Hydrodynamic and geotechnical controls of scour around offshore monopiles

Abstract

Marine monopiles can suffer from removal of sediment around their foundations by waves and currents, a process termed scour, which can negatively affect structure stability and integrity of associated infrastructure. Scour is a function of the interaction of local hydrodynamics with the geotechnical properties of the seabed, the feedbacks of which are not well understood. Using the largest prototype scour data base available to date, assembled from field data routinely collected during the consents and design phase of wind farms, this study aims to offer a detailed characterisation of marine monopile scour and conduct critical testing of the current, experimentally-derived, state-of-the-art knowledge and practices. Scour research has been hampered by a dearth of prototype scour observations and much of the existing knowledge is derived from physical and numerical work which has had very little validation with field data. This study addresses the dearth of prototype scour analysis and by adding observations from 281 monopiles more than doubles the size of the currently existing knowledge base on marine monopile scour. Furthermore, the scope, variety and quality of data available in this study have enabled a wider-ranging and more in-depth and problem-focussed analysis of scour to be conducted. The data used in this study comes from a "natural offshore laboratory", consisting of three offshore wind farms in the Outer Thames Estuary, which were strategically chosen to minimise the flow variability within the data set in order to focus on identifying and quantifying the controls on scour exerted by the sea bed substrate. The effect of geotechnical conditions on scour is, so far, little understood as most scour research has focused on unconsolidated sandy sediments. Nevertheless, scour experiments in cohesive substrates have revealed the great complexity of the scour response in such materials. For this reason, quantitatively scrutinizing prototype scour in various substrate types and attempting to establish causal links between geotechnical properties and scour development from real data is important. In order to fulfill the remit of this study, the research is guided by a set of questions, derived from a review of the current scour framework, which pose testable hypotheses and identify knowledge gaps which will be evaluated throughout the course of the analysis. The outcomes of the study include an extensive quantitative description and contextualisation of observed scour with existing prototype observations, a critical validation of current knowledge and methods and an investigation of hydrodynamic and geotechnical controls on scour. Some key findings include improved predictive models for scour depth based on mean water depth as well as secondary relationships for lateral extent and scoured volume. For scour in consolidated and cohesive materials, equations for the estimation of scour-limiting material strength as a function of erosion depth are also proposed. The study concludes with a discussion of temporal, survey resolution and geotechnical issues and recommendations for optimised field data collection and survey strategy, alongside suggestions for additional research to fully resolve some of the findings of this research.

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ORCID for J.K. Dix: orcid.org/0000-0003-2905-5403

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