Compliant Offshore Structure

Compliant offshore structure VIMAL KUMAR 2 9 TH B A T C H DEPT OF SHIP TECH CUSAT.

INTRODUCTION y Offshore structures can be designed g for installation

in 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 off up tto 2 kil depths kilometers t . Th These structures t t may be made of steel, reinforced concrete or a combination of both. both

Major use Offshore structures y Oil and g gas exploration p y Navigation aid towers y Bridges g and causeways y y Ship loading and unloading facilities y Drilling g of oil well.

Consideration for design and analysis y Environmental ((initial transportation, p , and in-place p y y y y

100-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 y Converted Jackup p barges 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 150 meters.

y Within this category g y there are 4 4-leg, g, 6-leg, g, and 8-leg g

towers and also minimal structures whose decks are supported by a single unbraced or pile-braced caisson. i y Stationary floating SPARs y The Th SPAR platforms l tf are used d iin very d deep water t exploration, The SPAR is a vertical floating cylinder attached by means of cables attached, cables, to anchors placed on the seafloor more than a kilometer away.

y Compliant p structure y are used in water depths greater than 300 meters or

even more than that. y The water depth at the intended location dictates platform height. y 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, tension and thus can be flexible without risk of a column buckling collapse failure

Compliant structure y A compliant p offshore structure for use in

hydrocarbon drilling and producing operations, comprising of y foundation template having skirt pile sleeves open at each end and a plurality of sockets closed at the bottom, bottom

y tower’s jjacket is composed p of four leg g tubular that

can range from 3 to 7 ft in diameter and are welded together with pipe braces to form a space-frame-like structure. t t

space-frame-like structure of compliant offshore structure

y The lower jjacket is secured to the seafloor by y weight g

and with 2- to 6-ft piles that penetrate hundreds of feet beneath the mud line. y Both the lower and upper jacket dimensions can range up to 300 feet on a side.

l lower jjacket k iis secured d to the h seafloor fl

y Deck adapted p to be attached to the top p tower,, y the drilling, y production, p , y and crew quarter y The surface facilities are smaller by y design g on

compliant towers than on fixed platforms because of the decreased jacket dimensions that support them.

Drilling ll ,production d and d crew accommodation d spaces

y A series of buoyant y tanks ((up p to 12)) located in the

upper part of the jacket places the members in tension, reducing the foundation loads of the structure. t t y The tanks can range up to 20 ft in diameter and up to 120 ft in length. length y The amount of buoyancy is computer controlled, keeping the appropriate tension in the structure members during wind and wave movements.

y compliant p towers in g general,, mooring g is onlyy used in

the guyed-tower design. y several mooring lines (up to 20 lines measuring 5 ½inch 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 line) y Clump weights (120 ft x 8 ft, up to 200 tons) may be attached to each mooring line and move as the tower moves with the wind and wave forces.

y To control the tower motions better,, the lines are

kept in tension during the swaying motions. y 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). tons)

mooring lines

A compliant p p piled tower design, g , it is 609.9 9 9 meters ((2,001 , feet)) high, 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

Compliant offshore structure

Important components of compliant structure y Vertically y extending g tower having g a bottom at sea

floor. y Piles extending upward from ocean floor y Coupling housing secured to the piles. y Elastomeric shear pad.

It s an elevation view of a compliant piled tower supported by resilient piles, 12 –resilient piles 30-pile attachment location on land during f b i ti fabrication 18-tower base from which centre of rotation passes. 4 frame 14-tower 16-tower legs

Cut away view of the upper section of resilient assembly 12 –resilient piles 36-elastomeric structure 38-housing rigidly secured to leg. 28-resillient coupling 34-cylindrical housing

Sectional view of at section3-3 in previous drawing. 16-tower leg 38-shear h plate l 28-Resillient coupling 26-pile 26 pile element 36-elastomeric structure

It shows sheer deformation of elastomeric structure of platform leg while tower sway 26 –pile element 16-tower 6 l leg

Elastomeric pads 360-stack of elastomeric disks.

Need of compliant offshore structure

y Oil industry y moves into deeper p water in the search

for additional supplies of oil and gas. y 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. methods

y Several concepts p have been p proposed p for enhancing g

y y y y

the 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 yg guyed y p platforms,, y tension leg platforms y articulated towers y Flex tube compliant offshore structure

Flex tube compliant offshore structure

y A compliant p offshore structure with flex tubes that

are not driven into the sea bottom. A foundation template has bottles with skirt pile sleeves and a closed l db bottom tt socket. k t A tower t and d ttower extension t i support the deck above the water line. The tower legs each enclose a flex tube

leg tension platforms

leg tension platforms

Comparison and similarity

y Compliant p towers are similar to fixed p platforms in

that they have a steel tubular jacket that is used to support the surface facilities. y compliant towers yield to the water and wind movements in a manner similar to floating structures. structures y Like fixed platforms, they are secured to the seafloor with piles

y The jjacket of a compliant p tower has smaller

dimensions than those of a fixed platform and may consist of two or more sections. y It can also have buoyant sections in the upper jacket with mooring lines from jacket to seafloor (guyedtower designs) or a combination of the two. two

Major feature of compliant structure y structures whose mooring g system y is constituted byy

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

y The vertical rigidity g y helps p to tie in wells for

production, while, the horizontal compliance makes the platform insensitive to the primary effect of waves. y Second-order, slowly varying drift forces at low frequency caused by waves may result in the low frequency, frequency resonant oscillation of structure.

y Economically y attractive for deep p water conditions

because of their reduced structural weight compared to conventional platforms. y The foundations of these kinds of structures do not resist lateral environmental loads forces; instead, restoring moments are provided by a large buoyancy force, a set of guy lines or a combination of both

y These structures have a fundamental frequency q y well

below the ocean wave’s lower frequency-bound. y 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 platforms

INSTALLATION y Tower legs g are fabricated in yyards in small module.

With normal shipyard facility ,it can be fabricated easily. Tower legs, accommodation module, are f b i t d and fabricated d ttransported t d tto th the site it with ith th the b barge. y Length of tower is up to 500m or even more, hence fabricated in considerable size. size

Fabricated compliant offshore structure

Fabricated module for production

Fabricated complaint offshore transported to site

y The water depth p at the intended location dictates

platform height. y Once the lower jacket is secured to the seafloor, it acts as a base (compliant tower) for the upper jacket and surface facilities. y Large L barge-mounted b t d cranes position iti and d secure th the jacket and install the surface facility modules.

Installation of accommodation module to compliant structure

References US Patent 5431512 543 5 US Patent 4721417 www.marinetalk.com Dynamic Response of Compliant Offshore Structures (R. Adrezin)

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