Mooring analysis and design for offshore WEC survivability and fatigue (MoorWEC)

Lead Research Organisation: University of Manchester
Department Name: Mechanical Aerospace and Civil Eng

Abstract

Wave energy globally has potential average power slightly less than wind but this has been unexploited to date. We are concerned here with wave energy converters (WECs) offshore, before the energy resource is reduced by shallow-water effects, which would be suitable for grid scale electricity generation. Individual WEC capacity has been considered to be much smaller than for wind turbines and cost of energy (COE) considerably larger. However, with multi-mode, multi-float systems, capacity may be similar to or greater than wind in some locations and COE has been estimated to be similar to offshore wind. Survivability in extreme waves needs to be established, along with reliability of components. The mooring is the most vulnerable structural component of an offshore wave energy converter. Snap loads are a particular problem in extreme waves, and also in intermediate waves affecting fatigue. There is a widespread consensus in the wave energy community that mooring system design and modelling is a major challenge that needs to be overcome. Although literature and design guidelines for conventional ocean engineering applications are abundant, in general they do not account for the requirements of wave energy conversion, where the mooring should not inhibit platform motion causing the energy generation. Design, optimization, and assessment of mooring systems require efficient hydrodynamic and dynamic mooring models, which should be fully coupled to represent all interactions. There are various mooring options: catenary slack moored, elastic taut moored, combinations with single point (buoy) moorings, and nylon/polyester ropes offer an economic option while reducing snap loads. While some progress has been made with nonlinear hydrodynamic WEC loading models for point absorbers, an efficient general nonlinear hydrodynamic loading model for multi-bodies, accounting for wave breaking, is presently not available. Computational fluid dynamics (CFD) simulations require days, even weeks, to run on multiple processors and is unreliable for complex dynamic problems. The intention here is generalise efficient linear hydrodynamic load models by including the fully nonlinear force component due to the pressure field in the waves, known as the Froude-Krylov force. This has improved predictions of response and mooring load, markedly in some cases. This will be advanced through comparison with experimental wave basin tests and formally generalised through system identification, for single and multi-bodies with a range of mooring configurations in representative, generally multi-directional wave fields and currents. The convenient simplification of linear wave input will also be assessed with a revised force formulation determined by system identification. These force formulations will be coupled with the general industry-standard mooring model Orcaflex accounting for dynamic and material properties enabling design optimization using multi-objective genetic algorithms. This will enable survivability, fatigue and reliability analyses.

Publications

10 25 50