Wave Loads on a Monopile in 3D Waves

During the last decades more and more wind farms have been erected offshore. Most of these wind farms are located at relatively shallow water. The majority of the offshore wind turbines are founded on monopiles. Many of these offshore wind farms are exposed to large and steep waves, in some cases even breaking waves. The loads on the piles caused by waves can be significant and a better knowledge of the forcing caused by real sea states, including irregular waves and directional spreading is required to optimize the design. The present physical model tests have been conducted in order to determine the effects of wave directionality and breaking of irregular waves. Piles with and without secondary structures have been tested. The waves were shoaled over a sloping bed and the pile was placed at two different positions with varying bed slopes. The three forcing components (Fx, Fy, Fz) were measured at the bottom of the pile during the experiments. Breaking waves occurred around the pile in most of the tests and significant slamming forces were observed in the cases where breaking waves hit the pile which is well known from the literature. The experimental results indicated that the slamming force may be reduced when the wave spreading is increased, similar to the case of non-breaking waves.