Proceedings of the Institution of Mechanical Engineers,Part M: Journal of Engineering for the Maritime Environment

Fluid structure response of vertical axis tidal turbine blades using NACA 0012 and periodic inflow equivalence model are predicted in this work. The response is investigated numerically by developing a two-dimensional computational fluid dynamics model at high Reynolds number (3.07 × 10⁶). The Periodic Inflow Equivalence Model is conducted by modeling the rotation of the turbine as a time-dependent incoming fluid velocity magnitude and angle of attack current entering the two-dimensional computational fluid dynamics domain. The blade response is modeled by a vibrational system with spring damper components which are attached at the blade fluid dynamic center point. The aim of this study is to predict a resonant condition or a lock-in frequency induced by wake generation at a vertical axis turbine blade during the turbine operation. The model is generated using a dynamic mesh construction in OpenFOAM 2.2, and the mesh is refined using snappyHexMesh utility. The mesh has seven added boundary layers around the blade surface and simulated using k-ω shear stress transport turbulence model. Drag, lift, and moment force coefficient are observed during 12 s, which is equal to 3.3 revolutions, and extracted using fast Fourier transform method to obtain its predominant frequency. The predominant frequency determines the dynamic condition of the blade and is used for predicting a resonance based on the turbine’s natural frequency. The result shows that the vertical axis tidal turbine which is manufactured from a composite material with pitch stiffness of 200 (N m)/rad, heave stiffness of 1000 N/s, and operates at tidal velocity of 0.656 m/s is found to experience a resonance or lock-in phenomena induced by wake generation in the pitch mode response.

Compared to onshore wind turbines, aerodynamics-induced dynamic characteristics of a floating wind turbine are more complicated due to its coupled aero-hydro-servo-elastic behaviours. The study investigates these aerodynamics-induced dynamic characteristics using an OC4 semi-submersible floating wind turbine. In this research, a high-fidelity wind field and a 1/50th scale model were tested. The dynamic characteristics induced by the aerodynamic effects were investigated in detail via the experimental results, including aerodynamic damping effects, gyroscopic effects, dynamic responses of the Rotor-Nacelle-Assembly and dynamic responses of the mooring system. It is found that aerodynamic damping is active in reducing surge and pitch resonant responses and probably increases with the inflow wind speed and the rotating-blades–induced induction coefficient; the gyroscopic effect intensifies the yaw motion and increases with the rotational speed of a rotor; the significant responses from the Rotor-Nacelle-Assembly may occur when the natural frequencies of the tower are close to aerodynamic periodic frequencies; and the mooring system is more sensitive to wave loads rather than to aerodynamic loads. This study presents the complexity of the dynamic characteristics induced by the aerodynamic effects in a floating wind turbine, and in the design code, using time-domain analysis software, which considers the coupling effects between the wind- and wave-induced loads and responses are suggested.

The fatigue damage sensitivity to wave directionality on large-diameter monopile foundations for use in the offshore wind turbine industry is investigated. A frequency-domain approach with the Dirlik method is used to estimate fatigue damage and to evaluate the effect of wave spreading and swell separation compared to a total sea representation. To evaluate the long-term impact, 30 years of hindcast data from the Dogger Bank area are used. Furthermore, a computationally efficient time-domain model of a 10-MW offshore wind turbine is used to compare with the frequency-domain results. Results show that benefits in terms of structural fatigue can be obtained with directional considerations.

Wave energy converter power production assessment, usually carried out using a power matrix, is essential for the appraisal of new wave energy converter technologies and for the planning of specific wave energy projects. Errors in power assessment may arise, both from an inaccurate description of the wave energy converter dynamics and from an excessively simplified representation of wave spectra in the power matrix approach. Ideally, the wave energy converter output should be computed in every individual sea state of the wave dataset considered, without the assumption of any parametric spectral shape. However, computationally efficient methods are necessary to achieve such extensive wave energy converter simulation. The non-linear frequency-domain technique is significantly faster than Runge–Kutta time-domain simulations, without affecting the representation of radiation forces and non-linear dynamics. In this article, the two main sources of errors in wave energy converter power assessment, namely the power matrix representation and wave energy converter modelling inaccuracies, are jointly studied and put into perspective, using four case studies (two wave energy converter systems in two locations). It is found that both types of errors can be of comparable magnitude. The non-linear frequency-domain technique simulation technique is shown to be a computationally efficient tool, retaining a realistic representation of the device dynamics while avoiding the use of a power matrix, thus preserving accurate representation of both the sea states and the wave energy converter, at little computational expense. Aside from those main results, the issue of the length and number of simulations, necessary to achieve average power estimates with sufficient accuracy in every sea state, is addressed in detail.

The wave loads and the resulting motions of floating wave energy converters are traditionally computed using linear radiation–diffraction methods. Yet for certain cases such as survival conditions, phase control and wave energy converters operating in the resonance region, more complete mathematical models such as computational fluid dynamics are preferred and over the last 5 years, computational fluid dynamics has become more frequently used in the wave energy field. However, rigorous estimation of numerical errors, convergence rates and uncertainties associated with computational fluid dynamics simulations have largely been overlooked in the wave energy sector. In this article, we apply formal verification and validation techniques to computational fluid dynamics simulations of a passively controlled point absorber. The phase control causes the motion response to be highly nonlinear even for almost linear incident waves. First, we show that the computational fluid dynamics simulations have acceptable agreement to experimental data. We then present a verification and validation study focusing on the solution verification covering spatial and temporal discretization, iterative and domain modelling errors. It is shown that the dominating source of errors is, as expected, the spatial discretization, but temporal and iterative errors cannot be neglected. Using hexahedral cells with low aspect ratio and 30 cells per wave height, we obtain results with less than 5% uncertainty in motion response (except for surge) and restraining forces for the buoy without phase control. The amplified nonlinear response due to phase control caused a large increase in numerical uncertainty, illustrating the difficulty to obtain reliable solutions for highly nonlinear responses, and that much denser meshes are required for such cases.

The combined wind and wave concept semisubmersible wind energy and flap-type wave energy converter was developed in the EU FP7 project MARINA Platform. It consists of a four-column semisubmersible with a 5-MW wind turbine placed on top of the central column and three flap-type wave energy converters on top of three pontoons that connect the four columns. Numerical and experimental studies have been performed to demonstrate the functionality and the survivability of the combined concept. In extreme conditions, both wind turbine and wave energy converters are set in a protection mode which reduces the dynamic loads and responses. In this article, different methods for predicting long-term (50-year) extreme responses considering the wind and wave conditions at two given European offshore sites are carried out, and structural response quantities are calculated, compared and presented. The full long-term analysis was performed and regarded as the reference method, and the corresponding results are compared with the modified environmental contour method and the environmental contour method. The response quantities studied here are the axial forces and bending moments of the semisubmersible wind energy and flap-type wave energy converter, including those of the wind turbine (blade, shaft and tower), arms of the flap-type wave energy converters and mooring lines, as well as the platform motion in 6 degrees of freedom. The extreme responses that are dominated by the aerodynamic loadings are effectively calculated either by the full long-term analysis or the modified environmental contour method. Compared to the full long-term analysis, the environmental contour method gives an under-prediction of the long-term extreme responses of quantities related to the wind turbine (e.g. internal loads of blades and tower). For the extreme responses that are dominated by the hydrodynamic loadings, all the three methods provide similar results.

This study presents an experimental and numerical investigation of a taut-moored wave energy converter system with a point-absorber type of wave energy converter. The wave energy converter system consists of a buoy, a unique three-leg two-segment mooring system with submerged floaters, and a power take-off system designed for the current experiment as a heave plate. The main objective of the study is to validate a numerical simulation model against experiments carried out in an ocean basin laboratory. Two physical models in model scales 1:20 and 1:36 were built and tested. The detailed experimental testing programme encompasses tests of mooring system stiffness, decay tests, and different sea state conditions for ocean current, regular, and irregular waves. A numerical model in the model scale 1:20 was developed to simulate coupled hydrodynamic and structural response analyses of the wave energy converter system, primarily using potential flow theory, boundary element method, finite element method, and the Morison equation. Several numerical simulations are presented for each part of the experimental testing programme. Results for the wave energy converter buoy motions under operational conditions from the experiments and the numerical simulations were compared. This study shows that the simulation model can satisfactorily predict the dynamic motion responses of the wave energy converter system at non-resonant conditions, while at resonant conditions additional calibration is needed to capture the damping present during the experiment. A discussion on simulation model calibration with regard to linear and non-linear damping highlights the challenge to estimate these damping values if measurement data are not available.

Marine renewable energy devices require mooring and foundation systems that suitable in terms of device operation and are also robust and cost effective. In the initial stages of mooring and foundation development a large number of possible configuration permutations exist. Filtering of unsuitable designs is possible using information specific to the deployment site (i.e. bathymetry, environmental conditions) and device (i.e. mooring and/or foundation system role and cable connection requirements). The identification of a final solution requires detailed analysis, which includes load cases based on extreme environmental statistics following certification guidance processes. Static and/or quasi-static modelling of the mooring and/or foundation system serves as an intermediate design filtering stage enabling dynamic time-domain analysis to be focused on a small number of potential configurations. Mooring and foundation design is therefore reliant on logical decision making throughout this stage-gate process. The open-source DTOcean (Optimal Design Tools for Ocean Energy Arrays) Tool includes a mooring and foundation module, which automates the configuration selection process for fixed and floating wave and tidal energy devices. As far as the authors are aware, this is one of the first tools to be developed for the purpose of identifying potential solutions during the initial stages of marine renewable energy design. While the mooring and foundation module does not replace a full design assessment, it provides in addition to suitable configuration solutions, assessments in terms of reliability, economics and environmental impact. This article provides insight into the solution identification approach used by the module and features the verification of both the mooring system calculations and the foundation design using commercial software. Several case studies are investigated: a floating wave energy converter and several anchoring systems. It is demonstrated that the mooring and foundation module is able to provide device and/or site developers with rapid mooring and foundation design solutions to appropriate design criteria.

Offshore renewable energy systems are generally required to operate in exposed offshore locations for long deployment periods at low cost. This requires innovative new mooring system solutions to go beyond the existing offshore industry designs. A number of novel mooring systems have recently been proposed which decouple mooring line compliance and minimum breaking load, offering multiple benefits to designers. Demonstrating reliability for such highly novel systems where standards do not yet exist is a common problem both for mooring systems specifically and in offshore renewable applications generally. A performance and reliability test method is proposed here and is applied to a novel mooring system, the Intelligent Active Mooring System. The line stiffness and damping properties of Intelligent Active Mooring System can be optimised to the prevailing metocean conditions without compromising minimum breaking load; the pre-tension is also adjustable for tidal range compensation or for service access. The article presents the results of a feasibility study for Intelligent Active Mooring System including detailed, large-scale physical performance tests that demonstrate load reductions under normal operating and extreme sea state conditions. The rationale and findings for an accelerated reliability test regime that quantifies the ultimate load capacity of the component and gives insight into the governing failure modes are also presented. The presented test approach provides assurance for the overall system integrity.