An investigation of wave impact pressures on an offshore wind turbine substructure subjected to breaking focused waves

Abstract

Offshore Wind Turbines (OWT) are very attractive candidates for renewable energy production for several obvious reasons such as stronger wind speeds and cleaner energy production. Among the two variants of the OWT's, namely, fixed and floating configurations, the former can be considered as an optimal solution considering the trade-off between factors such as economics, proximity to land and efficiency. The motivation is to get a deeper understanding of the interaction of breaking focused waves with a bottom fixed vertical cylinder by performing lab experiments. In shallow waters, one of the primary factors governs the design of a fixed OWT substructure is hydrodynamic loads acting on it due to breaking waves. The main objective of the study is to investigate wave impact pressures on a monopile substructure subjected to breaking focused waves. Laboratory experiments are conducted in a 30m long, 2m wide and 1.8m deep wave flume, at Department of Ocean Engineering, Indian Institute of Technology Madras, India. The generation of breaking focused waves is accomplished by linear superposition of wave components. Following the simulation procedure, the desired signal to the wave maker was computed by combining 28 sinusoidal wave components within the frequency range from 0.42Hz to 1.10Hz. The cylinder model is made of acrylic material of 0.2m diameter, 0.005m thickness and length of 1.3m. The cylinder is placed at the center of 2m wide flume. The wave induced dynamic pressures were measured at eight different positions along the depth of the vertical cylinder for different intensities of plunging breaking waves by using pressure transducers. The pressure transducers are connected along the front line of the cylinder facing the wave. There are six pressure transducers positioned above the Still Water Level (SWL). Then, the seventh transducer and the eighth transducers are positioned at and below SWL. The top six pressure transducers are uniformly spaced at an interval of 0.04m and the last two transducers are spaced at an interval of 0.06m. The pressure measurements are recorded in the front of the cylinder in order to investigate the wave induced impact pressure on the monopile due to breaking focused wave. The kinematics, breaking characteristics, focused wave surface elevation, pressure-time profile, pressure rise time, vertical pressure profile, are measured for four different incident wave characteristics and environmental conditions.