Second Order Wave Diffraction Around a Fixed Ship-Shaped Body in Unidirectional Steep Waves

Abstract

The objective of this research, part of the EU FP5 REBASDO Program, is to examine the effects of second order wave diffraction in wave run-up around the bow of a vessel (FPSO) in random seas. In this work, the nonlinear wave scattering problem is solved by employing a quadratic boundary element method. A computer program, DIFFRACT, has been developed and recently extended to deal with unidirectional and directional bichromatic input wave systems, calculating second order wave diffraction loads and free surface elevation under regular waves and focused wave groups. The second order wave interaction with a vessel in a unidirectional focused wave group is presented in this paper. Comparison of numerical results and experimental measurements conducted at Imperial College shows excellent agreement. The second order free surface components at the bow of the ship are very significant, and cannot be neglected if one requires accurate prediction of the wave-structure interaction; otherwise a major underestimation of the wave impact on the structure could occur.

Phase manipulation and the harmonic components of ringing forces on a surface-piercing column

“…NewWave-type focused wave groups correspond to the average shape of the free surface in the vicinity of large crests occurring in a random sea state and are used throughout the analysis to follow. As shown by [5,6], the NewWave model captures quite accurately the linear contribution to large ocean waves. In this paper, we seek to understand the effects of nonlinearity in the interaction between focused waves and a surface-piercing cylinder on the wave-induced loads.…”

An experimental decomposition of nonlinear forces on a surface-piercing column: Stokes-type expansions of the force harmonics

“…In order to fit the coefficients of the perturbation series (3.6), we need both the undisturbed incident wave field η and the total hydrodynamic load F. Given that each harmonic for a wave group is spread over a range of frequencies, we make use of the 'phase-inversion' method of Baldock et al (1996), Hunt et al (2004), Borthwick et al (2006) and Zang et al (2006). The crest and trough focused combinations (C − T)/2 for the odd harmonics and (C + T)/2 for the even harmonics now have the nonlinear harmonics better separated in frequency.…”

“…The Morison term hereafter implies the inertia contribution from the Morison equation, unless otherwise stated. A frequency domain potential-flow solver DIFFRACT has also been applied to obtain first-and second-order harmonic wave loading on the cylinder that is located on the flat seabed (Eatock Taylor & Chau 1992;Zang et al 2006).…”

“…It has been widely used in the field of offshore engineering for studying extreme events from a given random sea state of known spectral content. The investigations presented by Jonathan & Taylor (1997), and Zang et al (2006) indicate that the linear and bound harmonic contributions to large ocean waves and their local interaction with a body can be captured quite accurately by using the focused wave group technique. Bredmose & Jacobsen (2011) investigated breaking wave loads on a monopile foundation numerically by application of the focused wave group technique consistent with a Joint North Sea Wave Project (JONSWAP) sea state.…”

“…The use of focused wave groups allows extraction of the higher harmonics in wave-structure interactions by using the 'phase-inversion' method described by Baldock, Swan & Taylor (1996), Hunt et al (2004), Borthwick et al (2006) and Zang et al (2006Zang et al ( , 2009, and more recently generalized by Fitzgerald et al (2014). The 'phase-inversion' method assumes that there is a generalized Stokes-like perturbation expansion in focused wave groups and responses.…”