Crane Pedestral Design

Engineering & Construction Sector

DESIGN REFERENCE

OFFSHORE STRUCTURES CRANE PEDESTAL DESIGN DR 352

Rev 0 L

MAY 1991

John Brown Engineers & Constructors Limited 20 Eastbourne Terrace, London W2 6LE

DR352.WP5

UNCONTROLLED COPY. DO DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.

Engineering & Construction Sector DESIGN REFERENCE

DR 352/0 L

OFFSHORE STRUCTURES CRANE PEDESTAL DESIGN

CONTENTS

1.

OBJECTIVE

2.

DEFINITION

3. 3.1 3.2 3.3 3.4 3.5 3.6

DESIGN SPECIFICATION

4. 4.1 4.2 4.3 4.4

CRANE SPECIFICATION DUTY FACTOR CRANE UTILISATION STATE OF LOADING LIFT-IMPACT FACTOR API-RP2A RECOMMENDATIONS MAXIMUM LOADING FOR DESIGN

LLOYDS'S METHOD AN INDEPENDENT METHOD COMPARISON OF THE METHODS RECOMMENDATION FATIGUE DESIGN

5. 5.1 5.2 5.3

CONVENTIONAL FATIGUE ANALYSIS COMMENTARY CRANE VIBRATIONS

6.

ACCIDENTAL LOAD

7.

REFERENCES

FIGURES AND TABLES REV 0

ISSUED

MAY 1991

PREPARED BY: OFFSHORE STRUCTURES

APPROVED BY:

........................... K. LOGENDRA, ASSOCIATE DIRECTOR OFFSHORE STRUCTURES

AUTHORISED BY: ................... H. THIRKELL, DIRECTOR OF ENGINEERING

DR352.WP5

UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.



1

DR 352/0 L

PAGE 1 OF 17

OBJECTIVE

The objective of the Design Practice is to give detailed guidance on the static and fatigue design of the pedestal structures of offshore platform cranes. An offshore crane is subject to significant shockloading which should be addressed in an adequate manner in the design of the crane pedestal. 2

DEFINITION

The crane pedestal is, in general, a vertical tubular structure spanning at least two topside deck levels (see Fig. 1-3). In some cases this tubular is also used for diesel storage. However, the safety aspect of diesel storage must be addressed as part of a Formal Safety Assessment. The slew ring of the revolving crane is mounted onto the top of the tubular with or without a transition cone called the pedestal adoption. The definitions of the symbols used in this Design Practice are given immediately after their introduction in design equations. 3

DESIGN SPECIFICATION

For the design of the crane pedestal two loading conditions will have to be considered: the maximum loading and the fatigue loading. No specific guidance on crane pedestals is given in the DEn Guidance Notes (Ref.1) despite the fact that a series of crane accidents occurred in the early 80's. The section on the design of the crane supporting structure can make effective use of API-RP2A (Ref. 2) as will be discussed in Section 4.2. It is also worth noting that some interesting field data on offshore crane behaviour are reported in Ref. 3. It is common practice in John Brown to use a combination of the Lloyd's Code on Lifting Appliances in a Marine Environment, Chapter 3, Sect. 3 on offshore cranes (Ref. 4), and the British Standard on rules for the design of cranes (Ref. 5). This Design Practice will review the adequacy of this method and will bring it in line with the Guidance Notes and API-RP2A (Ref. 1 and 2) as applied to other parts of the topsides and substructure design. 3.1

CRANE SPECIFICATION The following crane specific information is required: -

a crane capacity curve (see Fig. 4) seastate dependent crane derating coefficients the weight of the boom and the hook

In some cases (e.g. Bruce) there is also an accidental load requirement which stipulates that the pedestal must be stronger than the crane strength. In that case the vendor supplied data should also contain a crane failure envelope.

DR352.WP5




3.2

DR 352/0 L

PAGE 2 OF 17

DUTY FACTOR According to Lloyd's Register it is logical and reasonable to reflect the harsh duty of an offshore pedestal crane by a Duty Factor (DF) = 1.2 This factor is to be applied to the lift-load only and in combination with Lloyd's dynamic amplification factors (see Ref 4 Ch. 3 Sect 2.3.1).

3.3

CRANE UTILISATION According to BS 2573 (Ref. 5) the crane operating life is reflected by two parameters (see Tables 1 and 2): -

the Class of Utilisation (U1-U9) the State of Loading (Q1-Q4)

Historically the following utilisations have been used: U3 for unmanned installations (N = 125,000 cycles) U5 for drilling/production platform (N = 500,000 cycles) The class can be revised based on Client supplied data. 3.4

STATE OF LOADING The previous sections only addressed the general working environment and the number of lifts in the course of the crane useful operating life. The aim of the parameter Q in BS 2573 is to reflect the average severity of the loading as a percentage of the maximum crane loading for the crane pedestal fatigue analysis. The State of Loading of offshore cranes is best reflected by Q2 - moderate state of loading In the fatigue analysis the state of loading is incorporated by a parameter Kp which is called the load spectrum factor. Its value is dependent on the state of loading and the value of Kp associated with Q2 is Kp = 0.63 It is a multiplication factor for the total pedestal bending moment. Following Section 2.3.2.1 of BS 2573 it could be derived independently based on Client supplied data but it is questionable if these additional calculations would effectively improve the accuracy of the fatigue analysis.

DR352.WP5




3.5

DR 352/0 L

PAGE 3 OF 17

Note 1:

The recommended value of m = 3 in the expression for Kp (see Sect. 2.3.2.1 in Ref. 5) corresponds to the slope of the DEn-SN curve in a log-log scale.

Note 2:

According to the expressions in Ref.5 a utilisation of 10, 60, 30% of the lifts at 100, 60, 40% of the crane capacity corresponds with a Kp = 0.63 equal to Kp for the Q2 state of loading.

LIFT IMPACT FACTOR The most onerous loading condition for an offshore crane will be experienced during the offloading of a supply boat. Since the hook-speed of the main hoist will be low in comparison with the supply boat heave motion there will be a significant impact on the crane when the load comes free from the supply boat for the first time. This impact factor can also be called dynamic amplification factor (DAF); it is a random variable because it will depend on the actual heave motion of the supply boat at the point of lift-off. The specific values for the DAF in a crane analysis can be derived from the equations in Ref 4, Ch. 3 Sect. 3.3.2; some specific numbers are given in Sect. 4.1. The DAF can also be found from computer simulations or from field measurements and can be as high as 3.0.

3.6

API-RP2A RECOMMENDATIONS API-RP2A Sect. 7.3.1 summarised the guidance on the crane supporting structures as follows: "7.3.1 Static Design. The supporting structure should be designed for the dead load of the crane plus a minimum of 2.0 times the static rated load as defined in API Spec. 2C and the stresses compared to the Par. 3.1.1 allowables with no increase." In the light of the discussion and analysis of Chapter 4 this design condition is significantly lighter than the recommendation of this DR. Therefore the API recommendations should not be used for crane pedestal design except for Gulf of Mexico type environmental conditions.

4

MAXIMUM LOADING FOR DESIGN

The maximum loading governing the design of the crane pedestal will be in accordance to the Lloyds's code on Lifting Appliances (Ref. 4), Ch. 3, Sect. 2.16.2. More specifically the governing load condition will be Case 2 for the crane in operating mode with wind. In particular the maximum moment in the pedestal will be governing. For this case the effects of the horizontal loads on the crane boom (which is so important for the crane design itself) can be ignored. 4.1 DR352.WP5

LLOYD'S METHOD UNCONTROLLED COPY. DOCUMENT VIEWED ON THE NETWORK TAKES PRECEDENCE.



DR 352/0 L

PAGE 4 OF 17

According to Ref. 4, Ch. 3 the design load for the pedestal will require the incorporation of a duty factor (DF) and a dynamic amplification factor (DAF) on the hook load. More specifically the following values can be obtained from Ref. 4, Chapter 3: Duty Factor (DF) = 1.2 (Sect. 2.3.1) DAF (for Hs = 1.6m) = 1.61 (Sect. 3.3.2) DAF (for Hs = 3.9m) = 2.07 (Sect. 3.3.2) Using these factors together with the vendor supplied data and after inclusion of the static moment due to the weight of the hook and the boom the most unfavourable moment for the design of the crane pedestal is: M = M +M + DF * DAF * M max hook boom hookload The maximum stress as a result of this moment is to be compared with the allowable bending stress p : a p = 0.57 p (Sect. 5.3.2) b y 4.2

AN INDEPENDENT METHOD It was noted in Sect. 3.2 that the maximum DAF on the hook load can be as high as 3.0. This value will be used in the independent method. Secondly the pedestal is a thin-walled tubular structure which should be designed in accordance to API-RP2A Section 3.2.3. The one-third increase in allowable stresses should not be applied to the crane pedestal. For p = 340 MPa and D/t = 75 the allowable y bending stress according to API-RP2A is: p = 0.65 p b y For values of D/t and p different from D/t = 75 and p = 340 MPa the equation for p in y y b API-RP2A Sect. 3.2.3 should be used. This value of p is to be combined with a dynamic amplification factor for the hookload b equal to DAF = 3.0 Finally p is to be obtained from the following equations for M b max

DR352.WP5




DR 352/0 L

PAGE 5 OF 17

M =M +M + DAF * M max hook boom hookload 4.3

COMPARISON OF THE METHODS Two comparisons between the two methods will be made base on the static part and the dynamic part of the crane loading. Using the equations in Sect. 4.1 and 4.2 and by setting the hookload equal to zero a direct comparison can be made between the allowable static bending stresses. The Lloyd's method (stat)

p = 0.57 p b y The alternative method (stat) p = 0.65 p b y Using the equations in Sect. 4.1 and 4.2 and by setting the boom weight and the hook weight equal to zero a direct comparison can be made between the allowable dynamic bending stress in the pedestal due to the hook-load. By dividing the maximum allowable bending stresses by the Duty Factor and DAF the following values are obtained. The Lloyd's method (dyn)

p = 0.23 p b y The alternative method (dyn) p = 0.22 p b y (These numbers are found as follows: 0.23=0.57/(1.2 x 2.07);0.22 = 0.65/3.0) 4.4

RECOMMENDATION The alternative method incorporates a well recognised model to accommodate the bending strength reduction for thin-walled tubulars. In addition the allowable static stress in the alternative method has been found to be 10% higher (using industry accepted practices) than the Lloyd's method. Therefore it is recommended to apply the alternative method of Section 4.2 for the ultimate design of pedestals for offshore cranes.

5

FATIGUE

The assessment of the fatigue strength of crane components should address the following load histories: -

DR352.WP5

the lift-off of a load from a supply boat the dynamics in the crane system as a result of the impact forces during lift off the setting-down of the load on the platform




DR 352/0 L

PAGE 6 OF 17

From a review of methods and consequences it is concluded that the BS 2573/Lloyd's procedure considering each lift-off and setting-down as a fatigue cycle is governing for the crane pedestal. This procedure will be further addressed in Sect. 4.1. It is noted that the API-RP2A recommendation on crane pedestal fatigue should not be used for North Sea conditions; it may form a simple basis for light cranes on platforms operating in Gulf of Mexico conditions. The crane vibrations will be discussed in Sect. 5.3. They are the result of impact forces during lift-off from a supply boat and forms a governing fatigue loading on many crane components. For the crane pedestal its inclusion leads to a small correction which can be disregarded within the accuracy of its overall fatigue analysis. 5.1

CONVENTIONAL CRANE FATIGUE ANALYSIS From the crane specific data the following is required for the fatigue analysis. -

Class of utilisation (U3 - U5) representing the number of lifts during the lifetime of the crane. (See Sect. 3.3) State of loading as reflected in Q2 = 2 and Kp = 0.63. (See Sect. 3.4)

The maximum stresses in the crane pedestal will directly depend on the bending moment and we are specifically interested in the maximum positive and negative bending moment. 5.1.1

The Maximum and Minimum Moment for Fatigue The lift-off moment to be considered for the fatigue analysis is : M = Kp * (M +M + DF x DAF x M ) max hook boom hookload This moment should be checked for the following conditions and seastates: (a) (b)

-

maximum reach associated to maximum load maximum load at the maximum associated reach

(i) (ii)

-

seastate 2-3 (Hs = 1.6m) with a DAF = 1.61 seastate 4-6 (Hs = 3.9m) with a DAF = 2.07

This leads to four cases (a-i, a-ii, b-i, b-ii). The opposite sign to the lift-off moment will occur during placing of the load on the platform. This moment is similar to the lift-off moment with one exception that the operation is seastate independent which can be reflected by a DAF = 1.0 or : M = Kp * (M +M + DF x M ) min hook boom hookload Comment 1: If the crane is optimally designed using Lloyd's recommendation for the DAF DR352.WP5




DR 352/0 L

PAGE 7 OF 17

then the maximum moment in the crane pedestal will be independent of the seastate. Comment 2: Due to the effect of the boom weight on the pedestal moment it is expected that the maximum pedestal moment will be concurrent with the maximum reach. Comment 3: Since the minimum pedestal moment (occurring while placing the load on the platform) is seastate independent it can be demonstrated that the minimum pedestal moment reaches its (absolute) extreme value for the highest load (i.e lowest seastate) and for the maximum reach. These three comments should be verified for the fatigue analysis. If confirmed then they form the immediate basis for obtaining the maximum and minimum moment to be used in the fatigue analysis of the crane pedestal as follows: M = K * (W + 0.5 W + DF x DAF * W ) *r max p h b l max M = K * (W + 0.5 W + DF x W ) *r min p h b l max where K = 0.63 (see Sect. 3.3) p W = weight of the hook h W = weight of the boom b W = weight of the hookload l DF = 1.2 (see Sect. 3.2) DAF = 1.61 (see Sect. 4.1) r = maximum reach max 5.1.2

The Allowable Stress The maximum and minimum moments also give the corresponding R-value which is defined as: R =M /M = min. stress/max. stress min. max. For crane pedestals the class F welding detail should be used. The corresponding table from BS 2573 for class-F welding details is copied as Table 3 in this DR. Using the R-value calculated above and the number of cycles in accordance to the class of utilisation (U3 - U5) the corresponding maximum allowable fatigue (tension or compressive) stress can be read directly from Table 3. The maximum stress together with the maximum pedestal moment determines the section modules or (if the diameter is specified) the material thickness of the pedestal.

DR352.WP5



DR 352/0 L


5.2

PAGE 8 OF 17

COMMENTARY Fatigue in the UK sector of the North Sea is in general addressed using the DEn Guidance Notes (Ref. 1) and therefore it is useful to make some comparative remarks on the procedure of Sect. 5.1. The only difference will be in the selection of the SN curve; all other aspects (the number of cycles, the F curve, the Kp - value) are found to be identical using the information contained in the Guidance Notes. The two differences between Ref. 1 and BS 2573 (Ref. 5) in the fatigue curves are: -

DEn do not recognise the R-dependency for R = -1 (fatigue with a zero mean) the allowable stress is different.

This is reflected in the following data for the maximum stress-amplitude in MPa at U5 (500,000 cycles). In this table, for completeness, the API-RP2A fatigue allowables have been included as well. Stress in MPa

BS 2570

DEnGN

API-X

API-X'

62 71

54 64

69 81

57 67

R = -1.0 R = -0.7

This difference between BS 2573 and the Guidance Notes is significant from a design point of view. But it should be noted that their difference is equivalent to doubling the estimated failure rate, (failure = through thickness crack) from 2% to 4% in the lifetime of the crane. Because of the ease of inspection of a crane pedestal as compared with underwater parts of the structure it is recommended to apply the slightly less conservative BS 2573 data. 5.3

CRANE VIBRATIONS The impact due to supply-boat offloading will result in vibrations in the crane system and these vibration gradually reduce in magnitude until the equilibrium condition is reached. The amplitude of the fatigue loading is governed by (DAF - 1.0) * hook-load and the total stress range is twice this amplitude; secondly the number of active cycles will depend on the system damping In general the damping of structural systems is small; for example a value of 2% of critical damping seems realistic implying that in 5 cycles the amplitude of the oscillations is reduced to 50%. It can then be demonstrated that for these conditions (with this damping) using Ref. 1 information the fatigue damage in the crane pedestal due to crane dynamics can be ignored. This is contrary to the findings of Ref. 3. It should be noted, though, that crane vibrations form an important aspect in the design of the crane boom and other components in the pedestal crane.

6

DR352.WP5

ACCIDENTAL LOAD




DR 352/0 L

PAGE 9 OF 17

For the design of the Bruce pedestal crane the following accidental load scenario was stipulated. "The crane pedestal, and supporting structure, will be analysed to demonstrate a minimum factor of safety against collapse of 1.5. The applied loading for this case shall be determined from crane failure envelopes supplied by the crane vendor. For this case, allowable stresses will be limited to the yield stress of the pedestal material." The accidental load scenario may well be governing for the design of the crane pedestal and its supporting steel work. The main reason for this scenario is the occurrence of accidents like the hook snatching the supply boat causing pedestal failure and fatalities in the early 80's. Without further data the following two changes are recommended for incorporation in the above description. a) b)

safety against collapse to be reduced from 1.5 to 1.3 yield stress to be replaced by allowable stress with a one-third increase.

Pt. 1 can be further reduced after review of the vendor data supporting the crane failure envelope. Pt. 2 is a reflection of API-RP2A on thin walled tubulars in line with the comments in Sect. 4.2. 7

DR352.WP5

REFERENCES

a)

Department of Energy; Offshore Installations: Construction and Certification (Fourth Edition) 1990.

Guidance

on

Design,

b)

API-RP2A API Recommended Practice for Planning, Designing and Construction Fixed Offshore Platforms 1989 (18th Edition).

c)

Shauschausen, J. and Gran, S. Supply Boat Motions; Dynamic Response and Fatigue of Offshore Cranes OTC 3795, 1980.

d)

Code for Lifting Appliances in a Marine Environment, Lloyd's Register of Shipping, Jan. 1987.

e)

BS 2573 Rules for the Design of Cranes Part 1: Specification for the Classification Stress Calculations and Design Criteria for Structures 1983.




DR 352/0 L

PAGE 10 OF 17

FIGURE 1. A TYPICAL CRANE PEDESTAL SUPPORT

STANDARDS\DR\DR352\FIG-1.WPG

DR352.WP5




DR 352/0 L

PAGE 11 OF 17

FIGURE 2a. PRIMARY STEEL ARBROATH DECK


DR352.WP5




DR 352/0 L

PAGE 12 OF 17

FIGURE 2b. PRIMARY STEEL ARBROATH DECK


DR352.WP5




DR 352/0 L

PAGE 13 OF 17

FIGURE 3. DETAILS OF THE CRANE PEDESTAL


DR352.WP5




DR 352/0 L

PAGE 14 OF 17

FIGURE 4. CRANE LOAD CAPACITY CURVE


DR352.WP5




DR 352/0 L

PAGE 15 OF 17



DR352.WP5




DR 352/0 L

PAGE 16 OF 17


DR352.WP5


Crane Pedestral Design

Recommend Documents