Macy 



cost. The second method is to provide scour coamings of the "cookie cutter" 

 type which extend 4 or 5 feet below the bottom of the hull. These are also of 

 benefit in preventing lateral shifting on bottom during storms. Care must be 

 taken to have vent pipes through the hulls to prevent trapping air behind these 

 coamings. The coamings are somewhat vulnerable to damage, and in very soft 

 bottoms such as encountered in the Gulf of Mexico off Louisiana they would have 

 to be uneconomically deep to do any real good. A third solution is to provide 

 the platform with a number of spuds which penetrate deep into the bottom of the 

 sea. While these do not acti^aUy prevent scouring, they will prevent lateral 

 shifting, and if the spuds are of sufficient number and capacity and are wedged 

 or locked into their guides, they will help to prevent undue settling of one 

 corner or one edge as a result of scouring. 



In soft bottoms such as in Louisiana it is not at all unusual for the lower 

 hulls of platforms to be completely buried in the mud after a long period on sta- 

 tion. To remove an imbedded hull from the bottom requires considerable pre- 

 cautions. Water jet piping is a necessity, but care has to be taken in pumping 

 out water ballast that the platform does not suddenly break loose from the bot- 

 tom and emerge at an excess velocity. It is ordinarily preferable to try to get 

 one end free before the other end. With a column stabilized platform disaster 

 could hardly occur, because as the platform starts to come up, the displace- 

 ment of the stability columns falls off rapidly, functioning as a natural hydro- 

 static brake. Old type platforms which had large buoyant lower hulls and only 

 small-diameter columns did not have this advantage of rapid change in dis- 

 placement on emerging and could conceivably come up to the surface of a 

 terrific rate. 



In cold areas it is necessary to consider the effect of ice on stability. 

 One platform had a loss of metacentric height of 1.5 feet as a result of 3 inches 

 of ice on the platform deck, not including ice elsewhere. To make matters 

 worse, ice formation is greater on the windward side resulting in a heeling 

 moment. Ice is not likely to form on the columns, as the waves would keep it 

 washed off, but it can form under the platform. As a result of snow, ice may 

 develop on high-center-of-gravity structures such as drilling derricks. Ref- 

 erence 2 contains an extensive bibliography on icing of ships. 



Ocean platforms exceeding 100 gross tons, other than those which rest on 

 the bottom, are subject to U.S. Coast Guard regulations, and rather complete 

 stability calculations are required. 



MOTION 



Aside from the usual considerations of comfort and structural design, mini- 

 mum motion is a necessity to facilitate the work assignment of a platform. Ex- 

 cessive angular motion or surge in an oil drilling platform will cause bending 

 stress in the rotating drill pipe, while heave will result in tension or compres- 

 sion in the drill pipe. Telescopic sleeve joints just above the bit prevent cyclic 

 lifting of the bit off the bottom. 



Experience has shown that if a flared structure like a curved inverted funnel 

 is fitted below the derrick substructure, that a half angle of as much as 20 



464 



