ABSTRACT

This study investigates the seismic performance of a suction caisson anchor with static vertical tensile loading using 3D finite element analysis, in the context of a floating offshore wind turbine tension leg platform. A non-linear dynamic analysis was conducted using the advanced UBC3D-PLM constitutive model calibrated against cyclic element tests and centrifuge experimental data. Orthogonal horizontal and vertical components of a strong ground motion were applied to the model with varying peak ground acceleration and vertical tensile loading applied to the anchor. The results suggest resilience to anchor pull-out during seismic loading in liquefiable soils, even with vertical tensile loading applied at twice the drained vertical tensile capacity of the anchor and subject to seismic loading at a PGA of 0.5g.

INTRODUCTION

Floating offshore wind is expected to generate 300GW by 2050 (DNV, 2023), unlocked by the development of vast areas of ocean considered too deep for fixed bottom foundations. Interest in the technology has been shown globally including in the UK, Europe, US, Brazil, Australia and Asia. Many of these regions are seismically active, and an important design consideration is the liquefaction potential of the seabed, which could reduce the capacity of the holding anchor and lead to pull-out failure.

The first arrays of floating offshore wind turbines (FOWTs) have been deployed in shallow water at significant cost (e.g. Hywind Scotland, Kincardine, and WindFloat Atlantic). To reduce the levelized cost of energy (LCOE) shared moorings (Hall and Connolly, 2018; Connolly and Hall, 2019; NYSERDA, 2021), shared anchors (Diaz et al, 2016; Hallowell et al, 2018; Fontana et al., 2018, Chen et al, 2021, Pillai et al. 2022), novel concepts for mooring (e.g. HoneymooringTM) and tension leg platform (TLP) designs are being proposed (e.g. Hexicon's TLP, Glosten's PelaStarTM). There is also consideration to transition from catenary to semi-taut or taut mooring (Catapult, 2022). In shallow or deeper water, many of these designs will require anchors that are axisymmetric and can sustain vertical tensile loading.

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