A general method for calculating hydrodynamic forces

Abstract

This paper presents the derivation of a general method for calculating wave forces on the cylindrical members of offshore structures. By means of the proposed method one can calculate the wave loading on cylindrical members of fixed or floating offshore structures orientated randomly in waves. This method of calculating wave forces is based on the linear Airy wave theory. Calculation procedure of wave force components is presented in great detail on the basis of wave particle kinematic properties obtained from the linear Airy wave theory. In the procedure of calculating wave forces presented, definitions of the wave reference system for propagating wave, the structure reference system for the platform and the member reference system for the tubular members of the structure are first established, and then the calculation of wave forces is given in terms of its components, which are pressure, acceleration and velocity forces, including current forces. At the end of the paper, expressions of total heave, sway and surge forces and total roll, pitch and yaw moments acting on the platform are given as a sum of these forces acting on each member of the platform. The calculation procedure derived in this paper provides a very efficient means of calculating wave forces and moments during the time-domain simulations of a floating platform experiencing large amplitude motion in intact, progressive flooding and damaged conditions. Comparisons of the predictions with the measurements which will be presented elsewhere reveal that the calculation procedure developed can predict large amplitude oscillatory and steady motion characteristics of an intact and damaged platform in waves with an acceptable degree of accuracy.