Local scour and flow-field evolution during monopile installation

Local scour during monopile installation is a critical issue in offshore wind engineering, yet its hydrodynamic mechanism remains insufficiently understood. This study combined flume experiments and OpenFOAM-based large eddy simulation to investigate scour around a large-diameter hollow monopile during installation, focusing on the pile-lowering stage as the pile approached the seabed. Experimental scour topography was imported into the numerical model to analyse mean velocity, vortex structure, turbulence intensity, and bed shear stress. Results show that scour beneath the pile is highly sensitive to pile-bed clearance. Almost no obvious scour occurred at a pile-bed clearance of 0.5 pile diameters (D), whereas distinct scour developed at 0.33D, indicating that 0.5D–0.33D corresponds to the initial stage of scour development. During the early stage, an upstream-scour and downstream-deposition pattern formed beneath the pile. As clearance decreased from 0.5D to 0, the depositional zone migrated downstream, the maximum scour position shifted laterally by about 0.1D, and the maximum scour depth reached about 0.2D. Compared with the fixed-pile case, pile lowering generated a gap jet, weakened the horseshoe vortex, altered the wake structure and vortex shedding, and enhanced internal circulation. Scour development was mainly governed by streamwise bed shear stress.