Extreme Wave Loads on Monopiles: Identification, Reproduction and Detailed Investigation

Research output: Book/ReportPh.D. thesis – Annual report year: 2018Research

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This thesis focuses on the extreme wave loads on monopiles from three different perspectives. Namely, reproduction, identification and local flow investigation of such wave impacts. A state of the art CFD model is used in addition to a fully nonlinear potential flow solver to validate cases compared to experimental measurements. The validation cases include breaking focused waves with and without directional spreading in different depths. Good consistency between the measurements and numerical results is found for the free surface elevation, in-line force and wave-induced pressures. The wave-structure interaction including the secondary load cycle and wave scattering around the structure are well reproduced using the coupled solver. The validation is then extended to the measured wave episodes which are associated with the two largest peak moments of a 3 hour test. It is observed that for small values of the inline force, the CFD results provides good agreement with the measurements. For the larger values of inline force, however, the consistency decreases largely due to different breaking processes in the numerical domain relative to the experiments. In addition, the vastly validated models are then used to investigate the source of the secondary load cycle and to demonstrate the applicability of an analytical slamming wave load model. The well known pressure impulse theory is used to calculate the slamming wave load for an incompressible and inviscid fluid. The geometry of the impacting wave is simplified as a block of water in Cartesian coordinates and as a wedge in cylindrical coordinates. The pressure impulse on a vertical flat plate and on a vertical circular cylinder is next calculated as analytical solutions to the Laplace equation. Parameter studies are performed for each case of slamming wave impact on a flat plate in the 3D domain and on a monopile. The results of the parameter studies clarify the behaviour of the pressure impulse distribution in relation to each parameter including the length of the impacting water block and the diameter of the cylinder. The pressure impulse distribution of a slamming wave on a monopile in the state of the art CFD model is compared to the results from the suggested model and good consistency is observed. To identify the expected extreme wave episodes that creates a target inline force on the monopile the First Order Reliability Method is used in combination with first-and second-order wave theories and Morison type force models. The calculated expected extreme wave episodes are validated against experiments and a good agreement is observed. Such wave episodes can be used in the design process for the Ultimate Limit State cases. The method is extended to incorporate a fully nonlinear potential flow solver (OceanWave3D) for the incident wave kinematics. Significant improvement relative to first-order and second-order results is observed. The average deviation between the model results and the wave averaged measurements is about 10In addition to the applicability of this method to extreme wave episode identification, it has the benefit of flexibility with topography of the bed. Hence, the effect of the bed slope on extreme wave episodes is investigated by combination of FORM and the nonlinear potential flow solver. It is observed that in the low Ursell number cases, force histories are very similar between flat bed and sloped bed. However, in the high Ursell number cases, larger skewness is observed in inline force time histories for flat bed. The exceedance probabilities for the same peak inline force are larger on sloped bed cases. The investigation is the most systematic investigation of the bed slope influence on the extreme wave loads to the author’s knowledge. The source of the Secondary Load Cycle is studied in large extends. The source of this event, which has been a matter of speculation for some time, is important for reduction of uncertainties in load calculation as some researchers claim that it can contribute to ringing of the structure. To conduct the investigation experimental and numerical results were extensively studied. Especially the state of the art CFD model is used to separate each important term in the momentum equation that creates the secondary load cycle. A thorough explanation for the source of the secondary load cycle is given which relates this phenomena to a suction region below the water column created behind the cylinder by diffraction as the outer wave disappears. The suction is created by the need for a sudden downward acceleration of the column. The author considers this part of the thesis the most detailed investigation of secondary load cycle event to date.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages191
Publication statusPublished - 2018
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