X-MED: EXtreme Loading of Marine Energy Devices due to Waves, Current, Flotsam and Mammal Impact

海洋能源应该为英国可再生能源目标做出实质性贡献，到2020年，可再生能源的发电量将达到30%。潮流水轮机是一种比波浪能装置更成熟的技术，但波浪能的潜力是可观的。随着设计的迅速发展，人们对海洋能源设备在运行条件下的负载和性能越来越有能力和信心。然而，关于极限载荷的知识还不够成熟，确实存在一些关于其来源的不确定性。潮汐条件在水位和平均流量方面有相对较好的定义，但大规模水轮机部署将产生不确定的影响(这里不考虑)。然而，潮汐流动，特别是在吸引能量提取的高流速地区，依赖于水深测量。例如，海角和岛屿造成了影响极端载荷的大规模非定常涡流结构。使情况更加复杂的是，水平面上的潮汐湍流的长度尺度大约是垂直方向的六倍，这使得水平长度尺度大约是水深的一半，类似于典型的涡轮机直径。这将在不确定的程度上影响极限载荷，因此无法理解。此外，叠加在海流上的波浪会引起渗透到水面以下的不稳定；这可能是由于长浪涌或破碎波造成的，在这些波浪中，集中的、通常是倾斜的涡旋结构向下传播。破浪效果是本工程的一个重要组成部分。破碎波对波浪能装置上的极端载荷也有重大影响，将从实验和建模中获得的物理知识应用于潮流和一般波浪装置是合适的。我们认为，只有系泊的、浮动的波浪能装置作为固定结构，成本很高，可能会抑制至少大规模的部署。浮式结构物也可用于潮汐涡轮机的部署。极端负荷也会受到残留物、碎片和海洋哺乳动物或鲨鱼的冲击的强烈影响。这种情况的发生具有很高的不确定性，但如果发生，影响将很大。风险通常被定义为损坏概率和成本的乘积，因此这对于潮汐涡轮机叶片特别令人担忧，因为它们必须是细长的，所以很容易受到伤害。在这个项目中，我们不会调查发生大规模撞击的可能性，但会确定当水流中有残留物或海洋生物时，撞击的可能性和程度。漂浮物一般略有浮力，漂浮在水面上，在正常情况下对涡轮机几乎没有危险。然而，在破碎条件下，会产生向下喷射的流动，夹带的残留物可能会冲击涡轮机。据我们所知，这还没有被研究过。这将通过实验和使用一种称为光滑粒子流体动力学(SPH)的数值模拟方法进行研究，这种方法非常适合于处理碎石(表示为水流中的小物体)。

Marine energy should make a substantial contribution to the UK renewable energy target of 30% electricity by 2020. Tidal stream turbines are a more mature technology than wave energy devices while the potential of wave energy is considerable. There is a growing capability and confidence in the loading and performance of marine energy devices in operating conditions as designs rapidly develop. However knowledge of extreme loading is less mature and indeed there is some uncertainty about their origin. Tidal conditions are relatively well defined in terms of water levels and mean flows but large scale turbine deployment will have an uncertain effect (not considered here). Tidal flows particularly in areas of high velocity attractive for energy extraction are however bathymetry dependent. For example headlands and islands cause large-scale unsteady eddy structures affecting extreme loads. To complicate matters further tidal turbulence in the horizontal plane has length scales about six times those in the vertical giving a horizontal length scale of about half the water depth, similar to a typical turbine diameter. This will affect extreme loading to an uncertain degree and is not understood. In addition waves superimposed on currents cause unsteadiness which penetrates below the water surface; this may be due to long swell waves or breaking waves where concentrated, generally oblique, vortex structures propagate downwards. The effect of breaking waves is an important component of this project. Breaking waves also have a major impact on extreme loads on wave energy devices and it is appropriate to apply physical knowledge obtained from experiments and modelling to both tidal stream and generic wave devices. We consider only a moored, floating wave energy device as fixed structures have high costs likely to inhibit at least large scale deployment. Floating structures may also be used for tidal turbine deployment. Extreme loading will also be strongly influenced by impacts due to flotsam, debris and marine mammals or sharks. Such occurrence is highly uncertain but the impact will be high if it occurs. Risk is normally defined as the product of probability and cost of damage and so this is of particular concern for tidal turbine blades which are vulnerable since they must be slender. In this project we will not investigate the likelihood of occurrence of impact at large scale but will identify the possibility and magnitude of impact when there is flotsam or marine life in the flow. Flotsam is generally slightly buoyant, floating at the water surface, and in normal conditions of little danger to turbines. However in breaking conditions downwards jet-like flow is generated and entrained flotsam is likely to impact turbines. This has not been researched to our knowledge. This will be investigated experimentally and using a numerical modelling method known as smoothed particle hydrodynamics (SPH) which is well suited to handling debris (represented as small bodies in the flow).