Introduction to Offshore Structures-PART-1

7/28/2019 Introduction to Offshore Structures-PART-1

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INTRODUCTION TO

OFFSHORE STRUCTURES

D.DHANASEKAR M.E.,Mem.A.S.M.E.,Design Engineer - mechanical


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Topics covered

World scenario Terminologies Types of Offshore structures Major Failures an historical perspective

Types of major loads Environmental loads Types of welded connections Types of Stresses Concept of Stress state

Theories of Static failure Stress Concentration


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Crude oil issold betweencountries in

quantitiescalled barrels

One barrel of oil is the same as

159 litres or 35 gallons (enoughto fit in the petrol tanks of about4 cars) 280 pints (a lot of bottlesof milk)

There are about 8 barrels in atonne.

You could fit nearly 2 millionbarrels of oil into a footballstadium - or one and a halftankers.

Crude oil is often mixed with gases,

water and sand. It forms anemulsion with the water

World scenario


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World scenario

ConditionInvestment costs

per Barrel per day

$/B/DProduction

costs per

Barrel$/BConventionalAverage 4000 - 8000 5Middle East 500 - 3000 1Non-Opec 3000 - 12000 8OffshoreNorth Sea 10000 - 25000 5 - 10Deepwater 15000 - 35000 10 - 15

World oil production in 1988 was 63 millionbarrel/day

Contribution of offshore oil production in theyear 1988 to the world energy consumption

was 9% and is estimated to be 35% in 2010


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World scenario

The economic feasibility of an offshore project depends on

many aspects: capital expenditure (CAPEX), tax, royalties,operational expenditure (OPEX)

In a typical offshore field development, one third of the CAPEXis spent on the platform, one third on the drilling of wells and

one third on the pipelines

Cost estimates are usually prepared in a deterministicapproach. The major elements in the CAPEX for an offshoreplatform are:

- project management and design- material and equipment procurement- fabrication- transport and installation- hook-up and commissioning.


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World scenario

Operational Expenditure (OPEX)

In a typical Offshore system, approximately 20percent of OPEX are required for offshoreinspection, maintenance and repair (IMR).

The amount to be spent on IMR over the projectlife can add up to approximately half the originalinvestment.

IMR is the area in which the structural engineer

makes a contribution by effort in design,selection of material, improved corrosionprotection, accessibility, basic provisions forscaffolding, avoiding jacket attachments dangerous

to divers, etc.


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World scenario

DEEP WATER DEVELOPMENTS Deep water introduces a wide range of extra difficulties for the

operator, the designer and constructor of offshore platforms.

Fixed platforms have recently been installed in water of 410 m. depth,i.e. "Bullwinkle" developed by Shell Oil for a Gulf of Mexico location.The jacket weighed nearly 500 MN.

Subsea wells are now feasible for 300 - 900 m deep water.

The deepest wells have been developed off Brasil in moderate weatherconditions.

The tension leg platform (TLP) seems to be the most promisingdeepwater production. It consists of a semi-submersible pontoon, tiedto the seabed by vertical pre-stressed tethers. The first TLP wasHutton in the North Sea and recently TLP-Jolliet was installed at a 530m deep location in the Gulf of Mexico. Norwegian Snorre and Heidrun

fields have been developed with TLPs as well.


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Terminologies

Terminology Description

G.B.S. Gravity based structure, sitting flatly on the

sea bottom, stable through its weight.

JACKET Tubular sub-structure under a topside,

standing in the water and pile founded.

LOAD-OUT The operation of bringing the object (module,jacket, deck) from the quay onto the

transportation barge.

CAISSONS or

SUMPS

Vertical pipes from topside down to 5-10 m

below water level for intake or discharge.

TOPSIDE or

SUPERSTRUCTURE

Compact offshore process plant, with all

auxiliaries, positioned above the waves


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Terminologies

Terminology Description

SLINGS Cables with spliced eyed at bothends, for offshore lifting, the upper

end resting in the crane hook.

SPREADER Tubular frame, used in lifting

operation

PADEARS (TRUNNIONS) Thick-walled tubular stubs, directly

receiving slings and transversely

welded to the main structure.

PADEYES Thick-walled plate with hole, receiving

the pin of the shackle, welded to themain structure.

PIPELINE RISER The piping section which rises from

the sea bed to topside level.


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Terminologies

Terminology DescriptionSUBSEA TEMPLATE Structure at sea-bottom, to guide

conductors prior to jacket installation.UP ENDING Bringing the jacket in vertical

position, prior to set down on the sea

bottom.WELLHEAD AREA Area in topside where the wellheads

are positioned including the valves

mounted on its top.HOOK-UP Connecting components or systems,

after installation offshore.SEA-FASTENING The structure to keep the object

rigidly connected to the barge during

transport.


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Oil can be transported by eitherpipelines or tankers.Under sea pipelines are made of

steel that is welded together andthen sealed to prevent corrosion.The pipeline is installed by beinglowered into a trench on the seafloor after either being uncoiled offa reel ship, towed behind a boat

from the shore or beingconstructed on a barge. On offshore barges there are several workstations. The pipeline isconstructed in stages at each ,ofthese work stations. The pieces of

pipe are welded together, x-rayed tolook for gaps, corrosion proofedand cleaned before being loweredinto the ocean. Tankers are alsoused to deliver these minerals whenthey are being exported to othercountries.

Off shore systems


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Types of Off shore Oil Platforms


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Jacket type

Platform

Fixed Platforms


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Gravity Base Platform

Fixed Platforms


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Gravity Base Platform

Fixed Platforms


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Types of Off shore Oil Platforms Tension Leg Platform

Mars tension leg platform, on location in Mississippi

Canyon block 807 in 2940ft of water
http://www.offshore-technology.com/projects/mars/


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Types of Off shore Oil Platforms Tension Leg Platform

Template held in place by piles driven into seafloor.

This method dampens the vertical motions of theplatform, but allows for horizontal movements.

Topside facilities (processing facilities, pipelines,and surface trees) of the TLP are same as for a

conventional platform

Tension Leg Platform(TLP) is a buoyantplatform held in place

by a mooring system.

Mooring system is aset of tension legs ortendons attached tothe platform andconnected to atemplate or foundationon the seafloor.


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Types of Off shore Oil Platforms Floating Platform


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A fixed platform may be described as consisting of two maincomponents, the substructure and the superstructure.

Superstructure:also referred as the 'topsides' supported on a deck,which is fixed (mounted) on the jacket structure. These consist of aseries of modules which house drilling equipment, productionequipment including gas turbine, generating sets, pumps,compressors, a gas flare stack, revolving cranes, survival craft,helicopter pad and living quarters with hotel and catering facilities. Itcan weigh up to 40,000 tonnes.

Substructure: is either a steel tubular jacket or a prestressed concretestructure. Most fixed offshore oil and gas production platforms have a

steel jacket although a small number of platforms have a concretefoundation.

Each platform is uniquely designed for the particular reservoircondition, location, water depth, soil characteristics, wind, wave andmarine current conditions. Fixed steel and concrete platforms can be

built in water depth from a few meters to more than 300 m.



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Fixed platform:

This platform design tackles the

challenges of offshore drilling in themost straightforward and industrialway imaginable.

A gigantic tower of concrete and steelis constructed and oil rig is mounted

on top.

Operates at depths of 1,500 feet (457meters) or less

These platforms are extremely stable,

despite the fact that the concrete baseisn't even attached to the seafloor. Itsimply stays in place due to all theweight above it. However, at depthsgreater than 1,500 feet, the designbegins to become more impractical

due to material costs.

A fixed platform (FP) is supportedby piles driven into the seabed andis economically feasible for water

depths up to 1,500 feet.


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Compliant tower:

These rigs take the basic idea of

the fixed platform and make itviable to operate in depths of1,500 feet to 3,500 feet (457meters to 914 meters).

The design achieves this byrelying on a narrower tower ofsteel and concrete.

But while fixed platform designsare rigid, compliant towers are

designed to sway and move withthe stresses of wind and sea --even hurricanes. In this respect,they're much like modernskyscrapers that are built to sway

with the wind.

The compliant tower (CT) is a narrow,flexible tower that can operate inwater depths of up to 3,000 feet. TheSea Star or floating mini tensionleg structure is suitable for smallerreservoirs and operates in waterdepths up to 3,500 feet.


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Sea Star platform:The Sea Star platform is basically alarger version of the semisubmersible

designProduction facilities sit atop a largesubmersible hull on a tower. Whenthe lower hull fills with water, it sinksto a lower depth, providing stability

while keeping the facilities high anddry. However, instead of giantanchors holding it in place, the SeaStar is connected to the ocean floorby tension legs.

These long, hollow tubes remain rigid at all times, preventing any up-and-down motion on the platform.

The legs are just flexible enough to allow side-to-side motion, which helpsabsorb the stress of waves and wind. These platforms operate from depths of500 to 3,500 feet (152 to 1,067 meters) and are typically used to tap smaller

reservoirs in deep waters.

Documents

Introduction to Offshore Structures-PART-1