Simulated wave hydrodynamics and loading on an offshore monopile

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Edesess, Ariel J.
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University College Cork
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Maintenance costs of offshore wind power, where fixed monopile support columns make up the majority of wind turbine types, are up to three times higher than those associated with onshore wind power. High costs are exacerbated by difficulties accessing the turbines in their marine environment. Safe transfer by crew transfer vessel (CTV) requires prediction of vessel motion whilst in contact with the turbine monopile. Future vessel motion prediction first requires analysis through analytical and numerical methods of the local hydrodynamic wave field and wave loading on the monpile turbine in ocean waves. A location-dependent unidirectional sea state is represented by superposition of periodic waves with amplitude components an, obtained from the spectral distribution of free surface displacement data from a single wave buoy located at the Teesside Offshore Wind Farm in the southern North Sea. Wave buoy data was obtained for each season during the 2015/2016 time period, providing a record of seasonal changes that occur in the spectral distribution. Wave loading in the local irregular sea state was predicted using the Morison equation and the linear diffraction formulation. Numerical predictions were obtained using OpenFOAM and a modification of the multiphase interFoam solver for generating free surface waves, where a boundary condition for inputting irregular waves based on the local wave spectra was developed for the purpose of this thesis. For unimodal spectral distributions, which occur in 50% of the data sets with a third data set displaying a small secondary peak, the analytical solutions for the diffracted hydrodynamics and wave loading show satisfactory agreement with the numerical predictions, provided a slip boundary condition is applied on the cylinder. Comparisons were made between analytical solutions and numerical predictions for each of the four data sets, where the irregular wave field was simulated first in a numerical wave tank and then interacting with a fixed cylinder representative of a monopile wind turbine. Simulations were run using both a slip and non-slip cylinder wall boundary conditions in order to determine the effects of viscosity. OpenFOAM can potentially provide better predictions of the diffracted water particle kinematics resulting from the interaction between the sea state at Teesside Offshore Wind Farm and the turbine monopiles, but with a significantly increased computational overhead. The analytical solutions provide satisfactory and relatively fast solutions, although at the expense of neglecting higher-order terms. Both methods presented in this thesis provide practitioners with enhanced knowledge of the seasonspecific local hydrodynamics and wave loading based on actual sea state data, rather than relying on a parametric location-specific representation. Enhanced knowledge of the hydrodynamic field affecting vessel motion will give a better prediction of vessel motion under operating conditions, and eventual determination of the limiting conditions under which the vessel will remain steady.
O&M , OpenFOAM , Offshore wind , Applied mathematics , Wave loading , Hydrodynamic
Edesess, A. J. 2018. Simulated wave hydrodynamics and loading on an offshore monopile. PhD Thesis, University College Cork.
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