We show experimentally that a stable wave propagating into a region characterized by an opposite current may become modulationally unstable. Experiments have been performed in two independent wave tank facilities; both of them are equipped with a wavemaker and a pump for generating a current propagating in the opposite direction with respect to the waves. The experimental results support a recent conjecture based on a current-modified nonlinear Schrödinger equation which establishes that rogue waves can be triggered by a nonhomogeneous current characterized by a negative horizontal velocity gradient.

The design of a flexible wave energy device with a spine shape diaphragm proposed by Sea Energy Associates Ltd. was analysed. The operation of the device involves reversible buckling of a diaphragm in both longitudinal and transverse directions. The design constraints of the diaphragm were identified and Cambridge Engineering Selection software was applied to select candidate materials for the diaphragm structure. Best candidates of materials were identified for both laboratory scale and industrial scale. The initial curvature of the diaphragm was analysed using the minimum energy principle. The theoretical predictions of transverse deflection and longitudinal radius of curvature were in good agreement with measurements taken on a 1/10th scale-model of the diaphragm structure.

The quest for exploiting the ocean resources and understanding its behaviour has been a challenge with increasing needs for innovation and technology. Model testing is an essential step in offshore renewable energy technology development. It involves challenges that require experience and guidance. Costly mistakes might arise with the subsequent waste of time and resources. This paper presents the model design and testing processes as part of wave energy projects and the results of experimental testing of two types of oscillating-water-column (OWC) wave energy converters (WEC). The model design aims at the creation of a reduced-scale model to simulate the physical phenomena found in full-scale devices. It is a process that requires several skills and an adequate compromise among all variables. This design involves several approaches as different physical phenomena do not follow the same similarity conditions, requiring adjustments in scale, materials, and other relevant properties. Besides, the model testing process comprises the necessary planning and actions to execute the tests and post-processing of data. This process is addressed here through model design and testing of two WECs: the coaxial-duct and the sparbuoy OWCs. The configurations have been designed and studied for large-scale energy production and small-scale power in oceanographic applications. Although the devices are both OWCs, the designs exhibit significant differences. The development process of the models and results are presented for the two OWC devices. Freedecay tests, hydrodynamic performance and mooring tension results are presented and discussed. These may serve as guidelines and numerical modelling validation.

The EU H2020 MaRINET2 project has a goal to improve the quality, robustness and accuracy of physical modelling and associated testing practices for the offshore renewable energy sector. To support this aim, a round robin scale physical modelling test programme was conducted to deploy a common wave energy converter at four wave basins operated by MaRINET2 partners. Test campaigns were conducted at each facility to a common specification and test matrix, providing the unique opportunity for intercomparison between facilities and working practices. A nonproprietary hinged raft, with a nominal scale of 1:25, was tested under a set of 12 irregular sea states. This allowed for an assessment of power output, hinge angles, mooring loads, and six-degree-of-freedom motions. The key outcome to be concluded from the results is that the facilities performed consistently, with the majority of variation linked to differences in sea state calibration. A variation of 5–10 % in mean power was typical and was consistent with the variability observed in the measured significant wave heights. The tank depth (which varied from 2–5 m) showed remarkably little influence on the results, although it is noted that these tests used an aerial mooring system with the geometry unaffected by the tank depth. Similar good agreement was seen in the heave, surge, pitch and hinge angle responses. In order to maintain and improve the consistency across laboratories, we make recommendations on characterising and calibrating the tank environment and stress the importance of the device–facility physical interface (the aerial mooring in this case).

Scale model testing in wave basins is a necessary part of the development of marine structures and marine renewable energy devices. Whilst many guidelines exist for the quality of experimentation and data acquisition, there are no standards for the basins themselves. We propose methodologies for assessing the quality of a wave field generated in a basin: a clustering parameter based on the variance of surface elevation at multiple gauges is used to score homogeneity and extended to a skill score for relative quality benchmarking. We use historic and recent data from the University of Plymouth's Ocean Basin as a case study for the methods. The quality metrics indicate that physical aspects of the basin itself, such as water depth, contribute the most to the accuracy and homogeneity with wave period performing better than height. Recommendations for experimentalists, such as using basins when operating in steady state, are presented and discussed.