Several types of mooring systems or devices may be used to 

 maintain the platform in its mean position. Examples are: 



(1) Vertical or slanted taut cables. 



(2) Catenary chain moorings, sometimes with buoys or 

 additional clump weights. 



(3) Dynamic positioning. 



In each of these cases, it is usually necessary to represent 

 the mooring device by a somewhat simplified analytical model in 

 order to develop a practical analysis procedure. As an example, 

 if the platform motions are expected to be small, the nonlinear 

 force/displacement characteristics of a catenary chain mooring 

 line may be replaced by a constant coefficient equal to the deri- 

 vative of the force versus displacement graph at the mean position 

 of the platform. 



In the case of the fluid forces, it is frequently possible to 

 consider the motion of both fluid and platform as small. In this 

 case, the forces caused by the waves can be computed independently 

 of the forces caused by platform motions and the latter are found 

 to be proportional to the motion variables and their derivatives. 

 This forms the basis for the linear spectral technique of deter- 

 mining the platform response to a random wave system by super- 

 position of the responses to a number of different elementary 

 regular wave systems. The procedure has found wide application to 

 such diverse problems as the prediction of sea state imposed 

 limitations on platform operations, or the long term cumulative 

 structural damage due to fatigue. 



In some cases, however, information concerning the effect of 

 specific nonlinear phenomena is required and the simplifications 

 noted above are not possible. Examples of such cases involve the 

 motion response of the platform to an extremely high wave, possibly 

 involving capsize or other hazard, or the effect on motion response 

 of a nonlinear dynamic position keeping system. It is seldom 

 possible to obtain a complete and exact nonlinear representation 

 of platform response including all relevant effects. Instead, it 



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