Hydrodynamic interaction analyses of very large floating structures

Abstract

A series of studies conducted at MIT over the past seven years for the analysis of hydrodynamic interactions among legs for the support of a very large floating structure is reviewed. For general three-dimensional leg geometries and arbitrary configurations, an algebraic method for exact hydrodynamic interaction, which includes the effects of evanescent waves, has been developed. This method produces identical results compared with direct computations for hydrodynamic coefficients, first-order wave exciting forces and second-order steady-drift forces. Motions with and without the presence of linear elastic constraints, such as moorings or deck structures, are also obtained. When the number of legs involved is extremely large, a matching idea is introduced which divides the total structure into an interior ‘core’ plus a relatively small number of legs near the outer boundary. The interior core is treated as part of an infinite array and the ‘end’ conditions are accounted for by considering the exact interactions between the interior core and the boundary legs. Finally, a method for the solution of the ‘inverse’ hydrodynamic interaction problem is presented. In this, optimal arrangements of legs for minimum wave forces, displacements, etc., are obtained, subject to constraints specified, say, for the total length of the array. Through comparison with results of other numerical methods and available experimental data, the validity and effectiveness of these methods are demonstrated.