sea floor. The cost of a fixed platform, especially the expense and 

 difficulty of moving it, reached a point where it could only be employed 

 for production purposes and a new type of mobile rig had to be developed 

 for exploration. 



A different type of platform was developed which entailed the 

 mounting of derricks on river barges which could be used in the shallow 

 coastal swamp areas of Louisiana. These platforms called bottom- 

 supported submersible platforms or simply submersibles proved to be 

 adaptable for shallow exploratory offshore drilling. The submersible 

 was generally towed to a well site and then sunk in shallow water. 

 After the drilling was completed, the submersible was pumped out, 

 refloated, and towed to a new location. Although developed more than 20 

 years ago during the infancy stage of offshore operations, there are 

 still about 20 of these rigs in use today. (However, submersibles are 

 of no value for exploratory drilling in the deeper waters of proposed 

 lease areas. ) 



Jack-up Rig : A type of bottom-supported rig which has evolved from 

 the submersible is the jack-up rig. By the end of 1976 approximately 

 180 jack-up rigs were in use worldwide. Figure 11 is a diagrammatic 

 illustration of this type of rig. The jack-up rig is essentially a 

 floating, barge-like hull that supports a platform. Drilling equipment 

 and crew quarters are mounted on the platform. Three legs, each up to 

 400 feet long, are fitted vertically through slots in the hull. While 

 the jack-up is being towed to a location, the legs are drawn up, but 

 when the rig is in place over the well site, the legs are lowered 

 mechanically or hydraulically until they reach the sea floor. The 

 platform is "jacked-up" until it has been elevated far enough out of the 

 water to be out of reach of most anticipated waves. 



The dimensions and designs of jack-up rigs vary according to 

 weather conditions and water depths. Most jack-up rigs operate in water 

 depths less than 300 feet in calm conditions; they are located in 

 shallower water in areas with rough winter conditions. Jack-up rigs are 

 built and serviced at existing ship yards and other coastal steel 

 fabrication facilities. A representative rig currently in use might 

 have a hull that is about 230 feet by 230 feet and about 25 feet deep 

 with crew accommodations for almost 80 crew members and a drilling 

 penetration capability of up to 25,000 feet. A towing draft of 20 to 30 

 feet is normally required. Jack-up rigs are extremely stable and provide 

 a secure drilling position when used in the appropriate depths. 



Semi -submersible Drilling Rig : The most recent development in 

 floating platforms is the semi-submersible; these have been operable for 

 more than 15 years. It floats, rather than rests on the sea bottom, and 

 is designed to minimize heave, pitch, and roll motions. In a semi- 

 submersible, the major buoyant support for the vessel is placed in 

 pontoons and risers which ride on and above the surface when a semi- 

 submersible is moving; when it is in the drilling mode, the pontoons are 

 sunk well below the water line by adding ballast. 



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