Comparative analysis of load reduction device stiffness curves for floating offshore wind moorings

Abstract

traditional mooring systems can be unsuitable and uneconomical for floating offshore wind turbines. Load reduction devices, which are extensible components installed along mooring lines, have been shown to reduce loads on anchors and mooring lines. This enables the use of smaller and lighter anchors and mooring components and reduces fatigue damage on the mooring system. Load reduction devices come in various forms, including ballasted pendulums, polymer springs, and hydraulic dampers, each with unique non-linear stiffness curves. These non-linear curves typically consist of either a progressively-increasing ‘single-phase’ stiffness, or a ‘three-phase’ stiffness which exhibit stiff first and third-phase responses with a low-stiffness second phase. Selecting the correct shape of stiffness curve is key to ensure optimal load reduction performance from the device. This study compares the impact of 4 different non-linear stiffness curves, including 2 single-phase curves and 2 three-phase curves, on tension reduction and platform motions through finite element modelling. Taut and catenary mooring configurations, in both shallow (75 m) and intermediate (150 m) water depths, during 50-year parked and 50-year operational load cases are considered. The IEA 15 MW reference turbine, on the reference Voluturn-US semi-submersible platform are adopted for the analyses. Of the 4 non-linear stiffness curves considered, those with three-phase stiffness offer the maximum load reduction compared to a base mooring with no load reduction device, and are most effective in reducing fatigue damage. All load reduction device stiffness curve types have little effect on out-of-plane motions of the platform and acceleration at the nacelle, but lead to an increase in horizontal offset, or surge, of the floating offshore wind turbine when compared to the base mooring system. The increase in surge is similar regardless of the load reduction device stiffness curve shape, and is shown to be mainly driven by the length and rated tension of the device.

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Identifiers

ORCID for Susan Gourvenec: orcid.org/0000-0002-2628-7914

ORCID for Adam Sobey: orcid.org/0000-0001-6880-8338

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