Semi-theoretical prediction for equilibrium scour depth of a mono-pile supported tidal stream turbine

Local scour around the mono-pile foundation of a tidal stream turbine is an essential issue regarding its structural safety and stability. A semi-theoretical framework for equilibrium scour depth prediction of the mono-pile foundation is proposed with consideration of effects from the rotating rotor. The equivalent mean velocity is obtained by fitting to the accelerated flow beneath the turbine rotor and used to derive the scour depth prediction equation based on the phenomenological theory of turbulence. Both clear-water and live-bed conditions are considered. Results show that the acceleration effect from the turbine rotor turns more pronounced with increasing thrust coefficient and tip clearance. Such acceleration generally leads to an increased scour depth around the mono-pile foundation, reaching the maximum scour depth when the equivalent mean velocity is equal to the critical velocity of sediment entrainment. In addition, a reduction factor of 0.74 is obtained for the bi-directional flow condition. Overall, the proposed prediction framework agrees with the experimental data in the literature with acceptable accuracy, although it could be improved with more data available in the future.