Experimental validation of empirical design of a scour protection around monopiles under combined wave and current loading
Loosveldt, N.; Vannieuwenhuyse, K. (2012). Experimental validation of empirical design of a scour protection around monopiles under combined wave and current loading. MSc Thesis. Universiteit Gent: Gent. xvi, 170 pp.
This master thesis is the first sequel to the PhD thesis of dr. ir. Leen De Vos in which experimental research on dynamic scour protections around monopile foundations was performed. De Vos developed the concept of a damage number which expresses the average height of stones which has dissapeared in the heaviest damaged subsection of a scour protection. Through an extensive parameter research, an empirical dynamic design formula was derived which is able to either determine the required stone size for a scour protection in typical North Sea conditions or to predict the developed damage to a scour protection after a design storm.In the first part of this master thesis, the design formula of De Vos is validated with a new series of scale model tests which extend the test conditions for which the formula was originally derived. It is found that the damage to the scour protection predicted with the formula of De Vos deviates from the measured damage for conditions outside the original test range. However, the predictions are mostly conservative leading to a first conclusion that the design formula can still be used outside the initial range but that it could lead to an oversized scour protection.The second part of this thesis is based on recommendations for future research on scour protections by De Vos. Firstly, the influence of the flow direction relative to the wave propagation is investigated and it is confirmed that the damage to the scour protection increases when waves are opposing the current compared to when they are following the current. Secondly, it is illustrated that the pile diameter does have an influence on the damage to the scour protection. Finally, some long duration tests are performed to investigate the long term behaviour of the scour protection under continued wave and current loading.In light of performing maintenance operations to scour protections, a final part is added to this master thesis in which first steps are taken towards a new concept of a global damage number. When further developed, this concept could lead to a new design formula which would make it possible to estimate the total amount of stones required to repair a scour protection damaged during a design storm.
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