Numerical Study for Liquefaction Response of Taut and TLP Mooring Anchors during Earthquakes-Paper

Paper

Pdf Summary

The study evaluates how earthquake-induced liquefaction affects anchor piles used in taut and tension-leg platform (TLP) moorings for floating offshore wind turbines. Unlike catenary moorings, these systems impose substantial tension on their anchors, making them potentially vulnerable to soil strength loss and upward movement during liquefaction.<br /><br />The authors performed coupled dynamic analyses in FLAC3D using the SANISAND constitutive model for saturated sand. Model parameters were checked against centrifuge-test results. The anchor-pile simulations represented a liquefiable sand profile and combined static mooring loads with either Kobe-L or Northridge earthquake motions. Three cases were considered: taut moorings under each earthquake and TLP moorings under Kobe-L shaking. The analyses tracked excess pore pressure and pile displacement, and estimated post-earthquake pull-out capacity using the residual strength of liquefied sand.<br /><br />Pore pressures inside the pile were lower than outside, likely because the pile provided additional confinement. Anchor movements varied with mooring type and earthquake. Under taut moorings, the pile could undergo upward displacement and tilt; under TLP moorings, it moved upward with little horizontal displacement or rotation. For the cases studied, these movements were not sufficient to cause pull-out during shaking.<br /><br />The greater concern was reduced post-earthquake capacity: estimates indicated that anchor pull-out capacity could fall to about 25% of its original value after liquefaction. The findings therefore suggest that evaluating anchor performance should include not only displacement during an earthquake, but also the substantial loss of holding capacity that may persist afterward.

Keywords

earthquake-induced liquefaction

anchor piles

taut mooring

tension-leg platform

floating offshore wind turbines

FLAC3D

SANISAND model

excess pore pressure

pull-out capacity

liquefiable sand