Coupled Cross-Flow and In-Line VIV Analysis of a Lazy Wave Dynamic Power Cable
Single Wake Oscillator and OrcaFlex Approach with Experimental Validation
Maria Christina Diamantopoulou (TU Delft - Mechanical Engineering)
J.O. (Oriol) Colomes Gene – Mentor (TU Delft - Civil Engineering & Geosciences)
A. Metrikine – Graduation committee member (TU Delft - Civil Engineering & Geosciences)
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Abstract
Floating offshore wind deployment requires dynamic power cables, as part of the system that transmits the generated electricity from the floating platform to the onshore grid. These cables are loaded continuously by waves and current and follow the platform motions, so they experience cyclic loading throughout their design life. Vortex-induced vibration adds a further dynamic contribution, which makes fatigue one of the governing limit states in their design. The industry prediction tools, however, are based mostly on rigid or straight cylinders, and field data, design standards and operational experience remain scarce, so their design relies on large safety margins.
To address this gap, a coupled cross-flow and in-line wake oscillator model was implemented to assess whether it predicts the response of a lazy wave power cable more accurately than the single-degree-of-freedom models common in industry. The model of Qu and Metrikine (2021) was adopted, in which a single wake oscillator drives both directions, forced by the cross-flow acceleration and by the in-line motion through a parametric term, so that the lift and the fluctuating drag originate from the same wake. In-line amplitudes are typically smaller than cross-flow ones, but for long flexible cylinders neglecting the in-line motion under-predicts the fatigue damage, which motivates the coupled formulation.
The model was implemented in a nonlinear finite element solver in Python and as an external function in OrcaFlex, the latter placing it within an industrial framework. The two implementations were verified against each other and validated against a tensioned vertical riser, where they reproduce the measured cross-flow and in-line response and recover the in-line motion that cross-flow-only models cannot capture. The model was then applied to an experimental lazy wave power cable in OrcaFlex and compared with the industry models in the same framework.
The coupled model reproduces the measured frequency content and the main responding modes, and recovers the shared in-line and cross-flow frequency that arises from the curved geometry. It over-predicts the in-line content at twice the cross-flow frequency, which appears only weakly in the experiment. Its Strouhal number can be set for each case, whereas the industry models are fixed at a single value above the measured one. The cross-flow amplitude is over-predicted by about a factor of two, mainly as a result of the selected tuning parameters, an over-prediction shared by the industry models. Overall, the coupled model predicts the response more accurately than the industry models, while the amplitude remains conservative.