loads. A particularly troublesome aspect of this is the evaluation of 

 the effects of the second order wave-induced drift forces. These forces 

 have two components: one at high frequency and one at low frequency. 

 The high-frequency component is generally of sufficiently high frequency 

 and low amplitude that it can be neglected. The low-frequency component 

 cannot be ignored since it can produce large-amplitude, low-frequency 

 excursions of the system, thereby involving the nonlinearities of the 

 mooring. Calculation of these second order forces is greatly complicated 

 by the fact that they depend on the motion of the moored body, which in 

 turn depends on the characteristics of the mooring, which change signif- 

 icantly as the system moves. Although it was emphasized that the major 

 interest was in designing and evaluating the adequacy of the mooring 

 system and not in the specific motion of the moored object, it is apparent 

 that you cannot get one without the other. 



The SEADYN/DSSM approach was discussed briefly. The setting of the 

 presentation (following an extensive discussion of possible approaches) 

 precluded much critical discussion of the DSSM approach. In general, 

 the feeling appeared to be that the approach was sound but did not go 

 far enough. Questions were raised about the spectra used, the nature of 

 the statistical calculations, and that it does not deal with the unsteady 

 part of the second order drift forces. 



Three general approaches to the problem emerged from the discussions. 

 There was not time in the seminar, nor was it the appropriate place, to 

 explore them enough to completely define them, but some rough outlines 

 were developed. These three approaches are briefly described below. 



APPROACH A - EXTEND THE PRESENT DSSM SOLUTION 



1. Calculate the initial static reference state with steady 

 components of wind, current, workloads, etc. 



2. Solve frequency domain dynamics of a coupled system with 

 appropriate wave spectra to estimate the steady part of 

 second order drift forces. 



3. Adjust the reference state for steady drift forces. 



4. Repeat the dynamic solution to evaluate changes (repeat step 3 

 if needed) and estimate the unsteady second order drift forces. 



5. Perform large displacement time domain solutions to get the 

 response to the unsteady drift forces. The small displacement 

 dynamics represented by the frequency domain solution are 

 neglected in this step. 



6. Locate the extreme states from step 5, and repeat the frequency 

 domain solution at each of these states to identify the worst 

 combined conditions and make statistical estimates. 



Changes required in SEADYN: 

 • New wave spectra form 



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