Increasing the Life of Marine Turbines by Design and Innovation

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Unsteady loads on tidal turbines are much larger than in wind turbines because of the high density of water and the high levels of unsteadiness in offshore marine environments. In addition the welded mild steel structures normally used to support marine turbines have a low fatigue life in salt water. This can lead to life of 30 months or less instead of the design requirement of 30 years. In addition fatigue life limits the number of locations where tidal turbine can be deployed, limiting the overall practical UK tidal resource. This project aims to develop innovative technologies which will reduce the unsteady loads which result from flow unsteadiness and thus increase the longevity of a marine turbine by an order of magnitude.Two technologies will be developed. The first uses innovative hydraulic drive trains, developed in the UK, to reduce the unsteady loads created by large length unsteady 'gusts', those larger in size than the machine diameter. The hydraulic drive train allows the speed of the turbine to respond quickly to the 'gust' ensuring that the load on the machine remains constant. The second innovative technology is designed to reduce the unsteady loads created by short length unsteady 'gusts', those smaller than the machine diameter. The technology is similar to that used in aircraft to stop sudden changes in aircraft lift as the wing is hit by an air 'gust'. The technology uses 'spoilers', 'fluid ejection' or 'flaps' on the wing to automatically hold the blade lift constant as the 'gust' moves over it. This technology will be employed on tidal turbine blades to reduce unsteady loading due to short length scale unsteady 'gusts'. Once developed this second technology could also be used as an alternative to variable pitch mechanisms to avoid peak loads being exceeded.The project approach will be to use experimental testing and computational modelling developed in the jet engine industry to understand how the unsteadiness in marine environment resulted in unsteady turbine loading. A simplified model describing the turbine response to unsteady flow will then be implemented in the tidal turbine design code of a number of UK tidal turbine companies. This will improve the UK tidal turbine industries capability to predict the effect of flow unsteadiness.Experiments and computation will be used to develop the new automated 'spoiler' and 'flap' control system for gust loading control. This control system in conjunction with a model for the innovative hydraulic drive train will be implemented in the tidal turbine design codes. This will allow the success of the unsteady load reduction systems to be modelled.Current published measurements of sea turbulence are limited in both their spatial and temporal resolution. It is important to this study that improved measurements are obtained. The project members will work in collaboration with the tidal turbines companies partnered in this project, and with the Supergen partners, to improve the resolution of the available measurements. A new method of using conventional acoustic doppler equipment has been developed and if necessary this will be used in the project to achieve improved measurement resolution.Finally the unsteady load reduction systems and the hydraulic drive train will be incorporated into a scale model of the tidal turbine and this will be tested in a water flume. The tests will be undertaken with flow unsteadiness characteristic of the marine environment. These tests will determine the project's success in achieving an unsteady load reduction.Successful application of the load reduction systems to full scale tidal turbines will provide benefits in terms of extended maintenance periods and increased service life. Furthermore as turbines become less sensitive to their environmental operating conditions a larger number of tidal sites will become available as viable for producing power thereby increasing the UK tidal resource.