Response of a monopile substructure due to breaking wave impacts

Offshore Wind Turbines (OWT) are very attractive structures 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 steep waves with a bottom fixed vertical slender cylinder by performing lab experiments. In Intermediate 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 experimentally wave impact forces and accelerations on a fixed OWT substructure subjected to steep 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 steep waves is accomplished by constructive interference (Chan & Melville, 1989) 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 and it is fixed firmly to the flume bottom by using the rigid frame. The top side of the model is kept free. In the present experiments, the measurements are recorded at the fixed cylinder location (7.8m from the wave maker) for different intensities of steep waves. Different intensities of steep waves are achieved by adjusting the gain (G) factor. Accelerations of the structure were measured by using accelerometer for different intensities of steep waves at focused location 7.8m from the wavemaker. Accelerations on a bottom fixed vertical slender cylinder which resembles a monopile substructure for OWT are measured at higher resolution.