Towing, Motion and Stability of Platforms 



drums to reduce the permeability. It is considered good practice to provide in 

 the stability columns a safety deck so located that in the event of inadvertent 

 total flooding the platform could not completely sink. An alternative procedure 

 is to make the upper platform structure of a watertight box configuration which 

 has the dual advantage of restricting the angle of heel in case of unsymmetrical 

 flooding or preventing foundering in case of total flooding of the lower hull. In 

 the latter alternative, however, each and every opening of every kind in the 

 upper platform must be made watertight, which means that drains must all be 

 connected into a centralized drain tank, and that watertight doors must be 

 avoided in the upper platform enclosure. 



The calculation of righting levers for complex platforms which not only 

 have stability columns but also secondary structural support columns which 

 may or may not be vertical is rather complex, and Ref. 5 describes a method of 

 facilitating such calculations by the use of optical projection in which a beam of 

 light is projected through a scale model of the platform onto a screen. 



In order to obtain the maximum metacentric height with a minimum of struc- 

 ture, it is obvious that the center of buoyancy should be as high as possible. An 

 optimum situation is to have a column stabilized platform which has no lower 

 hull of any kind and the entire displacement is afforded by the columns them- 

 selves. In this case the center of buoyancy is at half the draft. This arrange- 

 ment has been proposed several times but not actually constructed due to the 

 fact that the draft would be too high for construction of a large platform in most 

 shipyards. Therefore column stabilized platforms usually have a buoyant lower 

 hull, and in keeping with this principle the displacement of this hull should be 

 at an absolute minimum. It is not even strictly necessary that there be any 

 positive freeboard at all in the surfaced condition. However, from the practi- 

 cal consideration of accessibility to tank manholes, a freeboard of several feet 

 is usually provided. As discussed in the preceding section, however, a greater 

 freeboard insures that the columns supported on the hull will be high enough 

 above the water so that the effect of waves on towing resistance is reduced. 



Unless the stability columns contain some watertight subdivisions, the flood- 

 ing of one column will usually result in the loss of the platform. This is espe- 

 cially true of a three -column platform. A four column platform may survive 

 actual loss when one column is damaged but will be inclined at a very great 

 angle. A platform having six columns can survive at a moderate angle the 

 damage to any one column; but for commercial purposes six columns are not 

 used due to economic considerations. The platform proposed for the Mohole 

 project, however, will have six columns. 



Ordinarily, the tops of stability columns are tapered in diameter somewhat 

 above the maximum water line. This reduces the area exposed to waves, al- 

 though it has a disadvantage of taking away what would otherwise be very de- 

 sirable reserve buoyancy in the event of damage. The savings in structural cost 

 is very minor. Platforms having lower hulls or mats which rest on the bottom 

 of the sea are subject to scouring due to current or, in fairly shallow water, 

 effects from storm waves. There are three principal methods of reducing the 

 effect of scouring. One is to slope the sides of the hull into a flat wedge. This 

 is no longer popular in that it results in increased dimensions and construction 



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