Wave-current-induced scouring processes and protection by widely graded material

Abstract

The expansion of offshore wind energy depicts an important contribution to the fulfilment of renewable energy targets. Seeking for higher load factors, offshore wind energy steadily expands towards greater water depths. In addition, considering the already showing effects of climate change on extreme weather events, intensified hydrodynamic loads acting on offshore structures have to be expected in the future. In order to further improve the scour prediction and consequently optimize the design of foundation structures in the marine environment, a wide range of hydraulic conditions should thus be considered, including directional flow aspects.Furthermore, a flexible and sustainable scour protection system is required that provides resilience against those hydraulic conditions. Due to its cost-efficient production and undemanding installation procedure, a scour protection system made of widely graded broken stone material might depict a suitable alternative to the typical multi-layer setup. However, no systematic research on the behaviour and stability of this material exposed to flow has been carried out so far, and consequently, no design guidelines for a scour protection made of widely graded material exist, leaving its promising potential as scour protection system unused. This thesis aims at contributing to the further expansion of offshore wind energy by improving the design process of scour protection for offshore foundation structures. This is accomplished by carrying out a series of novel laboratory experiments addressing two major elements of scour protection design. At first, experiments on the scouring process around a monopile structure induced by complex marine flows were conducted. To advance the understanding of effects of flow directionality on the progression of scour, these experiments included tests with realistically represented tidal currents and multidirectional waves. The findings of these experiments emphasize the importance of selecting suitable reference flow velocities to reliable predict the scour development at offshore structures. Secondly, systematic experiments regarding the fluid-sediment interactions and stability affecting processes of widely graded broken stone material exposed to different flow conditions were carried out. An assessment of the material’s protective performance as scour and bed protection system is given and application-oriented design recommendations are provided for the dimensioning of a widely graded granular scour protection. The design recommendations link available and newly gained knowledge on the erosional behaviour of widely graded materials to present design methods for scour protections around offshore structures.