Evaluation Of Drift Force Computation Methods.

Abstract

ABSTRACT Moored-ship and/or platform system designers are often faced with the problem of deciding between different drift force computation methods. This paper is intended to assist in this decision making by providing a comparative evaluation of the two major categories of computation methods. The methods for calculating drift forces (second order) on vessels fall into two (2) major categories. In one method, the problem is reduced to the computation of the potential far away from the vessel. This is known as the far-field approximation as described and developed by Maruo (1960), Newman (1967) and Faltinsen and Michelsen (1974). In the other method, known as the near-field or close-in approximation, the integration is performed directly on the vessel hull as developed by Salvesen (1974) and Pinkster and Van Oortmerssen (1977). Following the approach of the first mentioned category, a computer program has been prepared which calculates drift forces and moments on vessels of arbitrary hull geometry. No assumptions regarding the hull geometry are included in the underlying theory. The second category, however, includes some mild slenderness assumptions (Salvesen (197). Based on the drift force program outlined in this paper, some selected results for various hull geometries are presented. These results are compared, where applicable, with existing results obtained from application of the near-field method. These comparisons permit:determination of the range of compatibility of the two methods, andthe influence of hull geometry on the applicability of the near-field approximation. Moored-ship system designers thus have a systematic procedure from which drift force calculations can be evaluated. INTRODUCTION Among the methods that are used for the computation of the second order (drift) forces on obstructions (including vessels such as floating drilling platforms and OTEC configurations) two different categories can be distinguished. The first, often referred to as the "near-field" approach, consists in integrating the pressure field over the immersed surface of the structure. The second, known as the "far-field" computation, replaces integration on the body surface by integration on a control surface surrounding the body and located far away from it. Both methods are intended to yield results suitable for engineering and operational purposes. The "far-field" approach is formulated in a less tedious and more elegant fashion; the "near-field" approach, on the other hand, lends itself more readily to computations of second order frequency component interactions (bichromatic problem). This, of course, is directly related to the description of the ocean environment where several frequencies are present. In order to evaluate statistics of second order non-linear responses of ocean structures, it is necessary to solve problems of the bichromatic type. This is currently accomplished in the field of naval architecture by employing near-field methods (usually based on strip theory) in combination with the socalled "weak scattering" assumption. The objective of this paper is to evaluate the validity of the "weak scattering" assumption.