In this paper, analytic diffraction and radiation solutions for arrays of bottom· mounted vertical circular cylinders are used to compute hydrodynamic forces on deep-draft-column-based offshore platforms, such as tension-leg platforms (TLP) and very large floating structures (VLFS). In order to account for the effects of the cylinder bottom and pontoon, rational bottom and pontoon corrections are also developed. The numerical results are validated through the comparison with those obtained from three-dimensional boundary element programs. The present method is especially powerful when the number of legs is large, for which the use of existing panel programs seems infeasible. The present method quickly produces reasonably accurate solutions with minimal memory requirement and is free of laborious grid generation and convergence test, hence is particularly suitable for the parametric study of large-volume multi-column structures in the preliminary design stage.

1. Introduction

Recently developed offshore structures often consist of a number of large-diameter vertical columns which are in many cases connected by pontoons at large depths., Typical examples are tension.leg platforms (TLPs) and deep draft floaters (DDFs) (Herfjord & Nielsen, 1988). This kind of multi-column system can also be used as an offshore airport or plant which can have up to an order of several hundred columns. As the size and number of columns increase, the interaction between waves and arrays of columns becomes increasingly important. The interaction of waves with multi-column structures can in principle be computed by three-dimensional boundary element or finite element programs. However, as the number of columns increases, the use of the three-dimensional codes becomes practically unrealistic. Recently, explicit diffraction solutions for arrays of bottom-mounted vertical circular cylinders, which was first investigated by Spring & Monkmeyer (1974), were obtained by Linton & Evans (1990), and the method was extended to the complementary radiation problem by Kim (1992).

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