Local scour around a porous surface-piercing square monopile in steady current

Local scour around a porous monopile with the square cross-section in steady current is investigated experimentally. A variety of porosity (P = 0-38.8%) is explored alongside with different attack angles ranging from alpha = 0 degrees-60 degrees in the live-bed flow condition. Time history of the scour development, equilibrium scour depth, time scale and the scour hole profile have been systematically studied. The increase of the streamwise porosity is found to largely weaken the horseshoe vortex in the upstream monopile and leads to a decrease of scour depth. However, the transverse porosity has limited contribution to the local scour around the monopile. A secondary-scour is revealed for the porous monopile with attack angles alpha = 30 degrees and 45 degrees, which is due to the sudden increase of the velocity amplifications at the corner of the monopile. The effectiveness of stone layer protection as countermeasure against local scour around a porous monopile has also been studied. The characteristics of the scour hole have been quantified. It is showed that the stone layers can effectively eliminate the scour hole. In particular, the larger the diameter of the protection layers or the height-to-diameter ratio, the greater the effectiveness of the countermeasure.