Torque-Transferring Characteristics of Offshore Tetrapod Piled Jacket Foundations in Dense Sand

Abstract

Tetrapod piled jacket (TPJ) foundations that support offshore wind turbines (OWTs) tend to be subjected to significant torque that is caused by eccentric loading from wind, waves, or accidental vessel impact. This paper presented a comprehensive three-dimensional (3D) numerical analysis to investigate TPJs performance under torsional loading, which considered the distribution of local torque and rotation over pile shafts and the underlying torsional load transfer mechanism. A modified Mohr–Coulomb (MMC) model for dense sands was implemented in ABAQUS/Standard that captured the evolution of the friction and dilation (ψ) angles against the accumulated plastic shear strain (γp). The results showed that the torque that was imposed at the head of a TPJ foundation could trigger soil reactions of torsional resistance (τs) and horizontal force (Hp) in individual piles; higher pile spacing-to-diameter ratios (S/D) mitigated the influence of torsional loads. A pressure-dependent torsional load transfer model was proposed that considered the effects of the pile–sand relative stiffness and sand’s relative density (DR). The model could be employed to predict TPJs response under torsional loading and improve their practical design.