Concurrent wind, wave and current loads on a monopile-supported offshore wind turbine

Abstract

Offshore wind energy harnessing is a major source of renewable energy. From the design perspective, it is crucial to properly determine environmental load effects on offshore wind turbines concurrently subjected to wind, wave and currents as reflected in a realistic offshore environment. Accordingly, the focus of the present study is on addressing this issue in the form of small-scale laboratory experiments in the Wind-Wave-Current Tank (WWCT) at Newcastle University, UK, using a scaled model of an offshore wind turbine. The hydrodynamic surge force exerted by currents represents a major contribution to the overall average loads. The waves are the main source of the unsteady loads. Higher amplitude waves cause an increase in the relative standard deviation of the integral loads, an effect which is more pronounced at greater wind and current velocities. While increasing the wind raises the dynamic loads at the natural frequency of the wind turbine, i.e., inertial loads, whereas currents introduce loads, which occur particularly at low frequencies due to nonlinear hydrodynamics, both observations were made in the absence of waves. If the waves are also present, the dynamic loads on the offshore wind turbines are primarily at the wave frequency, i.e. wave-induced dynamic loads on the offshore wind turbines are predominant. Higher modes of the surge and sway force fluctuations are more pronounced when all three load components act concurrently than is the case when the current or the wind acts in isolation. These trends may be further amplified in the dynamic response of floating wind turbines as noted in previous computational studies involving tension leg platforms.