Towing, Motion and Stability of Platforms 



pads at the bottom of each leg. At least one platform has been lost due to the 

 presence of buoyancy in such footings. If these spaces are insufficiently filled 

 with water ballast, during raising and lowering, the center of buoyancy of the 

 entire assembly may be too low and the platform may capsize. Where it is nec- 

 essary to have ballast in the lower hull, arrangements should be made so that 

 this ballast is added automatically by an open stand pipe extending up to a pre- 

 determined height. This insures that such tanks will be filled at an early stage. 



The stability of jack-up platforms when actually resting on the bottom is a 

 matter of overturning moment due to wind and wave versus the resisting moment 

 provided by the bottom of the sea. This is a matter of soil mechanics and is 

 outside of the scope of this paper. However, platforms have capsized in this 

 manner and have been completely lost. The provision of a buoyant upper hull 

 would have in several cases resulted in the platform at least not disappearing 

 out of sight, and salvage would have been possible. 



Column stabilized platforms are one of the most successful and safest types 

 of platforms; yet their stability presents some very unusual problems. The sta- 

 bility of such platforms when floating on the surface is tremendous but as soon 

 as they are submerged to the point where the hull is awash, the moment of in- 

 ertia of the water plane drops so that the metacentric height of hundreds of 

 feet becomes only 5 to 25 feet (Fig. 18). Exceptional care has to be taken in the 

 sequence of ballasting in order to provide proper trim as well as to restrict 

 free surface. The trim problem is aggravated with certain forms in that the 

 center of flotation may abrubtly jump a considerable distance when the hull be- 

 comes awash. For example, a configuration having a catamaran hull would have 

 its center of flotation at midships when surfaced but if this platform has three 

 stability columns, which is entirely feasible, the center of flotation, once the 

 hull were submerged, would be at one -third the distance from one end. 



It can be shown (4) that for a symmetrical arrangement of stability col- 

 umns, the moment of inertia of the water plane is the same about any axis. 



The diameter and spacing of stability columns are usually a compromise. 

 Economical construction calls for relatively large columns closely spaced so 

 that the connecting structures at the hull and at the platform are of minimum 

 proportions. However, considerations of motion and wave transparency dictate 

 small diameter columns very widely spaced. On the other hand, to prevent 

 heaving motions from being too short, small columns are desirable, but this 

 results in a very low value of tons per foot of immersion, so that considerable 

 changes in draft and trim occur when adding or removing loads. This partic- 

 ularly applies to the drilling load, which in some arrangements is at one end. 



In general drilling should be performed at the center of flotation in order 

 to provide minimum motion and minimum change in trim. One of the drilling 

 platforms of the Ocean Drilling and Exploration Co., the Ocean Queen (Fig. 9), 

 has some interesting features — one of which is enlargements on some of the 

 secondary structural support columns which provide an increase in water plane 

 area at the normal drilling draft. In the event of severe motions and synchro- 

 nism the barge could be ballasted or deballasted to bring these deeper in the 

 water or out of water and therefore change the heaving period. Also, presumably 



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