The force exerted on a cylindrical pile by ocean internal waves derived from nautical X-band radar observations and in-situ buoyancy frequency data

Abstract

Although large amplitude ocean internal waves (IWs) may induce strong underwater currents and cause a severe threat to offshore drilling platforms, little research attention has been paid to the issue. This paper proposes a method for estimating the force and torque exerted by ocean IWs on a submerged vertical cylindrical pile using nautical X-band radar observations (collected by standard ship equipment) and in-situ buoyancy frequency data. Radar image signatures associated with IWs will be used to infer characteristics of the interior ocean dynamics. The nonlinear velocities of IWs are retrieved based on the use of the Radon Transform technique applied to the radar images. Given the vertical distributions of buoyancy frequency, the amplitudes of IWs are determined based on the Korteweg-de Vries (K-dV) equation for a continuous stratified finite-depth system. With the above parameters, the horizontal velocities induced by IWs and their acceleration can be calculated. Finally, the forces and torques exerted by IWs acting on a pile are estimated using Morison's empirical formula. As an example, the method is applied to data acquired on June 2009 in the northeastern South China Sea.