The jack-up rig is the only bottom-supported platform that may be 

 used in the OCS. Drilling rigs of this type are readily available as 

 they make up some 40 percent of the world's offshore exploratory rig 

 fleet. One of several important factors the operators will consider in 

 their selection of rigs is whether jack-ups are sufficiently mobile for 

 the job. Unlike other platforms, a jack-up rig is secured to the sea 

 floor to enhance its stability and to increase its resistance to wave 

 action. Preparing the jack-up for a move to a new location and then re- 

 securing it to the sea floor can take several weeks, depending on sea 

 and sea-floor conditions. 



Cost factors aside,, the choice of rig for a particular OCS site is 

 based on a tradeoff between the demands of mobility and the desired 

 limits of vertical variation between the drilling platform and the 

 wellhead. If oply a few wells or wery deep wells are to be drilled, 

 mobility might be sacrificed for the greater stability of jack-ups. 

 However, if numerous wells are to be drilled, a floating platform may be 

 more feasible. Table 10 provides a comparative account of the three 

 major types of mobile exploratory drilling rigs--jack-up, drill ship, 

 and semi-submersible--by four major variables in selection (depth, 

 capability and other factors are judgemental and therefore vary from 

 source to source). 



Operations 



The exploration operations employed offshore, at sometimes great 

 depth, are an extension of the land methods that have developed over the 

 past seventy-five years. The only real difference is the specialized 

 hardware and the associated industries which developed in response to 

 that particular type of drilling. 



All exploratory rigs have the necessary equipment on board for 

 drilling, but they must be supplied from service bases on the shore by 

 service boats and helicopters. The boats usually bring drilling muds 

 and drilling pipes on a regular basis if the distance from shore is not 

 excessively great; helicopters may be employed when distance to the rig 

 is a factor and too much time would be consumed in boat transit. 

 Helicopters are also utilized for interim trips, providing a quick, 

 efficient means of contact with the rig. Crewboats or possibly helicopters 

 are employed to change the drilling rig crews; this occurs typically 

 once every seven days or two weeks, but it varies with projects and 

 companies. Food is brought out at these changes, and solid waste is 

 collected from the rigs. Sewage is treated on board the rig or drill 

 ship and discharged into the sea. 



70 



