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Bottom-founded structures, with the notable exception of the Gravity Base

Structures (GBS e.g. Condeeps), are typically constructed from welded steel

tubular members. These members act as a truss supporting the weight of the

processing equipment, and the environmental forces from waves, wind and current.

Bottom-founded structures are called “fixed” when their lowest natural frequency

of flexural motion is above the highest frequency of significant wave excitation.

They behave as a rigid body and must resist the full dynamic forces of the

environment. “Compliant” bottom-founded structures are usually designed so that

their lowest natural frequency is below the energy in the waves. Waves, wind and

current cause these structures to deflect, but the magnitude of the dynamic loads is

greatly reduced. This allows economical bottom-founded structures to be designed

for water depths, which would not be practical for fixed structures.

Another type of bottom-supported structure behaves like a fixed structure in a
mild environment. Such a structure is designed with the means to behave both as a

fixed and as a compliant structure. Compliancy is achieved using options such as
the taut wires connected to heavy chains on seabed or disconnectable pile

connections. Thus when the applied lateral wind, wave and current forces exceed
the design limit, chains are lifted off the seabed or the pile connections released, to

turn the fixed structure into a rotationally compliant structure (Le. from zero

degrees of freedom to two degrees of freedom about the seabed).

Floating structures have various degrees of compliancy. Neutrally buoyant

structures, such as semi-submersibles, Spars and Drillships are dynamically

unrestrained and are allowed to have six degrees of freedom (heave, surge, sway,

pitch, roll and yaw). Positively buoyant structures, such as the Tension Leg

Platforms (TLPs) and Tethered Buoyant Towers (TBTs) or Buoyant Leg Structures

(BLS) are tethered to the seabed and are heave-restrained. All of these structures

with global compliancy are structurally rigid.

Compliancy is achieved with the mooring system. The sizing of floating

structures is dominated by considerations of buoyancy and stability. Topside
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