Scour characteristics of an offshore umbrella suction anchor foundation under the combined actions of waves and currents

Abstract

An umbrella suction anchor foundation (USAF) is a novel foundation for offshore wind turbines. Compared to the local scour occurring around deep foundations such as monopiles, the scour around an USAF can significantly influence the stability of wind turbine systems. Therefore, research on the local scour around an USAF induced by waves and currents is of great value to coastal engineering projects. In this study, a series of scour tests of an USAF were carried out and the authors evaluated the influences of a steady current, waves, and the combination of waves and currents. The flow field and wave field were captured by an acoustic Doppler velocimeter (ADV) current meter and wave gauge, respectively. Moreover, the equilibrium depth, scope, and shape of the scour holes are determined using an ultrasonic topographic surveying meter. The results demonstrate that the scour profiles are definitely distinctive for the three environmental conditions. A standard k-ω turbulent model is used to simulate the turbulent flow field. The volume of fluid (VOF) method is used in computational fluid dynamic (CFD) software to deal with the free water surface and moving mesh to explore the changes in the topography over time. A dynamic three-dimensional numerical model of the local scour around the USAF is developed and validated by comparing the results with previous scour tests and other published experimental data. The scour characteristics of the USAF and the suction anchor foundation (SAF) are determined by solving the fully three-dimensional transient Navier-Stokes equations in a fixed Eulerian grid. The results indicate that compared with a conventional SAF, the USAF can significantly prevent scour; moreover, the scour depth is significantly different for the two foundations under the combined wave-current action and is approximately twice as large for the SAF than the USAF.