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Understanding the hydrodynamic performance of floating energy converters is a complex challenge. Hence, physical modelling is necessary to evaluate the performance of innovative designs and validate them. The present paper shows the experimental work performed to validate a new floating semisubmersible structure which combines wave energy converters (3 Oscillating Water Columns, OWC) and wind harvesting (5 MW wind turbine). To characterize the global response of the platform, as well as the OWCs’ performance, an innovative wave tank testing campaign was carried out at the Cantabria Coastal and Ocean Basin (CCOB). The multi-use platform was characterized under the incidence of regular wave tests (with and without wind), operational sea states and survival sea states (combining waves, currents and wind). During the tests wind was reproduced with a portable wind generator and the wind turbine was simulated as a drag disk. The OWC air turbines were experimentally conceptualized by different diameter openings on the upper part of each OWC. This paper describes the experimental testing campaign carried out at the CCOB and presents the most significant experimental results obtained, such as natural periods, movements, loads on the mooring system or accelerations, which are representative of the performance of the multi-use platform presented.

This paper explores the role of wave spectral characteristics and wave time history on the estimation of extreme mooring loads on floating offshore wind turbines. This research is applied to the DeepCwind semi-submersible platform located at the BiMEP test site in the North of Spain. Extreme sea states are selected using the inverse first-order reliability method (I-FORM). Mooring loads are modelled by quasi-static and dynamic numerical approaches. Different wave time series are generated numerically for each sea state to investigate the variability in predicted peak loads. All cases simulated incorporate the combined effect of wind and waves. Differences of approximately 30% in peak loads are found for the mooring system, reaching 40–79% for the most extreme sea states. Safety factors are proposed to account for sensitivity to wave groupiness in modelling loads under extreme work conditions of the DeepCwind platform (e.g., pitch and velocity control). A comparison between theoretical and real-sea wave spectra is also modelled to investigate possible differences due to the presence of multiple spectral peaks associated with swell and wind seas. In general, results show differences below 12% in the prediction of loads between both assumptions.

This paper presents a methodology to assess the walk-to-work accessibility of a floating wind turbine. The system composed by the vessel and the platform is modelled in the frequency domain as a rigid, possibly constrained multibody system. Non-linear actions, such mooring and viscous forces are linearised. Extreme maxima for the response variables are calculated assuming that crests are Rayleigh distributed. Two vessels are studied: a catamaran equipped with fender, and a supply vessel mounting a motion-compensated gangway. For the catamaran, accessibility is possible when no-slip conditions between the vessel fender and the ladder landing platform are ensured. For the supply vessel, accessibility is possible when the gangway motions are below the hydraulic system compensation limits. The catamaran is able to handle wave heights up to 2 m, provided that it can work under head sea conditions and take advantage of the shielding effect of the platform. The supply vessel allows personnel transfer with wave heights up to 5 m, but it is important that roll motions are not excited. The proposed methodology and the calculated maps are a valuable source of information for decision-making during personnel transfer to and from offshore floating platforms.

Mooring systems are under a cyclic loading process caused by the randomness of metocean conditions, which could lead to a fatigue failure of the station keeping system. The present paper presents an innovative methodology for the assessment of floating offshore wind turbine mooring system fatigue considering the full lifetime of the structure. The method integrates the impact of the life cycle metocean conditions over the dynamic performance of the platform thanks to coupled numerical models, selection and non-linear data interpolation techniques and commonly accepted fatigue approaches. One of the benefits of using this methodology is that there are no uncertainties due to the selection of a reduced set of sea states. The methodology is applied to a set of moorings with different properties in the DeepCwind platform to evaluate the solution which offers the best compromise between size and fatigue damage. Results show that the best long-term mooring behaviour is achieved with a weight of approximately 300 kg/m. A comparison is conducted between the fatigue damage obtained through the life-cycle method and conventional methods. The mean differences observed between the standard and the new method proposed are between 13% and 49% depending on the use of the S–N or T-N curves.

A new solver for wave and structure interaction is used for three-dimensional simulation of an Oscillating Water Column (OWC). The CFD model solves the Reynolds Averaged Navier–Stokes equations for two incompressible phases (water and air). Laboratory experiments are conducted on a small scale to validate the numerical results. Air and water pressures and velocities as well as the free surface evolution inside and outside the chamber are modeled with notably good agreement. The model is further used to improve understanding of relevant processes and shows potential for use detailed analysis. Even if the experimental problem analyzed had a two-dimensional behavior, the three dimensional domain of the wave flume was numerically simulated in order to prove the capabilities of OpenFOAM ® .

This paper presents a study of the factors that influence array layout on wave energy farms. The WEC considered is a two-body heave converter extracted from Babarit et al. (2012) . Simulations were run through a time domain model from de Andres et al. (2013) with irregular waves considering different sea states. Factors analyzed in this paper are array layout, WEC separation, number of WECs and wave directionality. Results show that wave directionality is very important in order to achieve constructive interference. When looking at the number of WECs the conclusion is that as the larger the number of WECs, more interactions are possible and therefore, the higher the interaction factor is. Regarding array layout, triangle and square configurations were found to be similar and the efficiency of each one depends on the most probable peak period. Separating distance was found to be a key factor and L 10 / 2 was set as the optimal one. Finally, wave climate was classified in different subtypes around the globe. The optimum layout in these sites was assessed. The influence of directionality was studied and the triangular configuration was found to be the most favorable for multidirectional climates while square configurations were most adequate in unidirectional climates.

A methodology to identify the state-space matrix and the vectors which solves the Cummins equations directly using free surface temporal series, obtained from laboratory or CFD codes, avoiding the use of potential flow solvers, is presented. The new methodology is applied to the solution of the piston-like movement of an Oscillating Water Column (OWC) moving in heave, assuming that sloshing is negligible, to incorporate flow effects that are not captured with traditional methods using potential flow solvers. The identification of the state-space components is done in two steps. The first step uses the solution of the state-space for decay tests, no forcing terms, obtaining a possible solution. This solution is used in the second step to solve regular waves, nonzero forcing term and no initial condition. The solution obtained in the second step is the component of the state-space that characterizes the movement of the OWC. The numerical series used to compute the solution are obtained using IH2VOF in this study, but they can be obtained by means of other sources, such as different numerical codes or laboratory measurements. Once the solution is found, the resulting state-space is validated against numerical results of IH2VOF for regular and irregular wave trains.

A simplified time-domain model for a fixed detached Oscillating Water Column (OWC) device is presented as a first step towards modeling a floating multi-chamber OWC device. The motion of a floating body in the time-domain is expressed by Cummins integro-differential equation, and based on it, water mass motion inside the chamber has been modeled here as a piston-like motion. Radiation, hydrostatic, excitation and viscous forces have been considered, as well as the added mass of the water in the chamber and the effect of the air pressure inside it. The equation of the floating body in the time domain has been approximated by a state-space method, which comes from the extension of the state-space system corresponding to the convolution integral of the radiation force. Experimental data have been used for model calibration and validation. Furthermore, the model has also been validated with a widely used Computational Fluid Dynamics (CFD) model (IH-2VOF). These show that the model presented is reliable and computationally efficient allowing for massive simulations for a statistical design or economic feasibility studies.

This study presents a methodology to obtain the power performance of a two-body heave converter. First, the methodology relies on a time domain model which represents the motion of the two bodies throughout the time. This time model was built substituting the entire equation system with a state-space system, thereby avoiding the convolution integral of the radiation force. This technique is demonstrated to be a reliable and very efficient method in terms of speed. Then, based on this model the instantaneous power of the device can be obtained. The performance of the device is shown through the power production matrix and the 60 year series power production statistics. This long series is obtained by means of using the MaxDiss selection technique in order to compute only the power of the most representative sea states and the Radial Basis Function (RBF) interpolation technique which interpolates in order to obtain the complete power series.

Using COBRAS-UC, a numerical model based on the Volume-Averaged Reynolds Average Navier–Stokes (VARANS) equations, 2-D wave interaction with low-mound breakwaters is analyzed. The model uses a Volume of Fluid (VOF) technique method to capture the free surface which allows the modeling of complicated processes such as breakwater overtopping. Furthermore, thanks to the VARANS equations, the flow in the permeable layers underneath the caisson is quantitatively correct. In order to validate the model's performance, a new set of experimental studies are carried out in a wave flume at a 1:20 scale using regular and irregular waves. Comparisons between numerical and experimental free surface, pressure time histories, and overtopping layer thickness for regular and irregular waves show a good agreement. Further comparisons of numerically predicted overtopping magnitudes with existing semi-empirical formulae and experimental data indicate that the model can be used as a complementary tool for the functional design of this kind of structures.