Non-linear Irregular Wave Impact on Monopile Structures
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- Institutt for marin teknikk 
This Master Thesis deals with non-linear wave impacts on monopile structures by introducing a potential flow solver named OceanWave3D (OCW3D) for simulating free surface waves and their kinematics under engineering consideration. A comparison to Rambøll s analytical tool WAVGEN is conducted with the aim of providing distinct recommendations on the application and suitability of both programs for the monopile design. As WAVGEN applies common wave theories it is able to generate single waves in linear or non-linear form but only linear irregular sea states. In contrary, OCW3D includes both the non-linearity of the wave shape and the randomness of the water surface, resulting in a fully non-linear sea state due to the numerical solution. The obtained wave kinematics from both tools are read by a finite element software which converts water particle velocities into wave loads. In order to reveal differences between both approaches an ultimate limit strength analysis of the foundation is performed, implementing wave kinematics by either WAVGEN or OCW3D. Here, the conventional approach with WAVGEN includes the principle of an embedded stream function wave into a linear irregular sea state to somewhat cover the non-linearity of the wave profile and the arbitrary surface elevation. As a result, the structural analysis yields a maximum overturning moment (OTM) which can be clearly affiliated to the inserted extreme wave represented by the stream function wave. On the other hand, the new approach with OCW3D generates a fully non-linear sea state in a numerical wave tank although without influencing the maximum wave. The already more realistic wave simulation by OCW3D is improved by activating a breaking filter which dissipates the energy of waves which would not exist in reality due to breaking. Multiple realisations give indications whether the non-linear sea state solution produces a single wave that exceeds the embedded stream function wave and the respective structural response. The final results in some cases confirm a greater OTM with OCW3D due to more aggressive and non linear wave kinematics although the wave height of the embedded stream function wave is not surpassed. However, considering the most realistic wave and kinematics after a certain distance along the numerical wave tank when the breaking filter has removed all the excess energy a saving of almost 4.0 % in the structural response with OCW3D is reached. Additionally, the work has provided an unprecedented validation of Rambøll s engineering procedure of defining an embedded stream function, i.e. this commonly used approach delivers representative wave loads compared to actual wave impacts induced by non-linear sea states.