Towing, Motion and Stability of Platforms 



can be expected. Column stabilized platforms can be designed to have natural 

 frequencies beyond these periods. 



Figure 19 shows typical heave response curves with an assumed maximum 

 wave period superimposed. This shows that a column stabilized platform can 

 be designed to have a natural frequency beyond the range of most anticipated 

 waves. The situation for roll and pitch is similar. 



10 13 



PERIOD OF WAVE.SEC. 



Fig. 19 - Typical heave response curves 



Normal single hulls cannot have a large radius of gyration due to their lim- 

 ited width. However, they have the benefit of reasonably low GM so that the roll 

 period may not be too short. Due to the large tons per foot the heave period is 

 short. Surface catamarans, very wide floating barges, and jack-up platforms 

 have large radii of gyration and high metacentric heights at the same time, and 

 even larger tons per foot, so that all motions are of a short period. 



Column stabilized platforms when submerged combine large radius of gy- 

 ration with medium GM to obtain long roll and pitch periods. The low tons per 

 foot of the columns coupled with a high displacement results in a long heaving 

 period. 



Platform design selection should include studies of its motion character- 

 istics in association with data on the wave periods expected at the contemplated 

 sites. For preliminary purposes, the roll and pitch formulas given seem to 

 agree well enough with actual results. The heave periods seem to be always 

 longer, by as much as 25 percent due to the damping of the hull or footings. It 

 is recommended that preliminary designs be later confirmed by model tests. 

 However, it should be pointed out that rolling and pitching tests are no better 

 than the precision of the calculation of the mass moment of inertia or radius of 

 gyration. This is a laborious calculation if done thoroughly. 



467 



