An integrated geological characterization of marine ground conditions for offshore wind foundations in the North Sea
The first part of the thesis deals with the sedimentary, geophysical and geotechnical characteristics of the Late Quaternary deposits at the Utsira Nord floating offshore wind site located within the deep Norwegian Channel ice stream trough. These interpretations are based on seabed and subsurface geomorphologic features studied using high resolution bathymetric data, legacy 2D seismic data, sub-bottom profiles and shallow sediment core data prior to the cruise. The findings are summarized as a conceptual geological model for ground conditions in former ice stream settings with four main units with contrasting geotechnical properties. This model was tested on the cruise and was found to be relatively accurate, with additional till wedges discovered within the main upper till unit in the southeastern part of the site.
A detailed regional study of the evolution of the Late Quaternary depositional environments that were present in the Dogger Bank-Ling Bank-Jutland Bank and Elbe Palaeovalley areas of the southern North Sea is then presented. This is accompanied by a preliminary ground model for the highly heterogeneous Sørlige Nordsjø II bottom-fixed offshore wind site, located on the Ling Bank. This model is based on the integration of geomorphic interpretations from legacy 3D seismic data with seismic facies variations identified on the high-resolution 2D sub-bottom profiler dataset and sedimentary facies variations observed in shallow sediment cores, collected in 2022.
The two sites can be considered end-members of the ground condition types that exist within the Norwegian sector of the North Sea, with 1) deep, clay-rich conditions in the Norwegian Channel, characterized by a semi-layer cake stratigraphy of subglacial tills and glacimarine to marine, fine-grained sediments and 2) the shallower, highly laterally heterogeneous, broadly sand-rich conditions on the North Sea Plateau, characterized by a wide range of depositional facies, from subglacial till to glacilacustrine and glacifluvial deposits, post-glacial fluvial and tidal channels, and shallow marine systems. In Articles 1 and 2, the applicability of the different available offshore wind foundation and anchor concepts to these end-member ground condition zones is investigated, with the aim of producing case studies for the North Sea that combine a detailed geological understanding of the North Sea ground conditions with the engineering implications for the different types of formerly glaciated marine areas. In general, it can be said that the Norwegian Channel is an area suitable for floating offshore wind turbines with suction anchors, because of its clay-rich stratigraphy, though it does contain features that pose potential issues to such infrastructure, such as pockmarks, ice-rafted debris deposits, boulders, stiff glacial diamictons and occurrences of shallow crystalline bedrock that must be assessed and mitigated for on a site-by-site basis. In contrast, the shallower, highly heterogeneous North Sea Plateau areas are best suited to the most common offshore wind foundation type, the monopile, in water depths less than c. 50 m, and multi-legged, supported structures (jackets) in water depths greater than c. 50 m. Key factors influencing foundation design variations on the North Sea Plateau are the distribution and thickness of the different depositional facies relating to the particular subglacial, proglacial and post-glacial processes that have impacted a particular site or turbine location, for example the presence of stiff glacilacustrine lake infills or weak, post-glacial channel infills.
- University of Bergen