How to read the seafloor and the importance to offshore wind projects. Examples from Sørlige Nordsjø II, Norwegian North Sea

Offshore wind projects benefit immensely from a good understanding of the seafloor. Together with the sub-bottom geology and geotechnical properties as well as geohazards and physical hazards, the seafloor conditions can be integrated in ground models and hazard maps as part of a holistic offshore wind site characterization, that enables successful wind farm development and reliable power production.

A full understanding of the seafloor requires “seafloor reading skills”: appropriate data mining, utilization, and interpretation. The seafloor is full of geological, environmental, and anthropogenic features which may affect a wind farm during its lifecycle, such as boulders, mobile sediment, escarpments/faults, wrecks, UXO, cables, seabed habitats and much more. However, when understood properly, we can comprehend the seabed conditions and processes and handle the different challenges at the seabed from installation to production, affecting e.g., foundation feasibility and HSE.

Sørlige Nordsjø II is an envisaged offshore wind site in the southern Norwegian North Sea. Following a multi-method approach, we utilized different geological and geophysical data, such as MBES bathymetry and backscatter, side scan sonar data, boreholes and vibrocores, sub-bottom profiler data, and 2DUHR seismic.

Key results that were achieved include:

• Detailed seafloor lithology map: Differentiation between different sand facies and glacial lag deposits.
• Understanding of sediment mobility: Sand waves and megaripples mapped; bedforms, grain size variations and anthropogenic features linked to zones of erosion, transport, and deposition.
• Determination of boulder locations: Boulder fields and individual boulders mapped; size of boulders determined to mostly up to 3 m.
• Update of cable and wreck positions, by utilizing backscatter and side scan sonar images, and magnetic anomalies.

We were able to pull out deep knowledge from the available data, to bring it in a coherent order and provide a holistic understanding of the site’s seafloor. This is a major step towards the aim of making informed cost-saving decisions throughout the offshore wind lifecycle.