Semi-submersible rigs

Semi-submersible rigs are often referred to as semis , and are a floating type of rig. Like the jack-up, a semi is self contained. The structure is supported by large pontoons which are ballasted with water to provide the required stability and height. The rig is held in position by anchors and mooring lines or dynamically positioned by thrusters. A large diameter steel pipe ( riser ) is connected to the sea-bed and serves as a conduit for the drill string. The blowout preventer (BOP) is also located at the sea-bed ( sub sea stack ). 

Figure 3.5 Semi-Submersible Rig (courtesy of Stena Drilling)...

Tension leg platforms (TLP) are used mainly in deep water where rigid platforms would be both vulnerable to bending stresses and very expensive to construct. A TLP is rather like a semi-submersible rig tethered to the sea bed by jointed legs kept in tension. Tension is maintained by pulling the floating platform down into the sea below its normal displacement level. The legs are secured to a template or anchor points installed on the seabed. 

The template will be constructed and fitted out at a fabrication yard and then transported offshore to the drilling location. The template is lowered to the seabed using a crane barge or, if small enough, lowered beneath a semi-submersible rig. Prior to drilling the first well, piles are driven into the sea bed to hold the template in place. 

Finally, the MMS-CG collaboration on workplace safety inspection had failed to become a coordinated and effective program because of jurisdictional confusion. As previously discussed, OCSLA assigned workplace safety to the CG despite its lack of qualification for this role, caused withdrawal of OSHA, which is more highly qualified, and assigned overall rig safety to MMS. Sorting out responsibilities between the agencies has been difficult and led to a series of interagency memoranda. More difficulty was created by maritime law and issues such as whether a semi-submersible rig is a vessel and thereby requires CG safely certification and a subordinate role for the MMS. 

The system uses two existing vessels The Helix Producer 1 and Q4000, a semi-submersible rig. A stack can, consisting of a well cap and intervention riser system, wiU transfer oil from the damaged blowout preventer (BOP), wellhead or tree to a riser connected to Q4000. From there the oil will be pumped through a flexible riser to Helix Producer I that can carry out some processing. Oil would then be transferred to a tanker that would take it to an onshore faciUty. 

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. 

A semi-submersible drilling rig in the North Sea. James WellsjGetty Images... 

The Alexander L. Kielland was ten thousand tons of steel welded into a five-footed monster of an oil rig capable of virtually walking on water. As originally conceived, the ungainly vessel known as a semi-submersible would move from place to place in the ocean, stopping here and there to straddle geologically likely spots and drill for oil beneath the sea. In places where it might strike it rich, a permanent production platform could be erected, as if a monument to the peripatetic rig s success. 

In deeper waters, exploration is conducted from floating rigs, including submersibles and drill ships. Drill ships with conventionally shaped hulls of seagoing vessels are not so stable as semisubmersibles (semis) in rough waters, but can to moved from location to location much faster-... 

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