Feasibility of an Innovative Methodology for Testing Marine Current Turbines in Unsteady Flow

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In the early years of the offshore oil industry, many design challenges were met by the incorporation of large factors of safety to allow for a lack of fundamental understanding of appropriate values of design loads. The high profitability of the oil business allowed the resulting high-cost solutions in the short-term; whilst occasional failures could not threaten such a well financed industry. In contrast, the economics of marine renewable energy do not allow such a high-cost approach, whilst early failures in high-profile projects could cause immense damage to the confidence placed in such a fledgling industry. In order to succeed technically and economically, MCT designs must meet stringent environmental legislation, must be efficient, cost effective, environmentally friendly and above all reliable. For this reason it is important to have a fundamental understanding not only of the performance of the rotor itself, but also of the global hydrodynamic loading on the system, in order to allow safe and cost-effective design of the mooring/anchoring system. There has been extensive research over many years aimed at finding approaches for determining drag, added mass and damping effects of fixed and floating offshore structures in unsteady flows - that is flows which vary with time, such as waves, or fluctuating currents - however the applicability of these approaches to the rotating blades of an MCT rotor is highly questionable raising uncertainties in perceived knowledge. The importance of the unsteady loading on the rotors is all the more apparent when their dimensions are considered; current proposals involve pairs of rotors of over 10m diameter, with a swept area which may be 50 times the projected area of the support structure. The global loading is therefore clearly dominated by the moving rotor. The importance of the unsteady hydrodynamic loads, often characterized by added mass and damping coefficients, is further emphasized by the relatively small real mass of these devices in comparison to conventional marine and/or offshore structures. For both fixed and compliant structures, maximum loading as well as fatigue performance are important issues; for compliant structures the influence of the unsteady rotor forces on the global dynamic response is also likely to be a critical parameter in the mooring design. MCTs are likely to operate in environments in which large currents and steep waves will be evident, and appreciable unsteady flows are inevitable. In such cases it is essential that the global added mass and damping of the rotors can be reliably evaluated for a wide range of unsteady flows.Whilst some progress towards the understanding of the unsteady response can be made with sea trials using large scale models, the lack of controllability and repeatability of the real-world environment presents substantial challenges to the development of fundamental understanding of these effects, particularly with regard to extreme conditions. A laboratory-based experimental technique is thus required which can be used to develop this fundamental understanding both directly, through experiment measurement, and indirectly, via the verification and validation of numerical simulation approaches. The study proposed here therefore aims to establish the feasibility of a model-testing methodology to predict these unsteady global loads as well as operational efficiency parameters on horizontal axis marine current turbines, addressing an interesting and challenging hydrodynamic problem of significant practical relevance.