Assessing Subsurface Hazards for Offshore Wind Farms in the North Sea

Thesis Abstract

The rapid expansion of offshore wind energy in the North Sea intensifies the demand for reliable subsurface characterisation to safeguard project timelines, reduce financial risk, and ensure structural integrity against geotechnical hazards. Subsurface hazards—including shallow gas pockets, seepage pathways, buried channels, sandwave dynamics, and fault-related deformations—pose significant uncertainty for installation, operation, and long-term reliability of wind turbine monopiles and jacket foundations. This study aims to quantify, map, and model subsurface hazards to support risk-informed siting, design, and mitigation strategies for offshore wind developments in the North Sea. The specific objectives are (1) to characterise subsurface stratigraphy and geotechnical properties across representative North Sea sites using integrated borehole logs, cone penetration tests (CPTu), and high-resolution borehole imaging; (2) to identify and spatially delineate hazard indicators (gas emissivity, shallow gas pockets, seabed liquefaction potential, and seabed subsidence risks) through seismic attribute analysis and controlled-source electromagnetic surveys; (3) to develop a probabilistic hazard model linking geophysical indicators to foundation performance metrics under design load cases; (4) to assess the mitigative efficacy of alternative foundation designs (monopile vs. jacket) and, where appropriate, scour protection and anchor strategies under identified hazard scenarios; and (5) to provide decision-support guidelines for site selection and risk management. The research adopts a mixed-methods design, combining quantitative geotechnical data synthesis with probabilistic risk modelling and qualitative stakeholder input. The population encompasses offshore sites within the North Sea that currently host or are planned for offshore wind developments. A stratified random sample of 15 sites is selected to cover a range of sedimentary environments (clay, silt, sand, and sand-dominated processes) and geomorphological features (sandwaves, ridges, and subsurface fault zones). Data collection integrates existing site investigation reports (n = 150 boreholes and CPTu datasets), newly acquired seismic-reflection and multi-channel seismic (MCS) data, high-resolution sub-bottom profiler outputs, and controlled-source electromagnetic surveys. Instrumentation includes boreholes for sediment sampling and in-situ testing, CPTu measurements, seismic attribute extraction using amplitude-versus-offset (AVO) and spectral decomposition, and electromagnetic responses for sand-to-clay contrasts. Data validation involves cross-verification of stratigraphic interpretations with core samples (n ? 200), grain-size distribution analyses, Atterberg limits, and standard geotechnical indices. Analytical methods include (i) Bayesian hierarchical modelling to quantify uncertainty in subsurface hazard probabilities across sites; (ii) logistic regression and random-forest classification to relate seismic and EM indicators to the presence of shallow gas, pore-fluid pressures, and liquefaction susceptibility; (iii) finite-element modelling (Abaqus) to simulate foundation response under predominant North Sea storm loading and varying pore pressure scenarios; (iv) time-series analysis of seabed subsidence indicators using historical bathymetric data (1990–2024) and satellite-derived surface deformation where applicable; and (v) scenario analysis comparing monopile and jacket foundation performance under hazard realizations. The theoretical framework integrates geotechnical hazard theory with risk governance, drawing on the Theory of Planned Behavior to capture stakeholder considerations in risk mitigation planning. Expected findings include (a) a spatially explicit hazard map highlighting high-risk footprints for shallow gas, overpressure, and seabed instability that correspond with specific stratigraphic units; (b) quantified hazard probabilities with credible intervals guiding design safety factors; (c) performance envelopes for monopile and jacket configurations under representative hazard scenarios, indicating key design thresholds and mitigation benefits of scour protection and foundation preloading; (d) robust decision-support criteria for site prioritization, including cost-risk tradeoffs. The study contributes to knowledge by integrating multi-disciplinary geophysical datasets into a probabilistic hazard framework tailored to offshore wind contexts, advancing site assessment methodologies, and informing policy and industry standards for geotechnical risk management in the North Sea. The main conclusion anticipates that integrating high-resolution geophysical characterization with probabilistic hazard modelling substantially narrows uncertainty in subsurface hazards, enabling more resilient foundation design and optimized site selection. Recommendations include adopting the hazard-informed design approach for future projects, expanding data-sharing platforms for offshore geotechnical information, and ensuring iterative monitoring during project execution to recalibrate hazard probabilities with new evidence.