Random Wave Forces Near Free Surface

Abstract

The present paper describes random wave forces acting on a slender structural member in the vicinity of the water surface, taking account of the intermittency of submergence. A brief review is initially given of available formulations for fully submerged portions of a member, these being based on the Morison equation applied in conjunction with linear wave theory. The Morison equation is generally linearized in order to estimate spectral densities, while its nonlinearity is retained in order to estimate probability densities and consequently extreme force values. For sections located near the free surface, the intermittency of submergence drastically alters the statistics of the water particle kinematics in the random wave field and consequently those of the wave forces. Expressions for the spectral density of the force and the probability density of the force maxima are derived. Numerical results relating to these and to the vertical distribution of extreme forces are presented, and the effects of including the intermittent submergence near the free surface are illustrated.