Nonlinear buoy motion under wave and current action



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In the last few years many different types of buoys have found wide use in the gathering of oceanographic environmental data. Many of the buoys used have been put on location with little or no past, results or data available with which to predict the buoy's dynamic behavior under varying sea conditions. The purpose of the research reported here was to develop a digital computer program which would simulate submerged buoy motion with two degrees of freedom. The equations of motion for submerged buoys are non-linear in nature and do not lend to exact analytical solution. Approximate methods must be used for the solution, but it is desirable for the results to approach the actual buoy behavior with as much accuracy as desired. The use of the digital computer provides the possibility for this accuracy. The SDS Sigma 7 computer was utilized to provide plots of buoy displacement, velocity and other appropriate results as functions of time. An incremental method of solution to the wave equation is satisfactory if the increments, in this case time, are made small enough. As a basis for choosing the size of the time increment, computer memory size and execution time must be considered. When a solution is found for a time increment of a given magnitude, and solutions for smaller time increments give essentially the same results, it can be safely concluded that the accuracy limit has been reached for the method used. If confidence in the solution method is accepted, then a limit can be reached on the accuracy of the solution. A computer program has been written in Fortran IV to simulate submerged buoy motion for buoys restrained by a single mooring line. An incremental method for solution was used. The program as developed will handle ten-component Fourier wave profiles, and allows for variations in twelve other physical parameters. Investigations were made on the effects of varying buoy weight, the drag coefficient, buoy submergence, and the magnitude of the wave profile.