Scour Forecasting for Offshore Wind Parks

In an effort to minimize the costs of offshore wind parks, the present research deals with optimizing a certain aspect of the support structure, namely the approach to scour. Scour is the phenomenon of seabed changes in the vicinity of the support structure that arises when the support structure disturbs the local flow sufficiently much. Scour is particularly evasive because in case of current, the flow disturbance can be intense and dig a hole comparable to the horizontal extent of the support structure. This usually implies a considerable loss of stiffness, ultimate strength or lifetime of the support and super structure. In case of waves, however, the flow disturbance can be much weaker and even backfill the hole with soil. The ability to accurately forecast this development of the geometry of the scour hole becomes central for obtaining both a safe and cost-effective solution. In practice, scour forecasts facilitate the comparison between a scour design based on either deployment of scour-protection or enhanced structural design. The broad goal is to develop a method that produces accurate scour forecasts for offshore wind parks. The present research investigates more specifically which parameters are suitable for characterizing the scour geometry during both scouring and backfilling and how the parameters develop in time for a given sea state. The present research is restricted to treat a monopile in sand since this is a common and potentially cost-saving case. The research leaves behind two legacies that deal with these two research questions in dialectical ways. The first legacy is a framework for the scour geometry based on epistemological considerations, theoretical concepts and model scale experiments. Relevant parameters are reviewed, defined and discussed. The combined use of the depth and the volume of the scour hole is recommended for characterizing the scour geometry and a scour shape factor is introduced to couple them together. Simple ordinary differential equations are reviewed and compared to the experiments and one set of equations involving the scour volume and the scour shape factor are found to fit excellent with the experiments. The present findings entail some degree of departure from the traditional focus in scouring research that only considers the depth of the hole. The second legacy is the reverse approach. This is a theoretical and numerical approach under development that relates the scour geometry development to the flow intensity. The aim of introducing the approach is to provide a supplementary way for estimating the parameter values for the mentioned equations. The reverse approach is detailed, discussed, compared to other studies and found to be in need of some improvement. The two legacies rely on a too limited basis to reach the broad goal of developing accurate scour forecasts. However, both legacies are thought to be novel and valuable contributions to the research community towards reaching it.