A Review of Hydrodynamic Effects on Bottom-Fixed Offshore Wind Turbines

Recent and historical literature regarding hydrodynamics has been reviewed, with offshore wind turbine support structures in mind. Under conditions of separated flow, several relevant phenomena have been noted which are not covered by the commonly-used Morison equation: 1. damping of structural vibration or slow-drift motion; 2. the interaction of structural vibration and vortex shedding; 3. loads near the free-surface; and, 4. burst motions, caused by impulse-like loading from steep waves. References have been given to books and articles that describe the phenomena in more detail. A form of the Morison equation is proposed which has separate empirical coefficients for each of the velocity and acceleration terms. The coefficients can be determined from existing test data with use of least-squares error minimization. A simplified form of the equation provides a means to obtain conservative bias on both the applied load (bias towards a high drag coefficient) and damping (bias towards a low drag coefficient). Further investigation into free-surface and burst motion (ringing) phenomena is recommended, considering a slender wind turbine monotower in 20 to 50 m water depth.