Golven rond een monopile voor maritieme operaties

Among renewable energy sources, offshore wind energy has shown a promising contribution to the clean energy transition, an attempt to prevent catastrophic climate change. The offshore wind energy is converted into electrical energy by offshore wind turbines, held by offshore wind structures. There are fixed and floating offshore wind structures, where the fixed structures currently become the most-built structures in the industry. Among many types of fixed structures, monopiles are the most preferred structures due to their simplicity, thus making them relatively cheaper than other fixed structure types. Monopiles were initially designed for shallow water depth, but currently have also been built in intermediate water depth and holding larger wind turbine capacities. Therefore, the diameters of the present monopiles have inevitably become larger than the initial offshore wind monopiles. Large monopiles are expected to modify the ocean waves more significantly than small monopiles. Therefore, the marine operability analysis around a large monopile cannot exclude the monopile. This thesis investigates the impact of the monopile on marine operations by observing the wave elevation around a monopile, defined as the Wave Field Around a Monopile (WFAM). Since the investigations are in the context of marine operations, the study is limited to moderate wave conditions. In addition, crew transfer operations are taken as a study case in this thesis to assess the importance of the monopile for the marine operability analysis. The WFAM is investigated through linear analytical solutions, numerical simulations, and physical experiments. For the analytical solutions, the linear theory is used, applying inviscid, irrotational, and incompressible flow assumptions (potential flow) along with the free surface linearization. For the numerical simulations, a high-fidelity numerical tool, OpenFOAM, is employed, solving incompressible Navier-Stokes equations. In this thesis, physical experiments were performed in the Coastal \& Ocean Basin in Ostend, Belgium. The physical experiments provide the WFAM dataset for regular, bichromatic, long-crested irregular, and short-crested irregular WFAMs. The regular and bichromatic WFAM datasets are investigated with the linear analytical solution and high-fidelity numerical simulations. Meanwhile, the short-crested and long-crested WFAM datasets are only discussed with the linear theory WFAM due to the expensive cost of performing the high-fidelity numerical simulations. In addition, linear numerical simulations with a low-fidelity tool (Capytaine) are also exploited to assess the WFAM in complex situations, such as a monopile with an installation vessel nearby. Capytaine is employed to perform the crew transfer operability analysis, investigating the effect of the large monopile on their wave height limitations. To simulate the regular and bichromatic WFAM efficiently with good accuracy, high-fidelity numerical wave tanks are initially investigated. Three wave generation and absorption systems, Relaxation zones, Sommerfeld, and Generalized Generating-Absorbing Boundary Conditions (GABCs), are assessed by their capability to generate uniform regular wave crest and trough envelopes along the numerical wave tank. Relaxation zones give flat crest and trough envelopes along the tank, indicating a proper wave generation and absorption system, but requiring larger numerical costs than the other wave tanks. The Generalized GABCs are seen as the most efficient way to generate and absorb accurate regular waves for a wider frequency range than the Sommerfeld GABCs, needed for bichromatic WFAM simulations. Nevertheless, spatial variations are seen in the Generalized GABCs, being more pronounced in nonlinear regular waves. This thesis explains that these spatial variations are due to the free waves generated from the absorbing side of the numerical wave tanks. A grading technique is proposed in this thesis to reduce the nonlinear spatial variations. Afterwards, the high-fidelity numerical tool is utilized to simulate the regular and bichromatic WFAM datasets. At first, the numerical simulations and the physical experiments were compared to each other, showing generally good agreements in the WFAM time series and crest envelopes around the monopiles. A Fast Fourier Transform (FFT) study is also performed to assess the higher harmonic components for regular and bichromatic waves, being more noticeable in the WFAM than in the wave field without a monopile. In addition, Wave Type-2 collision at the downwave side of the monopile is also captured by the numerical tools, appearing in every zero-crossing of the bichromatic WFAM time series. Wave Type-2 is a wave type where two lumps of water travels around the monopile in the clockwise and anti-clockwise directions, colliding either in the upwave or downwave side of the monopile. In the present bichromatic WFAM study, the largest Wave Type-2 collision also causes the highest crest of the bichromatic WFAM. This thesis also shows that the crest deviations are inherited in the long-crested and short-crested irregular waves. In the long-crested irregular waves, Linear Transfer Functions (LTFs), i.e. a method to compute the WFAM within the linear theory, are seen to work the best in the dominant frequencies of the irregular waves. This is evaluated based on the linear coherence evaluations of the physical experiments. Wave height and crest distributions are also evaluated where the linear theory shows good agreement in the height but crest distributions. A practical correction on the crest is also offered in this thesis where it is based on the ocean wave crest corrections, seen to be limited to estimate the WFAM at certain locations. The linear theory with LTFs is also exploited to evaluate the short-crested WFAM. The long-crested LTFs are extended into directional frequency domain analysis, where it manages to capture the standard deviation of the elevation and the wave height but the crest properties. Free surface mean, skewness, and kurtosis are also evaluated, showing noticeable deviation from the linear theory. After identifying the performance of the linear theory for the WFAM evaluations, Capytaine, a free open-source numerical tool, is used to evaluate the WFAM when an installation vessel is nearby. Capytaine suffered from the irregular frequency which this thesis managed to fix the problem by applying a well-known method of lid addition on the internal free surface of the floating structures, bringing Capytaine into the state-of-the industry. The results are also verified with a commercial numerical solver in this thesis. It is found that including the monopile and the vessel in the hydrodynamic analysis modifies the WFAM, especially in the velocity field. The developed Capytaine is then used to assess the wave height limitation of crew transfer operations, employing the fender-slip boundary conditions. It appears that the monopile inclusion is needed to evaluate the wave height limitations at short peak period waves, especially for short crew transfer vessels. The crew transfer operations are also used to demonstrate the valorization of the present thesis. Despite having only demonstrated the need for monopile inclusion in the crew transfer evaluations, the present study also promotes that the large monopile effect should not be overlooked in the other marine operation analysis. This thesis has offered the comprehensive knowledge of the wave field around a monopile that is needed to assess marine operations near a large monopile.