Dynamic characteristics of an offshore wind turbine with breaking wave and wind load

Abstract

In this paper, the response characteristics of an offshore wind turbine (OWT) structure under breaking wave forces and wind forces are studied. A 3D numerical model, based on solving the viscous and incompressible Navier-Stokes equations and the volume of flmethod, is employed to estimate the breaking wave forces on an OWT structure (6.0-m diameter monopile). The calculated wave forces are then applied with the wind forces on the OWT structure modeled in the computer program HAWC2 to understand the nature of its response. The effects from the aerodynamic damping and the foundation fl exibility on the structure's response are also discussed. In recent years, a large number of fioffshore wind turbines (OWT), which are supported by monopiles, tripod structures, and jacket structures, have been planned or constructed in shallow waters to capture the abundant wind energy source. Among them, a 'monopile-transi- tion piece-tower' type of structure seems to be preferred to support an OWT in shallow waters. For the design of a monopile structure installed on a fl at bottom, the Morison equation has generally been used to estimate wave forces on the structure. Moreover, as the frequencies of the environmental loads on a structure generally stay away from the structure's natural fre- quency, the vibration of the structure is not a 'major' problem in the designing stage of the structure. However, in the case where an OWT structure is installed in a submerged shoal, the water waves may experience severely nonlinear wave deformations on their propagation, and the nonlinear waves generated toward the structure can give rise to higher local pressures and impulsive forces on the structure. Because the breaking wave impact forces normally act in a very short time, this can also cause large horizontal accelerations at the nacelle. Furthermore, the repeated occurrence of breaking waves in every season can potentially affect the fatigue life of the structure. Therefore, the accurate estimation of breaking wave impact forces and dynamic responses induced by the forces is of great importance. Several researchers have investigated breaking wave impact forces and the dynamic responses induced by the forces throughout the numerical and experimental approaches. Hu and Kashiwagi (1) applied the Constrained Interpolation Profi le (CIP) method for studying the wave impact phenomena and violent wave-structure interactions. Christiansen et al. (2) studied the wave run-up and the extreme wave forces on an OWT foundation under the plunging breaker by using Navier-Stokes solver. Marino et al. (3) presented a numerical procedure by using Boundary Element Method (BEM)-MEL (Mixed Eulerian-Lagrangian) to simulate the extreme response of an OWT structure. Bredmose et al. (4) used a focused wave technique in Open-Foam to estimate the breaking wave impact forces on a cylindrical pile. Mokrani et al. (5) used a NS-VOF approach to study the slamming forces on a vertical wall. The wave impact on a rectangular column using smoothed particle hydrodynamics (SPH) was studied by Cummins et al. (6). However, although various researchers have stud- ied the breaking wave impact forces on the structure, little information is currently available