Macy 



To some extent motion periods or amplitudes may be adjusted. The obvious 

 means of controlling roll and pitch motions is by changing the radius of gyration. 

 This can be changed in wide hulls or catamarans by shifting ballast between out- 

 board and inboard locations. The shift must be done horizontally to prevent 

 modifying the metacentric height. Merely adding water ballast does little good 

 as the gain in radius of gyration tends to be offset by the increase in the meta- 

 centric height. 



The metacentric height may be adjusted in several ways to change the roll- 

 ing period. Figure 18 is the ballasting curve of a four -column grid hull plat- 

 form, and shows the wide choice of GM available with only moderate draft 

 adjustments. 



The metacentric height may also be changed by increasing or decreasing 

 the free surface in the ballast tanks. The free surface can be increased only 

 up to the limits imposed by stability requirements. The metacentric height may 

 also be adjusted on column stabilized platforms by providing enlargements or 

 bulges on some or all of the columns near the working water line (Fig. 9) so 

 that the draft may be set on the enlargements, or above or below them. Also, 

 if stability permits, the water plane area of the columns may be decreased by 

 reducing the stability column diameter over a portion of the length of the col- 

 umns. Usually holes are provided in the columns and a smaller inner core is 

 provided to maintain watertight integrity. 



The devices outlined in the preceding paragraph also change the tons per 

 foot and are therefore effective in changing the heave period. Another obvious 

 way to modify the heave period is to change the displacement, but increasing the 

 draft to do so results in an increase of surge force and mooring line tension. 

 Heave damping plates have been tried and are sometimes effective if placed 

 deeply enough. Otherwise, if they are in the larger wave particle orbits near 

 the surface, they could actually augment heave. 



It seems appropriate to introduce a word of caution about these motion con- 

 trol devices; a patent study should always be made before adopting any of them. 

 Many devices used or proposed on ocean platforms for stability or motion con- 

 trol based on principles known for many years now seem to be covered by re- 

 cent patents. 



MOORING 



Motion and mooring are closely related in that minimum motions may be 

 obtained from high tension in mooring lines, literally "tieing down" the rig. 

 This is at the expense of greatly increased mooring equipment, which on a large 

 platform can easily reach $1 million in cost. 



Three-column stabilized columns are usually moored at each column with 

 a total of 3, 6, or 9 anchor lines on the platform. Four-column platforms as 

 well as jack-up platforms in a catamaran have four to eight lines. The anchor 

 lines should have a moderate degree of pretension, and while constant tension 

 winches are desirable, their high cost is usually prohibitive. However, in any 

 case line tension recorders should be installed. 



468 



