Gridded cemented riprap for scour protection around monopile in the marine environment

Abstract

A novel gridded cemented riprap (GCRP) composed of loose riprap stones and cemented stone clusters is proposed, which combines high stability of large stones and excellent flexibility to follow ground deformation. A series of physical experiments were performed in a large-scale flume, and two extreme wave-current conditions of an offshore wind farm with a return period of 50 and 100 years, respectively, were selected. The anti-scouring performance of riprap and GCRP is studied in detail. It is found that the magnitude of wave leads to downstream movement of the maximum scour point. The armor stones around monopile cannot maintain their stability under the extreme wave-current condition, and riprap layer gradually disintegrates during scour process. The stability of riprap layer is significantly improved by fully or gridded grouting. Fully cemented riprap (FCRP) and GCRP remain intact even under the combined wave-current condition of return period of 100 years. For FCRP, scour occurs at the downstream edge and the erosion of underlying sand bed takes place. The anti-scouring performance of GCRP is better than riprap and FCRP, the maximum depth of scour hole around GCRP is reduced 80% than unprotected monopile.