Developing a Coupled CFD Model for Evaluating Floating Tidal Stream Concepts

Developing a Coupled CFD Model for Evaluating Floating Tidal StreamConcepts

S. Brown, E. Ransley, D. Greaves, E. Guerrini

School of Engineering, Computing and Mathematics, University of Plymouth

Modular Tide Generators Ltd

Floating systems provide an opportunity to expand the available tidal stream energy resource and reduce theFloating systems provide an opportunity to expand the available tidal stream energy resource and reduce thelevelised cost of energy (LCOE) by increasing the number of viable deployment sites; simplifying theinstallation, maintenance and decommissioning, and; by accessing greater flow speeds near the free surface.However, the proximity of the free surface raises concerns over both the power delivery and the survivability ofthese systems, due to the presence of waves and the associated excitation of the floating structures. Without anaccurate prediction of the power output and greater confidence in the resilience of these systems, the risk toinvestors is too high to gain significant support for the industry. This has led to the development of a coupled andfully-nonlinear numerical model within the open-source CFD environment, OpenFOAM, capable of evaluatingthe performance and behaviour of full floating tidal systems [1]. The model solves the incompressible Reynolds-Averaged Navier-Stokes equations for a two-phase fluid [2], tracks the motion of the system in six degrees offreedom, and uses expression-based boundary conditions for wave generation [3]; a two-way coupled, actuatormethod to represent the turbine [4]; and a static catenary formulation for the moorings [5]. The model haspreviously been shown to agree with industry standard codes in relatively benign conditions, but hasdemonstrated additional complexities are present in more realistic conditions and these are not captured bysimpler approaches.This work details the continuing development of the numerical model, and, in particular, focuses onvalidation against the Modular Tide Generator’s (MTG) floating tidal platform concept, which consists of acatamaran style hull, catenary mooring system and a submerged horizontal axis tidal turbine. The simulationresults are compared with a series of 1:12 scale physical experiments, conducted in the COAST laboratory’sOcean Basin at the University of Plymouth. The behaviour of the full system has been explored in a range ofwave, current, and wave-current conditions, both with and without the turbine. In each case, the accuracy of thenumerical model predictions has been assessed against multiple criteria, including the motion of the barge,tension in each of the mooring lines and the thrust on the turbine. The results imply that the model successfullycaptures some of the key coupled properties of the problem, including relative increases in turbine load due tothe motion-thrust coupling. However, further work is required to improve turbine load calculations in reversingflows, and to introduce dynamic catenary mooring line functionality.

REFERENCES

[1] Brown, S., Ransley, E., Xie, N., Greaves, D., Johanning, L., Guerrini, E., “Validation of a coupled CFDmodel for evaluating floating tidal systems”, in Proceedings of the 29th Ocean and Polar EngineeringConference (ISOPE), Honolulu, Hawaii, USA.

[2] Rusche, H. (2002). “Computational fluid dynamics of dispersed two-phase flows at high phase fractions”Ph.D. Thesis, Imperial College of Science, Technology & Medicine.

[3] Jacobsen, N., Fuhrman, D., Fredsøe, J. (2012). “A wave generation toolbox for open-source CFD library:OpenFOAM®”, Int. J. Numer. Meth. Fluids, 70, 1073-1088.

[4] Brown, S., Ransley, E., Greaves, D. (2020). “Developing a coupled turbine thrust methodology for floating tidal stream                                                                                                                                                      concepts: verification under prescribed motion”, Renew. Energ., 147, 529-540.

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