title: Vortex-induced vibrations of dynamic power cables for floating offshore wind turbines: influence of currents, waves and cable properties
Creators: name: Elrick, Peter James Mcleod
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: vortex-induced vibrations
subject: VIV
subject: dynamic power cables
subject: floating offshore wind turbines
subject: offshore wind turbines
subject: OrcaFlex
subject: Shear7
subject: Iwans and Blevins Wake Oscillator
subject: Milan Wake Oscillator
subject: deployed cable configuration
subject: RMS crossflow displacements
subject: wave height
subject: stress

description: This research investigates the influence of vortex-induced vibrations (VIV) on dynamic power cables for floating offshore wind turbines. This was conducted through the use of two different numerical modelling tools, OrcaFlex and Shear7. Shear7, a frequency domain VIV prediction model was coupled with OrcaFlex, a time-domain three-dimensional finite element analysis software. Two popular numerical methods, Iwans and Blevins Wake Oscillator (IBWO), Milan Wake Oscillator (MWO) are employed for predicting cable’s VIV. These models are validated against scaled down experimental results for a dynamic power cable. Shear7 produces the most accurate prediction and is chosen for further analysis of additional parameters. MWO seriously struggles and is deemed unsuitable for use in this context. IBWO predicts Root Mean Square (RMS) crossflow displacements up to four times greater than Shear7. The environment in which dynamic power cables will be deployed will potentially be in locations with extreme subsea currents and waves. As such it is important to understand the influence these will have on the VIV of these cables. Through this research multiple parameters were investigated to determine the impact these have on the VIV behaviour. The current the cable was exposed to was varied to account for known current behaviour at sites likely to host FOWTs. It was found that the profile, direction relative to the plane of the cable, and speed of the currents all have significant effects on the VIV behaviour. Different current directions result in drastically altering the VIV profile of the dynamic power cable by changing the maximum predicted amplitude and location along the cable length. A change from uniform current of 1m/s to a shear profile based on site data at Hywind, with a surface current speed of 1m/s, resulted in a decrease in RMS displacement of 10% at angle 0, 3% at angle 90, and 21% at angle 180. The deployed cable configuration was studied to observe how this impacted the VIV behaviour and under what conditions would a certain configuration be preferred. A lazy wave, double wave, steep wave, and tethered lazy wave were analysed. It was found that different configurations lead to substantially different RMS crossflow displacements and locations of largest displacement. Current directions and profiles were also shown to impact each configuration uniquely. The structural properties of an object undergoing VIV are known to greatly impact the response. With various cable designs still being investigated there is no preferred properties in operation. As such, multiple cables are used to determine the scale of this influence on the VIV response. The bending stiffness, diameter, and mass ratio are all shown to alter the VIV response of the cable. This is not only due to fundamental VIV influence but also changes on the global scale of the cable in terms of sensitivity to deformation and influences this has on the relative velocity. The cable density influences the extent of cable deformation which in turn changes the relative velocity resulting in more extreme variations in amplitude along the cable length the less dense the cable is. The bending stiffness of the cable has a similar impact, where the greater the stiffness the lower the variation in amplitude over cable length. Regarding diameter, the greater the diameter the lower the predicted frequency and less range in amplitude. Waves are known to influence the VIV behaviour of structural bodies. In offshore sites with high wind speeds, it is likely that the dynamic power cables will be exposed to waves of significant magnitude. The impact of the wave height, period, and direction are studied to assess the importance of this when considering the VIV response for dynamic cables. The wave height was shown to result in large stresses on the cable as it increases. The direction of the waves are also shown to alter the VIV behaviour, this was elevated when compared against current direction as well. Wave height and direction were found to greatly influence the RMS stress the cable was exposed to. A wave height of 8m at angle 0 resulted in stress over three times larger at certain locations compared to just current. For angle 90 a 50% increase could be seen. For angle 180, the mean stress over the full cable length was found to show minimal change. The influence of all the parameters discussed shows how crucial further investigation and analysis is to fully understand and predict the VIV influence before wide scale deployment.