Comparative Analysis of Scour in Riprap-Protected Monopiles and Hybrid Foundations

Abstract

As the demand for new sustainable solutions for harvesting energy increases, the offshore energy sector focuses on optimising well-established state-of-the-art solutions while striving for new innovative approaches. Hybrid foundation designs have introduced new challenges and raised questions regarding scour and effective countermeasures. To further improve the knowledge regarding scour prediction, this paper presents and analyses the results from an experimental study behaviour of a riprap protection system for a monopile that determines and characterises scour on a flexible arrangement of overlapping sub-areas. The study was complemented by a novel series of tests using a hybrid foundation, where an oscillating surge wave energy converter (OSWEC) type was coupled to the monopile. Despite being submitted to similar hydrodynamic conditions, distinct differences in the scour rate and damage number (S3D) were observed for both models. Although the OSWEC type contributed to local wave height attenuation (up to a 30% reduction on the leeward side of the hybrid monopile), its oscillatory motion severely aggravated scour, with measured damage rates two to three times higher than those observed in isolated monopiles. These results raise relevant questions about the applicability of existing design formulas for scour protection and underscore the necessity for revised criteria tailored to hybrid offshore foundations.

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