Wave-current-induced scouring processes around complex offshore structures

Abstract

The expansion of the offshore wind energy sector is important for the fulfilment of renewable energy targets. An accelerated growth of the worldwide offshore wind capacity will be required to meet the target of a climate neutral economy. In search of available space and higher load factors in the contested coastal areas, offshore wind parks are more often planned and installed in deeper waters or have to sustain intensified hydrodynamic loads. In consequence, complex offshore foundation structures such as jacket-type foundations are adopted, as they have an increased structural stability. However, the literature review of the present thesis reveals large knowledge gaps regarding the prediction of scouring processes around complex offshore foundations. These knowledge gaps are highlighted by the intensified demand of the expansion of offshore wind energy and show the requirement of an improved prediction and design of scour and scour protection systems. A reliable prediction of scouring processes is an important aspect of sustainable and economically optimised foundation designs. An improved understanding of morphological processes around offshore foundations is also important to evaluate and predict the man-made impact of offshore foundations on the previously unaffected marine environment. This thesis aims at contributing to the understanding of morphodynamical processes around offshore structures, with a focus on complex offshore foundations. This is accomplished by addressing different research questions related to: (I) the scouring processes induced by a realistic representation of hydrodynamic conditions around monopiles and jacket-type structures; (II) the spatial scour depth changes and deposition patterns around offshore foundation structures; (III) the damage assessment and stability of scour protections. A series of novel hydraulic model tests were carried out to systematically work on different aspects of these topics. (I) In order to further improve the understanding and prediction of scouring under realistic hydraulic conditions, the influence of the hydrograph shape on tidal current induced scour as well as scouring induced by multidirectional waves and currents is investigated around a monopile. Further investigations are conducted regarding the scour development, scouring rate and final scour depth in combined waves and current conditions at a jacket-type foundation structure. (II) In the next step, an approach is developed to quantify and assess spatial changes of the seabed around marine structures. The systematic application of this approach allows the derivation of empirical formulations to predict complex erosion or deposition patterns. This is demonstrated by the investigation of global scour patterns in combined waves and current conditions around jacket-type foundations. In a further study, geostatistical methods are developed for the introduced methodology to further improve the spatial prediction and quantification of complex sediment displacement patterns. (III) In the final step, the characterisation and assessment of damage patterns of scour protection layers is improved by the derivation of a new approach to quantify damage patterns. Large scale experiments under combined wave-current conditions are conducted in this context for uniformly and widely graded scour protection material in a joint group.