Effects of tidal range on mooring systems of wave energy converters

Wave energy converters are currently proposed to be deployed near coastal area for the closeness to the infrastructure and for ease of maintenance in order to reduce operational costs. The motivation behind this work is the fact that the deployment depths during the highest and lowest tides will have a significant effect on the mooring system of WECs. In this paper, the issue will be investigated by numerical modelling (using ANSYS AQWA) for both catenary and taut moorings to examine the performance of the mooring system in varying tides. The case study being considered is the ¼- scale wave energy test site in Galway Bay off the west coast of Ireland where some marine renewable energy devices can be tested. In this test site, the tidal range is macro-tidal with a range of approximately 6 m which is a large value relative to the water depth. In the numerical analysis, ANSYS AQWA suite has been used to simulate moored devices under wave excitation at varying tidal ranges. Results show that the highest tide will give rise to larger forces. While at lower depths, slackening of the mooring occurs. Therefore, the mooring lines must be designed to accommodate both situations

Tidal flyer; innovation, design & evolution (TIDE)

Open Ocean Energy (OOE) Ltd. is a tidal energy company developing a vertical hydrofoil based device, the Tidal Flyer. Following the initial years proving the concept and progressing the design of the system for the past 3 years OOE have been working intensively with Black & Veatch (B& V) on the techno-economic optimisation of the Tidal Flyer configuration. This paper will outline recently conducted Phase 1 physical testing of various aspects of the technical design evolving from the study. CFD work investigating critical features of the system dynamics was also undertaken but is not reported here. The Phase 1 testing was constructed to inform the design of a full, dynamic model due for testing at the combined wave and current tank, Flowave in Edinburgh, Scotland. The latter trials will be undertaken as a Phase 2 test programme. At the end of the physical test schedule, OOE will have completed up to TRL 4 and satisfied a series of predefined stage gates criteria. The programme of empirical testing involved two visits (8 testing days) to the IFREMER flow flume in Boulogne-sur-Mer, France. The CFD analysis is being undertaken at UCC as part of the SFI funded Marine Renewable Energy Ireland [MaREI] scheme. Initially the empirical setups were simulated to verify the test plan

Numerical hydrodynamic modelling of a pitching wave energy converter

Two computational methodologies – computational fluid dynamics (CFD) and the numerical modelling using linear potential theory based boundary element method (BEM) are compared against experimental measurements of the motion response of a pitching wave energy converter. CFD is considered as relatively rigorous approach offering non-linear incorporation of viscous and vortex phenomenon and capturing of the flow turbulence to some extent, whereas numerical approach of the BEM relies upon the linear frequency domain hydrodynamic calculations that can be further used for the time-domain analysis offering robust preliminary design analysis. This paper reports results from both approaches and concludes upon the comparison of numerical and experimental findings

Numerical hydrodynamic modelling of a pitching wave energy converter

Two computational methodologies – computational fluid dynamics (CFD) and the numerical modelling using linear potential theory based boundary element method (BEM) are compared against experimental measurements of the motion response of a pitching wave energy converter. CFD is considered as relatively rigorous approach offering non-linear incorporation of viscous and vortex phenomenon and capturing of the flow turbulence to some extent, whereas numerical approach of the BEM relies upon the linear frequency domain hydrodynamic calculations that can be further used for the time-domain analysis offering robust preliminary design analysis. This paper reports results from both approaches and concludes upon the comparison of numerical and experimental findings

Numerical hydrodynamic modelling of a pitching wave energy converter

Two computational methodologies – computational fluid dynamics (CFD) and the numerical modelling using linear potential theory based boundary element method (BEM) are compared against experimental measurements of the motion response of a pitching wave energy converter. CFD is considered as relatively rigorous approach offering nonlinear incorporation of viscous and vortex phenomenon and capturing of the flow turbulence to some extent, whereas numerical approach of the BEM relies upon the linear frequency domain hydrodynamic calculations that can be further used for the time-domain analysis offering robust preliminary design analysis. This paper reports results from both approaches and concludes upon the comparison of numerical and experimental findings

Evaluation of the Viscous Drag for a Domed Cylindrical Moored Wave Energy Converter

Viscous drag, nonlinear in nature, is an important aspect of the fluid−structure interaction modelling and is usually not taken into account when the fluid is assumed to be inviscid. Potential flow solvers can competently compute radiation damping, which is related to the radiated wave field. However, the drag damping primarily related to the viscous effects is usually neglected in the radiation/diffraction problems solved by the boundary element method (BEM), also known as the boundary integral element method (BIEM). This drag force can have a significant impact in the case of structures extending much deeper below the free surface, or for those that are completely submerged. In this paper, the drag coefficient C d was quantified for the heave and surge response of a structure which consists of a moored horizontally oriented domed cylinder with two surface piercing square columns located at the top surface. The domed cylinder is the primary part and is submerged. The drag coefficient is estimated using the experimental measurements related to harmonic monochromatic wave−structure interaction. Finally, this estimated drag coefficient was used in the modified time domain model, which includes the nonlinear viscous correction term, and the resulting device response in heave and surge directions is presented for an irregular incoming wave field. The comparison of the numerical model and the experiments validates the estimated C d values obtained earlier. Prior to the time domain model, frequency-dependent parameters such as added mass, radiation damping, and excitation force were computed using three mainstream potential flow packages (that is, ANSYS AQWA, WAMIT, and NEMOH), and a comparison is presented. The effect of free surface on the drag coefficient is investigated through differences in C d values between heave and surge modes

Tidal flyer; innovation, design & evolution (TIDE)

Open Ocean Energy (OOE) Ltd. is a tidal energy company developing a vertical hydrofoil based device, the Tidal Flyer. Following the initial years proving the concept and progressing the design of the system for the past 3 years OOE have been working intensively with Black & Veatch (B& V) on the techno-economic optimisation of the Tidal Flyer configuration. This paper will outline recently conducted Phase 1 physical testing of various aspects of the technical design evolving from the study. CFD work investigating critical features of the system dynamics was also undertaken but is not reported here. The Phase 1 testing was constructed to inform the design of a full, dynamic model due for testing at the combined wave and current tank, Flowave in Edinburgh, Scotland. The latter trials will be undertaken as a Phase 2 test programme. At the end of the physical test schedule, OOE will have completed up to TRL 4 and satisfied a series of predefined stage gates criteria. The programme of empirical testing involved two visits (8 testing days) to the IFREMER flow flume in Boulogne-sur-Mer, France. The CFD analysis is being undertaken at UCC as part of the SFI funded Marine Renewable Energy Ireland [MaREI] scheme. Initially the empirical setups were simulated to verify the test plan

Potential Time Domain Model with Viscous Correction and CFD Analysis of a Generic Surging Floating Wave Energy Converter

International audienceThe state of the art tools to assess the efficiency of the wave energy converters comprise the boundary element method (BEM) codes which are based on the potential linear approach whereas computational fluid dynamics (CFD) is still considered to be relatively computationally expensive. An attempt to enlarge the scope of the state of the art computational tools for wave energy converter applications is made in order to account for the viscous effects. This is achieved via the viscous damping term of the Morison equation which relies on a coefficient Cd – to be estimated prior force calculation. The state of the art wave to wire model together with additional viscous term is termed as potential time domain viscous model and is employed for evaluation of the power efficiency of a generic surging type wave energy conversion system. Finally a comparison of CFD and the viscous time domain model is conducted which concludes that the Morison equations’ drag term does offer an improvement

Optimum power capture of a new wave energy converter in irregular waves

This paper presents the optimum power capture of a new point-absorber wave energy converter, in irregular waves. A stepwise control system for the wave energy converter (WEC) is developed. The control system is used to efficiently extract power from irregular waves where amplitudes vary from wave to wave. The Bretschneider spectrum is used in the experiment and the device is 'tuned' to the peak period of the sea state. It is shown that this WEC has a reasonable capture width in irregular waves. However, the optimum mean power depends on the wave spectrum, the shape of the collector body, its freeboard and the device pivot depth