Towing, Motion and Stability of Platfornas 



Mooring lines are commonly of the following types: all wire rope, wire 

 rope with a nylon section, wire rope with a chain section, all chain, or wire rope 

 from the winch to the buoy and chain from the buoy to the anchor. The resil- 

 iency afforded by the second and the last types are attractive but complicate the 

 winding of lines on drums or wildcats. This is less serious than would appear 

 as most platforms are not maneuverable enough to set their own anchors, and 

 this has to be done by auxiliary supply vessels. Anchors are usually of the 

 Navy LWT or Danforth types. 



Model tests show that for minimum motion the mooring lines must not be 

 placed too low. A test of a platform 140 feet high at 70 feet draft showed that 

 raising the point of attachment of mooring cables from 30 feet above the bottom 

 to 100 feet resulted in a reduction of heel due to aerodynamic lift from wind 

 under the platform deck from 8 degrees to 1 degree. This was accompained by 

 a reduction of mooring line tension of 40 percent. 



This aerodynamic effect is important, and various types of "spoilers" have 

 been tried with little or no gain and should be the subject of further research. 

 Care should be taken to avoid overhanging platforms such as cantilevered pipe 

 racks or helicopter platforms which might be subject to such lift. 



A disadvantage of the multiple anchor system is the difficulty in changing 

 heading in order to improve the motion or kill synchronism. The anchor lines 

 originate at fixed points on the platform and only a few degrees of swing can 

 ordinarily be made. The Offshore Company's platform Discoverer is a single- 

 hull platform with a rotatable annular ring in the center of the hull, having the 

 anchor lines attached to the bottom of the ring. By the use of thruster propel- 

 lers, the hull can be rotated 360 degrees. There are, however, some formi- 

 dable problems of maintenance and servicing the anchor lines under the hull. 



Position can be maintained by rotatable propellers instrumented for auto- 

 matic position sensing and control. Conventional mooring arrangements become 

 too expensive in water more than about 600 feet deep, and dynamic positioning 

 will likely be a necessity in very deep water. 



PLATFORM SELECTION 



Table 2 gives an indication of the suitability of several common platform 

 types. Water depth is a relative term; water deep enough for a platform 50 by 

 SOfeet is shallow for one 2 50 by 2 50 feet. However the table is based on relatively 

 large platforms. While spar platforms are shown, their primary field seems 

 to be in oceanographic research, where a great load capacity is not required. 

 The principal conclusions drawn from this table are (a) that below 50 feet plat- 

 forms should rest on bottom, (b) between 50 and 300 feet platform types are 

 available which can float, rest on the bottom, or be semisubmerged, and (c) above 

 300 feet the floating or semisubmerged platforms seem the best. 



New developments in details of existing types as well as new types of plat- 

 forms are being developed. Unfortunately model test and actual performance 

 data does not exist to nearly the degree as in the case of ship data. Further- 

 more, the majority of platforms in use are for the oil industry, and data is 



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