MOORING CABLE DYNAMICS SUMMARY 

 By 

 Dr. R. L. Webster 



The two-day mooring dynamics seminar sponsored by GEL on 10 and 11 

 January 1980 covered various disciplines related to mooring vessels and 

 platforms in unprotected waters. The participants were well chosen to 

 provide broad coverage to the problem from practical as well as theoretical 

 aspects. Attempts at ranking the presentations appear futile since they 

 covered such diverse topics and points of view. The presentation by 

 Bruce Muga pointed out some categories of approach to or features of 

 mooring analysis that are useful. His classifications included (not 

 necessarily in his order): 



Ship Dominated vs. Mooring Dominated 



Linear vs. Nonlinear 



Frequency Domain vs. Time Domain 



Stochastic vs. Deterministic 



Muga further pointed out that the majority of mooring analyses presume 

 ship-dominated linear systems that are treated either in the time or 

 frequency domain with essentially deterministic methods. Treatment of 

 nonlinearities requires time domain methods at present. Probabilistic 

 data about the mooring responses are obtained from the time domain data 

 by statistical evaluation of the output using a knowledge of the statis- 

 tical nature of the input. Frequency domain solutions typically generate 

 response spectra from superposition of discrete responses for waves 

 selected from a wave spectrum or set of spectra. 



Webster's paper emphasized the role of the mooring lines and their 

 effect on the system response. It was suggested that ship-dominated 

 systems are much less common than they are assumed to be. Both Paulling's 

 and Webster's papers emphasized the importance of the geometric stiffening 

 effect of the preloads in the mooring lines. The common assumption that 

 mooring line djraamics have little consequence in ship-dominated systems 

 may be justified in situations where small excursions occur that do not 

 significantly reorient or stretch the lines. However, even in these 

 situations, the effect of the mooring line preload on motions transverse 

 to the line must be accounted for in the forces applied to the vessel. 

 It is not correct to represent a mooring line as a single-force member 

 (simple spring) with stiffness in only one line of action. 



The major focus of the seminar was on the methods available for 

 treating the mooring-dominated situation (be it in shallow or deep 

 water) where the dynamics and nonlinearities of the lines cannot be 

 ignored. Dynamic effects in the lines refer to mass- and stiffness- 

 related phenomena such as resonances in the lines themselves as well as 

 in the coupled system. Nonlinearities include material nonlinearities 

 and slack/snap phenomena, bottom interaction, and geometric nonlinearities 

 (large displacements). 



All present appeared to agree that realistic analysis of mooring- 

 dominated responses of a moored vessel or platform is a very formidable 

 problem. The major difficulty is the development of the equations of 

 motion for the vessel or platform and the expressions for the wave-induced 



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