structure having ship-like hulls . In the case of a platform having 

 one or more ship like hulls floating either at the surface or sub- 

 merged, a technique termed "strip theory" has been developed prin- 

 cipally in connection with the prediction of the wave induced 

 motions of surface ships. In this procedure, the solution of the 

 potential flow problem for the pressure distribution and, therefore, 

 the fluid loading on the three-dimensionaJ ship hull i.s obtained 

 from the solution for the flow about two-dimensional bodies having 

 cross sections similar to the transverse cross sections of the 

 ship. The two-dimensional problem is relatively easy to ?olve and 

 several methods have been developed which are suitable for either 

 single or multiple hulls. The damping, added mass and wave exciting 

 forces are first obtained for two-dimensional shapes similar to 

 the transverse sections of the ship, for the wave- frequencies of 

 interest, and these two-dimensional forces are then integrated 

 over the length to obtain the three-dimensional forces and moments. 

 The usual procedures are based upon assumptions of small motion 

 amplitude and inviscid irrotational fluid theory and yield results 

 suitable for inclusion in a linear motions analysis procedure. 



An example of the two-dimensional forces, expressed as damping 

 and added mass coefficients, is shown in Figure 3. In this case 

 the geometry consists of twin circular sections having proportions 

 typical of some twin-hulled semisubmersible platforms. The figure 

 illustrates two features of the behavior of these forces. First, 

 the dependence of damping and added mass on the motion frequency 

 is clearly seen. Second, in the lower part of the figure, an 

 additional viscous damping is shown for comparison with the wave 

 damping which is predicted by the two-dimensional potential flow 

 theory. The viscous damping is assumed proportional to the square 

 of the velocity and, therefore, in this nondimensional plot, 

 depends on the amplitude of motion. Several different combinations 

 of drag coefficient and amplitude of motion are shown in order to 

 illustrate the relative importance of viscous and wave damping 

 for such a configuration. 



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