The different development options for offshore fields can be broadly classified into three categories:
- Fixed Platforms,
- Floating systems
- Subsea Systems
1.0 Fixed Platforms
These are the structures that physically sit on the bottom of the sea. Being held by the sheer weight of the structure or by piles driven into the seabed to hold them in place. Fixed platforms are economically feasible for installation in water depths up to about 1,700 feet (520 m). They include:
Jacket-based platforms: These consist of a jacket and a deck. The jacket is tall, vertical
section built from tubular steel members and is locked to the seabed by driven piles. The
deck (or topside) is where crew quarters, production facilities and drill rigs are located.
Unmanned installations: These are small platforms, consisting of little more than a well bay,
helipad and emergency shelter. They are designed for operate remotely under normal
operations, only to be visited occasionally for routine maintenance or well work. They
include wellhead platforms.
Compliant towers: These are made of tubular steel members and are fixed to the bottom
with piling, and supports a deck. Compliant towers are designed to sustain significant lateral
deflections and forces, and are typically used in water depths ranging from 1,500 and 3,000
feet (450 and 900 m).
Gravity platforms: These are built from reinforced concrete. They rely on gravity to hold
them in place. Gravity platforms are used in up to 1000ft of water, but the seabed has to be
especially firm to ensure no creep over time.
2.0 Floating Production Systems
These include TLPs, FPS’s and spars. All three have to be moored in place with tendons or wire rope and chains in order to stay connected to the wells below.
Tension leg platforms (TLPs);- these have floating hulls made of buoyant columns and
pontoons. Steel pipe tendons hold the hull below their natural level of flotation, keeping the
tendons in tension and the hull in place eliminating most vertical movement of the structure.
TLPS are used in water depths up to about 6,000 feet (2,000 m).
Spar platforms;- these get their flotation from large diameter cylinders, weighted at the
bottom to keep them upright. Eight to 16 wire or synthetic rope and chain combinations
moor the hulls to the seabed. Spars have little vertical heaves because of their huge
Floating production systems (FPS’s);- these consists of ship shape, TLP-like, or a
semisubmersible hulls with production facilities on board.
Floating production, storage and offloading systems (FPSOs): These have large ship shape, made either converted tankers or new construction. They have no drilling capability and are moored to place with rope and chain. They process production from subsea wells and store large crude oil volumes, accumulated for later transport by shuttle tankers.
3.0 Subsea Systems
These can have single or multiple wellheads on the sea floor connected directly to a host platform or to a subsea manifold. The systems include connections by flowlines and risers to fixed or floating systems that could be miles away. It can be set in any depth water.