semi-submersibles, drill-ships, jack-ups, and fixed platforms. 

 Four of these types — barges, semi-submersibles, drill-ships, 

 and jack-ups — are primarily used in the exploration facet 

 of the oil industry, but may also be used, after oil or gas 

 discovery, for drilling development wells. 



Barges can be used to drill in water depths up to 600 ft. 

 This depth limitation is primarily imposed by the anchor and 

 chain systems used for maintaining position. Other disadvan- 

 tages are that they are easily affected by adverse weather 

 conditions and lack a self-propulsion system (Kash et al . , 

 1973: 37). While barges were used extensively in the early 

 years of OCS operations in the Gulf of Mexico, they are not 

 normally used there now. 



Semi-submersibles are self-contained and supported by 

 either lower displacement type hulls or by large caissons 

 (Figure 1.2). After towing to the drilling site, the caissons 

 are flooded, causing a portion of the rig to be below the sea 

 level. This procedure causes the rig to be virtually unaffected 

 by wave action and generally more stable than a drill ship. 

 Another advantage is that they act as fixed platforms when 

 drilling in shallow water (Jenner et al . , 1973: 127). 



Jack-ups are platforms with legs that can be extended or 

 retracted, depending upon the depth of water at which the drill- 

 ing will take place (Figure 1.3). With the legs retracted, it 

 becomes a floating platform, thus allowing it to be moved. It can be 

 used either for exploratory drilling when only one well is desired or 

 for multiwell production platform drilling. Its advantages are 



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