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Conductor pipes on bottom-founded structures effectively extend well casings

to the deck of the structure. Drilling and well operations are identical to those on

land. Floating structures require dynamic risers to connect with wellheads on the

seafloor. Drilling and production require a tieback at the mudline to the subsurface

casing. Well control can require expensive subsea control systems (wet trees), or

special low-motion vessels, which can support vertical risers in all weather

conditions with well controls at the surface (dry trees).

Fixed platform jackets are constructed on their side, loaded out on to a barge

(except for jackets with flotation legs), transported to the installation site, launched

and upended (or lifted and lowered) and secured to seabed with driven or drilled

and grouted piles. Floating structures, except for Spars, TBTs and BLSs, are

constructed upright, either dry or wet towed to installation site and connected to

the mooring system or secured to the seabed with tethers.
Fixed platform jackets need to have adequate buoyancy (Le. more than their

own self-weight) to stay afloat during installation. Thus, they are typically
constructed of small diameter tubulars that form a space frame. Floating structure

hulls need to have adequate buoyancy to support the deck and various other
systems. Thus, they are typically constructed of orthogonally stiffened large-

diameter cylindrical shells or flat plates. Smalldiameter tubulars are susceptible to

local instability and column buckling, while orthogonally stiffened systems are

designed to meet hierarchical order of local, bay and general instability failure

modes.

Fixed platform design is typically controlled by their functional gravity loads

and the lateral forces and overturning moments due to wind, wave and current. For

a preliminary design, wind, wave and current forces can be applied quasi-statically

to a structure along with the dead loads from the deck and structural self-weight. A

single load case defined by a “design wave” can, in most cases, be adequate to

determine the required strength of a fixed structure.

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