Abstract
Monopiles are large-diameter open-ended cylindrical piles under the action of the axial loads from the weight of the structure and lateral loads from the environmental actions. To accurately estimate the long-term dynamic behaviour of an offshore wind turbine (OWT) supported on monopile foundations, it is necessary to estimate the system damping. Precise damping measurement is critical for assessing dynamic response and fatigue in monopile-supported OWTs. This chapter reports the physics observed in controlled model tests and codifies them into a zonal soil damping curve (ZSDC) framework suitable for design. Instrumented small-scale piles installed in dry silica sand isolated hysteretic (material) damping from other sources and mapped its variation along the embedment together with its dependence on slenderness (L/D), cyclic amplitude, forcing frequency, and cycle count. The measured damping profile was distinctly non-uniform, with a peak near 0.1–0.2L, a minimum around 0.46–0.70L, and a toe-ward increase driven by the toe-kick. A three-zone linearised representation reproduced this behaviour, with LSRL ≥0.90, and the locations of maximum/minimum damping coincided with maxima/minima in soil reaction. Complementary cyclic element tests established D(γ,N) relationships (damping increasing with cyclic shear strain and decreasing with cycle number), providing a practical pathway to populate depth-weighted ζ(z) for 1D beam-on-non-linear-foundation models, PISA-style formulations, and macro-elements.