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weight for these structures is more critical than it is for a bottom-founded structure.

Semi-submersibles and ship-shaped hulls rely on waterplane area for stability. The

centre of gravity is typically above the centre of buoyancy. The Spar platform is

designed so that its centre of gravity is lower than its centre of buoyancy, hence it

is intrinsically stable. Positively buoyant structures depend on a combination of

waterplane area and tether stiffness to achieve stability.

Floating structures are typically constructed from stiffened plate panels, which

make up a displacement body. This method of construction involves different

processes than those used in tubular construction for bottom-founded structures.

Neutrally buoyant floating structure motions can be accurately determined as a

single six-degrees of freedom system subjected to excitation forces. Positively

buoyant floating structures in deep water will have restraining systems with

substantial mass, and the restraining systems are subjected to excitation forces as
well. The motions of the platform are coupled with the dynamics of the mooring

system. The coupling of motions between the platform, risers and mooring systems
becomes increasingly more important as water depth increases.

Fixed and floating platforms are very different not only in their appearance but
also in their structural members. They are unique in how they are constructed,

transported and installed, what kind of excitation forces they are subjected to, how

they respond to these excitation forces and how they are decommissioned and

reusedirecycled at the end of their design lives. The major common characteristic

of each type of structure is that they provide deck space and payload capacity (Le.

real estate) to support equipment and variable weights used to support drilling and

production operations.

The fixed platform deck loads are directly transmitted to the foundation

material beneath the seabed. Thus, fixed platform jackets supporting the deck are

typically long, slender steel structures extending from seabed to 20-25 m above the

sea surface. Floating structure deck loads are supported by the buoyancy forces of

the hull supporting the deck.
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