of California. In the case of both of the platforms depicted 

 here, model test data have been published for structures of similar 

 although not identical configuration to those depicted. For the 

 semisubmersible , data for the pitch and heave motions in head seas 

 are shown in Figures 6 and 7. The computed response is shown by 

 the solid line and the correlation with experimental data may 

 be considered typical of that obtainable using the tv;in-hull strip 

 theory procedure. In the case of the tension leg platform, data 

 are shown in Figure 8 for the surge motion and in Figure 9 for 

 the wave induced tension variations in an anchor line. In these 

 figures, the theoretical predictions were performed using the 

 slender member-space frame procedure. Here, an excellent prediction 

 of the motion is obtained, but the mooring tension prediction is 

 not quite in so good agreement with experiments . This behavior 

 has been observed rather generally and indicates that the mooring 

 system may be more sensitive to nonlinear effects that are the 

 motions. 



Effects of high waves . It was noted earlier that a numerical inte- 

 gration of the nonlinear equations of motion may be performed in 

 order to investigate certain specific nonlinear effects. Such an 

 effect, which may be of importance in the operation of a tension 

 leg platform, is the mean offset from the initial position caused 

 by high waves. To visualize the effect in question, refer to 

 Figure 10 and recall that the vertical tensioned mooring system 

 will effectively suppress the heave motion of the platform, one 

 column of which is shown in this figure. It is clear, that when 

 a wave crest is centered at this column, the length of member 

 immersed is much greater than it is in a trough. If the wave is 

 moving from left to right in the figure, the result will be that 

 a considerably greater drag force will act on the member when in 

 a wave crest, than when the member is in the trough, solely because 

 of the difference in the immersed length. In the wave crest, the 

 fluid velocities, thus the drag force is to the right, while in the 

 trough, the velocity and drag force are directed to the left. As 

 a result, an average force directed to the right will act on this 

 member as well as others comprising the platform, causing a mean 



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