MOORING DYNAMICS SEMINAR 

 By 

 Dr. S. Calisal 



INTRODUCTION 



A computer program to obtain a computer solution to sizing and 

 locating mooring tackle based on moored ship dynamics at sea is under 

 development at the U.S. Naval Academy. The important features of this 

 program are that it should require a minimum of input data and that the 

 output should be easily understood by users who are not necessarily 

 specialists in the field of ship or cable dynamics. 



A survey of the expected input variables was first done. To our 

 surprise, most of the variables required for input for a classical ship 

 dynamics problem can at best be based on "guessed" quantities. This 

 fact forced us to study possible sets of assumptions not normally required 

 for the solution of a ship dynamics program. During the early stage of 

 the model development, the major problem we faced was selecting the 

 assumptions that could be accepted. For a successful model, the assump- 

 tions had to be: 



a. realistic 



b. consistent 



A flow diagram of the calculation can be represented as in Figure 1. 

 The assumptions in each block had to have the same implication and same 

 order of importance. The following is an attempt to point out some of 

 the assumptions observed in the literature and used to build the total 

 model. 



SHIP GEOMETRY AND WEIGHT DISTRIBUTION 



The first question encountered was: "how accurately should the 

 ship impermeable boundary be represented?" In Naval architecture, a 

 ship is defined with a table of offsets and continuous curves; surfaces 

 are assumed to exist between the defined points. Ship surface definition 

 is still a continuous field of research and development, and Figure 2 

 shows one such effort from the Abkowitz paper (1966). The purpose is to 

 have a mathematical function to define the stations. Van Oortmerssen 

 (1976), on the other hand, used plane surface elements, while Raichlen 

 (1965) and Bomze (1974) used "equivalent displacement" rectangular 

 blocks for frequency and time domain solutions. 



For wind-induced resistance, the geometry of the ship above water 

 is certainly important. The general trend is to have a representation 

 based on projected area and the centroid of this area only. 



For simplicity of input requirements, and noting that the coefficients 

 in the dynamic equations require integration and moments of distributed 

 quantities and also the good correspondence reported between experimental 

 and theoretical values, an equivalent box representation is adopted. 

 This is not a limitation, however, as the program has a modular design 

 and this subroutine can be replaced by more sophisticated ones. 



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