Macy 



Fig. 17 - Towing resistance of mobile 

 platforms in smooth water. The data 

 for these composite curves were ob- 

 tained from actual towing perform- 

 ances or from model tests. 



and this load can be fully effective as soon as the bit is raised off the bottom 

 when pulling it out of the hole. For this reason, high-capacity single-hull 

 drilling rigs ordinarily drill in a well on the center line. 



Catamaran hulls or the outrigger type vessel have tremendous metacentric 

 height, and stability is no problem at all, though motion and mooring are. 



Likewise, jack-up platforms usually have a hull of substantial length and 

 width and static stability presents no difficulty at least in the upright condition. 

 The range of stability however is sometimes a problem, as the great width of 

 such hulls means that at a relatively small angle the deck is immerged or the 

 bilge is emerged, resulting in a loss of range of positive righting lever. There- 

 fore, it is best to have the freeboard equal to the draft so that both occur 

 simultaneously . 



The stability of jack-up platforms may be a problem if there is a common 

 buoyant lower hull at the bottom of the legs or if there are individual buoyant 



460 



