Mooring System Excitation and Analysis 



• Second-Order Drift Forces Dr. W. McCreight 



• Random Wave Characteristics Dr. M. K. Ochi 



• Spectral Analysis Dr. J. S. Bendat 



• Mooring System Analysis Dr. J. R. Paulling 



• Mooring Dynamics Model 



Development Example Dr. S. Calisal 



Each attendee delivered a short paper concerning his particular 

 subject; these are included in a separate section in this report. Each 

 of these papers gives a concise overview of the fields important to 

 mooring simulation by emphasizing assumptions, limitations, and applica- 

 tions. These papers are subjective in nature, and as such they are 

 easily read and understood. 



Session II 



The discussions on the second day were intended to give CEL better 

 insight into existing state-of-the-art mooring models and to allow the 

 participants to delve into new ideas and approaches to mooring analysis 

 problems. Both of these goals were achieved. 



Much of the discussion centered on the calculation and importance 

 of the slowly varying drift force. Although this force is small, it can 

 become very important because it exists at frequencies close to the 

 natural frequency of many moored systems. When this dynamic force is 

 negligible, a linearized frequency-domain dynamic analysis can be used 

 at a very small computation cost. However, if the force has a signifi- 

 cant effect on the mooring, a nonlinear time-domain model is required. 

 This model requires a great deal of computer time because the statistics 

 of the system behavior must be calculated indirectly from several brute- 

 force simulations. Thus, the magnitude of the slowly varying drift 

 force determines whether an inexpensive frequency-domain or an expensive 

 time-domain dynamic computer model is required. Other topics of discussion 

 are included in the CONCLUSIONS FROM SEMINAR DISCUSSIONS section. 



The discussions throughout the second day did result in progress 

 toward extending the state-of-the-art in mooring analysis. As Dr. Muga 

 points out in his paper, a nonlinear stochastic model would be the ideal 

 analysis tool for moorings. This imaginary model would give statistical 

 information directly and would eliminate the expensive intermediate 

 results necessary with current time-domain nonlinear dynamic models. 

 Dr. Bendat stated that he felt the time was right to extend linear time 

 series analysis techniques and modify existing nonlinear techniques to 

 obtain the necessary mathematics to describe nonlinear system behavior. 



Session III 



The final session began with an explanation by Dr. Webster on the 

 DSSM computer model and a short summary by CEL on Navy mooring appli- 

 cations. This was followed by discussions on the strong and weak points 

 of the DSSM model and suggestions on how to improve it. 



