Seismic analysis of a model tension leg supported wind turbine under seabed liquefaction

Abstract

The paper presents numerical analyses of a pile supported Tension Leg Platform (TLP) wind turbine, during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when subsoil liquefaction is extensive, leading to degradation of the pseudo-static safety factor against pile pullout well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance. Pile head displacements develop steadily both during shaking and dissipation, but only while the static pullout safety factor remains less than unity. Due to the high tensional stiffness of the tendons relative to the buoyancy stiffness of the platform considered in this study, the pile head pullout is mostly transmitted to the platform with a relatively small part corresponding to reduction of tendon elongation. The resulting loss of buoy stability and tendon pretension may prove detrimental for the short-term operation of the platform. However, except possibly from the rare scenario of concurrent extreme environmental loads, both effects are recoverable, as they are unlikely to cause irreparable damage to the foundation and the superstructure.