedure for locating the intermediate wind girder considers the variation in shell course thickness. As illustrated in Figure 18, the API-650 procedure mathematically converts the actual tank shell height to a "transformed shell" height. As detailed in Work Aid 2, the shell transformation is done by accounting for the actual individual course thicknesses. The transformed shell then has the same stiffness throughout its height. Locating the intermediate wind girder at the mid-height of the transformed shell results in equal shell stiffness both above and below the intermediate wind girder. The intermediate wind girder is then located on the actual tank shell in the same course and in the same relative position within that course as it is on the transformed shell. Using this approach, the intermediate wind girder is located much higher than the mid-height on the actual tank shell.


48



Figure 18. Transformed Shell and Intermediate Wind Girder

Work Aid 2 contains a procedure for determining the need for an intermediate wind girder, locating it, and sizing it, in accordance with Saudi Aramco and API-650 requirements. Sample Problem 4: Need, Location, and Size of Intermediate Wind Girder For the same tank as in Sample Problem 3, determine if an intermediate wind girder is needed. If an intermediate wind girder is needed, determine its required section modulus and locate it on the tank shell. Reference Figure 16 for the necessary tank data. Work Aid 2 is used to solve this problem.

 100t  3  100  2 H1 = 6(100t )  D   V 


49



H1 = 6[100(0.25 − 0.0625)]

3

 100 (0.25 − 0. 0625)  100  2    85  100

H1 = 12.6 ft.

The results of the transformed shell calculation are summarized in Figure 19.

Course

W

1

8

t  W tr = W  uniform   tactual 

8 2

3

8

8 4

8

5

8

 1/ 4 − 1/ 16  5  7 / 16 − 1/ 16  = 1.41

 1/ 4 − 1/ 16  5 8  3 / 8 − 1/ 16 

8

= 2.23

 1/ 4 − 1/ 16  5  5 / 16 − 1/ 16  = 3.90  1/ 4 − 1/ 16  5 8  1/ 4 − 1/ 16  = 8 8

8

5

 1/ 4 − 1/ 16  5  1/ 4 − 1/ 16  =8

W tr −total 23.54 > 12.6 ft.

Figure 19. Sample Problem 4 Transformed Shell

Since the height of the transformed shell is greater than H 1 (i.e., Wtr-total = 23.54 ft. > 12.6 ft.), an intermediate wind girder is required.


50



23. 54 = 11.77 ft. ≤ 12.6 ft. 2

Since (Wtr-total/2) ≤ H1, only one intermediate wind girder is required.

Locate Intermediate Wind Girder - The intermediate wind girder should be located approximately 11.77 ft. down from the top wind girder or up from the bottom of the tank on the transformed shell. The transposed widths of the top two courses are the same as their actual widths. Therefore, the intermediate wind girder should be located 11.77 ft. below the top wind girder because the widths of the top two courses are the same on both the transformed shell and the actual shell (i.e., 16 ft. total width for both courses together). Since the top wind girder is located at or near the top of the tank, the intermediate wind girder is located approximately 3.77 ft. below the top edge of the fourth course (i.e., 11.77 - 8 = 3.77 ft.). This location is more than 6 in. from the horizontal joint between the courses and is therefore acceptable. With the intermediate wind girder placed at 3.77 ft. below the top of the fourth shell course, the transposed height of the shell between the intermediate wind girder and the top wind girder is 11.77 ft., and the transposed height of the shell between the intermediate wind girder and the bottom of the tank is 11.77 ft. Therefore, no segment of the transposed shell exceeds H 1 (i.e., 12.6 ft.).

Size Intermediate Girder - The intermediate wind girder is sized by calculating its minimum required section modulus.

ZI = 0. 0001D 2H1

2  V   100 

ZI = 0. 0001(100 )

2

85  (11.77 ) 100 

2

= 8. 5 in.3 The corroded thickness of the fourth course (i.e., where the intermediate wind girder is located) is 3/16 in. Based on Table 3-22 of API-650, one angle with dimensions of 6 x 4 x 3/8 in. that conforms to Detail C of Figure 3-18 of API-650 is acceptable because its section modulus is 9.02 in.3 Selection of the required intermediate wind girder in this manner is illustrated in Figure 20, which is an excerpt from Figure 3-18 of API-650.


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Shell Thickness (inches) Member Size (inches)

3/16

1/4

5/16

3/8

7/16

One Angle: Figure 3-18, Detail c 2 1/2 x 2 1/2 x 1/4 2 1/2 x 2 1/2 x 5/16 4 x 3 x 1/4 4 x 3 x 5/16 5 x 3 x 5/16 5 x 3 1/2 x 5/16 5 x 3 1/2 x 3/8

1.68 1.98 3.50 4.14 5.53 6.13 7.02

1.79 2.13 3.73 4.45 5.96 6.60 7.61

1.87 2.23 3.89 4.66 6.25 6.92 8.03

1.93 2.32 4.00 4.82 6.47 7.16 8.33

2.00 2.40 4.10 4.95 6.64 7.35 8.58

6 x 4 x 3/8

9.02

10.56

11.15

11.59

11.93

Figure 20. Sample Problem 4 - Intermediate Wind Girder Selection

To summarize, a top wind girder that matches Detail 3 with a dimension "A" of 20 in. is required. An intermediate wind girder is also required. The intermediate wind girder must be located 11.77 ft. below the top wind girder, and it must have a section modulus of at least 8.5 in. 3 A 6 x 4 x 3/8 in. angle provides the necessary section modulus for the intermediate wind girder.


52



DETERMINING WHETHER CONTRACTOR-SPECIFIED DETAILS FOR opening Design ARE ACCEPTABLE The primary purposes that are served by openings into tank shells and roofs are as follows: •

Fill and empty the tank.

•

Provide access to the tank interior.

•

Permit tank cleanout.

•

Provide connections for items such as instrumentation, mixers, heaters, and water drawoffs.

Fill and discharge nozzles are located as low on the tank shell as possible, consistent with maintaining acceptable spacing between adjacent welds. Locating these nozzles as low as possible maximizes utilization of the total tank volume. Manholes are located in the tank roof and shell to provide access to the tank interior for inspection and maintenance when the tank is taken out of service. Accumulated dirt and sludge must be periodically cleaned out of the tank interior in order to maintain maximum possible storage volume, to prevent localized preferential corrosion that could occur under accumulated deposits, and to avoid possible floating roof damage that can be caused by uneven support when the roof is landed. Large-size cleanout fittings are typically installed in the tank shell and are designed to be flush with the tank bottom to facilitate cleanout. Nozzle connections that are required for instrument connections, heaters, and mixers must meet the same design requirements as are used for the filling and emptying nozzles. This section discusses Saudi Aramco and API-650 requirements for shell openings and covers the following topics: •

General Requirements

•

Reinforcement and Welding

•

Thermal Stress Relief

•

Manholes, Nozzles, and Flush-Type Cleanout Fittings


53



DETERMINING WHETHER CONTRACTOR-SPECIFIED DETAILS FOR TANK OPENING DESIGN ARE ACCEPTABLE, CONT'D Work Aid 3 provides an overall procedure to use to determine whether contractor-specified design details for openings meet Saudi Aramco and API requirements. The sections that follow elaborate on several of these requirements and briefly discuss several other points.

General Several general API-650 requirements that relate to openings are as follows: •

Attachments must be made by full-penetration welds except for insert-type reinforcement that meets API-650 details. Full-penetration welds develop the full strength through the thickness of the attachment and are thus better able to resist any loads that are applied by connected piping.

•

Connections and appurtenances that meet the requirements of API-620 are acceptable. The opening design procedure that is contained in API-620 is much more detailed than the procedure that is contained in API-650. Therefore, although acceptable, an API-620 nozzle design approach is only used in special cases for an API-650 tank.

•

Sheared or oxygen cut surfaces on manhole necks, nozzle necks, reinforcing plates, and shellplate openings must be uniform and smooth, with rounded corners, except when fully covered by attachment welds. These surface condition requirements and corner requirements minimize local stress concentrations that could act as crack-initiation points during operation.

Reinforcement and Welding API-650 requires that the cross-sectional area of the reinforcement at openings must equal or exceed the product of the vertical diameter of the opening that is cut in the shell, and the nominal plate thickness. However, API-650 simplifies the nozzle design process by specifying standard nozzle sizes and locations, together with associated reinforcing pad sizes and thicknesses, nozzle neck thicknesses, and attachment weld sizes. These specified nozzle details will satisfy the reinforcement requirements for most nozzle installations. Typically, design calculations are only required for cases where high loads are applied to a nozzle (such as from connected piping), an API-650 Appendix F tank (designed for a small internal pressure in addition to hydrostatic head), or if nonstandard nozzle design details are required.


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API-650 specifies minimum spacing requirements between welds that are made at connections, and other welds in the tank. These weld spacing requirements are specified to ensure that weld shrinkage stresses at one weld do not result in unacceptably high residual stresses at another weld. Excessive residual stresses could reduce weld reliability by making the weld more prone to the formation of cracks upon application of the design loads. The weld-spacing requirements are based on the size and type of weld, the thickness of the shell plate, and whether thermal stress relief has been done. Para. 3.7.3 in API-650 specifies weld-spacing requirements.

Thermal Stress Relief API-650 contains specific stress relief requirements that are not related to weld spacing requirements at openings. These postweld heat treatment (PWHT) requirements include minimum temperatures and hold times. API-650 requires PWHT in the cases that follow: • All flush-type shell connections and flush-type cleanout fittings. The entire nozzle assembly must be stress-relieved, and the stress relief must include the bottom reinforcing plate (or annular plate) and the flange-to-neck weld. This stress-relieved assembly is then welded into the shell and bottom. These flush-type connections and fittings are discussed further in the next section.

Flush-type nozzle assemblies have more reinforcement and welding than other nozzles that are of compatible size. These assemblies are also installed into the bottom-to-shell junction area of the tank. This area is critical for tank integrity and is already subject to a complicated combination of loads and stresses. Therefore, stress-relieving the assembly prior to welding it into the shell eliminates the introduction of more stresses into this area.

• When the shell material is in Material Group I, II, III, or IIIA, all openings of 305 mm (12 in.) or larger in diameter in shell plates of 25.4 mm (1 in.) or more in thickness must be prefabricated into the shell plate and the prefabricated assembly stress-relieved. Flange-to-neck welds and other nozzleneck and manway-neck attachments may be excluded if specified conditions are met.


55



•

When the shell material is Group IV, IVA, V, or VI, all openings that require reinforcement in shell plates more than 12.7 mm (1/2 in.) thick must be prefabricated into the shell plate and the prefabricated assembly stress-relieved. Flange-to-neck welds and other nozzle-neck and manway-neck attachments may be excluded if specified conditions are met.

Manholes, Nozzles, and Flush-Type Cleanout Fittings Standard-size shell and roof manholes are specified in API-650. Standard shell manholes range in size between 508 mm (20 in.) and 900 mm (36 in.) in diameter. Standard-size roof manholes are typically 508 mm (20 in.) and 610 mm (24 in.) in diameter. Shell and roof manholes are illustrated in Figures 3-4A and 3-13 respectively in API-650. API-650 specifies standard design details for three basic nozzle configurations: "regular-type," "low-type," and "flush-type." The low-type nozzle is located lower on the tank shell than the regular-type. Any reinforcement that is required for a low-type nozzle will typically extend to the tank bottom in order to avoid weld spacing problems with the bottom-to-shell junction weld, and to achieve a more uniform local stress distribution. The flush-type nozzle is another type of shell connection that is used for filling and emptying large diameter tanks. In the flush-type nozzle, the bottom of the nozzle is even, or flush, with the tank bottom. This configuration maximizes tank volume utilization. Shell nozzles are illustrated in Figures 3-5 and 3-11 in API-650. Figure 21 illustrates regular- and low-type nozzles, and Figure 22 illustrates a flush-type nozzle. In the flush-type cleanout fitting, the bottom of the nozzle is also even, or flush, with the tank bottom. This configuration simplifies personnel and equipment entry into the tank, and the removal of sludge and debris which have built up. A standard flush-type cleanout fitting is illustrated in Figure 3-9 in API-650.


56



Figure 21. Typical Regular- and Low-Type Nozzle Details


57



Figure 22. Typical Flush-Type Nozzle Details


58



32-SAMSS-005 requirements supplement those that are contained in API-650. Several of these requirements are highlighted as follows: •

Reinforcing pads must be welded to the shell for all pipe supports, gauges, and sample lines. The addition of reinforcement spreads the applied local load over more of the tank shell, and reduces the probability of excessive local stresses.

•

All attachments to the shell must be seal-welded all around. The continuous seal welding prevents water or dirt from getting between the pad and shell and causing accelerated local corrosion.

•

Reinforcing pads and all other external pads must have rounded corners of at least 50 mm (2 in.) radius. Rounded corners reduce the local stress concentration effects that could result in local crack initiation.

•

Nozzle reinforcing pads and pads that cross shell seams must have a 6.35 mm (1/4 in.) tapped hole. This hole is used for a compressed air-soapsuds test of the fillet welds attaching the pad to the shell.

Sample Problem 5: Opening Evaluation The tank that was used in Sample Problems 1 and 3 requires a flush-type cleanout fitting. Design information for this tank is contained in Figure 16 that was used for Sample Problem 3. The cleanout fitting size was specified to be 24 in. high by 24 in. wide. The contractor's proposal for this cleanout fitting specifies that the opening will be cut in a standard bottom course plate, and will be reinforced with a reinforcing plate. The cleanout fitting is located away from all shellseam welds. The reinforcing plate that was proposed by the contractor is illustrated in Figure 23. The contractor also specified that the completed assembly will be given PWHT in accordance with API-650. Determine if the contractor's proposal for this cleanout fitting is acceptable. If the proposal is not acceptable, state why.


59



Figure 23. Reinforcing Plate for Sample Problem 5


60



Solution Work Aid 3 is used to evaluate this proposal The design is deficient in the following areas: •

The width at the base of the reinforcing plate must be 72 in. instead of 60 in. This width is based on Table 3-11 of API-650. This width would widen the flat length at the top of the plate to 14 in.

•

The thickness of the shell plate that contains the opening must be 1/2 in. instead of the standard shell thickness for the bottom course of 7/16 in. This thickness is based on Table 313 of API-650.

•

The thickness of the reinforcing plate must be 1/2 in. instead of the 1/4 in. that was specified by the contractor. This thickness is based on Table 3-13 of API-650.


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DETERMINING WHETHER CONTRACTOR-SPECIFIED Design details For Tank Roofs ARE ACCEPTABLE The subjects that are discussed in this section are as follows: •

Design considerations for tank roofs based on roof type

•

Saudi Aramco and API design requirements

•

Sizing inlet diffusers

API-650 provides rules and guidelines to achieve roof designs that are within specified allowable stress and load limits. API-650 also provides minimum requirements for the fabrication of tank roofs and their connection to the tank shell. The various types of tank roofs that will be discussed were illustrated in MEX 203.01. API-650 classifies roofs by their shape, operation, and support as follows: •

Cone roof

-

Supported

-

Self-supporting

•

Self-supporting dome roof

•

Internal floating roof

•

External floating roof

Cone Roofs Supported Cone Roof A supported cone roof gets most of its support either from rafters that are positioned on top of girders and columns, or from rafters that are positioned on top of trusses (either with or without columns). Rafters are oriented radially from the tank shell toward the center, and girders are oriented as circumferential chords around the tank. Columns support the roof structural members from the tank floor. The number and size of the rafters, girders, and columns are based on the tank size and external loading. Figure 24 illustrates the relationship among the rafters, girders, and columns.


62



Figure 24. Rafters, Girders, and Columns in Cone Roof Tank


63



Some special design considerations for supported cone roofs are as follows: •

Lateral bracing may be required for rafters or other roof support members if the support members are not adequately restrained by the roof or by the inherent stiffness of their design.

•

Roof plates are welded to each other and to the tank top angle.

•

Roof plates are not attached to the rafters in order not to detract from the design intent of the frangible joint between the roof and shell. The frangible joint is discussed later in this module.

•

When pipe is used for support columns, the pipe is sealed or is designed such that it may be drained and vented. This design measure minimizes the likelihood that corrosion will occur inside the pipe columns.

•

Any required roof corrosion allowance must be added to the minimum nominal roof plate thickness. As previously noted, a corrosion allowance is normally not specified for the roof plate, but a corrosion allowance may be specified for roof support members.

Requirements are also specified for the slope of the roof, top angle size, and rafter spacing. These requirements are provided in Work Aid 4.

Self-Supporting Cone Roof A self-supporting cone roof is formed to the approximate shape of a right circular cone, and is only supported at its periphery by the tank shell. Requirements are specified for the slope of the roof, roof thickness, and the minimum area of the roof-to-shell junction region. These requirements are provided in Work Aid 4.

Self-Supporting Dome Roof A self-supporting dome roof is formed approximately to a spherical shape, and is only supported at its periphery by the tank shell . API-650 specifies requirements for the roof radius of curvature, roof thickness, and the minimum area of the roof-to-shell junction region. These requirements are provided in Work Aid 4.


64



Internal Floating Roof Internal floating roofs are often installed inside existing fixed roof tanks for specific product storage applications in order to minimize vapor losses and/or oxygen entry (e.g., MTBE service). An internal floating roof has some of the same design considerations as an external floating roof. However, since an internal floating roof is shielded by a fixed roof, an internal floating roof does not have to resist the same external environmental conditions (e.g., rainfall). Since internal floating roofs are not exposed to rainfall, they do not require drain systems and do not have to be designed for the weight of rainwater. Consequently, the internal floating roof may be made from materials that would be unsatisfactory for an external floating roof, such as aluminum, stainless steel, or plastic. Although an internal floating roof that is made from these materials may be less rigid, it will still provide the required reliability. Also, internal floating roofs do not have to be designed for the 122 kg/m 2 (25 lb./ft.2) roof live load requirement of an external roof. However, the internal floating roof must be able to support a reasonable personnel and equipment load without damaging the roof or causing product leakage onto the roof. In addition, the roof supports and attachments must be designed for a uniform live load of 61 kg/m 2 (12.5 lb./ft.2) unless the roof has drains or other means to automatically prevent liquid accumulation. API-650 Appendix H contains design requirements for internal floating roofs. These requirements focus on the following general areas: •

Material specifications for the roof itself and the peripheral seal

•

Flotation requirements

•

Design for the joints between the roof components

•

Minimum thickness for roof components based on the material that is used

•

Peripheral seals

•

Roof penetrations and supports

•

Fabrication, erection, welding, inspection, and testing

External Floating Roofs External floating roofs may be either single-deck type or double-deck type and present special design challenges. External floating roofs must be free to rise and lower as the liquid level inside the tank changes, must also resist the same environmental conditions as closed-top tanks, and must provide a reasonably tight seal at the junction between the roof and shell. Some of the primary design considerations for external floating roofs are as follows:


65



•

The roof must float properly under all design load conditions. The design load conditions must consider the density of the stored liquid, the roof weight, and the external loads that are applied to the roof. Rainwater accumulation and its associated weight is a major external load that must be considered in the roof design.

•

The roof must not bind as it rises and lowers. Binding could cause the roof to tilt. This tilted condition would allow stored liquid to get on top of the roof and cause an unbalanced load. An unbalanced load could damage or sink the roof.

•

The liquid cannot push the roof off the top of the tank. There must be enough tank shell height (freeboard) to permit the liquid level to rise to its maximum design level while keeping the roof entirely contained within the shell.

•

The design must permit the tank to be safely emptied and filled completely.

•

Tank appurtenances cannot be damaged as the roof rises and lowers through its entire movement range. There can be no internal projections that would hamper roof movement or damage the peripheral seal.

•

The roof drain system must work properly and not be damaged as the roof rises and lowers through its entire movement range. This system must also prevent stored liquid from getting on top of the roof should the drain hose or pipe become damaged. A check valve is installed in the drain line and is located inside the roof sump in order to prevent the stored liquid from getting on top of the roof in case the drain line fails.

Single-Deck Floating Roof Single-deck floating roofs are equipped with pontoons, as discussed in MEX 203.01. The pontoons float on the stored liquid and support the floating roof deck plate. The most common design uses a continuous pontoon structure at the roof periphery, with the roof deck plate welded to the pontoon structure. Other designs use both the peripheral pontoon and a central pontoon. The pontoon system is designed as a compartmented structure. With the compartmented-structure design, a local leak at a cracked weld or at a punctured plate will not result in complete flooding of the pontoon, and the roof will still maintain some buoyancy.


66



Double-Deck Floating Roof Double-deck floating roofs have both a top and bottom steel deck, as discussed in MEX 203.01. Thus, the entire roof is really a pontoon structure. The area between the two decks is divided into compartments. The compartment system is designed so that some of the compartments can fail and the roof will still maintain some buoyancy. The double-deck type floating roof will typically be used for larger diameter tanks over (90 m [300 ft.]) in order to provide sufficient buoyancy and stability. Special Considerations for External Floating Roofs Appendix C of API-650 contains specific design requirements that apply only to external floating roofs. These design requirements focus primarily on roof flotation and structural details. The material that follows presents a general discussion of these areas. Further details are provided in Work Aid 4. 32-SAMSS-005 also contains requirements that supplement Appendix C of API-650.

Flotation - A floating roof must float on the liquid that is contained within the tank. The lift or buoyancy that is provided by the liquid is affected by the liquid's density (specific gravity). The buoyancy that is provided by a liquid increases as its density increases. The tank could store a variety of liquids over its design life. Therefore, the roof is designed to provide flotation with a reasonably light liquid (Specific Gravity = 0.7) to ensure that the roof will float on top of any liquid that the tank is likely to store. The roof can suffer a minor mishap that would damage it and allow liquid to enter its pontoon compartments. The roof is designed so that a failure of any two adjacent pontoon compartments will not sink the roof. The roof is also designed so that any penetrations through the roof will not permit the stored liquid to flow on top of the roof. For example, the support leg sleeves must be long enough so that liquid will not exit through them when the deck sags under the load due to accumulated rainfall. External floating roofs are particularly susceptible to damage due to the accumulation of rainwater. If too much water accumulates on the roof, the roof can be damaged and sink under the weight of the water. External floating roofs are equipped with sumps and drains to collect rainwater and remove it safely from the roof. The drain system is designed to operate automatically with the roof at any level. Care is taken in the design of the drain system to ensure that the drain system is not damaged as the roof rises and lowers while the tank is being filled or emptied. The drain system is designed to drain the roof under the heaviest anticipated rainfalls for the tank's location.


67



The roof must be designed to float with a reasonable amount of rainwater accumulation on it. This provision allows for a possible temporary failure of the drainage system, or a reasonable delay in the ability of personnel to drain the accumulated rainwater off of the roof. The usual design allowance for a pontoon-type roof is for a rainfall of 254 mm (10 in.) in a 24 hour period. Double-deck type roofs may be designed for a lesser water volume, if emergency drains are installed to ensure that the rainwater level is kept to a lower design value.

Structural Requirements - Floating roof tanks are equipped with rolling ladders that provide access from a platform at the top of the shell to the top of the roof. These ladders must adjust automatically to changes in roof level and remain functional as the tank fills and empties. Floating roofs are equipped with support legs that can be adjusted to two positions from the top of the roof. The low-leg position will hold the roof at a safe height above the bottom when the tank is empty to permit the entry of personnel. The high-leg position permits the roof to travel lower during operation to maximize tank volume use, and to keep the roof from traveling below the filling and emptying nozzles, manways, and other appurtenances that are located below the tank roof. Special attention is given to the points where the legs attach to the roof to ensure that the stresses induced in the roof when the legs are supporting the roof are properly distributed, to avoid damaging the roof. Adequate protection must also be provided for the tank bottom to prevent it from being damaged by the legs with a full weight load. The tank bottom is typically protected by steel pads that are welded to the bottom at the locations where the legs will rest when the roof is landed. The tank and roof system is provided with a mechanism to keep the roof centered and prevented from rotation. If a roof were to move off-center, it could bind on the tank shell while the tank is being filled and emptied. This binding could damage the tank shell, roof, or roof seal, and cause the roof to sink. If a roof were to rotate, it could damage the roof access ladder, fire-fighting systems, roof seal, and other appurtenances. Also, when the roof lands, the roof support legs would not be aligned with their corresponding bottom reinforcing pads, and the legs could damage the bottom.

Saudi Aramco and API Design Requirements 32-SAMSS-005 supplements the roof design requirements that are contained in API-650, especially for external floating roofs. Work Aid 4 contains procedures that may be used to determine if contractor-specified roof requirements are acceptable. Figure 25 summarizes the locations within 32-SAMSS-005 and API-650 where information that is related to tank roof design may be found. Standard 32-SAMSS-005

Topic Seal welding of self-supported roofs

Location Para. 3.10.2

Gauging requirements Para. 3.10.2 Top-angle attachment for self-supporting roofs

Para. 3.10.7

External Floating Roofs

Para. C.3


68



API-650

General requirements

Para. 3.10.2

Frangible joint

Para. 3.10.2.5.1

Allowable stresses

Para. 3.10.3

Supported cone roofs

Para. 3.10.4

Self-supporting cone roofs

Para. 3.10.5

Self-supporting dome and umbrella roofs

Para. 3.10.6

Top-angle attachment for self-supporting roofs

Para. 3.10.7

External floating roofs

Appendix C

Internal floating roofs

Appendix H

Figure 25. Roof Design Requirements


69



32-SAMSS-005 Requirements Roof requirements that are contained in 32-SAMSS-005 relate to the topics that are listed in Figure 25. Refer to the referenced paragraphs in Figure 25 and Work Aid 4 for specific details.

API-650 Requirements General - API-650 roof requirements relate to the topics that are listed in Figure 25. Refer to the referenced paragraphs in Figure 25 and Work Aid 4 for specific details.

Frangible Roof-to-Shell Joint - A fixed roof atmospheric storage tank may experience a higher than expected internal pressure during operation or emergency conditions. This high pressure might occur in spite of all design and operational precautions. If this pressure is high enough, a tank failure could occur. If the failure occurs at a shell seam that is below the liquid level, a significant and possibly complete loss of the tank contents could take place. API-650 requires that one of the following two design precautions be taken: •

Tank emergency venting is provided and designed in accordance with API-2000, Venting Atmospheric and Low-Pressure Storage Tanks. The emergency vents are designed to relieve the excess pressure, and the tank is then adequate for any internal pressure that can occur.

•

A "frangible joint" is provided at the roof-to-shell intersection.

Figure 26 shows typical attachment details between a cone roof and the top angle on the shell. When a high internal pressure occurs, the pressure will tend to lift the roof up, which in turn pulls the top portion of the shell inward. The roof-to-shell intersection can be designed to be the "weak link" in the tank structure and fail before any other shell joint or the shell-to-bottom joint. This preferential failure is achieved by using a relatively small size fillet weld to attach the roof to the top shell angle, and by limiting the overall strength of the roof-to-shell junction region so that it cannot resist the inward buckling load that is caused by a high internal pressure. The roof-to-shell junction is called a "frangible joint" when it is designed as the weak link in the tank structure.


70



Figure 26. Cone Roof Attachment to Top Angle


71



When the frangible joint fails, a local region of the roof-to-shell junction will buckle inward and a portion of the roof-to-shell weld will open. This additional opening provides more venting capacity. Thus, the excess pressure is vented and failure of a lower tank seam is prevented. Since the failure occurs above the liquid level, the stored liquid will not be released in this situation. The damaged portion of the roof and shell will require repair, but this repair will typically not be a major job and a significant tank spill is prevented. Work Aid 4 contains the required evaluation procedure for a frangible joint.

Sample Problem 6:

Frangible Joint for an Atmospheric Storage Tank

You are reviewing the design of an atmospheric fixed cone roof storage tank. The contractor has said that the roof-to-shell junction is a frangible joint and no emergency venting capacity is required. Is he correct? The roof-to-shell junction details are shown in Figure 27. Additional tank design information is as follows: Shell and framing weight: Corrosion allowance:


332 777 kg 1.5 mm

72



Figure 27. Sample Problem 6 - Frangible Joint Details


73



Solution Work Aid 4 is used to solve this problem. It can be immediately stated that the junction is not a frangible joint because the roof attachment fillet weld exceeds a 4.76 mm size. Therefore the fillet weld size must be reduced to 4.76 mm. However, the other frangible joint criteria must also be checked to see if any additional changes are needed. The roof slope cannot exceed 1:6, or tan θ ≤ 0.167. Based on Figure 28, the roof slope is 8°. tan 8° = 0.141 Therefore the roof slope is acceptable.

A≤

217 W 217 × 332 777 = 30 800 tanθ 30 800 tan8°

A ≤ 16 682 mm2 to be frangible

Rc = 48 800 × 0.5 = 24 400 mm R2 = Rc / sin θ = 24 400 / sin8° = 175 321mm wh = 0.3 R2t h = 0. 3 175 321 × 6. 35 = 316. 5 mm However wh can only be a maximum of 305 mm w c = 0. 6 Rc t c = 0.6 24 400 × 8 = 265.1mm A roof = th wh = 6. 35 × 305 A shell = t cwc = 8 × 265.1

= 1 937 = 2 121

A angle = 9.5[75 + (75 − 9.5)] = 1 335 Available area

= 5 393 mm2

Since the available area is less than the maximum permitted area, the junction meets all the requirements for a frangible joint, except for the fillet weld size. The fillet weld must be reduced to 4.76 mm size in order for this to be a frangible joint.


74



Sizing Inlet Diffusers Liquid that is being pumped into a tank can damage or sink a floating roof (external or internal), and possibly force liquid onto the roof if the liquid flow velocity is too high. The possibility of such damage is increased if water slugs are also present in the inlet flow. The damage is caused when the high velocity liquid stream hits the relatively thin roof plates and structural members. Therefore, it is common to install a diffuser on the inlet pipe inside of the tank. Such an inlet diffuser reduces the velocity of the liquid that enters the tank and directs the inlet flow streams away from the tank roof. Therefore, the diffusers prevent damage and possible liquid "bubble over" onto the floating roof. Figure 28 illustrates a typical diffuser installation for a horizontal inlet pipe. Diffusers are designed using pipe that is the same size as the inlet nozzle, and that has slots cut in its bottom quadrant. These slots are sized so that the velocity of the liquid entering the tank is approximately 1 m/sec (3 ft./sec). The diffuser is positioned on a radial line to the center of the tank, and it is supported from the tank bottom.


75



Figure 28. Inlet Diffuser


76



DETERMINING WHETHER CONTRACTOR-SPECIFIED Design DETAILS FOR TANK BOTTOMS ARE ACCEPTABLE This section discusses Saudi Aramco and API requirements for tank bottoms. It covers the following topics: •

Minimum thickness

•

Cone up or down

•

Annular ring

•

Water withdrawal

•

Saudi Aramco and API design requirements

For the tanks that are discussed in this course, the bottoms are continuously supported by the ring wall and foundation pad. Therefore, stress in the bottom plates themselves is not a factor during initial tank design. Stress is a design consideration for bottom annular rings.

Minimum Thickness Work Aid 5 provides the minimum thickness and other requirements for storage tank bottoms. Since stress is not a major factor in the design of a storage tank bottom, minimum thickness requirements are primarily to ensure that there is adequate allowance for bottom corrosion. The minimum thickness requirements for annular rings (discussed below) are based on both stress and corrosion considerations.

Cone Up or Down Tank bottoms may be designed as either cone up or cone down. A cone-up configuration slopes up from the tank periphery to the center of the tank. A cone-down configuration slopes down from the periphery to the center of the tank. For flat-bottomed tanks, SAES-D-100 requires that a coned-down bottom with a 1:120 radial slope be used for tanks that are in services where water draw-off is required, unless otherwise stated in the Tank Data Sheet. Otherwise, 32-SAMSS-005 requires a cone-up bottom with a 1:120 radial slope. A coneup bottom permits less foundation grading and provides for future foundation settlement, which will be greater in the center of the tank.

Annular Ring The outer ring of bottom plates on which the tank shell rests is called an annular ring when the bottom plates are joined by butt-welded construction. Otherwise, this region of the bottom is called a sketch plate if normal lap-welded bottom construction is used in this region. Figure 29 illustrates lap-welded bottom plates under the tank shell. Work Aid 5 provides the minimum requirements for use of an annular ring.


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Figure 29. Lap-Welded Bottom Plate Under Shell

A butt-welded annular ring is typically required when the stress in the bottom shell course is relatively high, which is indicative of a high bending moment in the bottom-to-shell junction region. The butt-welded annular ring provides a more reliable construction to resist a high bending moment than lap-welded sketch plates. API650 also specifies the minimum required width and thickness of the annular ring. As the loads in this region increase, the minimum values for both of these dimensions also increase in order to keep the local stresses in the bottom-to-shell junction region within acceptable limits. SAES-D-100 requires that the predicted tank settlement be reviewed by the Consulting Services Department to determine if additional requirements for tank bottom design or soil improvement are needed. High predicted tank settlement could be another reason to use an annular ring, or to use an annular ring that is wider or thicker than API-650 would require. 32-SAMSS-005 requires that a 12 mm (1/2 in.) thick, asphalt-impregnated board be installed on top of the foundation ring wall under the annular ring, as illustrated in Figure 30. This board helps to minimize the likelihood of corrosion on the underside of the annular ring. Corrosion of the annular ring would reduce its effectiveness. The board also helps accommodate foundation imperfections, uneven weld surfaces, and backing strips.


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Figure 30. Asphalt-Impregnated Board Under Annular Ring


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Water Withdrawal Water that may be contained in the liquid being stored tends to settle out to the bottom of the tank. The slope of the tank bottom causes the water to flow to one general location in the tank, where it collects in a sump and can be periodically withdrawn. Work Aid 5 contains sump and water drawoff pipe requirements for atmospheric storage tanks.

Saudi Aramco and API Design Requirements Figure 31 summarizes the primary topics that are related to tank bottom design, and the locations within 32SAMSS-005 and API-650 where information that is related to these topics may be found.

Standard 32-SAMSS-005

API-650

Topic

Location

Bottom plates

Para. 3.4

Annular bottom plates

Para. 3.5

Bottom plates

Para. 3.4

Annular bottom plates

Para. 3.5

Figure 31. Bottom Design Requirements


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Work Aid 1:

PROCEDURE FOR DETERMINING WHETHE R CONTRACTORSPECIFIED shell thicknesses ARE CORRECT

This Work Aid is to be used with the copy of API-650 that is in Course Handout 1 and with the copy of 32SAMSS-005 that is in Course Handout 2. Work Aid 1A is used when the One-Foot Method is used to determine the required tank shell thicknesses. Work Aid 1B is used when the Variable-Design-Point Method is used to determine the required tank shell thicknesses. Shell course thickness information that is contained in the Contractor Design Package must be verified using the appropriate procedure.

Work Aid 1A:

Procedures (One-Foot Method) and Additional Information for Calculating the Required Shell Thickness for Atmospheric Storage Tanks

Note: This method is not to be used for tanks that are over 61 m (200 ft.) in diameter. Refer to Work Aid 1B for larger diameter tanks. This procedure must be modified for application to API-650 Appendix F (small internal pressure) and Appendix M (elevated temperature) storage tanks. Appendix F and Appendix M tank types are not discussed in this course.

1.

Using the following formula, calculate the required shell thickness for the design case: SI Units td = Where:

English Units

4. 9D(H − 0. 3)G + CA Sd

td =

2. 6D(H − 1)G + CA Sd

td = Design shell thickness, mm (in.) D = Nominal tank diameter, m (ft.) H = Design liquid level, m (ft.). The design liquid level is the height from the bottom of the course under consideration to the top of the shell, including the top angle, if any, or to the bottom of any overflow that limits the tank filling height, or to any other level specified by the Purchaser, restricted by an internal floating roof, or controlled to allow for seismic wave action.


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G

Plate Specification

=

Design specific gravity of the liquid to be stored

Sd =

Allowable stress for the design case, MPa (psi). To be obtained from Table 32 of API-650 for the specified course shell plate material. An excerpt from Table 3-2 is provided in Figure 34.

CA =

Corrosion allowance, mm (in.). To be obtained from the Tank Data Sheet or from other provided information.

Grade

A 283 A 285 A 131 A 36 A 131

C C A, B, CS EH 36

A 442 A 442 A 573 A 573 A 573

55 60 58 65 70

A 516 A 516 A 516 A 516

55 60 65 70

Minimum Yield Minimum Tensile Strength Strength ASTM Specifications 55,000 30,000 55,000 30,000 58,000 34,000 58,000 36,000 71,000 a 51,000 30,000 32,000 32,000 35,000 42,000

55,000 60,000 58,000 65,000 70,000 a

30,000 32,000 35,000 38,000

55,000 60,000 65,000 70,000

Product Design Stress Sd

Hydrostatic Test Stress St

20,000 20,000 22,700 23,200 28,400

22,500 22,500 24,900 24,900 30,400

20,000 21,300 21,300 23,300 28,000

22,500 24,000 24,000 26,300 30,000

20,000 21,300 23,300 25,300

22,500 24,000 26,300 28,500

Figure 34. API-650 Table 3-2 (Excerpt)


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2.

Using the following formula, calculate the required shell thickness for the hydrostatic test case: SI Units tt =

English Units 4. 9D(H − 0. 3) St

Where:

3.

tt =

2. 6D(H− 1) St

tt

=

Hydrostatic shell thickness, mm (in.)

St

=

Allowable stress for the hydrostatic test case, MPa (psi). Obtain from Table 3-2 of API-650 for the specified course plate material.

D and H

=

As defined in Step 1.

The required minimum shell thickness for the course is the greatest of the following: •

The design shell thickness calculated in Step 1.

•

The hydrostatic test shell thickness calculated in Step 2.

•

The minimum shell thickness value based on the tank diameter given in Figure 35. NOMINAL TANK DIAMETER

MINIMUM PLATE THICKNESS

m

ft.

mm

in.

<15.2

<50

6.35*

1/4*

15.2 to <36.6

50 to <120

6.35

1/4

36.6 to 61

120 to 200

7.94

5/16

>61

>200

9.53

3/8

* Minimum nominal thickness of 6.35 mm (1/4 in.) required by 32-SAMSS-005.

Figure 35. Minimum Tank Shell Plate Thickness

4.

Repeat Steps 1 through 3 for each shell course.

5.

Verify that no shell course is thicker than the one beneath it.


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Work Aid 1B:

Procedure (Variable-Design-Point Method) and Additional Information for Calculating the Required Shell Thickness for Atmospheric Storage Tanks

Note: This method may only be used for tanks that satisfy the following equation:

SI Units

English Units

0.268 Dt ≤2 H

Where:

6Dt ≤2 H

D

=

Tank diameter, m (ft.)

t

=

Bottom course shell thickness, mm (in.)

H

=

Maximum design liquid level, m (ft.)

Note: Since the One-Foot Method of Work Aid 1A cannot be used for tanks that are larger than 60 m (200 ft.) in diameter, the variable-design-point method is the most common design approach that is used for large diameter tanks. A detailed elastic analysis must be used for tanks that do not satisfy the above equation or for banded tanks. Note: Figure 36 provides an overall flow chart of the variable-design-point method.


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Figure 36. Variable-Design-Point Method Flow Chart


85



1.

Use Work Aid 1A to calculate a preliminary thickness for the bottom shell course for the design condition, tpd, and for the hydrostatic test condition, tpt, using the One-Foot Method.

Note: Throughout the rest of this procedure, calculations for both the design case and hydrostatic test case are carried out independently. Do the calculations for either the design case or for the hydrostatic test case for all shell courses. Then do the calculations for the other case for all shell courses. In addition, all subsequent equations are based only on English units of measure because several contain constants that are based on English units.

Calculation of Bottom Shell Course Thickness

2.

Use the following formula to calculate the bottom course thickness for the design case:  0. 463D HG   2. 6HDG   t1d =  1.06 − + CA H  Sd   Sd 

Where:

t1d = Minimum thickness of the bottom shell course for the design condition, in. D

= Tank diameter, ft.

H

= Design liquid level, ft.

G

= Design specific gravity of the liquid to be stored.

Sd

= Allowable stress for the design condition, psi. Obtain from Table 3-2 of API-650 (Reference Figure 34) for the specified shell course plate material.

CA = Corrosion allowance, in. information.


Obtain from Tank Data Sheet or other provided

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3.

Use the following formula to calculate the bottom course thickness for the hydrostatic test case:  0. 463D t1t =  1. 06 − H  Where:

  2 .6HD      St 

t1t

=

St=

Allowable stress for the hydrostatic test case, psi. Obtain from Table 3-2 of API650 (Reference Figure 34) for the specified course plate material.

H and D =

4.

H St

Minimum thickness of the bottom shell course for the hydrostatic test case, in.


Identify the greater of the shell thicknesses determined in Step 2 or Step 3. The required bottom shell course thickness is the lesser of this value (i.e., the larger of Step 2 or Step 3) or the shell thickness that was determined by the One-Foot Method in Step 1. Calculation of the Second Shell Course Thickness

5.

Calculate the following ratio. h1 rt1 Where:

h1

=

Height of the bottom shell course, in.

r

=

Nominal tank radius, in.

t1

=

For the design case, actual bottom shell course thickness minus any corrosion allowance, in.

=

For the hydrostatic test case, actual bottom shell course thickness, in.


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6.

The procedure that is used to determine the required thickness for the second shell course depends on the value of the ratio that was calculated in Step 5. Based on the calculated ratio, select the appropriate choice from the selections that follow: •

If Ratio ≤ 1.375: Required second shell course thickness is the same as the thickness required for the first shell course. Proceed with the third shell course thickness calculation at Step 7.

•

If Ratio ≥ 2.625: Required thickness of the second shell course is calculated using the same procedure as for the third and higher shell courses. Proceed with the second shell course calculation at Step 7.

•

1.375 < Ratio < 2.625: Required second shell course thickness is calculated using the equation that follows:

  h1 + CA t 2 = t 2a + (t1 − t2 a ) 2.1− 1.25 rt1  

Where:

t2 =

Minimum required thickness of the second shell course for the case being considered, in.

t2a =

Minimum thickness for the second shell course calculated using the method that applies to higher shell courses, in. See Step 7 to calculate this value. Then return here to complete the calculation of the second shell course thickness. For the design case, subtract the corrosion allowance first before entering values into this equation.

t1, h1 and r

= As previously defined.


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Calculation of Third and Higher Shell Course Thicknesses 7.

Use Work Aid 1A to calculate a preliminary thickness for the case being considered (i.e., design or test) using the One-Foot Method.

8.

Use the following formulas to calculate the three possible values (i.e., x1, x2, and x3) for the point of maximum stress. The smallest of x1, x2, or x3 is the value for x that is used in Step 9:   t t L  L − 1   t t  C= u u  1.5  t    1+  L     tu  x1 = 0.61 rtu + 3. 84CH x 2 = 12CH x 3 = 1. 22 rtu

Where: C

=

Constant used in the following expressions.

tL

=

Thickness of the course immediately below the course being considered, in. For the design case, subtract the corrosion allowance first.

tu

=

Thickness of the shell course being considered at the girth joint, in. Use the value that was calculated in Step 7 as the initial value for t u. For the design case, subtract the corrosion allowance first.

x1, x2, x3

=

Three possible points of maximum stress in the shell course being considered, measured as the distance from the bottom of the shell course, in.

r and H

=

As previously defined.


89



9.

Calculate the required shell course thickness for either the design or hydrostatic test cases (depending on which case is being considered) using the equations that follow: •

Design case calculation:

 x  G 2. 6D H −  12  t dx = + CA Sd Where:

tdx = Minimum thickness for the shell course for the design case, in. x

= Smallest of the three values for the possible point of maximum stress as calculated in Step 8.

D, H, G, Sd and CA = As previously defined. •

Hydrostatic test case calculation:

t tx

Where:

ttx

 x 2.6D H −   12  = St

= Minimum required thickness for the shell course for the hydrostatic test case, in.

D, H, x, and St = As previously defined. 10. Use the value that was calculated for tx in Step 9 as the value for tu. Repeat the procedure from Step 8 to this Step until there is little difference between successive calculated values of tx. Two additional times through the calculations are usually sufficient.


90



Work Aid 1B:

Procedure (Variable-Design-Point Method) and Additional Information for Calculating the Required Shell Thickness for Atmospheric Storage Tanks, cont'd

11. Once the thicknesses for each shell course for one condition (design or hydrostatic test) have been obtained, repeat Steps 2 through 11 for the other condition. When the thicknesses for each shell course for both conditions have been obtained, proceed to Step 12. Conclusion

12. For each shell course, the minimum required thickness is the greater of the minimum required thickness for the design condition, or the minimum required thickness for the hydrostatic test condition, and in no case less than the thickness specified in Figure 35. 13. Verify that no shell course is thicker than the shell course beneath it.


91



Work Aid 2:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED wind girder REQUIREMENTS for open-top API-650 tanks ARE CORRECT

This Work Aid is to be used with the copy of API-650 that is in Course Handout 1, and Saudi Aramco Drawing AD-036211 that is in Course Handout 3. All needed tank design information should be obtained from the Contractor Design Package.

Top Wind Girder Evaluation 1.

Use the following formula to calculate the minimum required section modulus of the top wind girder. SI Units ZT

English Units

 V 2 = 0.0578 D H2  161 

Where:

2

 V  Z T = 0.0001D H2  100 

2

2

ZT = Required minimum top wind girder section modulus, cm 3 (in.3) D= Nominal tank diameter, m (ft.) H2 = Height of tank shell, including any freeboard that is provided above the maximum filling height, m (ft.) V

2.

= Design wind speed, mph Typically, this is 137 km/h (85 MPH) for Saudi Aramco applications.

Determine the appropriate standard top wind girder design details and dimensions to be used for the size of tank from Figure 37 and Figure 38. Figure 38 is an excerpt from Saudi Aramco Drawing AD-036211 that is in Course Handout 3 for reference. Use the class reference copy of drawing AD-036211 for additional information that is not shown in Figure 38.


92



Figure 38. Saudi Aramco Drawing AD-036211 (Excerpt) 3.

If the contractor has not used a standard Saudi Aramco wind girder, refer to Figure 3-18 of API-650 to determine which API-650 standard design was used. Then refer to Table 3-22 of API-650 to determine the section modulus. An excerpt from Table 3-22 is contained in Figure 39. Note that the corroded shell thickness is to be used to determine the section modulus from this table.


95



Shell Thickness (inches) Member Size (inches)

3/16

1/4

5/16

3/8

7/16

-------------

-------------

1.93 2.32 4.00 4.82 6.47 7.16 8.33 11.59

2.00 2.40 4.10 4.95 6.64 7.35 8.58 11.93

25.61 32.36 39.53 47.10 55.07 63.43 72.18 81.30 90.79 100.65 110.88 121.47 132.42 143.73 155.40 167.42

26.34 33.33 40.78 48.67 56.99 65.73 74.89 84.45 94.41 104.77 115.52 126.66 138.17 150.07 162.34 174.99

Curb Angle: Figure 3-18, Detail b 2 1/2 x 2 1/2 x 1/4 2 1/2 x 2 1/2 x 5/16 3 x 3 x 1/4 3 x 3 x 3/8 4 x 4 x 1/4 4 x 4 x 3/8

1.61 1.89 2.32 2.78 3.64 4.17

1.72 2.04 2.48 3.35 4.41 5.82

-------------

One Angle: Figure 3-18, Detail c 2 1/2 x 2 1/2 x 1/4 2 1/2 x 2 1/2 x 5/16 4 x 3 x 1/4 4 x 3 x 5/16 5 x 3 x 5/16 5 x 3 1/2 x 5/16 5 x 3 1/2 x 3/8 6 x 4 x 3/8

1.68 1.98 3.50 4.14 5.53 6.13 7.02 9.02

1.79 2.13 3.73 4.45 5.96 6.60 7.61 10.56

1.87 2.23 3.89 4.66 6.25 6.92 8.03 11.15

Formed Plate: Figure 3-18, Detail e b = 10 b = 12 b = 14 b = 16 b = 18 b = 20 b = 22 b = 24 b = 26 b = 28 b = 30 b = 32 b = 34 b = 36 b = 38 b = 40

---------------------------------

23.29 29.27 35.49 42.06 48.97 56.21 63.80 71.72 79.99 88.58 97.52 106.78 116.39 126.33 136.60 147.21

24.63 31.07 37.88 45.07 52.62 60.52 68.78 77.39 86.35 95.66 105.31 115.30 125.64 136.32 147.35 158.71

Figure 39. Section Moduli for Several Standard Wind Girders From API-650


96



4.

Verify that the section modulus for the selected standard API-650 wind girder is equal to or greater than the required modulus that was calculated in Step 1. If it is not, a stronger standard design should be used, or a "special" wind girder should be designed. This latter approach should rarely be required.

5.

In some situations, it may be necessary to verify or calculate the section modulus of the wind girder that is actually used. This necessity may occur if a nonstandard design is used, or to verify that a standard design is acceptable after shell corrosion allowance is considered. The following procedure and equations are provided as an example of how the section modulus is calculated for a wind girder geometry that looks like Detail 3 of Saudi Aramco Drawing AD-036211. A similar approach is used for other wind girder geometries. Refer to Figure 40 for the wind girder reference geometry.


97



Figure 40. Wind Girder Reference Geometry

Note •

Numbers indicate the separate areas to be combined to obtain section modulus.

•

Effective length of shell included in calculation.


98



Where: Ts

=Nominal top shell course thickness minus corrosion allowance, cm (in.)

Tw =Wind girder thickness, cm (in.) A

=Wind girder width, cm (in.)

Hw =Wind girder height, cm (in.) Lw =Wind girder lip, cm (in.)

a.

A2

b.

Calculate the area of each section: A1

=

(32Ts + Tw) Ts

=

ATw

A3

=

HwTw

A4

=

LwTw

Calculate the distance from the tank inside diameter to the centroid of each individual area. x1

=

0.5 Ts

x2

=

(Ts + 0.5 A)

x3

=

(Ts + A - 0.5 Tw)

x4

=

(Ts + A - 0.5 Lw)


99



c.

Use the results of Steps 5a and 5b and Figure 41 to locate the centroid of the combined area, x , cm (in.).


100



Section

A

x

(Step 5a)

(Step 5b)

Ax

1 2 3 4 ∑A

x=

∑Ax

∑ Ax ∑A Figure 41. Centroid of Combined Area

d.

For each section, calculate the distance between the centroid of the combined area and the centroid of the individual section, d, cm (in.).

e.

Calculate the moment of inertia of each area, I, about its centroid, cm4 (in.4).


101



I1 =

(32 T s + T w )T 3s 12

TwA 3 12 H w T 3w I3 = 12 T L3 I4 = w w 12 I2 =


102



f.

Use the results of Steps 5a, 5d, and 5e and Figure 42 to calculate the moment of inertia of the combined areas, Io, cm4 (in.4).

Section

A

d

(Step 5a)

(Step 5d)

Ad2

I (Step 5e)

1 2 3 4 ∑Ad2

∑I

Io = ∑ Ad2 + ∑ I Figure 42. Moment of Inertia of Combined Area

g.

Determine the distance from the centroid of the combined area to each edge of the combined area, c3 and c2, cm (in.). c1 = T s + A − x c2 = x

h.

Determine the maximum of either c, or c2 determined in Step 5g, c, cm (in.).


103



i.

Calculate the section modulus of the combined area, Z, cm3 (in.3). Z=

Io c


104



Intermediate Wind Girder Evaluation 6.

Use the following formula to calculate the maximum height of the unstiffened shell: SI Units

English Units

 12t  3  161 2 H1 = 72t  D   V  Where:

H1

=

Vertical distance between the intermediate wind girder and the top angle of the shell or the top wind girder of an open-top tank, m (ft.)

t

=

Thickness of the top shell course excluding corrosion allowance, cm (in.)

=


D and V

7.

H1 = 6(100 t )

 100 t  3  100  2  D   V 

Use the following formula to calculate the transposed width of each shell course:

W tr = Wcourse Where:

 tuniform     t actual 

5

Wtr

=

Transposed width of each shell course, m (ft.)

Wcourse

=

Actual width of each shell course, m (ft.)

tuniform

=

Design thickness of the top shell course, excluding corrosion allowance, cm (in.)

tactual

=

Design thickness of the shell course for which the transposed width is being calculated, excluding corrosion allowance, cm (in.)

Add up the transposed widths of all the shell courses that were obtained from these calculations to determine the height of the transformed shell, Wtr-total. The conceptual relationship between the actual shell height and the transformed shell height is illustrated in Figure 43. The table in Figure 44 may be used to summarize the results of the transformed shell calculations. Saudi Aramco DeskTop Standards

105



Figure 43. Actual Shell and Transformed Shell


106



Course

W

t  W tr = W  uniform   t actual 

5

1 2 3 4 5 6 7 8 W tr −total =

∑ Wtr

Figure 44. Transformed Shell Calculation Summary

8.

If the height of the transformed shell is less than or equal to H1 as calculated in Step 6, no intermediate wind girder is required. If the height of the transformed shell is greater


107



than H1, an intermediate wind girder is required. If (Wtr-total/2) >H1, a second intermediate wind girder is needed. Note that H1 need only consider the distance between the intermediate wind girder and the top wind girder (not the top of the shell). Therefore, the top wind girder should be located first before concluding that a second intermediate wind girder is required. Only one intermediate wind girder is usually required.


108



9.

If an intermediate wind girder is required, it should be placed at the mid-height of the transformed shell. The location of the wind girder on the actual shell should be at the same course and same relative position as on the transformed shell. See Figure 42. This location is determined using the thickness relationship that was used in Step 7. The intermediate wind girder must not be placed within 150 mm (6 in.) of a horizontal joint of the shell. When the preliminary location of an intermediate wind girder is within this distance from a horizontal joint, the girder shall preferably be located 150 mm (6 in.) below the joint.

10. Verify that no segment of the transformed shell between any wind girder and another wind girder or the bottom of the tank exceeds H1. 11. Calculate the minimum required section modulus of the intermediate wind girder using the following formula: SI Units

English Units

 V 2   0 . 0577 D H ZI = 1  161  2

Where:

 V   Z I = 0 . 0001 D H 1   100  2

2

ZI

= Required minimum intermediate wind girder section modulus, cm 3 (in.3)

H1

= Vertical distance between the intermediate wind girder and the top angle of the shell or the top wind girder of an open-top tank, m (ft.)

D and V =As previously defined.

12. A standard structural steel shape may be used for the intermediate wind girder. Refer to Table 3-22 of API-650 (excerpted in Figure 39) to select a shape that has a section Saudi Aramco DeskTop Standards

109



modulus that is at least as large as that calculated in Step 11. In using Table 3-22, use the corroded thickness of the shell course at which the intermediate wind girder is located.


110



Note that the use of this table will typically be conservative. API-650 permits including a portion of the tank shell for a distance of 1.47 Dt above and below the attachment of the intermediate wind girder to the shell in determining the available section modulus. Table 3-22 uses a smaller distance along the tank shell. Section modulus calculations may be made for the geometry of the intermediate wind girder that is actually used. These calculations may be done in situations where a nonstandard intermediate wind girder is used, if the corroded shell course thickness differs from values that are shown in Table 3-22, or if the total permissible portion of the tank shell is considered. The general calculation approach that is described in Step 5 may be used to make this section modulus calculation.


111



Work Aid 3:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED DETAILS FOR Opening design ARE ACCEPTABLE

This Work Aid is to be used with the copies of SAES-D-100 and 32-SAMSS-005 that are in Course Handout 2, and with the copy of API-650 that is in Course Handout 1. All needed tank design information should be obtained from the Contractor Design Package.

1.

If the opening is not attached to the storage tank by full penetration welding, reject the design. Failure to meet this requirement is not expected to be a problem for new tank construction. The only exception to the requirement is that partial penetration welds may be used for insert-type reinforcement that conforms to API-650 details.

2.

If the opening is for a 50 mm (2 in.) or smaller standard weight coupling, no additional reinforcement is required.

3.

Determine which of the following categories matches the opening being reviewed and proceed accordingly. •

Shell manhole, proceed to Step 4

•

Shell nozzle and flange, proceed to Step 5

•

Flush-type cleanout fitting, proceed to Step 6

•

Flush-type shell connection, proceed to Step 7

•

Roof manholes, proceed to Step 21

•

Roof nozzles, proceed to Step 22

Except for flush-type openings and connections, the reinforcement must be located within a distance above and below the centerline of the opening equal to the vertical dimension of the opening in the tank shell plate. The reinforcement can be provided by any one or any combination of the following:


112



•

Attachment flange of the fitting

•

Excess plate thickness beyond that required by Work Aid 1

•

Portion of the fitting neck that may be considered as reinforcement

•

Reinforcing plate

Detailed evaluation of nozzle reinforcement for only hydrostatic loads will normally not be required as long as the nozzle design details and dimensions conform to a standard API-650 design. Shell Manholes 4.

Verify that the manhole conforms to API-650, Figures 3-4A and 3-4B, and the dimensional requirements contained in Tables 3-3 through 3-7 for the specified manhole size. If no problems are detected, the design is acceptable. Shell Nozzle and Flange

5.

Verify that the nozzle and flange conform to API-650, Figure 3-5 and the dimensional requirements contained in Tables 3-8 through 3-10 for the specified nozzle size. If no problems are detected the design is acceptable. Excerpts from these portions of API-650 are contained in Figure 45 through Figure 48.


113



Figure 45. Shell Nozzles


114



Column 1 Size of Nozzle

Column 2 Outside Diameter of Pipe

48 46 44 42 40

48 46 44 42 40

c c c c c

48 1/8 46 1/8 44 1/8 42 1/8 40 1/8

24 22 20

24 22 20

0.50 0.50 0.50

24 1/8 22 1/8 20 1/8

18 16 14 12 10

18 16 14 12 3/4 10 3/4

0.50 0.50 0.50 0.50 0.50

8 6 4 3 2f 1 1/2f

8 5/8 6 5/8 4 1/2 3 1/2 2 1/8 1.90

0.50 0.432 0.337 0.300 0.218 0.200

3g 2f

1 1/2f 1f 3/4f

4.00 2.875 2.200 1.576 1.313

Column 3 Column 4 Diameter of Nominal Hole in Thickness of Flanged Reinforcing Plate Nozzle Pipe Walla DR tn

Coupling Coupling Coupling Coupling Coupling

Column 5 Length of Side of Reinforcing Plateb or Diameter L=Do

Column 6 Width of Reinforcing Plate W

Column 7c Minimum Distance from Shell to Flange Face J

Column 8c

Column 9c

Minimum Distance from Bottom of Tank to Center of Nozzle Regular Low Type Typed HN C

Flanged Fittings 96 3/4 117 92 3/4 112 88 3/4 107 1/4 84 3/4 102 1/2 80 3/4 97 3/4

16 16 15 15 15

52 50 48 46 44

48 3/8 46 3/8 44 3/8 42 3/8 40 3/8

49 1/2 45 1/2 41 1/2

60 55 1/4 50 1/2

12 11 11

28 26 24

24 3/4 22 3/4 20 3/4

18 1/8 16 1/8 14 1/8 12 7/8 10 7/8

37 1/2 33 1/2 29 1/2 27 23

45 1/4 40 1/4 36 33 28 1/4

10 10 10 9 9

22 20 18 17 15

18 3/4 16 3/4 14 3/4 13 1/2 11 1/2

8 1/4 6 3/4 4 5/8 3 5/8 2 1/2 2

19 15 1/4 12 10 1/2 __ __

23 1/4 19 1/2 15 1/4 13 1/2 __ __

8 8 7 7 6 6

13 11 9 8 7 6

9 1/2 7 7/8 6 5 1/4 3 1/2 3

Threaded Fittings 11 1/4 14 1/4 __ __ __ __ __ __ __ __

__ __ __ __ __

9 7 6 5 4

5 3/8 3 3 3 3

4 1/8 3 2 1/8 1 11/16 1 7/16

Figure 46. Dimensions for Shell Nozzles


115



Column 1 Thickness of Shell and Reinforcing Platea t and T

Column 2 Minimum Pipe Wall Thickness of Flanged Nozzlesb tn

3/16 1/4 5/16 3/8 7/16 1/2

1/2 1/2 1/2 1/2 1/2 1/2

Column 3 Maximum Diameter of Hole in Shell plate (Dp) Equals outside Diameter of Pipe Plus 5/8 5/8 5/8 5/8 5/8 5/8

Column 4 Size of Fillet Weld B

Column 5 Column 6 Size of Fillet Weld A Nozzles 2-, 1 1/2-, 1-, Larger Than and 1/4 inch 2 Inches Nozzles

3/16 1/4 5/16 3/8 7/16 1/2

1/4 1/4 1/4 1/4 1/4 1/4

1/4 1/4 1/4 1/4 1/4 5/16

9/16 5/8 11/16 3/4

1/2 1/2 1/2 1/2

3/4 3/4 3/4 3/4

9/16 5/8 11/16 3/4

1/4 5/16 5/16 5/16

5/16 5/16 5/16 5/16

13/16 7/8 15/16 1

1/2 1/2 1/2 1/2

3/4 3/4 3/4 3/4

13/16 7/8 15/16 1

3/8 3/8 3/8 7/16

5/16 5/16 5/16 5/16

Figure 47. Dimensions for Shell Nozzles, Pipe, Plate, and Welding Schedules


116



Figure 48. Dimensions for Shell Nozzle Flanges


117



Flush-Type Cleanout Fittings 6.

Verify that the flush-type cleanout fitting and its support conform to API-650, Figures 39 and 3-10. Verify that the dimensions conform to Tables 3-11 through 3-13. Verify that PWHT in accordance with API-650 paragraph 3.7.4 will be provided on the fitting assembly. If no problems are detected, the design is acceptable. Flush-Type Shell Connections

7.

Verify that calculations are provided to confirm that the design prohibits shell uplift at the cylindrical-shell-to-flat-bottom junction.

8.

Verify that calculations are provided to confirm that the vertical membrane stress in the cylindrical shell at the top of the opening for the connection does not exceed one-tenth of the circumferential design stress in the lowest shell course that contains the opening.

9.

Verify that the maximum width of the flush-type connection does not exceed 914 mm (36 in.) and that the maximum height of the opening does not exceed 305 mm (12 in.).

10. Verify that the thickness of the bottom transition plate in the assembly is at least 13 mm (0.5 in.). 11. Verify that the shell plate that contains the flush-type shell connection is given PWHT in accordance with paragraph 3.7.8.3 of API-650.


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12. Verify that the flush-type shell connection conforms to API-650 Figure 3-11 and the dimensional requirements that are contained in Table 3-14 for the specified opening size. 13. The cross-sectional area of the reinforcement over the top of the connection must satisfy the following equation: Ar ≥ Where:

Ar

K 1ht 2

= Cross-sectional area of reinforcement, mm 2 (in.2)

K1 = Area coefficient based on tank diameter and height, and opening height, from API-650 Figure 3-8 h

= Maximum height of the opening, mm (in.)

t

= Thickness of the shell plate that contains the flush-type connection, mm (in.)

14. Verify that the shell plate that contains the flush-type connection is the proper thickness. Except for 203 mm (8 in.) by 203 mm (8 in.) flush-type connection openings, the shell plate that contains the flush-type connection must be at least 1.6 mm (1/16 in.) and not more than 3.2 mm (1/8 in.) thicker than the adjacent plates in the bottom shell course. For a 203 mm (8 in.) by 203 mm (8 in.) flush-type connection opening, the shell plate that contains the flush-type connection may be the same thickness as the shell plates in the remainder of the bottom shell course. 15. Verify that the reinforcing plate is the same thickness as the shell plate that contains the flush-type connection. 16. Verify that the following equation is satisfied.


119



L ≤ 1. 5h Where:

L

= Height of the reinforcing plate above the bottom of the opening, mm (in.)

h

= Height of the opening, mm (in.)

If L must exceed this limit in order for [(L – h) ≥ 150 mm (6 in.)], only the portion of reinforcement that is within 1.5h can be considered effective. 17. Verify that the width of the tank bottom reinforcing plate at the opening centerline is at least 254 mm (10 in.) plus the combined thickness of the shell plate in the connection assembly and the shell reinforcing plate. 18. Verify that the thickness of the bottom reinforcing plate conforms to the following: SI Units tb =

English Units

h2 + 0. 00584b H 355 600

Where:

tb =

h2 b + H 14 000 310

tb

= Minimum thickness of the bottom reinforcing plate, mm (in.)

h

= Maximum height of the opening, mm (in.)

b

= Maximum width of the opening, mm (in.)

H

= Maximum design liquid level, m (ft.)

In no case shall the thickness of the bottom reinforcing plate be less than: •

16 mm (5/8 in.) for H = 14.6 m (48 ft.)

•

17.5 mm (11/16 in.) for H = 17 m (56 ft.)

•

19.1 mm (3/4 in.) for H = 19.5 m (64 ft.)

19. Confirm that the minimum thickness of the nozzle neck and transition piece, tn, is 16 mm (5/8 in.). Externally applied loads may require this area to be thicker.


120



20. If no problems are detected in Steps 7 through 20, the flush-type connection design is acceptable. Roof Manholes 21. Verify that the roof manhole conforms to API-650 Figure 3-13 and the dimensional requirements contained in Table 3-15 for the specified manhole size. If no problems are detected, the design is acceptable. Roof Nozzles 22. Verify that flanged roof nozzles conform to API-650 Figure 3-14, and the dimensional requirements contained in Table 3-16 for the specified nozzle size. Verify that threaded roof nozzles conform to API-650 Figure 3-15, and the dimensional requirements contained in Table 3-17 for the specified nozzle size. If no problems are detected, the design is acceptable.


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Work Aid 4:

PROCEDURE FOR DETERMINING WHETHER CONTRACTORSPECIFIED DESIGN DETAILS FOR TANK ROOFS ARE ACCEPTABLE

This Work Aid is to be used with the class reference copies of SAES-D-100 and 32-SAMSS-005 that are in Course Handout 2, and the copy of API-650 that is in Course Handout 1. All needed tank design information should be obtained from the Contractor Design Package.

General

1.

Verify that the roof plates have a minimum nominal thickness of 4.76 mm (3/16 in.). Any corrosion allowance should be added to this nominal thickness for supported roofs.

2.

Verify that the roof and its supporting structure are designed for the dead load plus a uniform live load of 122 kg/m2 (25 lb./ft.2) times the projected area.

3.

Verify that all internal and external structural members have a minimum nominal thickness of 4.3 mm (0.17 in.) plus corrosion allowance.

4.

If the roof is a fixed roof and is specified to have a frangible joint, proceed to Step 5. If the roof is not specified to be frangible and is a fixed roof, proceed to Step 9. If the roof is an external floating roof, proceed to Step 28. If the roof is an internal floating roof, proceed to Step 46. Frangible Fixed Roof

5.

Verify that the following formula is satisfied: SI Units A≤

217W 30 800 tanθ


English Units A≤

0.153W 30 800 tanθ

122




123



Where: =

Area resisting the compressive force, mm 2 (in.2). All members in the region of the roof-to-shell junction, including insulation rings, are considered to contribute to the cross-sectional area. Refer to API-650 Figure F-1 for determination of the contributing areas of the roof, shell, and top angle. An excerpt from Figure F-1 is contained in Figure 49.

=

Aroof + Ashell + Aangle

=

Roof contributing area, mm2 (in.2)

=

thwh

wh

=

t Smaller of 0.3 R2 h or 305 mm (12 in.)

Ashell

=

Shell contributing area, mm2 (in.2)

=

tcwc

wc

=

0.6 Rc tc

Aangle

=

Cross sectional area of top angle, mm 2 (in.2)

R2

=

Length of normal to the roof, mm (in.)

=

Rc / sinθ

Rc

=

Inside radius of tank shell, mm (in.)

th

=

Nominal thickness of roof plate, mm (in.)

tc

=

Nominal thickness of top shell course, mm (in.)

W

=

Total weight of the shell and any framing, but not the roof plates, that are supported by the shell and roof, kg (lb.)

θ

=

Angle between the roof and a horizontal plane at the roof-to-shell junction, degrees

tan θ

=

Slope of the roof, expressed as a decimal quantity

A

Aroof

If the formula is not satisfied, the joint is not frangible.


124



Figure 49. Permissible Compression Ring Details


125



6.

Verify that the shell-to-roof compression ring details conform to one of the details in API-650 Figure F-1, details a through d (See Figure 49). If the details do not conform, the joint is not frangible.

7.

Verify that the continuous fillet weld between the roof plates and the top angle does not exceed 4.76 mm (3/16 in.). If the fillet weld exceeds 4.76 mm (3/16 in.), the joint is not frangible.

8.

Verify that the slope of the roof at the top angle does not exceed 1:6. If the slope of the roof exceeds 1:6, the joint is not frangible. General Fixed Roof Tanks

9.

For a fixed roof that is not frangible, verify that proper protection against excessive pressure in accordance with API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks, is provided. If the protection is inadequate, the roof is unsatisfactory.

10. If lateral loads for the roof support columns have been specified, verify that the columns are designed to satisfy the following formula. If the formula is not satisfied, the roof support columns are unsatisfactory. fby f fa + bx + ≤1 F bx F by Fa Where:

fa

= Computed axial stress, kPa (psi)

Fa

= Allowable axial stress, if the axial force alone existed, kPa (psi)

fb

= Computed compressive bending stress at the point under consideration, kPa (psi)

Fb

= Allowable compressive bending stress, if the compressive bending moment alone existed, kPa (psi)


126



x and y =

Axis of bending about which the stress applies, kPa (psi)

11. Verify that the seams of the roof plates are welded on the top side with continuous fullfillet welds. If the seam welds are not continuous full-fillet welds on the top side, the roof is unsatisfactory. 12. Verify that the roof plates are attached to the top angle of the tank with a continuous fillet weld on the top side only. The fillet weld size shall be 4.76 mm (3/16 in.) unless a smaller size is specified. If the roof plates are not attached in the prescribed manner, the roof is unsatisfactory. 13. For a fixed roof tank with a supported cone roof, proceed to Step 14. For a fixed roof tank with a self-supported cone roof, proceed to Step 18. For a self-supporting dome roof proceed to Step 23. Supported Cone Roof Tank 14. Verify that the slope of the roof is at least 1:16. If the slope is less than 1:16, the roof is unsatisfactory. 15. Verify that the tank shell is equipped with a top angle that is at least the size given in Figure 50. If the top angle is too small, the roof is unsatisfactory. TANK DIAMETER, D

MINIMUM TOP ANGLE SIZE

m

ft.

mm

in.

D≤10.7

D≤35

50.8 x 50.8 x 4.76

2 x 2 x 3/16

10.7
35
50.8 x 50.8 x 6.35

2 x 2 x 1/4

D>18.3

D>60

76.2 x 76.2 x 9.53

3 x 3 x 3/8

Figure 50. Minimum Top Angle Size Saudi Aramco DeskTop Standards

127



16. Verify that the rafters that are located in the outer ring are spaced at no more than 1.91 m (6.28 ft.) center-to-center, and that the rafters in the inner rings are not more than 1.68 m (5-1/2 ft.) center-to-center. If the rafters are spaced too far apart, the roof is unsatisfactory. 17. If a supported cone roof design has been satisfactory in all steps to this point, the roof is satisfactory for the purposes of this Work Aid. Self-Supporting Cone Roof 18. Verify that the top-angle sections are joined by butt welds that have complete penetration and fusion. If the sections are not joined in the prescribed manner, the roof is unsatisfactory. 19. Verify that the slope of the roof is at least 2:12 and no more than 9:12. If the slope of the roof is outside the allowable range, the roof is unsatisfactory. 20. Verify that the roof thickness is at least the thickness that is calculated by the following formula, but does not exceed 12.7 mm (1/2 in.). If the thickness is outside the specified range, the roof is unsatisfactory. SI Units t roof =

Where:

English Units

0. 208D ≥ 4.76 mm sin θ

troof =

t roof =

D ≥ 3 / 16 in. 400 sin θ

Thickness of the roof, mm (in.)


128



D

=

Nominal diameter of the tank shell, m (ft.)

θ

=

Angle of cone elements to the horizontal, degrees

21. Determine the participating area at the roof-to-shell junction, based on API-650 Figure F-1, and the nominal material thicknesses minus the corrosion allowance. See Step 5 for calculation of the participating area. Note that the corroded roof and shell plate thicknesses are used for this calculation. However, the nominal thicknesses are used for the calculations done in Step 5. The amount of participating area must be at least the area calculated by the following formula. If the participating area is less than the amount specified, the roof is unsatisfactory. SI Units

English Units

2.314D2 Ap ≥ sinθ Where:

Ap ≥

D2 3 000 sinθ

Ap = Participating area at the roof-to-shell junction, mm 2 (in.2) D

= Nominal diameter of tank shell, m (ft.)

θ

= Angle of cone elements to the horizontal, degrees

22. If a self-supporting cone roof has been satisfactory in all steps to this point, the roof is satisfactory for the purposes of this Work Aid. Self-Supporting Dome Roof

23. Verify that the top angle sections are joined by butt welds that have complete penetration and fusion. If the sections are not joined in the prescribed manner, the roof is unsatisfactory.


129



24. Verify that the radius of curvature of the roof satisfies the following formula. If the radius of curvature does not satisfy the following formula, the roof is unsatisfactory. 0. 8D ≤ rr ≤ 1. 2D Where:

D

= Nominal tank diameter, m (ft.)

rr

= Roof radius, m (ft.)

25. Verify that the thickness is at least the thickness calculated by the following formula, but does not exceed 12.7 mm (1/2 in.). If the thickness is outside the specified range, the roof is unsatisfactory. SI Units

English Units t roof ≥

t roof ≥ 0. 4166r r ≥ 4.76 mm Where:

troof =

Thickness of roof, in.

rr

Radius of roof, ft.

=

rr ≥ 3 / 16 in. 200

26. Determine the participating area at the roof-to-shell junction, based on API-650 Figure F-1, and the nominal material thicknesses minus any corrosion allowance. See Step 5 for calculation of the participating area. Note that the corroded roof and shell plate thicknesses are used for this calculation. However, the nominal thicknesses are used for the calculations done in Step 5. The amount of participating area must be at least the area that is calculated by the following formula. If the participating area is less than the amount specified, the roof is unsatisfactory. SI Units


English Units

130



A p ≥ 4.627Drr

Ap ≥

Drr 1500

Ap =Participating area of the roof-to-shell junction, mm 2 (in.2)

Where:

D

=Nominal tank shell diameter, m (ft.)

rr

=Roof radius, m (ft.)

27. If a self-supporting dome roof has been satisfactory in all steps to this point, the roof is satisfactory for the purposes of this Work Aid. External Floating Roof

28. Verify that the top of the roof has a minimum slope of 1:64. If the slope is less than 1:64, the roof is unsatisfactory. 29. Verify that the deck of a pontoon type roof will be in contact with the liquid during normal operation. 30. Verify that the deck plates are joined by continuous full-fillet welds on the top side. Confirm that full-fillet welds that are at least 50 mm (2 in.) long on 250 mm (10 in.) centers are used on the underside of any lap plates that are located within 300 mm (12 in.) of stiffening girders or support legs. 31. Verify that vendor calculations confirm the roof has sufficient buoyancy to remain afloat when the tank contains a liquid with a specific gravity of 0.7 under the following conditions: •

250 mm (10 in.) of rainfall in a 24 hour period, with the roof intact. However, double-deck roofs with emergency drains that are designed to prevent the stored liquid from flowing onto the roof may be designed for a lesser rainwater volume as determined by the drain sizing.

•

In single-deck pontoon roofs, single-deck and any two adjacent pontoon compartments punctured with no water or live load.


131



•

In double-deck pontoon roofs, any two adjacent compartments punctured with no water or live load.

If sufficient buoyancy does not exist, the roof is unsatisfactory.

32. Verify that vendor calculations confirm that the pontoons of single deck roofs will not be permanently distorted under the loads that are specified in Step 31. Also confirm that the roof sag will not permit any stored liquid to get on the roof through any roof penetrations (e.g., leg sleeves or vents). 33. Verify that each pontoon compartment has a liquid-tight manway. The top edge of the manway necks must be at an elevation that prevents liquid entry into the compartment under the conditions specified in Step 31. If the manways are not provided or are inadequate, the roof is unsatisfactory. 34. Verify that each pontoon compartment has vents that protect against internal or external pressure. The vents must be at a level that prevents liquid entry into the compartment under the conditions specified in Step 31. If the vents are not provided or are inadequate, the roof is unsatisfactory. 35. Verify that a ladder has been provided that automatically adjusts to any roof position. If the ladder is a rolling ladder, verify that it has full-length handrails on both sides and has been designed for a 454 kg (1 000 lb.) mid-point load with the ladder in any operating position. If a ladder is not provided or is inadequate, the roof is unsatisfactory.


132



36. Verify that roof drains are adequate to prevent backflow of stored liquid in case of drain pipe leakage, and to prevent the roof from accumulating a water level greater than design at the maximum rainfall rate when the roof is floating at the minimum operating level. Verify that a check valve is installed in the drain line at the roof sump to prevent backflow. The drain size must be at least 76.2 mm (3 in.) for roofs that are no larger than 36.6 m (120 ft.) in diameter, and at least 101.6 mm (4 in.) for roofs that are larger than 36.6 m (120 ft.) in diameter. If the drains are not provided or are undersized, the roof is unsatisfactory. 37. Verify that adequate automatic bleeder vents and rim vents are provided to prevent overstressing of the roof deck or seal membrane. Vents that are capable of evacuating air and gases from underneath the roof during initial filling must be provided. If the vents are not provided or are inadequate, the roof is unsatisfactory. 38. Verify that roof support legs are provided. The support legs must be capable of supporting the roof and a uniform live load of 122 kg/m2 (25 lb./ft.2). The length of the legs must be adjustable from the top of the roof. The low-leg position must provide at least 2 m (6.5 ft.) clearance between the lowest portion of the roof and the tank bottom. All tank appurtenances must be cleared by the roof in its lowest position. If the support legs are not provided or are inadequate, the roof is unsatisfactory. 39. Verify that at least two roof manholes with a minimum inside diameter of at least 610 mm (24 in.) have been provided. Verify that all roof manholes that were specified in the


133



purchase specification are provided. If the manholes are not provided or are inadequate, the roof is unsatisfactory. 40. Verify that an adequate sealing device is provided between the periphery of the roof and the tank shell. If the device is not provided or is inadequate, the roof is unsatisfactory. 41. Verify that all bulkheads and rims inside the pontoon or double deck roofs are continuously welded along their upper edges to the top deck plates. 42. Verify that inlet pipes have been equipped with diffusers, if specified in the purchase specification. 43. Verify that roof centering and anti-rotation devices have been provided.


134



44. Verify that the additional requirements that are contained in Appendix C of 32-SAMSS-005 are satisfied. 45. If an external floating roof has been satisfactory in all steps to this point, the roof is satisfactory for the purposes of this Work Aid. Internal Floating Roof 46.

Verify that the design complies with API-650, Appendix H.


135



Work Aid 5:

PROCEDURE FOR determining whether contractor-specified Design details for Tank Bottoms are acceptable

This Work Aid is to be used with the copies of SAES-D-100 and 32-SAMSS-005 in Course Handout 2, and with the copy of API-650 in Course Handout 1. All needed tank design information should be obtained from the Contractor Design Package.

1.

Verify that the tank bottom has a minimum nominal thickness of 6.35 mm (1/4 in.) plus any specified corrosion allowance. In general, any specified tank corrosion allowance must be specifically stated as also applying to the bottom to be considered. A tank bottom corrosion allowance is normally not used.

2.

Verify that the tank bottom plates will project at least 25 mm (1 in.) beyond the outside edge of the weld that attaches the bottom to the shell.

3.

If the bottom shell course of the tank is designed based on the allowable stress for materials in Group IV, IVA, V, or VI, proceed with Step 4. If the bottom shell course of the tank is designed using other materials, proceed with Step 11.

4.

5.

Verify that butt-welded annular rings are used based on the criteria that follows: •

Butt-welded annular rings must be used if the material allowable stress governs the bottom shell course thickness. However, the use of butt-welded annular bottom plates are always acceptable.

•

Lap-welded annular bottom plates (i.e., sketch plates) are acceptable if the product stress for the first shell course does not exceed 160 MPa (23 200 psi) and the maximum hydrostatic test stress for the first shell course does not exceed 172 MPa (24 900 psi).

Use the following formula to calculate the hydrostatic test stress in the first shell course: SI Units Sh =

4.9D(H− 0.3) t


English Units Sh =

2.6D (H − 1) t

136



Where:

6.

Sh

= Hydrostatic test stress in the first shell course, MPa (psi)

D

= Nominal diameter of the tank, m (ft.)

H

= Design liquid height, m (ft.)

t

= Nominal thickness of the bottom shell course as constructed, mm (in.)

Verify that butt-welded annular rings have at least the thickness that is specified in Figure 51.

t, in. t Š 0.75 0.75 < t Š 1.00 1.00 < t Š 1.25 1.25 < t Š 1.50 1.50 < t Š 1.75

Š27 000 1/4 1/4 1/4 5/16 11/32

Sh, psi Š 30 000 Š 33 000 1/4 9/32 9/32 3/8 11/32 15/32 7/16 9/16 1/2 5/8

Figure 51. Minimum Permissible Annular Ring Thickness

7.

Verify that the butt-welded ring has a radial width that provides at least 600 mm (24 in.) between the inside of the shell and any lap-welded joint in the remainder of the bottom and at least a 50 mm (2 in.) projection outside the shell.

8.

Verify that the butt-welded annular ring has a radial width that is at least that required by the following formula: SI Units r=

English Units 215 t b HG


r=

390 tb HG

137

Š 36 0 11/3 7/16 9/16 11/1



Where:

r

= Radial width of the annular bottom plate, mm (in.)

tb

= Thickness of the annular bottom plate, mm (in.)

H

= Maximum design liquid level, m (ft.)

G

= Design specific gravity of the liquid to be stored

If this formula results in a radial width that is less than the requirement of Step 7, Step 7 governs the radial width. 9.

Verify that the ring of annular bottom plates has a circular outside circumference, and either a circular or regular polygonal shape with the number of sides equal to the number of annular plates inside the tank.

10. Verify that the bottom of the tank is a cone up (unless the Tank Data Sheet specifies a cone down, in which case it must be a cone down) with a slope of at least 1:120. 11. Verify that the bottom-to-shell junction conforms to Para. 3.1.5.7 of API-650 (reference Figure 52).


138



Figure 52. Bottom-to-Shell Junction Details


139



12. If water drawoff is specified, verify that the draw-off sump and outlet pipe comply with the requirements of Figures 53 through 55 (taken from Saudi Aramco Drawing AD36500) and the dimensional requirements that are contained in Table 3-18 of API-650. TANK DIAMETER

WATER DRAW-OFF PIPE SIZE

m

ft.

mm

in.

3 to 6

10 to 20

50

2

6 to 27

20 to 90

100

4

27 to 48

90 to 160

150

6

Figure 53. Required Water Draw-Off Pipe Size

13. Verify that the top of the foundation ring under the annular plate is provided with a 13 mm (1/2 in.) thick asphalt-impregnated board. 14. Verify that a foundation drip plate has been provided in accordance with Saudi Aramco Standard Drawing AB-036050.


140



Figure 54. Water Draw-off for Tanks Over 6 m (20 ft.) Diameter


141



Figure 55. Water Draw-off for Tanks 3 m (10 ft.) to 6 m (20 ft.) Diameter


142



glossary annular ring

A butt welded outer ring of bottom shell plates on which the tank shell rests.

cone roof

A fixed roof formed approximately to the surface of a right cone.

dome roof

A fixed roof formed approximately to a spherical surface.

frangible roof

A fixed roof designed such that the roof-to-shell junction will fail due to high internal pressure prior to failure of any other shell or bottom seam.

NPS

Nominal pipe size in inches.

obround

A shape formed by two parallel sides with semicircular ends.

PWHT

Postweld heat treatment is bringing the temperature of a metal object up to a high enough level for a prolonged duration of time to remove stress that is caused during the fabrication process.

self-supporting roof

A fixed roof designed to be supported only at its periphery at the tank shell.

sketch plates

A lap-welded outer ring of bottom plates on which the tank shell rests.

supported roof

A roof that has its principal support provided by rafters that are mounted on girders and columns, or by rafters on trusses with or without columns.


143

Mechanical Design of Tank Structure

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