This paper is dedicated to the study of non-linear wave forces and free surface motions in the vicinity of a multicolumn structure. This topic, which is of the utmost interest for designers of offshore platforms, is addressed using an original time domain simulation method for non-linear potential flows with a free surface. The model is based on a formulation of the fully non-linear diffraction problem in which the incident wave is given explicitly, with sensible advantages in terms of efficiency and accuracy over more conventional formulations. Original results using this model have already been published, with emphasis on higher order forces in long waves ([10], [11]), or non-linear wave-current-body interactions ([1][9]). In these calculations, the incident wave was given by the stream function model of Rienecker g∼ Fenton [16], and waves were diffracted by a vertical circular cylinder. This latter restriction allowed an easy implementation of bi-cubic splines interpolations, for accurate estimations of slopes and fluid velocity at free surface markers. In this paper, applications of this fully nonlinear diffraction model are extended to multi-column structures. This extension is based on the implementation of more flexible schemes for the computation of slopes and fluid velocities around free surface piercing structures of complex shape.
The accurate calculation of forces and free surface motions in the vicinity of offshore structures is a topic of permanent interest for offshore engineers. Especially, free-surface motions, including wave runup on cylindrical columns, as well as resonant interaction effects between columns have a direct incidence on the airgap (or deck clearance). The airgap estimation is a major design parameter, with considerable consequences on the building and operational costs of structures. Beyond linear diffraction theory, the usual practice for estimating loads and free surface motions is to rely on frequency domain second order theory.