An approximate method to compute the drift force on an array of vertical cylinders

Abstract

An approximate method to compute the drift forces on semisubmersible platforms is presented. The platform is supposed to have vertical, circular columns. It is assumed that the drift forces is governed by the scattering of waves from vertical columns only. The interaction between the columns is treated in a simplified manner; only the effects of phase shifts in the wave are included. Under these assumptions the drift forces may be estimated without use of, i.e. general programs based on diffraction theory. If the case of no platform motions is considered, the method is self-contained. If the effects of platform motions are to be included when estimating the drift forces, the motions can be found by any standard computer program. A diffraction theory approach is not needed. The method is thus suited for estimating the drift forces on floating platforms at an early design stage. Before more advanced numerical tools are applied. The drift forces as computed by this simplified approach are compared with the drift forces obtained by use of a panel method approach. For the two platforms used in the comparison the agreement between the two methods is quite satisfactory.