Hannah Petrie

An integrated geological characterization of marine ground conditions for offshore wind foundations in the North Sea

The geological characterization of marine ground conditions for wind turbine foundations and anchors at large offshore wind sites is a relatively new field. As such, the establishment of best practices for the integration of geological, geophysical and geotechnical (G, G & G) data is a work in progress. While existing methods give a broadly sufficient basis for the design of offshore wind turbines and their foundations or anchors, current workflows do not give enough weight to understanding how depositional and post-depositional processes have influenced mapped geophysical units and their measured geotechnical properties. This limits experience transfer from developed sites to new ones and the degree of predictability of certain geotechnical conditions.Together with data acquisition costs, foundation fabrication and installation can constitute up to a third of the overall cost of an offshore wind project. Thus, a better understanding of how geological factors influence ground conditions at offshore wind sites could contribute to significant cost reductions and help countries achieve their renewable energy goals. This is particularly true in offshore wind areas with complex geology, such as formerly glaciated terrains like the North Sea. In this thesis, an integrated geological characterization of the marine ground conditions within the North Sea is presented, based on legacy conventional 2D and 3D seismic data, sub-bottom profiles, borehole reports and literature, in addition to an extensive marine geological dataset acquired at two contrasting offshore wind sites in the Norwegian North Sea during a ten-day cruise with the research vessel G.O. Sars in 2022.

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.

In Article 3, a broader look is taken at the ground conditions within the North Sea and other previously glaciated marine areas, the ways in which the Late Quaternary glaciations impacted the geotechnical conditions of the soils and the resulting engineering considerations that are relevant today. Through a range of case studies, we highlight how site characterization methods are and should be evolving as the importance of an integrated approach becomes increasingly apparent through the publication of geological studies from complex wind sites.
Overall, it was found that in previously glaciated terranes, depositional setting and geotechnical conditions are intrinsically linked. It is becoming increasingly apparent to a range of academic and industry players that giving more weight to geological data could enhance the predictability of the geotechnical heterogeneities that inform cost-effective foundation and anchor design processes. However, work practices and guidelines for offshore wind site characterization still have a way to go to become reflective of this.