Second order, slowly varying wave forces . In addition to the 

 wave-frequency forces described above, there will be a system of 

 forces which depend upon wave reflection and interference between 

 the ship motions and the incident wave system. These forces are 

 proportional to the squares and products of wave height and ship 

 motions, and consequently are neglected in a linearized motion 

 analysis. 



For a moored ship or platform, they may be of considerable im- 

 portance to the mooring response and must usually be included. In 

 regular waves, the wave reflection force is constant and merely 

 causes a mean offset. In random waves, however, this effect gives 

 rise to a slowly varying force having important frequencies equal 

 to the frequencies of the envelope of the wave time history. These 

 low frequency forces may excite resonance of the plat form/ mooring 

 system which in turn may lead to high loads in mooring lines. 



The computation of the low frequency wave loads is beyond 

 the scope of this paper, but a paper by Newman (1974) discusses 

 the important fluid dynamics aspects. 



IV. APPLICATION OF ANALYSES PROCEDURE 



There are a number of design or operational problems to which 

 the hydrodynamic loading and motions analysis procedures described 

 in the preceeding section may be applied. We shall describe several 

 of them here. 



Performance prediction . A vital element in any design procedure is 

 the ability to predict the performance of the system being developed 

 at an early phase of the design process. Two performance parameters 

 of great importance in the design of offshore platforms are the 

 motions of the platform and the forces in the mooring system. 

 Figures 4 and 5 illustrate two different types of stable platforms 

 with which we are concerned here. The first type is the conventional 

 twin-hulled semisubmersible, and Figure 4 depicts a somewhat simpli- 

 fied version of an actual design. The second. Figure 5, is a tension 

 leg platform which is an innovative concept currently attracting 

 considerable interest as a candidate for deep water drilling and 

 production. The tension leg platform illustrated here is a small 



experimental platform which has been tested at sea off the coast 



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