The effect of breaking wave induced currents on an offshore wind turbine foundation

Abstract

Several studies (Chun et al., 1999; Yoon et al., 2004) have shown that the occurrence of breaking waves over a submerged shoal can induce a jet-like strong current on the lee side of the submerged shoal, and the forces on structures following breaking waves can be perceived as wave forces being superimposed on strong current forces. In the present paper, the strong current was assumed to be induced by a breaking wave, a so-called breaking wave induced current. A MIKE 21 BW model (MIKE 21 BW, 2009) was established to quantify wave heights and current velocities at structural positions over a submerged shoal, and the results were compared with the results of a hydraulic model test. The wave plus current forces, the wave forces without the current, and the wave forces based on the deepwater design wave height were calculated and compared, using the wave heights and current velocities at selected structural positions. The results of numerical analysis did show that in spite of the fact that the wave heights on the lee side of the submerged shoal appeared to be small (0.084~ 0.086 m) compared with the deepwater design wave heights (0.205 m), a strong current (0.277 ~ 0.448 m/s) surely occurred on the lee side of the submerged shoal due to radiation stress differentials given by the breaking of the incident waves. The comparison of the total forces on the structure without the current and with the current showed that the wave plus current forces in this area increased by an average of 280 % to 300 % compared with the wave forces without current. Moreover, the wave plus current forces rather exceeded the wave forces which were calculated for the deepwater design wave height (0.205 m).