Mooring and Positioning of Vehiales in a Seaway 



appropriate measured inputs can then be evaluated in detail by com- 

 puter system simulation to deternnine the optimum gains to bemused 

 in the control rule in Eq. (139). The only guidance that can be given 

 for this selection, based on sinaple dynamic principles, is to select 

 the value of the gain C| that will produce a resultant frequency that 

 lies between the frequency associated with the mcixinnum spectral 

 energy of the predominant wave system and the very low (near zero) 

 frequency for large responses to drift forces. The value of the gain 

 Cg should be such that," when added to the normal ship damping, 

 the resulting response of the system will be relatively "flat through- 

 out the nnajor band of disturbing frequencies for the drift force. 

 All of these characteristics can be refined in the course of control 

 system analysis and design, as well as from the simulation results, 

 and further discussion lies beyond the scope of the present paper. 



XI. CONCLUDING REMARKS 



All of the preceding problem areas discussed in this paper 

 have illustrated the application of a specific area of Naval Hydrody- 

 namics, viz. hydrodynamics of ship naotion in waves. The utility 

 of presently existing techniques of analysis for solution of practical 

 problems in ocean engineering, with emphasis on mooring and posi- 

 tioning of vessels and other systems in a seaway, has been shown 

 within the limits of the present state of development of this field. 

 Greater emphasis toward consideration of certain nonlinear hydro- 

 dynamic forces for application to^these problems has been indicated, 

 especially in view of their predominant effect in certain modes of 

 motion. Possible directions for future research and development 

 activities in this field will involve consideration of better techniques 

 of representing the form of these forces in terms of body geometric 

 parameters, more concentration of basic model measurements for 

 comparison with theory, and techniques of simulation? in dynaxnic 

 analyses of motion behavior. This information will be fundamental 

 in establishing computer models for determining many aspects of 

 system performance at sea prior to actual construction, thereby 

 providing insight as to expected problem areas and methods of solu- 

 tion. The methods of applied hydrodynamics for these puTrposes are 

 generally available now, and it remains for the ocean engineering 

 profession to determine the utility or applicability of these tools 

 to the particular practical problems that they face in their own 

 operations. 



1079 



