McClure and Hove 



Fig. 1 - Drilling platform model 



design of the vessel with small water plane area and deeply submerged main 

 hulls. These characteristics make it difficult to obtain low resistance which is 

 a desirable characteristic for station -keeping. The photograph, Fig. 1, shows 

 that the configuration chosen will yield relatively low drag for the hulls without 

 sacrificing desirable wave transparency (1). 



It is mandatory to the life of the drill string and riser that the platform be 

 kept within a prescribed distance of the hole, dependent upon water depth. At 

 the Mohole site, in 14,700 ft of water, the maximum safe distance off the hole is 

 400 ft. A circle of this radius is clearly too small for conventional ship-type 

 maneuvering. The platform is equipped with propulsion equipment which enables 

 it to turn at any radius, or to move in a straight line in any direction. While on 

 station it is intended that the vessel will be headed into the current if the cur- 

 rent is one knot or more, or into the wind if wind predominates. If both wind 

 and current are strong, a heading between the two forces will be chosen to min- 

 imize propulsive power. At times it will be necessary to move the platform 

 transversely through the water or at some large angle to the centerline in order 

 to hold station in a weak current from one direction while a strong wind is blow- 

 ing from another direction. As conditions change, a new heading will be se- 

 lected where propulsive power and resistance can be most favorably matched. 

 The platform will not automatically change heading but rather will maintain a 

 predetermined gyrocompass heading. Environmental conditions for design of 

 the positioning system were of necessity selected before the Mohole site was 

 chosen. Wind and current conditions were established which would permit oper- 

 ation in any of the potentially suitable areas. The design conditions do not 



416 



