CFD simulation of wet-towing operation of semi-submersible platforms
Floating Offshore Wind Turbines
K.H. Chen (TU Delft - Mechanical Engineering)
W. Yu – Mentor (TU Delft - Wind Energy)
David Kristiansen – Mentor (Norwegian University of Science and Technology (NTNU))
Remmelt van der Wal – Mentor (Royal Boskalis Westminster)
Saghy Saeidtehrani – Mentor (Royal Boskalis Westminster)
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Abstract
Floating wind is a proven concept to harvest wind energy in deeper water. Most technology of floating wind is transferred from the oil and gas industry. However, a commercial wind farm can consist of 50 wind turbines, and an efficient transport method is needed. Wet-towing is an efficient transport method, but it introduces fluid-structure interaction and should be carefully investigated. For wet-towing operations, the expected drag load is of interest, so a vessel with the right capacity can be chosen. Vortex shedding can occur since most semi-submersible platforms consist of several cylinders. This can cause vortex-induced motion and cause undesired motions such as sway motion.
While previous studies have examined fluid-structure interaction of semi-submersible platforms, two key gaps remain. First, most are conducted at model scale, where the lower Reynolds number leads to different flow regimes and may not reflect full-scale behavior. Second, many vortex-induced motion studies use single-phase simulations with a double-body assumption, neglecting the free surface and its influence on wake dynamics. These gaps highlight the need for full-scale, multiphase simulations to capture realistic wet-towing hydrodynamics.
This study carries out full-scale multiphase simulations to investigate three key aspects. It examines the influence of towing configuration, specifically heading and draft, on the hydrodynamic performance of the platform. It evaluates the impact of the free surface on vortex-induced motion of semi-submersible platforms. Finally, it compares the accuracy and efficiency of drag prediction and vortex behavior across three modeling approaches: full-scale multiphase, full-scale single-phase, and model-scale multiphase simulations.
The results indicate that 180 degree heading is preferable to 0 degree, as it produces a more stable wake, lower drag, and avoids the constant offset lift forces observed in the 0 degree case. A shallower draft of 10 m is also favored, as both the free end and free surface effects help suppress coherent vortex shedding, reducing the risk of vortex-induced motion and lowering drag. Comparing models, full-scale single-phase simulations exhibit vortex shedding, while multiphase simulations develop a steady wake. This discrepancy arises from the unrealistic symmetry constraint at the free surface in single-phase cases. Finally, while model-scale simulations scaled by Froude’s method can offer a conservative estimate for drag, they lack the fidelity needed to capture accurate vortex behavior.
These findings highlight the importance of using appropriate CFD models when evaluating towing strategies, and provide a foundation for optimizing semi-submersible transport operations in future floating wind projects.