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FRINATEK-Fri mat.,naturv.,tek

HYDROMORE – HYDROdynamic Mooring analysis for Ocean Renewable Energy

Alternative title: HYDROMORE - Hydrodynamisk analyse av forankringssystemer for flytende offshore havenergi

Awarded: NOK 8.0 mill.

HYDROMORE seeks to establish new best-practice approaches for designing future mooring systems used in floating ocean renewable energy devices. This will ultimately enable the design of leaner and safer mooring solutions and reduce the cost of energy from these technologies. In line with the Paris Climate Agreement, the European Commission estimates to install 230-450 GW of new offshore wind farms by 2050. As accessible shallow waters of the North Sea become saturated with wind farms, around a third of this capacity will need to be installed in deep water on floating structures. A major obstacle to minimise cost of electricity from both floating wind and wave energy, is the mooring system cost. Traditional moorings are heavy and designed using methods suitable for large floating oil rigs, but not for small, dynamic floating wind structures or wave energy converters. Mooring forces are influenced by low-frequency effects, such as slow drift and the interaction of variable wind and wave conditions, coupled with structure response and adaptive control systems. These interactions are not readily assessed using high fidelity methods, such as computational fluid dynamics. As such, there is a knowledge-gap between fast, low-fidelity methods and slow, high-fidelity methods. Hence the design of leaner, more advanced mooring solutions is restricted. Through four work-packages, HYDROMORE will address this knowledge-gap, to accurately model nonlinear and irregular seas, whilst allowing rapid evaluation of multiple design condition parameters. HYDROMORE will assess reliability and limitations of state-of-the-art methods for accurate hydrodynamic modelling over long-periods, as well as highly nonlinear breaking waves. Methods for modelling the combined effects of wind loading, e.g. a turbine moving in and out of its own wake, with hydrodynamics will also be studied. All modelling will be validated against controlled laboratory-scale tests to help inform new industry standards.

The mooring is a vulnerable structural component for ocean renewable energy platforms. Snap loads are a particular problem in extreme waves, and also in intermediate waves affecting fatigue. There is a widespread consensus that mooring system design and modelling is a major challenge that needs to be overcome. Design, optimisation, 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), elastic (taut), 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 loading models, an efficient general nonlinear hydrodynamic loading model, accounting for wave breaking, is presently not available. CFD simulations require very long run times (days) even on multiple processors and can be unreliable for complex dynamic problems. The intention here is to generalise efficient linear and second-order hydrodynamic load models by including the fully nonlinear force component due to the pressure field in the undisturbed waves, known as the Froude-Krylov force. This has improved predictions of response and mooring load, markedly in some cases. The aerodynamic loads acting on the rotor are also required and will be incorporated using blade element momentum theory and computationally efficient actuator line modelling, which allows the effects of the turbine moving in and out of its own wake to be incorporated. All the numerical tools developed here will be advanced through comparison with experimental wave tank tests for a range of mooring configurations in representative, multi-directional wave fields. These force formulations will be coupled with the general industry-standard mooring model Orcaflex to account for the dynamic and material properties of the mooring system, enabling design optimisation.

Funding scheme:

FRINATEK-Fri mat.,naturv.,tek