its economy of operation, stability, and the speed with which 

 it can begin operation after arrival at the drilling site 

 (Jenner et al . , 1973: 121-125). 



Drill ships (Figure 1.4) are self-propelled and therefore 

 capable of moving from one drilling location to another with- 

 out the assistance of ocean tugs or other propulsion units. 

 Two methods, a mooring system with anchors and chains or a 

 dynamic positioning system which uses propellers or thrusters 

 coupled to sensors to detect and compensate for movement, are 

 used to keep the vessel stable during drilling operations 

 (Kash et al . , 1973: 39). 



Drill ships are usually replaced by fixed platforms 

 (Figure 1.5) following the initial discovery; normally by one 

 of the other four types of rigs. However, they may be used 

 in the developmental stage of a field when it becomes necessary 

 to drill (from 10 to 40 individual) wells directionally for 

 production (Jenner et al . , 1973: 121). 



Production 



Once exploratory drilling has located sufficient quanti- 

 ties of oil or natural gas, production and development efforts 

 begin. Due to the complexity of the entire operation, many 

 exploration and field development activities may overlap. 

 Figure 1.6 gives a general breakdown of the overlapping 

 which can occur, beginning before the lease sale and extending 

 through the installation of permanent production facilities 

 (Kash et al . , 1973: 49-50). 



19 



