Towing, Motion and Stability of Platforms 



degrees of roll can be tolerated. The sleeve joints in the drill string consists 

 of multiple sections, and enough of them can be used to permit tolerating a 

 heave of around 20 feet. The platform must be reasonably restrained laterally, 

 and a rough rule of thumb is that a drift from the vertical of 10 percent of the 

 water depth is acceptable. Even so, all these motions must be kept to a mini- 

 mum, and platforms of various types have greatly differing motion properties. 



Broadly speaking, the usual types of platforms may be separated into three 

 categories as shown in Table 1. 



Table 1 

 Platform Categories 



Jack-up platforms have motion problems when raising and lowering. Since 

 the jacking speed is rather slow, when lowering the legs at a stage when the legs 

 are close to the bottom, there is an appreciable interval of time when the leg 

 bottoms are touching or nearly touching the bottom. This also applies when the 

 legs are being raised and have just come off the bottom. The leg bottoms are 

 then subject to pounding on the bottom of the sea even in a very moderate sea 

 condition, due to the heaving motion of the floating platform. This effect is very 

 much less if there is a bottom mat, due to its inertia and damping effect. A jack 

 or motor failure just at this time would be serious. 



Another critical condition exists from leg bending stress when the legs are 

 hanging almost to the bottom, due to rolling acceleration. Even in moderate 

 sea conditions, the combination of a short rolling period and a long radius to the 

 leg bottom results in a considerable length of arc of movement of the leg. The 

 acceleration is sufficient to cause very large stresses in the legs unless they 

 are designed with this in mind. The use of a bottom hull instead of individual 

 legs tends to dampen this acceleration. 



Similarly with the platform on the surface, legs may extend several hundred 

 feet in the air and during bad weather, which might be encountered during towing, 

 the top of the legs will be moving through a considerable arc. 



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