Offshore Engineering - An Overview

Offshore Structural Engineering – An Overview

2011

A SEMINAR REPORT ON

OFFSHORE ENGINEERING- An Overview SUBMITTED UNDER THE PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER OF TECHNOLOGY IN STRUCTURAL ENGINEERING

SUBMITTED BY

KHARADE AMIT S. (P10ST525)

GUIDED BY

Dr. A.K.DESAI

2010-2011 DEPARTMENT OF APPLIED MECHANICS SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY SURAT-395007

SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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This is to certify that,

Mr. Kharade Amit Suryakant (P10ST525) Has successfully submitted a Credit Seminar Report in, “OFFSHORE

ENGINEERING – An Overview”

In Partial fulfillment of the requirement for award of the degree in Master of Technology in Structural Engineering as per the rules and regulations of the National Institute of Technology, Surat For the academic Year 2011-12 this report represents the bonafied work of the student and matters submitted here is not been submitted elsewhere for award of any degree or diploma. th

Date : 6 October 2011

Dr. A.K Desai Associate Professor AMD SVNIT, Surat

Place: Surat

Dr. C. D. Modhera Professor In-Charge P.G.Centre AMD SVNIT, Surat

Prof. S. N. Desai Associate Professor and Head of Department, AMD SVNIT, Surat


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2011

This is to certify that,

Mr. Kharade Amit Suryakant (P10ST525) Has successfully submitted a Credit Seminar Report in, “OFFSHORE

ENGINEERING – An Overview”

In Partial fulfillment of the requirement for award of the degree in Master of Technology in Structural Engineering as per the rules and regulations of the National Institute of Technology, Surat For the academic Year 2011-12 this report represents the bonafied work of the student and matters submitted here is not been submitted elsewhere for award of any degree or diploma. th

Date : 6 October 2011

Dr. A.K Desai Associate Professor AMD SVNIT, Surat

Place: Surat

Dr. C. D. Modhera Professor In-Charge P.G.Centre AMD SVNIT, Surat

Prof. S. N. Desai Associate Professor and Head of Department, AMD SVNIT, Surat


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Abstract

Since the 1970s a need for deep water structure that would exploit energy resources such as oil and natural gas has arisen. Various types t ypes of platforms are designed as per the requirement and depending upon the depth of sea water. When deep water combines with hostile weather condition, conventional fixed offshore structures required excessive physical dimensions to obtain the stiffness and strength needed. Study involves some important forces such as wind, ocean wave‘s buoyant forces, current loading and marine growth etc. Accurate prediction of the wave loadings on the structures is extremely important for design purpose so that various software‘s are introduced. These software analysis the structure in all manner and give a desirable results which helps in designing the structure. Each part of the structure is simulated by considering actual and environmental loads on it.


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Acknowledgement

With

profound

sense

of

regard

and

gratitude,

I

thank

my

Guide

Dr. A. K. Desai. For his invaluable guidance, incessant interest and constructive suggestions

during the course of the Seminar. The Seminar report preparation would not have been possible without the zeal and interest shown by Dr. A.K Desai sir throughout the task. I thank him for his immense knowledge and timely help which helped in making this seminar at completion. I appreciate and wish to thank Prof. S. N. Desai, Head of the Department of AMD, Sardar Vallabhbhai National Institute of Technology, Surat. for providing the required facilities available in department for the seminar work. Finally I would like to thank our P.G.Incharge Dr. C. D. Modhera and the college for providing us with the platform to excel in curriculum.

Mr. Kharade Amit Suryakant (P10ST525)


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CHAPTER 1 INTRODUCTION

1.1 General..........................................................................................................1 1.2 Historical Perspective....................................................................................2 1.3 Objectives......................................................................................................2 CHAPTER 2 LITERATURE REVIEW

3

CHAPTER 3 OFFSHORE ENGINEERING

3.1 Types of Offshore Structures........................................................................6 3.1.1 Fixed Platforms.............................................................................6 3.1.2 Compliant Structures.....................................................................9 3.1.3 Floating Structures......................................................................11 CHAPTER 4 LOADS ON OFFSHORE STRUCTURES

4.1 Types of Loads............................................................................................13 4.2 Detailed Study of Loadings.........................................................................13 4.2.1 Gravity Loads..............................................................................13 4.2.2 Environmental Loads..................................................................15 CHAPTER 5 SIMULATION OF STRUCTURE (ANALYSIS)

5.1 General........................................................................................................22 5.1.1 for Structural Analysis................................................................22 5.1.2 for Hydrodynamic Calculation...................................................22 SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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5.1.3 for Pile Foundation Analysis......................................................22 5.2 Process of Analysis.....................................................................................22 5.2.1 Structure Geometry Selection.....................................................23 5.2.2 Geometry Simulation..................................................................23 5.2.3 Foundation Simulation................................................................24 5.2.4 Load Simulation..........................................................................27 5.3 Nature of Analysis.......................................................................................28 5.3.1 Dynamic Analysis.......................................................................28 5.3.2 Fatigue Analysis..........................................................................28 5.3.3 Ship Impact Analysis..................................................................29 5.3.4 Pushover Analysis.......................................................................29

CHAPTER 6 CONCLUSIONS

30

CHAPTER 7 REFERENCES

7.1 Research Papers...........................................................................................31 7.2 Search Engines............................................................................................31


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LIST OF FIGURES

FIGURE NO

DESCRIPTION

PAGE NO

3.1

Steel Jacket Structure

7

3.2

Jacket up Ring

7

3.3

Operational Sequence of Jack up Ring platform

7

3.4

Foundation Pattern of Gravity Structure

8

3.5

Components of Gravity Structure

8

3.6

Guyed Tower

9

3.7

Compliant Tower

9

3.8

Tension Leg Platform

10

3.9

Articulated Tower

11

3.10

Semi-Submersible offshore platform

12

3.11

Floating Production, Storage and offloading System

13

4.1

Current Profile on Structure

17

4.2

Wave Loads on Jacket Structure

19

4.3

Buoyancy Calculation methods

20

5.1

Computer Model of a Wellhead Jacket and Deck

23

5.2

Computer Model of a Jacket with Axis system

24

5.3

Pile Simulation for an offshore jacket

25

5.4

Pile Group arrangements for 4 legged platform

26

5.5

Pile Group arrangements for 8 legged platform

26


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Chapter 1 INTRODUCTION 1.1 GENERAL

Offshore Structures constructed on or above the continental shelves and on the adjacent continental slopes take many forms and serve a multiple purpose. Such as Towers for microwave transmission, installations for power generation, portable pipeline systems for mining the ocean floor and a few platforms and floating islands that serve as resort hotels. Most offshore structures however have been built to support the activities of petroleum industries. Exploratory drilling is done from mobile platforms or carefully positioned ships. Production and storage operation involve more permanent structures. Offshore platforms have many uses including oil exploration and production, navigation, ship loading and unloading, and to support bridges and causeways. These offshore structures must function safely for design lifetimes of twenty-five years or more and are subject to very harsh marine environments. Some important design considerations are peak loads created by hurricane wind and waves. The platforms are sometimes subjected to strong currents which create loads on the mooring system and can induce vortex shedding. Offshore platforms are huge steel or concrete structures used for the exploration and extraction of oil and gas from the earth‘s crust. Offshore structures are designed for installation in the open sea, lakes, gulfs, etc., many kilometers from shorelines. These structures may be made of steel, reinforced concrete or a combination of both. Offshore platforms are very heavy and are among the tallest manmade structures on the earth. The oil and gas are separated at the platform and transported through pipelines or by tankers to shore. The design of marine structures compatible with the extreme offshore environmental condition is a most challenging and creative task for the ocean engineers. The marine engineer‘s goal is to conceive and design a lasting structure that can withstand the adverse conditions of high winds and waves, earthquakes, tsunami and ice effect. These structures are analyses in all possible manner to avoid the loss of property and life of workers as they are situated long away from shore line and constructed at a depth more than 200m.


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1.2 HISTORICAL PERSPECTIVE

The earliest offshore structure for oil drilling was built about 1887 off the coast of southern California near Santa Barbara. This was simply a wooden wharf outfitted with a ring for drilling vertical wells into the sea floor. more elaborate platforms supported by timber piers were then built for oil drilling, including installation for the mile deep well in Caddo lake, Louisiana (1911) and the platform in lake Maracaibo, Venezuela (1927) soon after these early pier systems were built, it become apparent that the lifetime of timber structures erected in lakes or ocean is severely limited because of attacks by marine organisms. For this reason Reinforced Concrete replaced timbers the supporting structure for many offshore platforms up to the late 1940s. Over the next 50 years about 12000 platforms structures were built offshore, usually of steel but more recently of precast concrete. Offshore mooring system has a variety of configurations all have anchors or groups of pipelines in the seabed with flexible lines. Leading from them to buoys, ship or platform structures. The function of mooring system is to keep the buoy, ship or platform structure at a relatively fixed location during engineering operations. When pipeline were first laid offshore, no extraordinary analyses or deployment techniques were needed since they were in shallow water and were of small diameter. As platforms were built in deeper or deeper water with multiple well slots, large diameter pipelines of higher strength were required during the 1960s. Engineers met this challenge with new design and with refined methods of analysis and deployment. Throughout the world there are at present about 90000 km of marine pipelines. Since 1986 the rate of building new marine pipelines has been about 1000 km per year. Pipeline varies from 1 km to 100 km in length and 7 cm to 152 cm in diameter. The pipelines of smaller diameter are used to transport oil and gas from wellhead and those of large diameter are used to load and unload oil from tankers moored at offshore terminals. At present Norwegian project has a 1000 km line extending from the Troll field to Belgium completed in 1992, Kuwait has the loading line of largest diameter 152 cm. 1.3 OBJECTIVE

Today requirement of oil worldwide is increases rapidly and for that need of offshore structure is more. This topic gives an overall knowledge of offshore engineering such as various types of structures, loadings on structure, construction and analysis procedure.


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Chapter 2 LITERATURE REVIEW Adrezin R. And Benaroya H (1996)

[15]

, this paper describes offshore compliant structures

such as Guyed platforms, Tension leg platforms and articulated towers are economically attractive for deep water conditions because of their reduced structural weight as compared to conventional platforms. Geometric nonlinearity is an important consideration in the analysis of such structure. Study of static and dynamic response of the structure due to various environmental conditions such as wind, waves and currents. Modeling and analysis techniques are common to the aerospace and ocean engineering communities due to similarities in structural and environmental complexities. Author focused on important class of offshore structure known as compliant structure. Such structures have been found primary offshore application in oil industry but also in case where a stable ocean platform is needed for communication and mooring. Ahmed A. Elshafey and Mahmoud R. Haddara (2009)

[1]

, Dynamic response of a scale

model of a jacket offshore structure is investigated both theoretically and experimentally. Model subjected to random loads, fraud‘s low of modeling was used to obtain the dimensions of scale model based on dimension of existing structure. A finite element model was designed to determine the dynamic response of the model. Reaction force at the foundation was estimated from strain measurements. Experiments interpret results as finite element model used for response prediction. However there is about 13% difference in the value of the reaction force estimated from strain measurement and value which was obtained numerically. Reaction at foundation decreases as mass of the model increases. Haritos. N (2007)

[11]

, this paper provides a broad overview of some of the key factors in the

analysis and design of offshore structures to be considered by an engineer in field of offshore engineering. Offshore structures have the added complications of being placed in an ocean environmental where hydrodynamic interaction effects and dynamic response become major considerations in their design. Hydrodynamics is concerned with the study of water in motion. The topography of the ocean bottom also has an influence on the water depth changes from deeper to shallower conditions, (Dean and Dalrymple, 1991). This influence is referred to as the ―shoaling effect‖. A number of regular wave theories have developed to


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describe the water particle kinematics associated with ocean waves of varying degree of complexity and levels of acceptance by the offshore engineering community, (Chakrabarti, 2005). [19]

Kabir Sadeghi (2001)

, this paper reviews the fundamentals behind all types of offshore

structures (fixed or floating). The overall objective is to provide a general understanding of different stages of design, construction load out, transportation and installation of offshore platforms. For different sea water depths, in which the Cyprus platforms are intended to be installed, suitable kinds of offshore platforms are proposed. These offshore structures must function safely for design lifetimes of twenty-five years or more and are subjected to very harsh marine environments. The platforms are sometimes subjected to strong currents which create loads on the mooring system and can induce vortex shedding. Philip Esper (1991)

[7]

, This paper discusses the major aspects that should be considered in

the evaluation of seismic response of offshore structures through a case study of a concrete gravity substructure supporting a conventional steel topside structure. It highlights the importance of selecting the most appropriate arrangements for the connection between the topside and the substructure and its effect on the seismic performance of the platform. The advantage of performing a detailed global 3-D non linear analysis of the whole structure in order to predict its dynamic performance during a seismic event is discussed. The seismic analysis showed that the seismic performance of the platform is satisfactory, with plastic hinges developing in a small number of elements in the topside. The global FE seismic non linear analysis was the tool that predicted the performance of the whole platform, including the CGS, the topside and the deck connection, under a DLE (Ductility Level Earthquake) event. R.G.Bea, Fellow, ASCE (1999)

[16]

, this paper describes the API (American Petroleum

Institute) guidelines to determine wave forces acting on the decks of platforms indicate that most platforms cannot survive such loadings. Several approaches have been developed to compute the worst crest loadings and the responses of the platforms to the loading. Many platforms have experienced sufficient wave loadings on their lower deck during hurricanes. The API procedure to determine wave in deck forces produces results that are not in conformance with observations of the performance of platform that have experienced hurricane wave crest in their lower decks.


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Offshore Structural Engineering – An Overview Stavros A. Anagnostopoulos (1982)

2011

[20]

, this paper states one of the main loadings for which

offshore structures are designed is caused by extreme waves generated during intense rare storms. The dominant periods of such waves are typically much longer than the fundamental periods of most fixed offshore structures and therefore static analysis are usually sufficient for obtaining the design response of these structures to extreme waves. For description a regular wave described by its height, period and direction is passed through the structure and forces on the various structural elements are computed for a wave cycle by summing up elemental forces predicted by the well known ‗Morison equation‘. As the development of oil and gas moves into deeper water, however taller platforms with longer periods are built that respond more dynamically to extreme waves. Thomas H. Dawson (1983)

[21]

, this paper describes various environmental loading

conditions and the resulting forces that are generated on offshore structures. Some of important forces are wind, ocean surface waves, buoyant forces and current loadings. Study involves ultimate capacity and response of base elements used to distribute loading of the structure over the seafloor. An offshore support pile is subjected to cyclic lateral forces and moments at the ground line from wave action on the overhead structures, surrounding soil exerts resisting forces along the pile. Work presents analytical procedures for evaluating the dynamic characteristics of an offshore structure. This is necessary as the natural frequency of a structure can coincide with the period of wave loading which can produce substantial dynamic amplification.


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Chapter 3 OFFSHORE ENGINEERING

3.1 TYPES OF OFFSHORE STRUCTURES

The offshore structures built in the ocean to explore oil and gas is located in depths from very shallow water to the deep ocean. Depending on the water depth and environmental Conditions, the structural arrangement and need for new ideas required. Based on geometry and behavior, the offshore structures for oil and gas development has been divided into Following categories. 1. Fixed Platforms A) Steel template Structures. B) Concrete Gravity Structures. 2. Compliant tower A) Compliant Tower. B) Guyed Tower. C) Articulated Tower. D) Tension Leg Platform. 3. Floating Structures. A) Floating Production System. B) Floating Production, Storage and offloading System.

3.1.1 Fixed Platforms A) Steel template Structures

The steel template type structure consists of a tall vertical section made of tubular steel members supported by piles driven into the sea bed with a deck placed on top, providing space for crew quarters, a drilling rig, and production facilities. The fixed platform is economically feasible for installation in water depths up to 500m. These template type structures will be fixed to seabed by means of tubular piles either driven through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the jacket. The principle behind the fixed platform design is to minimize the natural period of the structure below 4 seconds to avoid resonant behavior with the waves (period in the order of 4 to 25 seconds. SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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The structural and foundation configuration shall be selected to achieve this concept.

Fig3.1 – Steel Jacket Structure

Fig – 3.2 Jack up ring Structure

Jack up ring Jacks up ring are similar to drilling barges, with one difference. Once jack up ring is towed to the drilling site, three or four ‗legs‘ are lowered until they rest on the sea bottom. This allows the working platform to rest above the surface of the water, as opposed to a floating barrage. However, jack up rings are suitable only for shallower waters, as extending these legs down too deeply would be impractical. This ring type can only operate 500 feet in the depth of water.

Fig3.3 – Operational Sequence of Jack up Ring platform


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B) Concrete Gravity Platforms

Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is remote. These platforms are very common in areas with strong seabed geological conditions either with rock outcrop or sandy formation. Some part of North Sea oil fields and Australian coast, these kinds of platforms are located. The concrete gravity platform by its name derives its horizontal stability against environmental forces by means of its weight. .

Fig – 3.4 Foundation Pattern of Gravity Structure These structures are basically concrete shells assembled in circular array with stem columns projecting to above water to support the deck and facilities. The main advantage of these types of platforms is their stability, as they are attached to sea floor so there is limited movement due to wind and water forces. Concrete gravity platforms have been constructed in water depths as much as 350m

Fig 3.5 – Components of Gravity Structure


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3.1.2 Compliant Structures

In addition to the developing technologies for exploration and production of oil and natural gas, new concepts in deepwater systems and facilities have emerged to make ultradeepwater projects a reality. With wells being drilled in water depths of 3000m, the traditional fixed offshore platform is being replaced by state-of-the-art deepwater production facilities. Compliant Towers, Tension Leg Platforms, Spars, Subsea Systems, Floating Production Systems, and Floating Production, Storage and Offloading Systems are now being used in water depths exceeding 500m. All of these systems are proven technology, and in use in offshore production worldwide. A) Compliant Tower

Compliant Tower (CT) is much like fixed platforms. They consist of a narrow, flexible tower and a piled foundation that can support a conventional deck for drilling and production operations. The compliant towers flexibility withstands large lateral forces by sustaining significant lateral defections, and is usually used in water depths between 300m and 600m. B) Guyed Tower

Guyed tower is an extension of complaint tower with guy wires tied to the seabed by means of anchors or piles. This guy ropes minimizes the lateral displacement of the platform topsides. This further changes the dynamic characteristics of the s ystem.

Fig3.6 – Guyed Tower Fig3.7 – Compliant Tower


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C) Tension Leg Platforms

A Tension-leg platform is a vertically moored floating structure normally used for the offshore production of oil or gas, and is particularly suited for water depths around 1000m to 1200 meters (about 4000 ft). The platform is permanently moored by means of tethers or tendons grouped at each of the structure‘s corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion and gives better control over the production from the oil or gas reservoir. Tension Leg Platform (TLP) consists of a floating structure held in place by vertical, tensioned tendons connected to the sea floor by pile-secured templates. Tensioned tendons provide for the use of a TLP in a broad water depth range with limited vertical motion. The larger TLP‘s have been successfully deployed in water depths approaching 1250m. Mini Tension Leg Platform (Mini-TLP) is a floating mini-tension leg platform of relatively low cost developed for production of smaller deepwater reserves which would be uneconomic to produce using more conventional deepwater production systems. It can also be used as a utility, satellite, or early production platform for larger deepwater discoveries. The worlds first Mini-TLP was installed in the Gulf of Mexico in 1998.

Fig3.8 – Tension Leg Platform


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D) Articulated Tower

Articulated tower is an extension of tension leg platform. The tension cables are replacing by one single buoyant shell with sufficient buoyancy and required restoring moment against lateral loads. The main part of the configuration is the universal joint which connects the shell with the foundation system. The foundation system usually consists of gravity based concrete block or sometimes with driven piles. The articulated tower concept is well suited for intermediate water depths ranging from 150m to 500m.

Fig3.9 – Articulated Tower

3.1.3 Floating Structures A) Floating Production System (Semi-Submersible)

Floating Production System (FPS) consists of a semi-submersible unit which is equipped with drilling and production equipment. It is most common type of offshore drilling rings, combining the advantages of submersible rings with ability to drill in deep water. The ring is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to ring. Semi-submersible rings are generally held in place by huge anchors with wire rope and chain, or can be dynamically positioned using rotating thrusters. Production from subsea wells is transported to the surface deck through


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production risers designed to accommodate platform motion. The FPS can be used in a range of water depths from 300m to 1500m.

Fig3.10 – Semi-Submersible offshore platform

B) Floating Production, Storage and offloading System

Floating Production, Storage and Offloading System (FPSO) consists of a large tanker type vessel moored to the seafloor. An FPSO is designed to process and stow production from nearby subsea wells and to periodically offload the stored oil to a smaller shuttle tanker. The shuttle tanker then transports the oil to an onshore facility for further processing. An FPSO may be suited for marginally economic fields located in remote deepwater areas where a pipeline infrastructure does not exist. Currently, there are no FPSO‘s approved for use in the Gulf of Mexico. However, there are over 70 of these systems being used elsewhere in the world.

Fig3.11 - Floating Production, Storage and offloading System.


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Chapter 4 LOADS ON OFFSHORE STRUCTURES 4.1. TYPES OF LOADS

Loads on offshore structures are gravity loads and environmental loads. Gravity loads are arising from dead weight of structure and facilities either permanent or temporary. Seismic loads are arising from gravity loads and are a derived type. Environmental loads play a major role governing the design of offshore structures. Before starting the design of any structure, prediction of environmental loads accurately is important. Various environmental loads acting on the offshore platform is listed below. 1) Gravity Loads A) Structural Dead Loads B) Facility Dead Loads C) Fluid Loads D) Live Loads E) Drilling Loads 2) Environmental Loads A) Wind Loads B) Wave Loads C) Current Loads D) Buoyancy Loads E) Ice Loads F) Mud Loads 3) Seismic Loads

4.2 DETAIL STUDY OF LOADINGS 4.2.1 Gravity Loads A) Structural Dead Loads

Dead loads include the all the fixed items in the platform deck, jacket, bridge and flare structures. It includes all primary steel structural members, secondary structural items such as boat landing, pad eyes, stiffeners, handrails, deck plating, small access platforms etc. The primary structural steel members will be calculated based on the structural information in the model automatically when a computer program is used to analyze the structure. But the SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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weight of secondary structural steel items shall be calculated applied to the structural model at appropriate locations. B) Facility Dead Loads

The structure built either for drilling or wellhead type platform or for process type platform supports various equipment and facilities. These are fixed type items and not structural components. They do not have any stiffness to offer in the global integrity of the structure and shall not be modeled. The weight of such items shall be calculated and applied at the appropriate locations according the plan of the structure. These items include a) Mechanical equipment b) Electrical equipment c) Piping connecting each equipment d) Electrical Cable trays e) Instrumentation items C) Fluid Loads

The fluid loads are weight of fluid on the platform during operation. This may include all the fluid in the equipment and piping. The weight of these items shall be calculated accurately and applied to the correct locations. D) Live Loads

Live loads are defined as movable loads and will be temporary in nature. Live loads will only be applied on areas designated for the purpose of storage either temporary or long term. Further, the areas designed for lay down during boat transfer of materials from boat shall also be considered as live loads. Other live load includes open areas such as walkways, access platforms, and galley areas in the living quarters, helicopter loads in the helipad, etc. These loads shall be applied in accordance with the requirement from the operator of the platform. This load varies in nature from owner to owner but a general guideline on the magnitude of the loads is given below. Tab4.1 – Design Live Load Intensity 2

Sl. No

LOCATION

LOAD (KN/m )

1 2 3 4

Storage/Lay down Walkway Access Platform Galley

10 5 5 10


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E) Drilling Loads

Drilling loads are due to drill rigs placed on top of the platform for drilling purposes. These are large equipment assembled together and placed on top. Normally, drilling rigs are as heavy as 500 Tones to 1000 Tones. These will deliver reaction forces on the deck and the stiffness of the drilling rigs are not considered in the structural analysis. Hence the weight of the structure shall be applied as load on the structure. Further, during drilling, additional loads will be developed due to drill string and pulling operations. These loads also shall be considered in the analysis. 4.2.2 Environmental Loads A) Wind Loads

The wind speed at 10m above LAT (Lowest Astronomical Tide) is normally provided (Vo).This wind speed shall be extrapolated to the height above for the calculation of wind speed. The extrapolation shall be calculated as below

Where Y is the elevation of point in consideration in m above LAT and V is the velocity at that point. Wind loads shall be calculated as per API RP2A guidelines. Sustained wind speeds (10min mean) shall be used to compute global platform wind loads and gusty wind (3 second) shall be used to compute the wind loads to design individual members. The wind pressure can be calculated as,

Where F is the wind pressure per unit area, ρ (0.01255 KN/m3) is the density of air, g is the gravitational acceleration (9.81 m/sec2) and V is the wind speed in m/sec. the above equation can be simplified by substituting the values and can be expressed as

The total wind load on the platform can be calculated using the wind blockage area and the pressure calculated as above. The shape coefficient ( Cs) shall be selected as per AP RP2A guidelines. But for the calculation of global wind load (for jacket and deck global analysis) shape coefficient can be 1.0. The total force on the platform can be calculated as,


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B) Wave and Current Loads

The wind speed at 10m above LAT (Lowest Astronomical Tide) is normally provided Methodology In applying design waves load onto the offshore structures, there are two ways of applying it. -

Design Wave method

-

Spectral Method

In design wave method, a discrete set of design waves (maximum) and associated periods will be selected to generate loads on the structure. These loads will be used to compute the response of the structure. In the spectral method, an energy spectrum of the sea-state for the location will be taken and a transfer function for the response will be generated. These transfer function will be used to compute the stresses in the structural members. a) Design Wave method The forces exerted by waves are most dominant in governing the jacket structures design especially the foundation piles. The wave loads exerted on the jacket is applied laterally on all members and it generates overturning moment on the structure. Period of wind generated waves in the open sea can be in the order of 2 to 20 seconds. Theses waves are called gravity waves and contain most part of wave energy. Maximum wave shall be used for the design of offshore structures. The relationship between the significant wave height ( H s) and the maximum wave height ( H max) is

The above equation corresponds to a computation based on 1000 waves in a record. The design wave height (in Meter) for various regions is tabulated below.

Table 4.2 - Maximum design waves in various regions Region

1 year

100 year

Bay of Bengal Gulf of Mexico South China Sea Arabian Sea Gulf of Thailand Persian Gulf North Sea

8 12 11 8 6 5 14

18 24 24 18 12 12 22


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API RP2A requires both 1 year and 100 year recurrence wave shall be used for the design of jacket and piles. Appropriate combination of loads with these waves shall be used in the design. A one-third increase in permissible stress is allowed for 100 year storm conditions. b) Spectral Method Instead of simulating the design wave environment by discrete maximum wave, a design seastate described by energy spectrum of for the given site will be used in the load simulation. A directional spectrum can also be used to simulate the changes design wave sea-state.

Current Profile Oceans currents induce drag loading on offshore structures. These currents together with the action of waves generate dynamic loads. Ocean currents are classified into few t ypes based on their nature e.g., tidal current, and wind driven current and current generated due to ocean circulation. Wind driven currents are small in nature and it varies linearly with depth where as tidal currents vary nonlinearly with depth. Similarly, the currents generated due to ocean circulation will vary nonlinear with depth and can be as much as 5 m /sec.

Fig 4.1 - Current Profile on Structure The current variation with depth is shown in Figures and can be expressed as below

Where VT is the tidal current at any height from sea bed, VoT is the tidal current at the surface, y is the distance measure in m from seabed nd h is the water depth.


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Where V W is the wind driven current at any height from sea bed, Vow is the wind driven current at the surface, y is the distance measure in m from seabed and h is the water depth.

Marine Growth Marine growth is an important part in increasing the loads on offshore structures. The growth of marine algae increases the diameter and roughness of members which in turn cause the wave or current loading to increase. Detailed discussion on the member roughness and its relationship with hydrodynamic coefficients can be found in AP I RP2A. The thickness of marine growth generally decreases with depth from the mean sea level and it is maximum in the splash zone. The thickness of marine growth in the splash zone can be as much as 20cm and will reduce below to 5cm. In deeper zones, the thickness may be negligible. Splash Zone is a region where the water levels fluctuate between low to high. The actual elevation of the bottom and top of these vary from location to location due to different tidal conditions. In general terms, the splash zone will vary from -3m to +5m. In structural analysis, the increased diameter of the member ( D = d + tm) shall be included so that the wave and current loads can be calculated correctly. D and d are the diameter of increased member and original member respectively and tm is the thickness of marine growth. The roughness of the marine growth is an important parameter in determining the drag and inertia coefficients. Reference shall be made relevant API RP2A clauses for more details. Morison Equation Wave and current loading can be calculated by Morison equation. Morison equation can be written as:

where FT is the total force, ρw is the density of water, CD and CM are the drag and inertia coefficients respectively, D is the diameter of the member including marine growth, V is the velocity and a is the acceleration.


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The first term in the equation is drag component ( FD) and the second term is the inertia component (FI ). This can be expressed as

Most of the time, current exists in the same direction of the wave propagation and hence the current shall be taken into consideration in the load calculation. However, algebraic sum of wave and current loads is different from calculation of load by adding the horizontal water particle velocity with the current velocity and computing the loads. This is because of nonlinear term in the drag equation. Current velocity shall be added using vector with the water particle velocity before computation of drag force, i.e. V = Vw + Vc where V is the total velocity, Vw is the Velocity due to waves and Vc is the velocity of current. This is required since there is a square term in the drag force equation.

Figure 4.2 - Wave Loads on Jacket Structure

D) Buoyancy Load

The offshore structural members mostly made buoyant by air tight sealing of the welds to avoid water entry. This is purposely planned so that the overall structure will have SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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adequate buoyancy during installation. Typical example is the jacket structure. This kind of structure requires at least a reserve buoyancy of 10% to 15%. The reserve buoyancy is defined as buoyancy in excess of its weight. To obtain this buoyancy, structural tubular members are carefully selected such that their buoyancy / weight ratio is greater than 1.0. This means that the member will float in water. On other hand, if the member is part of a structure supported at its two ends and forced to be submerged by weight of other members; this member will experience an upward force equal to the displaced volume of water. This is called buoyancy force. The buoyancy force can be calculated by two methods. -

Marine Method

-

Rational Method

a) Marine Method The marine method assumes that the member in consideration considered to have rigid body motion. This means that the weight of the member is calculated using submerged density of steel and applied to the member vertically down as an uniformly distributed load. b) Rational Method The rational method takes in to account this pressure distribution on the structure, results in a system of loads consisting of distributed loads along the members and concentrated loads at the joints. The loads on the members are perpendicular to the member axis and in the vertical plane containing the member.

Figure 4.3 - Buoyancy Calculation methods


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E) Ice Loads

For structures located in Polar Regions and cold countries, ice loading shall be considered in the design. In these regions, the ice sheets of varying thicknesses can move from one location to other due to tide and under water current. These ices sheets when come closer and hit the o ﬀ shore structures, large impact force are experienced by the structure. This kind of force cannot be calculated by means of analytical tools. However, based on Experimental studies, an empirical equation is available and can be used to estimate the Force (F ice) F ice = C f A

Where, f ice

= Crushing strength of ice vary between 1.5 MPa to 3.5 MPa

C ice

= Ice force coefficient vary between 0.3 to 0.7

A

= Area struck by ice (Diameter of member x ice sheet thickness)

F) Mud Loads

Platforms located in the vicinity of the river mouth (shallow water platf orms) may experience the mud flow loads. The river flow brings sediment transport and nearby mud towards the platform and may slide through the location. Sometimes over a long period of time sediment settlement at the location of the platform may have sloping surface and mud slides can also generate mud loads. These loads can be calculated using F mud = C mud τ D

Where, C mud

= Force Coefficient vary from 7 to 9

τ

= Shear strength of soil 5 KPa to 10 kPa

D

= Diameter of pile or member)


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Chapter 5 SIMULATION OF STRUCTURE (ANALYSIS) 5.1 General

With the advancement in computer and software technology and availability of computers, the structural analysis of structures has been made easy and fast. There are a number of commercial computer programs available specifically coded to carry out three dimensional structural analyses for offshore structures (Sadeghi 2001). Few programs are listed below. 5.1.1 For Structural analysis 1. SACS - Structural Analysis Computer System (USA) 2. FASTRUDL, MARCS, OSCAR, StudCAD and SESAM 5.1.2 For Hydrodynamic Calculation 1. Maxsurf, Hydromax and Seamoor 5.1.3 For Pile Foundation analysis 1. GRLWEAP, PDA and CAPWAP

The modern day offshore development project schedules do not permit designers to carry out hand calculations due to faster requirement of design and drawings for fabrication. Usually, the first discipline to produce documents and drawings is structural so that the materials can be ordered to mill for production. Hence the structural designers are under very high pressure from fabricators to produce the structural material take off for order placement. The use of structural analysis programs with fast computers has made possible some of the largest structures to be designed in 6 to 8 months.

5.2 Process of analysis

Following preparatory activities are required before analysis and design can be carried out. 1) Structure Geometry Selection 2) Geometry Simulation 3) Foundation Simulation 4) Load Simulation


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5.2.1 Structure Geometry Selection

Structure geometry shall be selected based on various requirements such as layout, water depth, environmental condition, installation methodology and topside loads etc.

Figure 5.1 - Computer Model of a Wellhead Jacket and Deck

5.2.2 Geometry Simulation

A geometric model of a structure contains a database of following information. -

Joints or Nodes

-

Members and Properties

-

Foundation

-

Loads

Each of the above information can be entered in a planned and systematic way so that the post processing and correlating the design drawings with analysis results becomes easier and faster.


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Figure 5.2 - Computer Model of a Jacket with Axis system

5.2.3 Foundation Simulation

Pile Modeling In an offshore structure, the piles hold them on to the sea bed. This needs to be simulated in the structural analysis involving there in place strength and stability. There are type of pile system that can be used in the offshore structures. -

Main Pile

-

Skirt Pile

As it can be seen from the fi gure 4.3, that the skirt pile is always grouted with the skirt sleeve of the jacket. But in the case of main pile, the annulus between the pile and the jacket leg may be grouted or not grouted depending on the design water depth. Like other structural elements of the jacket structure, pile is also a structural member and shall be modeled according to the diameter, wall thickness and material properties. It is the load transfer mechanism between the jacket leg and pile that requires special care in simulation of actual load transfer.


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Figure 5.3 - Pile Simulation for an offshore jacket For the case of grouted skirt piles and main piles, the model becomes much easier by simply specifying the cross section as a ‖Composite Section‖ containing jacket leg, pile and the annulus filled with cement grout. The equivalent axial area, shear area and bending stiffness can be calculated using the equivalent section concept and used in the analysis.

But for the case of main pile, this cannot be done. The pile and jacket are two parallel members physically connected at the top of jacket by means welded connections and elsewhere no welding but spacers are placed inside the jacket leg to provide contact points for load transfer. These spacers are specially located at the horizontal framing such that the lateral loads from the wave and current can be easily transferred to the piles

Soil Simulation Piles below seabed shall be modeled in the structural analysis to reflect the vertical and lateral behavior of pile soil system. This is very essential to simulate the jacket and deck deflections and pile stresses. SARDAR VALLABHBHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT

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Pile Group Effect The skirt piles for very large jackets normally arranged in cluster at each corner to resist the forces from gravity and environmental loads. These pile clusters can be arranged in various ways but due to construction limitations usually they will arranged in closed manner as shown in the Figure 4.4 and 4.5 below. The distance between the jacket leg and the farthest pile shall be kept to a minimum possible for fabrication to avoid unnecessary bending on jacket legs as well on the pile sleeves

It is a good practice to space the centre to centre of adjacent piles at a distance of 3D where D is the diameter of the pile. This will prove a clear distance between the pile face of 2D.Even with this separation, the effect of load on one pile will affect the behavior of the adjacent pile.

Figure 5.4 - Pile Group arrangements for 4 legged platform

Figure 5.5 - Pile Group arrangements for 8 legged platform


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5.2.4 Load Simulation

A) Dead Loads The dead loads of primary structural members such as deck beams, braces, jacket legs and Braces, piles etc shall be calculated by the program automatically based their dimensions and unit weight of material supplied.

B) Equipment Loads Generally, the equipment weight is manually entered based on VENDOR supplied information. The weight of the equipment shall be distributed to the deck beams or plating depending the load transfer method adopted for the design of the equipment skid.

C) Fluid Loads The fluid loads are based on equipment operating weight. This can also be obtained from the equipment manufacturer. The contents of the equipment can be calculated as below.

W fluid = W Oper – W dry

Where, W oper and W dry are the weight of equipment in operating and dry conditions.

D) Drilling Loads Drilling equipment include rig, drill strings, mud tanks, etc. These equipments are also similar to the other equipment described above except that the drilling rig is not fixed equipment. Normally, these shall be applied as point loads on the skid beams. There may be several load cases to cover all the well positions. The complication is due to application of wind loads on this drilling rig structure.

E) Live Loads The live loads shall be applied on open areas not occupied by equipment or facilities. This can be applied as member loads,


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F) Wind, Wave and Current Wind

Wind loads are normally calculated manually and applied to deck edge usually on nodes at the periphery. Diagonal or non-orthogonal wind load cases can be generated from loads from orthogonal cases Wave and Current

The wave and current shall be simulated using software contained modules. Manual calculation and application of these loads will lead large errors and approximations since the number of members are very high.

5.3 NATURE OF ANALYSIS 5.3.1 Dynamic Analysis

Basically, the dynamic analysis is carried out to determine the natural periods, mode shapes etc for further use in the seismic analysis, spectral fatigue analysis. Further, the natural period will be used for the calculation of Dynamic Amplification Factor (DAF) for bot inplace storm analysis and fatigue analysis

5.3.2 Fatigue Analysis

Fatigue analysis can be carried out using the following two methods.

A) Deterministic Method – In the deterministic method, the sea state energy is simulated using discrete frequencies and wave heights with corresponding number of occurrences. Structural responses and hot spot stresses are generated for each of these discrete waves. The summation of fatigue damages due to these discrete wave load cases are then summed up to obtain the total damage during the life of the structure.

B) Spectral MethodSpectral method uses the sea state energy spectra us used to generate the transfer function for the structural response. This transfer function is the used to generate the hot spot stresses in the joints.


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5.3.3 Ship Impact Analysis

In an offshore development, often service boats and supply vessels have to serve the offshore operation. During their trips, due to harsh weather conditions, it may sometime drift and hit the jacket legs or braces. These vessels during their normal approach to the platform may arrive in with normal operating speed or may arrive at accidental speed depending on the weather conditions at the time of arrival. API RP2A specifies a operating a speed of 0.5 m/sec and accidental speed of 2 m/sec. The jacket legs and braces in the splash zone shall be designed of such loads to avoid premature failure and collapse of the platform. Where such impacts are not allowed, a properly designed boat impact guard (sacrificial) shall be provided. For example, the risers located outside the jacket perimeter shall be protected with riser guard or riser protector and this kind of riser guard shall be located sufficiently away (at least a 1m) so that during vessel impact, a riser does not experience large deflection.

1. The purpose of the boat impact analysis is

A) Normal Impact - To ensure the adequacy of the jacket leg and brace members in the splash zone such that they can absorb the energy imparted by a design vessel travelling at normal operating velocity. B) Post Impact Strength - To ensure the compliance of the damaged platform for operating (1-year wave) design requirements after the boat i mpact.

5.3.4 Push over analysis

Push Over analysis is carried out to check the global integrity in terms of collapse behavior. The analysis will be performed using the full non-linear structural model described in Previous Section. The finite element program ABAQUS will be used to carry out the analyses. ABAQUS uses full Newton-Raphson iteration to determine the non-linear response of the platform.


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Chapter 6 CONCLUSION 1) Each platform/ ring type is chosen mainly due to water depth considerably and due to the deck equipment necessary to perform its service. 2) Selection of Offshore structures based on water depthsA) The jack up ring platforms may be used in relatively shallow water depths up to 90m -150m B) The fixed platforms vary in size and height and can be used in water depths up to about 600m C) Semi-submersible platforms are used in water depths up to 1000m D) The tension leg platforms are used in water depths greater than 300m to 1200m. E) The spar platform technology is used for large water depths may be more than 1300m. 3) Forces on these structures due to the ocean and atmosphere include ocean waves and currents, wind, buoyancy and friction at the base. 4) Various loading parameters are described here, which are key factors that must consider in the analysis and design of offshore structures. 5) The present of marine growth, has significant effects on the hydrodynamic loading of offshore structures and should be taken into consideration in t he design and analysis of structures. 6) Compliant structures has found primary offshore application in the oil industry, but in case where a stable ocean platform is needed for communication and mooring. 7) Whole structural platform is analyzed by various ways such as Dynamic analysis, Impact analysis, Seismic analysis, fatigue analysis and push over analysis. 8) Corrosion in offshore structure is main influencing factor in reducing the strength of structure; this can be treated properly for good stability. 9) New software developed for analysis of offshore structure predicts str uctural behavior in all possible manners and it reduces human stress.


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Chapter 7 REFERANCES 7.1 RESEARCH PAPERS

1. Ahmed A. Elshafey, Mahmoud R. Haddara, ―Dynamic Response of Offshore Jacket Structure Under Random Loads‖,Faculty of Engineering and Applied Science Memorial University of Newfoundland, St. John‘s, NL, Canada A1B 3X5, Marine Structures 22 (2009) 504-5212 2. API, 1984. Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms, American Petroleum Institute, API RP2A, and 15th Edition. 3. American Petroleum Institute [API], 1996. (RP-2A 20th edition, and Supplement 1, dated December 1996. 4. API (American Petroleum Institute), Recommended Practice for Planning, Designing and Construction Fixed Offshore Platforms, API Recommended Practice 2A (RP 2A) th 19 Edition, Washington, 1991. 5. Barltrop, N.D.P and Adamd, A.J, Dynamics of Fixed Marine Structures, Third Edition, Butterworth-Heinemann Ltd and Marine Technology Directorate Ltd, London, 1991. 6. Chakrabarti, S.K. 1994. ―Hydrodynamics of Offshore Structures‖, Southampton: Computational Mechanics Publications. 7. Esper, P., Duan, X., and Willford, M., Numerical Modelling in the Seismic Design of Oil and Gas Platforms, 1st International Symposium on Earthquake Engineering, Riyadh, Saudi Arabia, 2000 8. Faltinsen, O., (1994), "Wave and Current Induced Motions of Floating Production System", J.Applied Ocean Research, 15, 351-370. 9. Gudmestad OT, Moe G. ―Hydrodynamic Coefficients for calculation of hydrodynamic loads on offshore structures‖. Marine Structures 1996:9(8):745 -58. 10. Hallam, M. G., Heaf, N.J and Wooton, L.R, Dynamics of Marine Structures: Methods of Calculating the Dynamic Response of Fixed Structures Subjected to Wave and Current Action, Report UR8. Construction Industry Research and Information Association Underwater Engineering Group, London, 1977. 11. Haritos, N. 1996. ―Analysis and Design of Offshore Structures – an Overview‖, Proc. 10th Aust. Conf. on Mechanics of Strut. & Materials, 2007, Adelaide, pp 253-258.


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Offshore Engineering - An Overview

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