Fixed Seabed Structures

The anodes can be arranged in different ways. On fixed offshore structures the anodes may be mounted on the structure itself (but of course insulated from it), they may be supported by special conduits more or less remote from the structure, or remote seabed anodes may be used. 

Tension leg and floating platforms can easily be released and towed away for service elsewhere, which is cheap and attractive. In the case of the fixed platforms, the topside modules are removed by lift barge and taken to shore for disposal. Gravity based structures can in theory be deballasted and floated away to be re-employed or sunk in the deep ocean, and steel jackets cut and removed at an agreed depth below sea level. In some areas jackets are cleaned and placed as artificial reefs on the seabed. The... 

Floating exploration and production facilities are sometimes provided on jackup rigs, semi-submersible vessels or ex-crude oil shipping tankers converted to production treatment vessels. These facilities are essentially the same as fixed offshore platform or installations except they are moored in place or provided with a temporary support structure instead of provided with fixed supports to the seabed. The major process fire and explosion risks are identical to the risks produced on offshore platforms. They have one addition major facility risk, that is the maintenance of buoyancy of the installation. Should fire or explosion effects cause a loss of buoyancy (or even stability) the entire facility is at risk of submergence. Adequate compartimization and integrity assurances must be implemented in these instances. 

Touchdown is when the structure s base first makes contact with the seafloor. The position and orientation of the platform is then held fixed so that any adjustments in position or orientation can be made before the skirts come into contact with the seabed. Touchdown can be recorded by instrumented dowels or any other appropriate system. 

Tension Leg Platform (TLP)—a floating structure held in place by tensioned tendons securely fixed with templates to the seabed. As long as vertical motion is limited, it can operate up to 1220 m. 

The platform deck is built as an integrated deck structure. This is located on the top of a substructure, consisting of four columns and a rectangular pontoon. The platform is fixed to the seabed by sixteen anchor chains. 

---