Offshore Wind Farm Displacement due to Submarine Landslide

Abstract

The fast expansion of offshore wind turbine (OWT) installations necessitates durable monopile foundation designs for saturated sandy seabed. This study investigates soil liquefaction and instability, as well as their impact on large-diameter monopile behaviour. NorSand was used to create a PLAXIS 3D numerical model that simulates non-linear, undrained soil responses. The monopile was modelled using Beam and Embedded Beam elements, with liquefaction-induced instability initiated by programmed displacement at the model base. The researchers observed pile head load settlement, pore pressure evolution, effective stress variations, and lateral displacement and rotation. Another finding reveal that excess pore pressure accumulates quickly, resulting in effective stress loss and extensive liquefaction, which has a major impact on pile performance. Load-settlement response was erratic and unstable, characterized by rapid settlement and sudden load-carrying capacity drops. The pile initially heaved 2mm before settling significantly. Sensitivity study demonstrated that increasing pile diameter had an effect on load and settlement, although L/D ratio variations showed comparable tendencies after liquefaction. For L/D = 5, the maximum lateral rotation occurred at mid-depth, but in other cases, the average rotation was -0.9° at the base. This work emphasizes the essential sensitivity of monopile foundations to liquefaction induced instability, highlighting the importance of improved soil-structure interaction modelling in offshore foundation design.