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Abstract
The continued expansion of offshore wind energy to deeper waters require extended knowledge on wave loads in these wave environments, including wave load imposed by steep and breaking waves. While many studies have investigated the total force on vertical, circular cylinders, only few have investigated the local force and the force distribution. The following thesis explores the force distribution on a vertical, circular cylinder exposed to steep and breaking waves. An experimental setup was designed which enabled the measurement of local wave loads on a cylinder. In the analysis, special care was given the local force near the free surface. Furthermore, measurements of wave loads were accompanied by measurements of wave kinematics with Particle Image Velocimetry (PIV). The experimental setup was applied to two different wave configurations: 1) steep, regular waves and 2) a spilling breaker. Local force coefficients were established from the measured kinematics and forces. A good correspondence with global force coefficients reported in the literature was seen, for both the steep, regular waves and the spilling breaker. For the steep, regular waves, the local force harmonics were examined. It was found that near the free surface, the local force was dominated by higher harmonics. In fact, in shallower water (kℎ ≤ 0.65), the second force harmonic had the same magnitude as the first harmonic. Further, the magnitude of the third force harmonic increased rapidly with wave steepness at the section closest to the free surface at these water depths. Investigations of the local forces also showed that the local maximum force occurred later at the section exposed to the free surface. For smaller Ursell numbers (Ur<20), the maximum force occurred 0.2T later near the free surface than at the bottom. For larger Ursell numbers, the time difference was smaller, as the maximum force at the vicinity of the free surface occurred 0.06T later than near the bed. For the spilling breaker, the same analysis showed that maximum force occurred 0.08T later near the free surface than near the bed. This suggests that assuming a simultaneous maximum force is a good approximation for highly nonlinear waves, while the approximation does not hold for waves with smaller Ursell numbers. For the spilling breaker, the influence of the breaking point relative to the cylinder was investigated. The largest total force was found when the wave broke immediately in front of the cylinder. Likewise, the largest local force was also found at this breaking location and was near the free surface. The influence of air entrainment was also explored. The breaking wave carried a roller, which was studied in detail by designated data treatment. For the local force imposed by the roller, a simple force model was tested. In a separate, idealized setup, the influence of air fraction on wave loads was investigated, by measuring the loads on a cylinder oscillating in a water tank with air bubbles. In the idealized setup, it was found that the inertia coefficient could be corrected by accounting for the effective density of the waterair mix. The application of this result to the roller of the spilling breaker showed promising first results.
Original language  English 

Place of Publication  Kgs. Lyngby 

Publisher  Technical University of Denmark 
Number of pages  178 
Publication status  Published  2019 
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 1 Finished

Extreme wave interaction and wave loads on offshore wind turbine structures
Vested, M. L. H., Christensen, E. D., Carstensen, S., Dixen, M., Madsen, P. A., Andersen, T. L. & Kristiansen, T.
01/09/2015 → 12/08/2020
Project: PhD