Loading Characteristics and Geotechnical Behaviour of Shared Caisson Anchors in Sandy Seabeds to Support Floating Offshore Wind Turbines
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2026
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Melbourne Authors
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Infrastructure Engineering
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PhD thesis
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© 2026 Bin Yan
Abstract
Floating offshore wind development is expanding into deeper waters and larger arrays. As water depth increases, fixed-bottom foundations become progressively less technically and economically viable, making floating systems an important option for exploiting deepwater wind resources. Floating turbines are held on station by mooring lines connected to seabed anchors. In conventional wind farm layouts, the number of anchors and the associated construction and installation costs increase with array size. Shared anchor systems reduce this demand by allowing mooring lines from several turbines to terminate at a common anchor. This arrangement, however, creates a more complex loading condition. Differences in turbine motion under wind, wave and current action result in mooring line tensions with different amplitudes and phases. Their vector sum at the shared anchor traces a load path whose magnitude and direction vary with time. Such path-dependent multidirectional loading is not adequately represented by conventional anchor assessments based on monotonic loading in a single direction.
This thesis investigates the geotechnical behaviour of shared caisson anchors in sandy seabeds. The research combines analytical load characterisation, three-dimensional finite element analysis, 1g biaxial model testing and mini-CPT measurements. The programme covers the generation of resultant load paths, changes in capacity under staged multidirectional loading, and anchor behaviour under unidirectional and rotational cyclic loading. The main findings are as follows.
(1) The analytical formulation relates environmental loads on an individual floating wind turbine to the corresponding mooring line tensions and derives the resultant load path at a shared anchor by vector superposition. Environmental load direction sets the principal orientation of the path, spatial phase differences between mooring line tensions determine its shape, and the equivalent load transfer angle governs the spatial components, mean value and fluctuation amplitude of the resultant load. Rotational paths, commonly taking eccentric elliptical forms, are representative of shared anchor loading. A fixed-direction cyclic path emerges only under specific conditions involving horizontal load transfer and ideal symmetry. The formulation provides representative load inputs for the subsequent numerical and physical investigations.
(2) The finite element analyses show that prior loading creates distinct zones of passive densification, shear dilation and active loosening around the caisson. When the loading direction changes, reloading capacity depends on which of these zones provides most of the resistance. Capacity is maintained or increases by no more than 5% when reloading mainly mobilises previously densified soil. When the mobilised soil has previously undergone shear dilation or active loosening, capacity decreases by a maximum of approximately 15%. A capacity modification factor is developed to account for the combined effects of prior load magnitude, change in loading direction and initial relative density of the sand.
(3) The unidirectional cyclic tests show how the mean load ratio and cyclic amplitude ratio affect accumulated displacement and rotation, unloading stiffness and hysteretic energy dissipation. Empirical expressions are developed for the initial unloading stiffness and initial hysteretic energy dissipation. Within the present test matrix, the post-cyclic capacity ratio exhibited an amplitude-dependent transition for reloading along the preceding loading direction, with an empirical critical cyclic amplitude ratio of 0.236. Mini-CPT measurements indicate that cyclic loading densifies the soil in front of the anchor while loosening the soil at its sides and rear. Frontal densification increases reloading stiffness, whereas loosening at the sides and rear reduces overall resistance. This contrast explains why reloading stiffness can increase even as post-cyclic capacity decreases.
(4) The rotational cyclic tests show that the phase difference between two orthogonal load components controls the geometry and extent of rotation of the resultant load path, as well as the in-plane response of the anchor. Within the test range, a phase difference of 90 degrees produces the largest transverse displacement and associated rotation amplitudes. Displacement increments are not generally aligned with the instantaneous resultant load, demonstrating a non-coaxial response. For comparable amplitudes of the resultant load, rotational cycling produces greater accumulated displacement and rotation than unidirectional cycling. Mini-CPT measurements show that the changes in soil state vary with azimuth after rotational loading. Post-cyclic reloading indicates that the capacity ratio measured in the prescribed reloading direction generally increases with phase difference. A mean path rotation index is introduced to quantify path rotation, and a quantitative relationship is established between this index and the corresponding post-cyclic capacity ratio.
Together, the findings show that shared anchor performance depends not only on load magnitude but also on load path geometry and loading history. The geotechnical assessment framework developed in this thesis connects system-level resultant loading with anchor serviceability, spatial redistribution of soil state and directional variations in subsequent capacity. The framework provides a basis for selecting representative load cases and evaluating the serviceability and bearing capacity of shared anchor systems in floating offshore wind farms.
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shared caisson anchors, sandy seabeds, resultant load paths, soil-state redistribution, post-cyclic capacity