Experimental study of collar protection for local scour reduction around offshore wind turbine monopile foundations

Scour protection is commonly designed to avoid local scouring of soil around monopile foundations in offshore windfarm engineering. Collar protection is widely used in bridge engineering as an effective flow-altering scour protection method. The present experimental study is focused on the efficiency of collars as a local scour countermeasure at windfarm monopile foundations. Collars with different sizes and elevations exposed to various flow intensities were tested. Three-dimensional digital elevation maps were generated for scour topography analysis using photogrammetry technology. According to the collar installation height, the scour patterns are categorised as collar placed above bed level, collar placed below threshold elevation, and collar placed between bed level and threshold elevation. The threshold elevation is the elevation where no scour could occur below the collar. The local scour and scour topography are illustrated in the present study. A more comprehensive equation for prediction of the equilibrium scour depth with collar protection, based on new data and data from previous studies, is presented. The new equation has good accuracy for equilibrium scour depth estimation. Relationships are presented for prediction of the plan dimensions of the scour hole in terms of the predicted scour hole depth. Also, an equation for collar protection efficiency evaluation is proposed. The proposed equations are only applicable under clear water conditions without general scour.