Hydrodynamic Loads on Offshore Wind Turbine Substructures due to Nonlinear Irregular Breaking, High Steep and Extreme Waves

Substructures for fixed offshore wind turbines (OWT) account for a significant portion of the total investment for this technology. Detailed knowledge of the waves and knowledge in how to predict waves interacting with different structures is essential to design the most cost-effective construction. This particularly applies to high, extreme breaking waves. The dynamic behaviour of wind turbine substructures is completely different from the oil and gas structures; therefore, design criteria need to be further evaluated for this type of structures. The research focusses on the numerical and experimental investigation of the hydrodynamic loads on offshore wind substructures. Maximum wave loads are expected from breaking waves. At the Department of Civil and Transport Engineering at the Norwegian University of Science and Technology (NTNU), the regular breaking wave impact on vertical cylindrical structures (monopiles) has been investigated by numerical modelling using the CFD code REEF3D (https://reef3d.wordpress.com/). Here, the breaking wave kinematics and dynamic pressures have been correlated, giving new insight in the breaking wave physics. The same topic has been investigated through laboratory tests performed at the Department of Ocean Engineering at Indian Institute of Technology Madras (IITM), India. Active cooperation between the two research communities at NTNU and IITM has been completed and the collaboration has led to new knowledge. For real sea states, waves cannot be expected to follow regular wave theory. Here, waves are irregular and they consist of many individual harmonic components of different amplitude and frequency. Through numerical simulations, the hydrodynamics of irregular waves and their impact on vertical and horizontal circular cylinders has been investigated. Since the different components of the irregular wave train travels at different speeds, extreme waves can occur for certain wave superposition. These extreme waves are characterized as focused waves, as several wave crests focus at a certain location at the same time. Through extensive numerical simulations, the hydrodynamics of such focused waves has been investigated. In addition, their interaction with vertical circular cylinders has been studied. In experiments in wave tanks or wave basins, the waves are generated either by flap or piston motion of a plate. These generation mechanisms are implemented in the numerical model. This means that the simulation becomes exact simulations of the physical model experiments. Not only the wave spectrum is reproduced, but also the exact time development of the waves. The simulations have been performed for various types of hydrodynamic loads, including breaking and non-breaking waves on monopile and jacket structures. The results produced as part of the project are published as open-source conference and journal articles. Offshore wind energy sector is expected to benefit from this project due to the detailed studies on nonlinear wave loads on such structures.

The project was carried out as planned. Researchers at IIT Madras carried out experiments in the wave flume to estimate the breaking wave loads for regular, irregular and focused waves. They provided valuable inputs for the development of the numerical model REEF3D at NTNU. The numerical investigation was conducted at NTNU by the PhD and Post-doctoral candidates, in order to validate REEF3D with experimental data and further investigate breaking wave characteristics and associated wave loadings on monopole and jacket structures. The team at SINTEF Ocean conducted validation studies with REEF3D using in-house experimental data. The results produced are published in open-access conference and journal articles, which could be used by the offshore wind energy for the substructure design. This project has helped in increasing the cooperation among IIT Madras, SINTEF Ocean and NTNU and have presented an opportunity to meet other potential project partners in the future.

Offshore wind turbine (OWT) substructures account for an important share of the total investment of this technology. Hence, the hydrodynamic knowledge concerning the wave kinematics and their interaction with different structures are of a great importance to design the most cost effective support structure. Understanding and predicting the shallow and intermediate water kinematics and in particular their interaction with OWT substructures are important for the global design of OWT. The dynamic characteristics of the wind turbine substructures are completely different from oil and gas structures. Hence, the design methods and guidelines need to be investigated in detail for offshore wind turbine substructures. Hence, the proposed research focuses on an accurate assessment of hydrodynamic loads on OWT substructures due to nonlinear irregular breaking waves, high steep and extreme waves. The laboratory experiments will be performed at the Department of Ocean Engineering at the Indian Institute of Technology Madras (IITM), India for selected OWT substructures in order to evaluate the wave-structure interaction characteristics due to breaking, steep and extreme waves. This will focus on the measurement of the dynamic pressures, wave-run up, wave height distribution and wave forces on the selected OWT substructures in intermediate and shallow water conditions. Numerical investigation will be carried out at the department of Civil and Transport Engineering at the Norwegian University of Science and Technology on the wave characteristics and associated forces on OWT structures due to irregular breaking, high steep and extreme waves. The outcome of the proposed research will give a great insight into the estimation of hydrodynamic loads and enhancing the understanding of the irregular breaking and extreme wave forces on OWT substructures and also useful in various stages of offshore wind farm development such as site selection, installation, commissioning and maintenance.