Background:

The deployment of floating offshore wind turbines (FOWTs) is expected to increase globally in the future. Power generated by FOWTs is transported to substations and the onshore grid via dynamic power cables, which are typically suspended between the FOWTs and the seabed. These cables are highly expensive to install and replace, making their safety-critical design essential to ensure reliable operation in the ocean. In FOWTs, a significant portion 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, while currents induce oscillations in the cable through vortex shedding. In the water column, the cable experiences increased dynamic loads and complex motions. The floating foundation's movements in surge, sway, and heave cause the power cable to undergo oscillatory motions, which in turn promote vortex-induced vibration (VIV)—similar to the vibrations experienced by long marine risers used in offshore oil and gas platforms. Consequently, large and complex deflections occur along the cable, altering its mechanical properties and strength, eventually leading to fatigue-induced failure. The dynamic forces produce cyclical motions and VIV, which can result in fatigue damage to the cable.

This research will contribute to the EPSRC CableDyn project (https://gow.epsrc.ukri.org/NGBOViewGrant.aspx?GrantRef=EP/W015102/1) and will investigate the 3-dimensional nature of VIV, dynamic loads, and motion of power cables subjected to combined waves, currents, and turbulence.

Research Methodology:

This research aims to investigate the dynamic loading, motion response, impact of vortex-induced vibration (VIV) and its suppression mechanisms, and fatigue failure of subsea power cables subjected to combined waves, currents, and turbulence. The approach will involve both numerical and physical modeling of the power cable's response. The Institute for Energy Systems has access to OrcaFlex and SHEAR7 software tools for modeling power cable dynamics. Additionally, the research student is encouraged to develop their own numerical tools if they wish. Advanced novel phenomenological wake oscillator models are under development by the project partner's institution and will be made available once calibrated and validated.

Experimental tests on two different scale models of the power cables have already been conducted at Edinburgh University’s FloWave wave-current facility. Measurements of the cable’s displacements and strain along its length at various locations, as well as dynamic forces, have been recorded for various current speeds and directions, and for regular and random waves. These data will be made available to the researcher for further analysis and validation with the numerical models.

This research will directly benefit the offshore wind turbine design and power cable manufacturing industries. The successful applicant will have the opportunity to collaborate with researchers at Newcastle, Southampton, and Exeter Universities who are part of the CableDyn project.