CableDyn: Subsea Power Cable Dynamics Under Complex Ocean Environment

浮式海上风力涡轮机（FOWT）的部署预计将在未来增加，未来10年全球将建造6.2 GW的FOWT （https://tinyurl.com/camyybxk）。高动态、自由悬挂的电力电缆将这些fowt产生的电力输送到变电站和陆上电网。考虑到这些电缆的安装和更换都非常昂贵，而且涡轮机电力输出的任何停机都会导致巨大的收入损失，为了使它们在海洋中无故障地运行，对这些电缆进行安全关键设计至关重要。在浮式基础中，从浮式基础底部到海底的大长度电力电缆直接受到海浪、海流和湍流的动载荷作用。波浪移动浮动的基础，而电流产生由漩涡脱落产生的索振荡。在水柱中，缆索承受着增强的动载荷和复杂的运动。动力电缆在深水中敷设时，电缆上部承受较大的机械载荷和疲劳，下部承受较大的静水压力。浮式基础在浪涌、摇摆和升沉中的运动导致电力电缆发生振荡运动，进而促进涡激振动（VIV），这类似于海上油气平台中使用的长立管所经历的振动。因此，沿其长度的不同位置会发生大而复杂的挠度，改变其力学性能和强度，最终导致疲劳破坏。动力产生索的周期性运动，当这些运动和载荷集中在刚性连接点时，索的刚度会发生急剧转变。重复上述过程和过度弯曲也会导致电缆的疲劳损伤。迄今为止，几乎没有任何研究进行了研究的三维性质的涡激振动，动态载荷和运动的电力电缆受到联合波，电流和湍流。此外，海上风电行业的设计标准中没有给出关于如何预测、评估和抑制波浪-电流湍流条件下动力电缆疲劳失效的详细指导。如果电力电缆的设计是建立在对电缆与海洋环境之间的水动力相互作用理解不足的基础上，那么电力电缆的故障就更有可能发生。本基础科学研究旨在研究海底电力电缆在三维波、流、湍流复合作用下的动载荷、运动响应、涡激振动影响及其抑制机制、疲劳失效等问题。本研究将采用电力电缆响应的数值和物理模型。爱丁堡大学的FloWave波流设施将对电力电缆的比例模型进行控制实验测试，在那里可以产生各种幅度、频率和方向组合的多向波和电流。先进的新型现象尾流振荡器模型，经过FloWave实验结果的校准和验证，将用于模拟电力电缆的水动力行为。由此产生的软件工具、实验数据、表征电缆动力学和VIV的分析技术、为疲劳分析建立的方法以及本研究的其他成果将增强具有成本效益的电力电缆的设计。通过减少不确定性，我们的研究将提高海上电力电缆的可靠性，有利于电力电缆制造业。

Floating offshore wind turbine (FOWT) deployments are predicted to increase in the future and the outlook is that globally, 6.2 GW of FOWTs will be built in the next 10 years (https://tinyurl.com/camyybxk). Highly dynamic, free hanging power cables transport power generated by these FOWTs to substations and the onshore grid. Safety critical design of such power cables in order for them to operate in the ocean without failure is of utmost importance, given that these cables are highly expensive to install and replace and any down-time of turbine electrical output results in huge revenue loss. In FOWTs, a large length of the power cable, from the base of the floating foundation to the seabed, is directly exposed to dynamic loading caused by ocean waves, currents, and turbulence. Waves move the floating foundation, and currents produce cable oscillations generated by vortex shedding. In the water column a cable experiences enhanced dynamic loads and undergoes complicated motions. When a dynamic cable is installed in deep water, the upper portion of the cable is exposed to high mechanical load and fatigue, and the lower part to substantial hydrostatic pressure. Motion of the floating foundation in surge, sway, and heave causes the power cable to undergo oscillatory motions that in turn promote vortex-induced vibration (VIV) - which is analogous to the vibration experienced by long marine risers used in offshore oil and gas platforms. As a result, large and complex deflections of the cable occur at various locations along its length, altering its mechanical properties and strength, and eventually leading to fatigue-induced failure. The dynamic forces produce cyclical motions of the cable, and a sharp transition in cable stiffness is expected in cases where these motions and loads concentrate toward a rigid connection point. Repetition of the foregoing process and over-bending can also lead to fatigue damage to the cable. To date, hardly any research has been undertaken to investigate the 3-dimensional nature of VIV, dynamic loads, and motion of power cables subject to combined waves, currents, and turbulence. Moreover, no detailed guidance is given in design standards for the offshore wind industry on how to predict, assess, and suppress fatigue failure of dynamic cables under wave-current-turbulence conditions. Power cable failure is much more likely to occur if the design of such cables is based on poor understanding of the hydrodynamic interactions between cables and the ocean environment.This fundamental scientific research aims to investigate the dynamic loading, motion response, impact of vortex induced vibration and its suppression mechanism, and fatigue failure of subsea power cables subjected to combined 3-dimensional waves, currents, and turbulence. This research will be approached by both numerical and physical modelling of power cable's response. Controlled experimental tests on scale models of power cables will be undertaken in Edinburgh University's FloWave wave-current facility where multi-directional waves and currents of various combinations of amplitudes, frequencies, and directions can be generated. Advanced novel phenomenological wake oscillator models, calibrated and validated with FloWave experimental results, will be used to simulate the hydrodynamic behaviour of power cables. The resulting software tools, experimental data, analysis techniques for characterising cable dynamics and VIV, methodologies established for fatigue analysis, and other outcomes of this research will enhance the design of cost-effective power cables. By reducing uncertainty, our research will lead to increased reliability of offshore power cables, of benefit to the power cable manufacturing industry.