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ANALYSIS OF A TYPICAL OFFSHORE BRIDGE

CONNECTING ADJACENT OFFSHORE PLATFORMS

SUMMER INTERNSHIP

AT

L&T VALDEL ENGINEERING LIMITED

SUBMITTED BY

CHINMOY PATHAK CHOUDHURY

27th JUNE, 2012 – 26th JULY, 2012

DEPARTMENT OF CIVIL ENGINEERING

ASSAM ENGINEERING COLLEGE, JALUKBARI

GUWAHATI – 781013

Table of Contents

1. INTRODUCTION 1

2. EXPLORATION, DRILLING AND EXTRACTION 2

3. OFFSHORE DEVELOPMENT 5

4. TYPES OF OFFSHORE STRUCTURES 7

5. PRELIMINARY REQUIREMENTS FOR DESIGNING 11

6. TYPES OF LOAD ON PLATFORM 12

7. INSTALLATION OF FEXED STEEL STRUCTURES 13

8. TYPES OF ANALYSIS 15

9. LOADOUT ANALYSIS 17

10. LIFT ANALYSIS 19

11. TRANSPORTATION ANALYSIS 22

12. DESIGN CONSIDERATIONS 26

13. APPENDIX A - PLATFORM POSITIONING AND BRIDGE SPECIFICATIONS 27

14. APPENDIX B - CODES, STANDARDS AND REFERENCE DOCUMENTS 34

15. APPENDIX C - DESIGN CALCULATIONS AND COMPUTER MODEL FOR IMPLACE AND

LIFT ANALYSIS 36

16. APPENDIX D - BASIC LOADING AND LOAD CASE SUMMARY 45

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Chinmoy Pathak Choudhury

Assam Engineering College, Jalukbari

Guwahati - 781013

1. INTRODUCTION

Oil a d gas a e o side ed to e a o g the o ld s ost i po ta t esou es. The oil a d gas industry plays a crucial role in driving the global economy. Oil is not only an essential raw material to

over 2,000 end products but also useful for transportation, heating, electricity and lubrication. It

supplies a out 5 % of the o ld s total e e g e ui e e ts. Deep ate oil ese es a e e pe ted to play an important role in the future of the world oil and gas energy. So, intensive activities for

exploration and exploitation of the oceans, especially the growing demands for hydrocarbons led to the

development of wide range of offshore structure during the last decades.

Throughout the world, estimated proved reserves of petroleum have been reported to be about

1.29 trillion barrels of oil and about 6,110 trillion cubic feet of natural gas. More than 90 percent of

reserves were based on land or near shore. About 90 countries produce oil, although a few major

producers account for the bulk of world output. Oil reserves are heavily concentrated in the Middle East

while gas reserves in the Russian Federation.

The variety of Offshore structure concerning the function, size, geometrical configuration and

material selection as well as the variability of the environment factors complicate the development of

a unique design procedure. Therefore, a separate investigation of the interaction between the actual

structure and the environment is necessary.

On the other hand, Offshore structures must have an acceptable margin of safety during all phases

of their life i.e. construction, transportation, installation, operation, and retrieval. The most adverse

conditions during the entire life of the structure have to be taken into consideration in its design

process.

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2. EXPLORATION, DRILLING AND EXTRACTION

The petroleum industry comprises of upstream and downstream sectors. The Upstream sector

involves exploration, development and production of oil and gas; Downstream sector covers transport,

refining petrochemicals, distribution and retail.

Oil and gas consists of the processes and methods involved in locating and discovering potential

sites for oil and gas drilling and extraction. This is the first stage of oil and gas production. Production

Platforms may have separate drilling platforms nearby while large production platforms may have their

own production drilling equipment. The different forms of drill rigs are as follows –

2.1. Jack-up Drill Rig – A Jack-up drill rig or a self elevating unit is a type of mobile platform that consists

of a buoyant hull fitted with a number of movable legs capable of raising its hull over the surface

of sea. These platforms are designed to move, under its own power or using tugboats, from place

to place and then anchor themselves by deploying the legs that are driven into the sea bed using

motors. Then the derrick slides outward into position and the drilling operation begins. There are

two ways to mount the drilling equipment on the hull which can be further categorized as –

Cantilevered Type - This is the most

popular design of jack-up platform.

The drilling derrick is mounted on

two arms, or cantilevers, that

extend away from the platform and

over the sea surface. This design

provides the derrick with a large

range of motion and therefore can

operate in a considerable area at

some instance of leg positioning.

Fig 1. A Model of Cantilevered type Jack-up Drill Rig

Slot-Type - In a slot-type jack-up, the platform has a slot through the floor of the hull. A drilling

derrick is positioned over it during operations. When the platform moves to another location,

the derrick is lifted above it before the transportation operation is started.

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2.2. Semi-submersible Drill Rig – These are rigs having legs of sufficient buoyancy to the structure to

float but of weight sufficient to keep the structure upright. It can be moved from place to place

and can be ballasted up and down by altering the amount of flooding in buoyancy tanks. They are

generally anchored by cable anchors during drilling operations. Semi-submersible drill rigs are the

most stable of any floating rig and chosen mainly for harsh conditions because of their ability to

withstand rough waters. Based on the way the rig is submerged in the water, there are two main

types of semi-submersibles.

Bottle-type – It consist of bottle-shaped hulls below the drilling deck that can be submerged by

filling the hulls with water.

Column-stabilized – Here, two horizontal hulls are connected via cylindrical or rectangular

columns to the drilling deck above the water. Smaller diagonal columns are used to support the

structure.

Fig 2. Semi-submersible Drill Rig Fig 3. A 3D model of Semi-submersible Drill Rig

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2.3. Drillships – It is a marine vessel that has

been fitted with drilling apparatus. It is

most often used for exploratory offshore

drilling of new oil or gas wells in deep

water but can also be used for scientific

drilling. It can conduct drilling operations

upto 2500 m deep. Drillships have

extensive mooring or positioning

equipment, as well as a helipad to receive

supplies and transport staff. Drillships are

differentiated from other offshore drilling

units by their easy mobility. However, they

are susceptible to being agitated by waves,

wind and currents.

Fig 4. A Typical Drillship

Fig 5. A 3D Model of Drillship

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3. OFFSHORE DEVELOPMENT

In Offshore development, different types of structures are used depending on the size and water

depth. Typical Offshore platforms consist of a substructure called Jackets supporting processing

facilities referred to as Topsides. Such Offshore platforms are installed offshore in water depths ranging

from 8 m to 600 m. Offshore platforms can be classified as below based on their purpose:

1. Wellhead Platforms - It is an offshore structure which is an assembly of fittings, valves and

controls providing facilities for the extraction of crude oil, gas and water composite from the sea

which is then transported by risers (pipelines) to the process platform. It stands either on a

tripod (3 legged) or a 4 legged jacket arrangements. The wellhead operation can be conducted

either on the process platform or underwater which feeds into the Process plants from where

the crude oil and gas composites are sent into separators.

2. Process platforms - It is a processing plant which generally stands on a 6 legged or 8 legged

jacket arrangements located on the sea which partially processes the crude oil and gas before

exporting the hydrocarbon onshore by means of pipelines or are stored so that it can be loaded

on tankers for transportation to onshore production facilities. Generally, there are 3 levels of a

Central Processing Plant viz. Cellar, Mezzanine and Main Deck.

3. Living Quarters Platforms – It is built to house personnel working on those Offshore platforms.

A typical Offshore complex consists of Process platforms and several Wellheads underwater

connected to the process platforms by risers or pipelines.

Fig 6. A Typical Process Complex

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Offshore structures can be classified into 3 broad categories based on their foundation concepts as –

3.1. Bottom Fixed Structures

Jacket

Compliant Tower

Gravity based Structures

3.2. Floating Type Structures

Tension Leg Platforms

Floating Production, Storage and Off-loading (FPSO)

Floating Production Systems (FPS), etc.

3.3. Subsea Systems

Each type of platforms carries some advantages and disadvantages. However, the foundation

concept selection depends upon various factors such as depth of installment, purpose of installment,

Topside information, Intensity of Environmental Loads, etc.

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4. TYPES OF OFFSHORE STRUCTURES

4.1. Bottom Fixed Structures

Fixed Platform (FP) is a platform extending above the water surface and supported at the sea bed

by means of piles, spread footings, etc. It consists of a jacket (a tall vertical section made of

tubular steel members) 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 upto 500 m deep.

Compliant Tower (CT) consists of a narrow, flexible tower and a piled foundation that can support

a deck for drilling and production operations. Unlike the fixed platform, the compliant tower

withstands large lateral forces by sustaining significant lateral deflections by flexible legs to

reduce resonance and de-amplify wave forces, and is usually used in water depths between 300

m and 600 m.

Fig 7. A Fixed Platform Fig 8. A Compliant Tower

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Gravity-based Structure (GBS) is a support structure held in place by gravity. It is constructed of

steel reinforced concrete often with tanks to control the buoyancy of the finished structure.

Fig 9. A Gravity-based structure

Fig 10. Bottom Supported and Vertically Moored Structures

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4.2. Floating Type Structures

SPAR Platform (SP) consists of a large diameter single vertical cylinder supporting a deck. It has

typical fixed platform topside (surface deck with drilling and production equipment), the three

types of risers (production, drilling and export) and a hull. These are presently used in water

depths upto 1000 m, although existing technologies can extend its use to water depths as great

as 2500 m.

Tension Leg Platform (TLP) is a vertically moored floating structure held in place by vertical and

tensioned tendons of relatively high axial stiffness (low elasticity) connected to the sea floor by

pile-secured templates. While a uo a t hull suppo ts the platfo s topsides, a i t i ate mooring system keeps the structure in place. The tension leg mooring system allows for

horizontal movement with wave disturbances, but does not permit vertical movement. The

larger TLPs have been successfully deployed in water depths approaching 1200 m.

Floating Production System (FPS) consists of a semi-submersible unit which is equipped with

drilling and production equipment. It is anchored in place with wire rope and chain. Production

from the subsea wells is transported to the surface deck through the risers. It can be used in a

water depth ranging from 600 to 7,500 feet.

Fig 11. A Tension-Leg Platform ` Fig 12. Floating, Production, Storage and Offloading

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Floating, Production, Storage and Off-Loading (FPSO) consist of a large tanker type vessel

moored to the seafloor and designed to process and stow production from nearby subsea wells.

It periodically offloads the stored oil to a smaller shuttle tanker which then transports the oil to

an onshore facility for further processing.

4.3. Subsea Systems – These can have single or multiple wellheads on the seafloor connected directly

to a subsea manifold. The systems include connection by flowlines and risers to fixed or floating

systems. It can be set at large depths.

Fig 13. Subsea Systems

Fig 14. Floating Type Structures

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5. PRELIMINARY REQUIREMENTS FOR DESIGNING

5.1. Platform Geometry – The geometry of the platform is influenced by -

1. Topsides Information

2. Geotechnical Data

3. Environmental Data

4. Seismic Data

5. Installation Equipments

The equipment layout is an engineering drawing representing the general layout of a platform viz.

pipelines, electrical wires, equipments, etc. which helps the structural engineers to formulate the

structural framework for achieving the required design particulars. It includes descriptions of Piping

equipments, Mechanical equipments and Electrical wiring.

5.2. Concept Selection

1. Selection of Topside Layouts – The layout will be greatly influenced by the type of facility such

as wellheads, processes, living quarters, bridge landing location, etc.

2. Selection Of Jacket Geometry – It will be Influenced by –

The water depth which will determine the number of legs.

Installation Methods such as lift or launch.

Geographical Location.

3. Evaluation of Gravity Loads such as structural dead loads, facility loads, etc

4. Evaluation of Environmental Loads such as Wind loads, Wave or Current Induced loads, etc.

5. Evaluation of Seismic Activity to determine the level of earthquake proneness of the area.

6. Evaluation of Geotechnical Parameters such as hardrock depth, borehole information, axial pile

capacity, etc.

7. Availability of Installation Equipments such as transportation and launch barges, lifting cranes,

grouting and leveling equipment etc.

8. Availability of Fabrication Facilities such as Yard Cranes, quality manpower, steel supply, etc.

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6. TYPES OF LOADS ON PLATFORM

Offshore structures are subjected to severe loading conditions. The different types of loads can be

classified as –

6.1. Gravity Loads

Structural Dead Loads – It includes all the fixed items in the platform deck, jacket, etc. which

includes all primary structural members, secondary structural items such as handrails, stiffeners,

etc.

Facility Dead Loads – The structure built either for drilling or wellhead type platform or for

process type platform supports various equipments like mechanical equipment, electrical

equipment, etc.

Fluid Loads – The loads of the fluid which are in the pipelines and equipments.

Live Loads – These are moving loads and are temporary in nature.

Drilling Loads – It includes the reaction from Jack-up Cantilever type rigs or Deck mounted rigs.

Setback Load – It is the weight of drill string pipes hanging from drill mast side before placing into

the drill assembly.

Hook Load – It is the weight of the drill string hanging from the drill mast during every stage of

the drilling.

6.2. Environmental Loads

Wind Loads

Wave and Current Induced Loads

Buoyancy Loads

Ice Loads

Seismic Loads

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7. INSTALLATION OF FIXED STEEL STRUCTURES

The Components of a Fixed Steel structure are described below –

7.1. Subsea Template – The first installation operation involves piling of the subsea template on the sea

bed in a location most favourable for extracting the hydrocarbon deposits. The templates provide

a guide frame through which wells can be drilled prior to the arrival of the jackets and it also assists

the accurate positioning of the jackets.

Fig 15. A Subsea Template

7.2. Jacket – Jacket is a structural framework which transfers the load of the topside/ deck to the

supporting sea floor. These are completely fabricated in a yard (onshore) from where it is transported

to the installation site (offshore) on a barge/ship. The jackets can be classified based on their depth

–

Shallow Water Jacket (<500 m) – These are lifted, due to their less weight within the carrying

capacity of cranes, and then installed vertically in the sea bed.

Deep Water Jacket (500-1500 m) – These may be either lifted or launched from barge depending

upon their weight and the installed accordingly.

Ultra Deep Water Jacket (>1500 m) – These are launched from a barge due to its high weight

beyond the lifting capacity of crane and then installed by upending supported by ballasting the

tanks.

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Fig 16. A Fixed Jacket Fig 17. A Topside

7.3. Topside – It is the upper part of a fixed installation which sits on top of the jacket and consists of

the decks, accommodation and process equipments. The weight of the topside may be as low as

1500 tonnes to as high as 35000 tonnes. The latter is clearly beyond the lifting capacity of any

crane. The lifting capacity that has been achieved till date is 14000 tonnes. Consequently, the larger

constraints must be constructed within the constraints of the lifting capacities available. Therefore,

the platform is divided into liftable packages which can be secured and welded one at a time. The

heavy lift cranes used for installing the rigs are mounted on semi-submersible barges which have

been built for offshore work.

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8. TYPES OF ANALYSES

8.1. Pre-service analysis – These are the analysis carried out on a jacket before it is welded to the

topside/deck. Assuming the following cases -

Case A -Shallow Water Jacket (<500 m)

Case B - Deep Water Jacket (500-1500 m)

Case C – Ultra Deep Water Jacket (>1500 m)

Yard Reaction – The jacket is placed horizontally and is simply supported. The supporting

members are to be analysed for bending moments, shear and deflection. (Required for Case A,

B and C)

Roll Out – The components of the jacket is lifted one by one in a specified sequence and welded

together to form the jacket. It is done in case of heavy jackets which cannot be manufactured in

one go. (Required for Case B and C)

Load Out – This analysis is done to determine the optimal method for loading the structure from

the fabrication yard to the transportation barge. (Required for Case A, B and C)

Transportation – It is to be checked whether the assemblage/assembly to be transported on the

barge/ ship is safe against severe environmental conditions that develops transportation forces.

The various transportation forces can produce motions such as Heave, Sway, Surge, Yaw, Pitch

and Roll. (Required for Case A, B and C).

Lift (Offshore) – Light Jackets are lifted by a barge crane and is installed vertically on the site.

The lift points shall be selected such that the Centre of Gravity of the structure is at the middle

of the lift points. This will give equal sling loads on the slings. (Required for Case A and B)

Launching (Offshore) – Heavy Jackets are mechanically pushed from the barge using mechanical

jacks/hydraulic jacks such that it floats on the water. It is to be analysed for coefficient of friction,

winch speed, etc. (Required for Case B and C)

Floatation (Offshore) – Heavier Jackets are often slided to the oceans from the barge by

ballasting one end of the ship with water. Buoyancy tanks are provided to enhance the floatation

properties of the jacket. (Required for Case B and C)

Upending (Offshore) – The floating jacket is lifted by a crane to install the jacket vertically on the

site. (Required for Case B and C)

On-Bottom Stability – Stability of the jacket after upending and before driving the piles into the

seabed is to be analysed.

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8.2. In-service Analysis – These are the analyses which are required to be carried out on the structure

by simulating the behavior of structure after its construction.

In-place Analysis

o Local – This is carried out to simulate the behavior of structures as close as possible and to

obtain the response of the structural members locally to individual or combined action of

different forces during its service.

o Global – This is carried out also to simulate the behavior of structures as close as possible and

to obtain the response of the entire structure as a whole due to service load conditions and

environmental forces during its service. In other words, these analyses are to check the global

integrity of the structure against premature failure.

Fatigue Analysis – Due to stress range and repeated loading and unloading operations, a

material loses its property which is called fatigue. For example – due to wave, ocean currents,

wind, etc.

Earthquake/ Seismic Analysis – This analysis is imperative to be carried out in areas of high

earthquake magnitude zone and the structure is designed for seismic load also.

Pushover Analysis – The structure is analysed and designed for safety against water current and

wave forces and wind forces on the jacket and topside/ deck so that it does not generate an

overturning moment on the entire structure.

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9. LOADOUT ANALYSIS

Loadout Analysis is done to safely transfer structure from the fabrication yard to the

transportation barge .The purposes of Loadout analysis are the following –

1. To determine the allowed displacement between the fabrication yard and the transportation

barge.

2. To determine the reactions at various support conditions during the operation.

3. To check the members and joints against the loads that they are carrying and transmitting to

the barge.

4. To simulate the inertial effects of motion due to movements of structure and barge system and

to obtain the motion induced loads.

The following are the methods of loading out of a structure/ jacket from the yard on to the

transportation barge -

1. Lifted Loadout - The jacket is lifted and is placed on the barge/ship for transportation and

installation in the site. It is only feasible in case of light jacket such that its weight is less than the

lifting capacity of the crane.

2. Skidded Loadout - When the jacket is too heavy to be lifted by the crane as its lifting capacity is

limited, it is made to slide along the yard over skid beams, which can be continuous or discrete,

using hydraulic or mechanical jacks to the barge for transportation. The surface of the barge is

brought to the same elevation as the yard by ballasting the tanks with water.

Fig 18. Skidded Loadout of a Jacket (Plan view)

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3. Trailer Loadout – The structure is loaded on a trailer which carries it to the barge and then the

trailer is removed from the barge.

Fig 19. Trailer Loadout of a Jacket

During Loadout, the barge is moored and the elevation of the skid rails on the barge is leveled

with the elevation of the yard by adequate ballasting of tanks so that there is no risk of uncontrolled

sliding and the load gradually increases on the barge. Launch Jackets will have launch truss and cradle

at predetermined spacing and hence Loadout is done only by skidding the jacket from the yard onto the

barge using pull wires and winches. For deck trailer Loadout, the deck is provided with temporary braces

to provide a suitable carrying point on a deck over a trailer.

The barge is loaded with the following equipments –

1. Crane – It is a lifting machinery which is used to displace structure which is to be installed in site. It

is to be considered that the weight of the structure to be displaced is within the safe lifting capacity

of the crane.

2. Trailer - Trailers are multi-axle load balancing wheels with appropriate spreader girders on top.

3. Ballast Pump – Its purpose is to pump water into the barge to control barge submergence.

4. Mooring – It is done to anchor the barge or ship so that its position does not change due to currents

and hence enabling it to operate in deep water.

Topsides and Jackets which are light in weight are only feasible to be displaced by lifting. If these

are heavy, skidded loadout or trailer loadout can be adopted.

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10. LIFT ANALYSIS

The purposes of carrying out Lift Analysis are as follows –

1. To safely lift the structure from the yard to the barge or from the barge to the installation site.

2. For selecting suitable lifting gears such as shackles, slings and its attachments with adequate factor

of safety.

3. To check the global integrity of the structure against premature failure.

4. To simulate the inertial effects of motion due to movements of structure and barge system and to

obtain the motion induced loads.

The various equipments for Lifting are as follows –

1. Crane – It is a lifting machinery which is used to move a structure. It is to be seen that the weight of

the structure to be moved is within the safe lifting capacity of the crane.

2. Sling – It is a high tension rope attachment between the crane and the structure to be lifted.

3. Shackle – It is an attachment enabling the sling to handle smoothly and transfer the loads without

damage.

4. Padeyes – These are plates welded and sufficiently stiffened to primary members at suitable location

to distribute the loads to the slings.

Fig 20. A Lifting Operation (Elevation View)

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Lift points are suitably selected points on the module by which the module can be safely lifted by

a crane. It is generally selected at a distance of 0.2 – 0.3 times the longitudinal length of the module

from each end longitudinally depending on the feasibility and weight distribution of the module. It is

done to decouple or nullify the moment generated due to bending between the supports. Lifting is done

such that the main hook is directly above the Centre of Gravity of the structure to distribute the loads

equally to the slings. The purpose of lift points can pointed out as follows –

1. To lift jacket or deck fabricated in a yard and install padeyes.

2. Upending is required for jackets fabricated in horizontal condition and it is required to be made

vertical either in the yard or in the final installation site. Usually at site, upending is done in water

but small water jacket may be upended in the yard and transported vertically.

Fig 21. Side Lift Operation (Plan View)

The lift capacity of the crane depends on –

1. Lift radius – As the lift radius increases, lift capacity decreases.

2. Lift Hook Height – Lift Hook height increases with decreases in lift radius. Emphasis should be such

that the radius selected brings the main hook directly above the Centre of Gravity of the module to

be lifted. It is to be ensured that the minimum angle between the slings and the horizontal is 60° so

that the beams connected to padeyes are subjected to lesser minor axis moments.

3. Lift Method – Whether the method is a side lift or a stern lift.

4. Sea-state – Whether the surface of the sea with respect to the wind and wave motions favour the

lifting operation in the site.

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For most of the time during lifting, the tensile stresses in the slings are the same during the

operation but during upending of jacket, the stresses in the sling changes due to the rotation of the

jacket.

Design Dynamic Factors for lift analysis as suggested by API RP 2A guidelines

Condition Members connected to

Padeyes

Other Members

Open Water 2 1.35

Shielded Water 1.5 1.15

On Shore 1 1

It is used to account for the following eccentricities –

1. Centre of Gravity Shift – When there is difference in position of the main hook and the Centre of

Gravity of the module in the X-Y plane, a moment is generated. Hence an allowance is made in the

initial design to account for the change.

2. Weight Variation – Similarly, a variation in weight is also expected from the calculated weight due to

acceleration/ deceleration of the barge. Allowance is also provided to account for this variation.

3. Sling Length – The Length of the Slings provided is not equal to the ideal sling length for equal load

distribution and therefore an allowance is provided to account for this difference.

4. Skew Load Effects – It distributes the loads of the module to the slings artificially.

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11. TRANSPORTATION ANALYSIS

The purposes of transportation analysis –

1. To check whether the barge is safe against stability, strength and is adequate to carry the structure

during its voyage.

2. To obtain the motion induced loads during its Tow from the yard to the final location.

3. To satisfy the code requirements against safety of the structure and supporting foundation on the

barge.

Transportation of the structure is done by different tow methods. There are 3 different Tow methods

which can be listed as the following –

1. Dry Tow – This is the most common method of towing. The structure is loaded on a dump barge and

then towed by a tug which normally moves at a speed of about 6 knots.

2. Wet Tow – In this method, self propelled vessels transport the structure without having the need of

a tug separately. The speed at which the vessel moves can reach upto 12 knots reducing the time of

transportation and risk during the voyage. Hence, this method is normally employed for trans-

continental tow.

3. Self Tow – In this method, the structure itself is towed directly i.e. without any barge support. Hence,

special care is taken for this method.

Fig 22. A Jacket being Dry towed

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Fig 23. Topside being Dry towed

The Tow route shall be selected such that the distance travelled is small. It is preferable that the tow

route is along the coast line. Therefore, barge selected for transportation and the environment must

also fulfill some criteria as the following –

1. Adequate space for the cargo items and sufficient stability during tow for the seastate during that

period.

2. The water depth at the quayside should be sufficient for the barge to approach the yard and to carry

out the Loadout operation.

3. The barge must not deflect excessively due to the cargo.

4. It must have the necessary emergency equipment and documents for certification.

For the purpose of spreading the large vertical load of the structure/ cargo directly of the barge,

grillage are used which acts like support stubs providing a larger surface area. The location of the grillage

is determined such that the barge does not deflect excessively. The structure is held by sea-fasteners

connecting it to the barge deck for translational restraining of the structure during transportation.

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Fig 24. Grillage for Deck Structure

Fig 25. Sea-fastening Details

Transportation is the most delicate operation as the barge carries the structure under severe

environmental condition and therefore a proper transportation analysis is imperative. In sea, it is

difficult to control the motion of a body than in land. Due to the various environmental forces in open

water, the most critical condition is developed mainly by roll motion of the barge.

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The motion of the barge can be defined in 6 parameters as follows.

Barge Motion Parameter Movement and Direction

Surge Translation along X - axis

Sway Translation along Y - axis

Heave Translation along Z - axis

Roll Rotation along X - axis

Pitch Rotation along Y - axis

Yaw Rotation along Z - axis

Fig 26. Barge Motion Parameters Definition

Fatigue occurs when a body is subjected to repeated loads depending on the intensity and direction

of load. Sea-fastenings are subjected to repeated loads of varying intensity and direction during

transportation. Hence for long tow periods, it is recommended that the critical joints should be checked

for fatigue.

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12. DESIGN CONSIDERATIONS

1. Marine Growth – Marine growth around the submerged structural members increases the wave/

current loads as the diameter increases adding to the total weight. The density of marine growth is

around 1300 kg/m3. The structure is to be analysed taking this consideration into account.

2. Cathodic Protection – Corrosion protection relying on sacrificial anodes to protect submerged steel

jacket components from corrosion by electrolytic action.

3. Corrosion Allowance – In addition to the cathodic protection to the jacket members, the corrosion

allowance shall also be provided. This is required since the cathodic protection does not protect the

structure effectively in the splash zone where the structure is not continuously submerged. Hence

sacrificial thickness shall be provided in addition to the design wall thickness of the member.

Typically, corrosion allowance is in the range of 6 mm to 12 mm depending upon the locality and

corrosiveness of the sea water.

4. Hydrodynamics factors

Wave Kinematics factor – This varies between 0.8 to 0.95. This is to be applied since the calculated

wave factor is based on 2-dimensional wave theory while the actual loading is from 3-dimensional

wave conditions.

Conductor Shielding factor – The presence of rows of conductors provide a shielding effect to the

conductors behind them. It depends on the spacing and number of conductors.

5. Dead Weight of Non-Structural members – It shall be calculated for each item and applied to the

deck/ jacket at appropriate locations. Since most of the values will not be available initially, their

reasonable values are needed to be assumed at the start of the analysis. Some examples of non-

structural members are Monorails, Handrails, Walkways, Grating, Piping, etc.

6. Buoyancy tanks - These are not required after the installation of the jacket is complete. But it is not

always possible to remove and if left permanently, the wave loads on these tanks shall be considered.

7. Load Combinations – The load combinations shall be formed such that the most critical section during

the design life of the structure covers maximum compression loads, maximum tension loads and

maximum moment.

8. Environmental loading – Offshore structures are subjected to considerable lateral and overturning

loads from the winds, waves and currents. Therefore, they are required to carry general load

combination of moment, horizontal and vertical loads.

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APPENDIX A - PLATFORM POSITIONING AND BRIDGE SPECIFICATIONS

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The above diagrams show a number of plans for the positioning of the non-existing HRD platform

with respect to the existing HRC platform. The direction of the prevailing wind is towards North. A

bridge will connect the two platforms in order to assist movement of personnel and to fulfill

departmental requirements. Each of the plans is observed and the plan which is found most suitable

for installation is accepted. The summary of the observations are tabulated below.

Plan 1 1. The installation of a bridge is not feasible.

2. The exhaust from the PCG modules on HRD platform will return back to the

modules due to the wind.

Plan 2 1. The exhaust from the PCG modules on HRD platform will return back to the

modules due to the wind.

2. The exhaust from the PCG modules on HRC platform will interfere with the crane

operation in HRD platform.

3. It will be inconvenient for supply boats to access the platforms as the clearance

line will be too narrow.

Plan 3 1. The installation of a bridge will be obstructed by the PCG module on HRC

platform.

2. The exhaust from the PCG modules on HRC platform will interfere with the crane

operation in HRD platform.

3. There will be an insufficient clearance for the barge crane to approach and install

the bridge in position. Moreover, the barge crane cannot approach from the

West as it cannot be moored.

Plan 4 No disadvantage

Thus, Plan 4 is adopted as it does not have any disadvantage. The bridge span is found as 64.5 m.

However for design purpose, the span of the bridge is considered as 74m. It will be pinned at the HRC

platform as there is a risk that it may collide with the PCG module on HRC platform if longitudinal

translation is allowed; and sliding at the HRD platform as there is some free space. It is designed to

carry, Piping, Electrical and Instrumentation cable trays and equipped with 2 monorails to assist

material movement. The bridge designed is a two level rectangular structure with the following

dimensions.

Length - 74.00 m

Height - 10.00 m

Width - 7.50 m

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It is to be installed by single crane lift (conventional four point lifting arrangement) and is simply

supported on the two platforms by support stubs of 0.5 m each. The bridge geometry includes

horizontal framings at:

Main Level - I - EL (+) 28.150 m (TOS)

Main Level - II - EL (+) 33.150 m (TOS)

Bridge Roof - EL (+) 38.150 m (TOS)

Note: All elevations are with respect to Chart Datum EL (+) 0.0. Chart datum in this case corresponds to

Mean Sea Level.

The Bridge is essentially made of tubular sections because of the following reasons.

These are comparatively lighter in weight than other sections. It is desirable that the self - weight

of the structure should be kept minimum as possible.

These have the same moment capacity in both x and y – axis. As the bridge will have to endure

environmental forces such as wind, it must have a high moment capacity in the minor axis.

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APPENDIX B –CODES, STANDARDS AND REFERENCE DOCUMENTS

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CODES AND STANDARDS

The bridge structure has been analysed and designed in accordance with the current editions and

related amendments of the following codes and documents:

1. API-‘P A, ‘e o e ded P a ti e fo Pla i g, Desig i g & Co st u ti g Fi ed Offsho e Platfo s W“D , 21st Edition ERRATA and Supplement 3, October 2007.

2. SACS (Structural Analysis Computer Software) Release 5.3 SP2.

. Desig of Tu ula Joi ts fo Offsho e “t u tu es –UEG

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APPENDIX C –

DESIGN CALCULATIONS AND COMPUTER MODEL FOR INPLACE AND LIFT ANALYSIS

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In order to arrive at the member sizes to be utilized in the computer model, a preliminary hand

calculation has been performed. This calculation has been based on the assumption that the bridge

operating weight (structure self-weight, discipline loading and live load) can be idealized as a uniformly

distributed load of intensity 13.5 MT/m. This loading is not sacrosanct but is typically between 10 – 15

MT/m for most bridges connecting Offshore platforms. The design calculations are summarized below.

Bridge Top and Bottom Chord : OD 660 x 20 WT

Elevation Bracing : OD 508 x 20 WT

Plan Brace : OD 406 x 12.7 WT

C.1. CALCULATIONS

Given, Span of the bridge = 74 m

Uniformly distributed load = 13.5 MT/m2

Width = 7.5 m

Height = 10 m

Fig 1. Bridge Elevation

Maximum bending moment on the two longitudinal frames, Momax = wL2/8

= (13.5 x 9.81 x 742)/8

= 90651 KNm

Maximum bending moment on each frame, Mmax = Momax /2

= 90651 /2 KNm

= 45325.5 KNm

Axial Compression on Top chord or Axial Tension on Bottom chord = Mmax /e

= 45325 /10

= 4532.5 KN

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Design of Top Chord

The Top chord is in compression

Therefore, assuming kL /r = 80 Cl. E.2 AISC

Critical Slenderness Ratio, Cc =�(2E/fy)0.5

= �(2 x 2 x 105 / 345)

= 106.97

Now, R = (kL /r)/Cc = 0.748

Since, R < 1

Therefore ,allowable stress, fa = (1 – R2 /2)fy ____

(5/3) + (3R /8) – (R3 /8)

= 131.16 N /mm2

Now, Cross – sectional area required = P /fa

= 4532.5 x 103 / 131.16

= 34557 mm2

Adopting ∅660 x 25.4, D/t = 25.98

Area provided = �(D – t)t

= �(660 – 25.4) x 25.4

= 50638.83 mm2 > 34557 mm2 Hence, it can be adopted.

Now, shear on each end on each member = (W/2)/2

= 9990 /4 = 2497.5 KN

Shear stress = V/(0.5A) Cl. 3.2.4-1 API-RP 2A

= 2497.5 x 103 /(0.5 x 50638.83)

= 98.64 N/mm

Allowable shear stress = 0.4fy

= 0.4 x 345

= 138 N/mm > 98.64 N/mm

Slenderness ratio is given by kL/r, where r = (I/A)0.5

For Tubular sections, I = �(Do4 – Di

4)/64

= �(6604 – (2 x25.4)4)/64

Therefore, Slenderness ratio = kL /r L = 7400 mm

= 32.95 < 80 Hence, it is safe.

For the bottom chords which will be in tension, the allowable stress is more as there is no

buckling. Therefore, we can adopt a smaller section than the top chord. However in design practice,

we provide the same section as the top chord.

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Design of Elevation Braces

Fig 2. Shear force description (Bridge Elevation)

α = tan-1 (7.4/10)

= 36.5°

We k o , ƩV =

Fcos 36.5°= 2497.5 KN

F = 3106. 9KN

which is the axial compression.

Therefore, Area required

= F/fa

= 3106.9 x 1000 /131.16

= 23687.86 mm2

Adopting ∅508 x 25.4,

Area provided = �(508 – 25.4) x 25.4

= 38509.77 mm2 > 23687.86 mm2

Similarly Slenderness ratio, kL/r = (74002 + 100002)0.5

(I/A)0.5

= 72.71 < 80

Calculation of Wind Forces (for extreme storm condition)

Maximum elevation of platform (HRD and HRC) = 27650 + Bridge Height Refer. Mumbai High

= 27650 + 10000 Field

= 37650 mm

= 123.492 ft

Extreme Wind Speed at 10 metres above sea level = 161.39 Km/hr

= (161.39 x 1000)/(3600 x 32.8) ft/s

= 147.04 ft/s

For bridge more than 50 m span, averaging tome period = 15 sec gust

C = 5.73 x 10-2 (1 + 0.0457 x Uo)0.5 Cl. 2.3.2-2 API-RP 2A

= 5.73 x 10-2 (1 + 0.0457 x 147.04)0.5

= 0.1592

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1 hour mean wind speed at elevation of z ft,

U(z) = Uo(1 + C.ln(z/32.8))

= 147.04 (1 + 0.1592 ln (123.492/32.8))

= 178.1 ft/s

Turbulence intensity, Iu(z) = 0.06 (1 + 0.0131 x Uo) x (z/32.8)-0.22 Cl. 2.3.2-2 API-RP 2A

= 0.06 (1 + 0.0131 x 147.04) x (123.492/32.8)-0.22

= 0.131157

Design Wind Speed, Cl. 2.3.2-1 API-RP 2A

u(z,t) = U(z) x [1 – 0.41 x Iu(z) x ln (t/to)]

= 178.1 [1 - 0.41 x 0.131157 x ln (15/3600)]

= 230.59 ft/s = 70.3 m/s

Now, wind force, Cl. 2.3.2-8 API-RP 2A

F = (ρ/2)μ2CsA = (0.0023668/2) x 230.592 x 3.282 [0.5(59.2 + 74) x 10]

= 450852.44 slug ft s-2

= 4.4482 x 450852.44 N

= 2005481.82 N = 2005.48 KN

Therefore, Total Force at each end = 2005.48/2 = 1002.74 KN

Fig 3. Shear force description (Bridge Bottom Plan)

β = tan-1 (7.4/7.5)

= 44.62°

Axial Compression of the bracing member

F = 1002.74 /cos 44.62°

= 1408.78 KN

Therefore, area required = 1408.78 x

1000/131.16

= 10740.93 mm2

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Adopting ∅406 x 12.7, D/t = 31.97

Area provided = �(406 – 12.7) x 12.7

= 15691.97 mm2 > 10740.93 mm2

Slenderness ratio = (74002 + 75002)

(I/A)0.5

= 75.73 < 80 which is provided to the Top and Bottom braces.

Check for Lateral Bending

As UDL = 2005.48 /74

= 27.1 KN/m

Maximum bending moment = wL2/8

= 27.1 x 742/8

= 18549.95 KNm

Axial Compression on Bottom Chord of the frame facing the wind = P /e

= 18549.95 /7.5

= 2473.32 KN

Area required = 2473.32 x 1000 /131.16

= 18857.324 mm2

Area provided = 50958 mm2 > 18857.324 mm2 Hence, it is safe.

Fig 4. Bottom Plan

Fig 5. Top Plan

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A 3D space frame model of the bridge utilizing the members mentioned above has been generated

for the Inplace and Lift analysis of the bridge with respective boundary conditions and loading. The

bridge structure has been analysed for different load combinations with applicable contingencies. The

structural analysis of the Bridge has been performed using SACS suite of programs. Isometric Views

of the bridge under consideration has been included in the figures below.

Fig 6. A 3D model of Bridge for Inplace Analysis

Fig 7. A 3D model of Bridge for Lift Analysis

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Material Properties

Material properties for tubular members are specified below:

You g s Modulus : 2x 105 MPa

Yield Strength : 345 MPa (for tubular with diameter >= 457 mm)

248 MPa (for tubular with diamater < 457 mm)

Shear modulus (G) : 0.8 x 105 MPa

Poisso s ‘atio : 0.30

Steel Density in air : 7850 kg/m3

The Inplace weight of the Bridge is 695 MT whereas the Lift weight of the bridge is 676 MT. The Bridge

structure has been analysed for the Inplace conditions during the extreme storm (100 year return period),

operating storm (1 year return period) with the pertinent loads envisaged during the operation of the

structure. ;a d fo the lift o ditio s ith the ele a t loads a d D a i A plifi atio Fa to s DAF s . The

following cases have been analysed during lifting of the module.

Balanced sling load distribution (50:50) with COG shift.

Skew sling load distribution-1 (75:25) with COG shift.

Skew sling load distribution-2 (25:75) with COG shift.

Codal and installation contractor requirements stipulate that the structure being lifted be analysed

for equal sling load distribution.

C.2. SOFTWARE USED

The structural analysis of the Bridge has been performed using SACS suite of programs.

Following SACS Program Modules have been used:

The `SACINP` input file, which contains model information of the analyzed structure viz., the geometry,

member sizes, materials, loads and load combinations and analysis options. The input file is generated

usi g the `P‘ECEDE a d DATAGEN odules i `“AC“`.

The `SEASTATE` module has been used to generate dead weight of the modelled members. It also

computes the environmental loads (wind) on the structure. This module will combine basic loads (user

input or generated) to form various load combinations specified in the input file.

SACS IV Module is used for the linear static analyses of the model.

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The `JOINT-CAN` module has been used to perform the joint punching shear checks according to API-

‘P A, ‘e o e ded P a ti e fo Pla i g, Desig i g & Co st u ti g Fixed Offshore Platforms

W“D , st Editio E‘‘ATA a d “upple e t , O to e 7.

The `PO“TVUE odule has ee used fo e ie i g the a al sis esults.

C.3. METHODOLOGY FOR LIFT ANALYSIS

Codal and installation contractor requirements stipulate that the structure being lifted be analyzed

for equal and skew sling load distribution. The equal sling load distribution requirement stipulates that

diagonally opposite pairs of slings carry equal loads. This is generally referred to as the 50:50 case in

offshore design practice. The skew sling load distribution requirement stipulates and diagonally opposite

pairs of slings carry 75% of the total load while the remaining pair carry the balance. This load

distribution is reversed as an additional load case for lift analysis. These two distributions are generally

referred to as the 75:25 and 25:75 case.

The distributions described above are simulated as follows.

One diagonally opposite pair of slings connects a diagonally opposite pair of lift points on the top

chord of the bridge to the hook with fixed (111111) boundary conditions (generally named HUK1). The

second hook joint (named HUK2) is created at the same location as the first (co-incident joint). The

remaining diagonally opposite pair of lift points are connected with slings to this hook joint. The

boundary conditions for this second co-incident joint are 110111.

To simulate the 25:75 load distribution, 25% of the lift weight of the bridge is applied to HUK2 in the

upward direction. This load is combined with the gravity load combination Since two diagonally opposite

slings are connected to this joint (HUK2), this sling pair will carry 25% of the bridge lift weight. By

equilibrium the other sling pair connected to HUK1 will carry the balance 75% of the lift weight.

To simulate the 50:50 load case, the gravity load combination is combined with two times the Hook

Load basic case. Since two diagonally opposite slings are connected to this joint (HUK2), this sling pair

will carry 50% of the bridge lift weight. By equilibrium the other sling pair connected to HUK1 will carry

the balance 50% of the lift weight.

To simulate the 75:25 load case, the gravity load combination is combined with three times the

Hook Load basic case. Since two diagonally opposite slings are connected to this joint (HUK2), this sling

pair will carry 75% of the bridge lift weight. By equilibrium the other sling pair connected to HUK1 will

carry the balance 25% of the lift weight.

Each of these load combinations is generated using the applicable Dynamic Amplification Factors

described in Section D.3.

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APPENDIX D – BASIC LOADING AND LOAD CASE SUMMARY

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D.1. BASIC LOADING

1. Computer Generated Loads

These loads account for the weight of the modelled primary and secondary members.

2. Module Non-Generated Dead Loads NGDL’s

These loads account for the weight of the non-modelled secondary and tertiary structural

items/appurtenances on the Bridge including grating, handrails, monorails, etc. These loads are applied

on the corresponding joints/members where they are supported.

3. Discipline Loads

Bridges carry piping, electrical and instrumentation cable trays. The Load intensities depend largely

on the extent to which the facilities on the two platforms are inter-connected. A typical weight summary

of individual discipline loads is included below for reference. These loads are applied on the

corresponding members/ joints where they are supported.

Piping (Dry) : 145 MT

Piping (Content) : 39 MT (applied only in Inplace analysis)

Electrical : 18 MT

Instrumentation : 102 MT

4. Live Loads (applied only in Inplace analysis)

Each level is designed to carry a Live load of 150 Kg/m2 on walkway area and is considered for the

analysis. The loading intensities are based on the values typically used in Offshore industry practice.

5. Environmental Loads

The parameters for the environmental loading for Bridge Inplace analysis are specified below.

Extreme Wind (all directions) : 161.39 Km/hr

Operating Wind (all directions) : 99 Km/hr

Wind velocity based on a 15 seconds gust period and eight (8) approach directions is considered for

the B idge I pla e a al sis. The elo it at the desig ele atio is auto ati all al ulated the “AC“ “EA“TATE odule. The fa es of the B idge ha e ee ep ese ted as projected areas with respect to the

principal directions (Global X and Y directions). Wind load in each direction is calculated based on

projected areas with the appropriate wind velocities. A sample calculation for wind loading included in

Appendix C for reference.

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6. Hook Load

This load is applied to the free hook joint in order to simulate the equal and skew sling load

distributions described in Appendix C.3 of this report. The magnitude of this load is 25% of the lift weight

applied at the free hook joint applied in the positive Z direction (upward).

7. COG Shift Load (applied only in Lift Analysis)

A 2% COG shift is simulated by vertically applying 132.63 KN load and opposite in direction to each other

at two diagonally opposite lift points.

Table 1. Basic Load Case Summary (Inplace)

BASIC LOAD CASES

LOAD CASE DESCRIPTION LOAD SUM (KN)

LL Live Load 359.34

611 Piping Dry Load 1422.43


621 Electrical Load 176.97

625 Instrumentation Load 1001.44

NGDL Non-Generated Dead Load 468.43

101 Self Weight 2999.56

201 Extreme Wind Load at 0˚ 222.60








301 Operating Wind Load at 0˚ 69.80








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Table 2. Basic Load Case Summary (Lift)

D.2. BOUNDARY CONDITIONS

For Inplace analysis, the bridge model is pinned at one platform end and sliding at the other platform

end. The plot below indicates the boundary conditions. The bridge is allowed to slide on one of the

platforms to account for relative platform movement.

For Lift analysis, the bridge model is connected at 4 conventional lift points at the top chord to the

hook point by means of slings. The hook is generally given fixed boundary conditions with releases

provided to the modelled slings so that they do not carry any moments. An additional hook point is

modelled and fixed in two translations and three rotations. It is left free in the vertical direction. This

second hook is used to simulate the equal sling load distribution and skew sling load distribution.

Table 3. Contingency applied in Loadings

CONTINGENCY FACTORS

BASIC LOAD CONTINGENCY

Structural modelled weight 13%

Non-Generated Dead Loads 13%

Piping 20%

Electrical 20%

Instrumentation 20%

Live Load No contingency (Applied only for Inplace Analysis)

BASIC LOAD CASES

LOAD CASE DESCRIPTION LOAD SUM (KN)


621 Electrical Load 176.97

625 Instrumentation Load 1001.44

NGDL Non-Generated Dead Load 468.43

HUK 25% of the Total Load 1660.17

101 Self Weight 2990.38

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D.3. LOAD COMBINATIONS

FOR INPLACE ANALYSIS

Table 4. Design Load Combinations Summary

DESIGN LOAD COMBINATIONS

LOAD CASE DESCRIPTION

CE1 to CE8 Self Weight + NGDL + Piping + E&I + Live Load + Extreme storm condition

CO1 to CO8 Self Weight + NGDL + Piping + E&I + Live Load + Operating storm condition.

FOR LIFT ANALYSIS

Table 5. Dynamic Amplification Factors

DYNAMIC AMPLIFICATION FACTORS


Skew Load

distribution

1.35 – Lift point connected members

1.15 – Other members

Equal Load

distribution

2.00 – Lift point connected members

1.35 – Other members

Using the above Dynamic Amplification Factors, the load combinations for Lift analysis is formed

accordingly.

Table 6: Design Load Combinations Summary

DESIGN LOAD COMBINATIONS


1000 Self Weight + Piping Dry Weight + E&I + NGDL

2575 1000*1.35 + Hook Load*1.35 + COG Shift Load* 1.35

2676 1000*1.15 + Hook Load*1.15 + COG Shift Load*1.15

5050 1000*2 + Hook Load*4 + COG Shift Load*2




Member Check Parameters

The effective buckling length of the member used in the calculation of axial allowable compressive

stresses is in accordance with the recommendation of API-RP 2A. These factors are input relative to

member local axes. The allowable stresses are also in accordance with API –RP 2A. An extract of the

allowable stresses has been included in the Appendix B for reference.