Numerical modeling of liquefaction and its impact on anchor piles for floating offshore structures

Anchor piles and suction anchors have been used for anchoring different types of offshore structure in the past four decades. The recent growing interest and demand for wind energy has motivated the industry to evaluate the use of Offshore Wind Turbines (OWT) in deep waters for which floating wind turbine is a good alternative to bottom-fixed solutions particularly in seismic regions with possibility of soil liquefaction. Extensive research has been carried out to assess the consequences of soil liquefaction for buildings and onshore structures; however, this phenomenon has not been sufficiently studied for offshore foundations. This paper aims at investigating the use of advanced liquefaction modeling in assessment of the performance of anchor piles for offshore facilities and in particular floating offshore wind turbines. The software FLAC3D is used to carry out the nonlinear dynamic analyses using SANISAND constitutive model for saturated sand. The analyses indicate that SANISAND model is capable of correctly simulating the excess pore water pressure in the free-field as observed in centrifuge tests. Pore pressure build-up due to earthquake shaking together with earthquake-induced displacements are computed at various points in the soil medium containing an anchor pile in different scenarios. The numerical results indicate that anchor piles may experience permanent lateral displacements and tilt due to the combined action of static mooring load and earthquake shaking leading to soil liquefaction.