compliant offshore structure

Compliant offshore structure

V I M A L K U M A R


V I M A L K U M A R

2 9 T H B A T C H

D E P T O F S H I P T E C H C U S A T .

INTRODUCTION

Offshore structures can be designed for installation gin protected waters, such as lakes, rivers, and bays or in the open sea, many kilometers from shorelines.

Offshore structures that can be placed in water d th f t kil t Th t t depths of up to 2 kilometers . These structures may be made of steel, reinforced concrete or a combination of bothcombination of both.

Major use Offshore structures

Oil and gas exploration g p

Navigation aid towers

Bridges and causeways g y

Ship loading and unloading facilities

Drilling of oil well.g

Consideration for design and analysis

Environmental (initial transportation, and in-place ( p , p100-year storm conditions)

Soil characteristics

Intensity level of consequences of failure

Purpose (drilling, production etc)

Life of structure.

TYPES OF OFFSHORE OIL/GAS EXPLORATION STRUCTURES

Converted Jackup bargesp g

The Converted jackup barges are the rarest, and may be used in remote areas with relatively shallow water depths.

Fixed tower structures

These structures vary in size and height, and can be used in water depths up to about 300 meters, although most commonly in water depths less than although most commonly in water depths less than 150 meters.

Within this category there are 4-leg, 6-leg, and 8-leg g y 4 g, g, gtowers and also minimal structures whose decks are supported by a single unbraced or pile-braced

icaisson.

Stationary floating SPARs

Th SPAR l tf d i d t The SPAR platforms are used in very deep water exploration, The SPAR is a vertical floating cylinder attached by means of cables to anchors placed on attached, by means of cables, to anchors placed on the seafloor more than a kilometer away.

Compliant structurep

are used in water depths greater than 300 meters or even more than that.

The water depth at the intended location dictates platform height.

They consist of a floating deck structure anchored to pile heads on the sea floor by means of long pipes which are always kept in tension and thus can be which are always kept in tension, and thus can be flexible without risk of a column buckling collapse failure

Compliant structure

A compliant offshore structure for use in phydrocarbon drilling and producing operations, comprising of

foundation template having skirt pile sleeves open at each end and a plurality of sockets closed at the bottombottom,

tower’s jacket is composed of four leg tubular that j p gcan range from 3 to 7 ft in diameter and are welded together with pipe braces to form a space-frame-like t tstructure.

space-frame-like structure of compliant offshore structure

The lower jacket is secured to the seafloor by weight j y gand with 2- to 6-ft piles that penetrate hundreds of feet beneath the mud line.

Both the lower and upper jacket dimensions can range up to 300 feet on a side.

l j k i d h fllower jacket is secured to the seafloor

Deck adapted to be attached to the top tower,p p ,

the drilling,

production,p ,

and crew quarter

The surface facilities are smaller by design on y gcompliant towers than on fixed platforms because of the decreased jacket dimensions that support them.

ll d d dDrilling ,production and crew accommodation spaces

A series of buoyant tanks (up to 12) located in the y ( p )upper part of the jacket places the members in tension, reducing the foundation loads of the t tstructure.

The tanks can range up to 20 ft in diameter and up to 120 ft in length120 ft in length.

The amount of buoyancy is computer controlled, keeping the appropriate tension in the structure keeping the appropriate tension in the structure members during wind and wave movements.

compliant towers in general, mooring is only used in p g , g ythe guyed-tower design.

several mooring lines (up to 20 lines measuring 5 ½-inch dia.) are attached to the jacket close to the waterline and are spread out evenly around it (up to 4 000 ft of line)4,000 ft of line).

Clump weights (120 ft x 8 ft, up to 200 tons) may be attached to each mooring line and move as the tower attached to each mooring line and move as the tower moves with the wind and wave forces.

To control the tower motions better, the lines are ,kept in tension during the swaying motions.

The portion of the lines past the clump weights are anchored into the seafloor with piles (as many as 20, each 72-inch dia., 115-ft long, penetrating 130 ft, and weighing up to 60 tons)weighing up to 60 tons).

mooring lines

A compliant piled tower design, it is 609.9 meters (2,001 feet) high, p p g , 9 9 ( , ) g ,arguably the tallest free-standing structure in the world, 75 meters of the platform are above water. The multi-deck topsides are 64 meters by 43

meters by 18.3 meters high

Important components of compliant structure

Vertically extending tower having a bottom at sea y g gfloor.

Piles extending upward from ocean floor

Coupling housing secured to the piles.

Elastomeric shear pad.

It s an elevation view of a compliant piled It s an elevation view of a compliant piled tower supported by resilient piles,

12 –resilient piles30-pile attachment location on land during

f b i tifabrication18-tower base from which centre of rotation

passes.14-tower frame416-tower legs

Cut away view of the upper section of resilient assembly12 –resilient piles12 –resilient piles

36-elastomeric structure38-housing rigidly secured to leg.

28-resillient coupling

34-cylindrical housing

Sectional view of at section3-3 in previous drawing.

16-tower legh l38-shear plate

28-Resillient coupling26-pile element26 pile element

36-elastomeric structure

It shows sheer deformation of It shows sheer deformation of elastomeric structure of

platform leg while tower sway26 –pile element

6 l16-tower leg

Elastomeric pads360-stack of elastomeric disks.

Need of compliant offshore structure

Oil industry moves into deeper water in the search y pfor additional supplies of oil and gas.

The size of conventional fixed leg platforms is approaching the economic limit due to the very large amount of steel required and limitations imposed by fabrication and installation methods fabrication and installation methods.

Several concepts have been proposed for enhancing p p p gthe water depth capability of platforms ,which are as follow.

Tension Leg Platforms (TLP),

guyed and articulated tower platforms.

Flex tube compliant offshore structure

These are types of compliant offshore structure

Types of compliant structures

guyed platforms, g y p ,

tension leg platforms

articulated towers



A compliant offshore structure with flex tubes that pare not driven into the sea bottom. A foundation template has bottles with skirt pile sleeves and a l d b tt k t A t d t t i closed bottom socket. A tower and tower extension

support the deck above the water line. The tower legs each enclose a flex tube each enclose a flex tube

leg tension platforms

Comparison and similarity

Compliant towers are similar to fixed platforms in p pthat they have a steel tubular jacket that is used to support the surface facilities.

compliant towers yield to the water and wind movements in a manner similar to floating structuresstructures.

Like fixed platforms, they are secured to the seafloor with pileswith piles

The jacket of a compliant tower has smaller j pdimensions than those of a fixed platform and may consist of two or more sections.

It can also have buoyant sections in the upper jacket with mooring lines from jacket to seafloor (guyed-tower designs) or a combination of the twotower designs) or a combination of the two.

Major feature of compliant structure

structures whose mooring system is constituted by g y yvertical tethers.

This characteristic makes the structure very rigid in the vertical direction and very flexible in the horizontal plane. Both these features result particularly attractiveparticularly attractive.

The vertical rigidity helps to tie in wells for g y pproduction, while, the horizontal compliance makes the platform insensitive to the primary effect of waves.

Second-order, slowly varying drift forces at low frequency caused by waves may result in the low frequency, caused by waves may result in the low frequency resonant oscillation of structure.

Economically attractive for deep water conditions y pbecause of their reduced structural weight compared to conventional platforms.

The foundations of these kinds of structures do not resist lateral environmental loads forces; instead, restoring moments are provided by a large buoyancy restoring moments are provided by a large buoyancy force, a set of guy lines or a combination of both

These structures have a fundamental frequency well q ybelow the ocean wave’s lower frequency-bound.

use of compliant towers in water depths ranging up to 3,000 ft. This range is generally considered to be beyond the economic limit for fixed jacket-type platformsplatforms

INSTALLATION

Tower legs are fabricated in yards in small module. g yWith normal shipyard facility ,it can be fabricated easily. Tower legs, accommodation module, are f b i t d d t t d t th it ith th bfabricated and transported to the site with the barge.

Length of tower is up to 500m or even more, hence fabricated in considerable sizefabricated in considerable size.

Fabricated compliant offshore structure

Fabricated module for production

Fabricated complaint offshore transported to site

The water depth at the intended location dictates pplatform height.

Once the lower jacket is secured to the seafloor, it acts as a base (compliant tower) for the upper jacket and surface facilities.

L b t d iti d th Large barge-mounted cranes position and secure the jacket and install the surface facility modules.

Installation of accommodation module to compliant structure

References

US Patent 5431512543 5

US Patent 4721417

www.marinetalk.com

Dynamic Response of Compliant Offshore Structures (R. Adrezin)

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compliant offshore structure

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