Case study: Sampling techniques and collaboration advanced offshore wind farm subsea investigations
By Dennis Van Den Bulk, Geoquip Marine Group
The offshore energy sector is under constant pressure to optimize efficiency, reduce costs and maintain safety, while conducting complex subsea site investigations.
In the German Baltic Sea, Geoquip Marine's recent geotechnical study for Skyborn Renewables on the Gennaker offshore wind farm is an example of how technology, proactive planning and collaboration can streamline offshore projects and achieve remarkable results.
The challenge
The Gennaker offshore wind project required extensive subsea data collection to inform the design and installation of the foundations for 63 offshore wind turbines. The challenge was conducting site investigations across 58 cone penetration testing (CPT) locations and five sampling sites, reaching depths of up to 60 m below sea floor.
CPT is a technique used to determine the geotechnical properties of soils, which involves pushing a cone-shaped instrument into the seabed to measure resistance and collect data on soil behavior. This method provides critical insights necessary for designing and installing offshore structures.
For this project, the presence of glacial till and chalk units posed further complexity, requiring precise extraction techniques to minimize soil sample disturbance and maximize recovery.
The approach
To overcome these challenges, Geoquip deployed Geoquip Seehorn, equipped with specialized sampling methods tailored to the seabed conditions of the wind project. The investigation employed both piggyback coring (PBC) and American Petroleum Institute (API) techniques, which are methods designed to capture high-quality samples in difficult seabed conditions.
PBC, while 7% more expensive per day than API techniques, achieved a higher recovery rate (1.06 m versus 0.51 m) by reducing chalk disturbance and remoulding. This strategic choice not only improved data quality but significantly reduced the overall project costs by avoiding additional rework of the site.
To enhance CPT performance in tough chalk conditions, the team switched to cones with double sleeve capacity that could handle pressures up to 3 MPa, replacing cones limited to 1.5 MPa. This adjustment allowed for smoother and more reliable testing.
The results
Fieldwork commenced in February 2024 and concluded in June 2024, seven days ahead of schedule. On average, CPT locations were completed in 30 hours, outperforming the tendered 33.5-hour estimate. Across 1,797 tests, only four cones were permanently damaged, demonstrating the robustness of the equipment and methodologies employed.
Emphasis on safety and adaptability allowed operations to continue within 6 m of magnetic anomalies (e.g., potential unexploded ordnance [UXO] are remnants of military munitions that can pose significant risks to offshore operations). Geoquip utilized detailed seabed assessments to mitigate risk, highlighting the importance of risk management in offshore geotechnical investigations.
Furthermore, shallow-water operations at 13.5 m depth were successfully executed using Geoquip’s Seehorn vessel and by aligning activities with favorable weather windows. These measures circumvented the need for additional costly equipment, reinforcing the ability to balance operational risk with financial prudence.
Collaboration and supply chain integration
Project owner Skyborn Renewables offered support by engaging in proactive collaboration and providing clear communication and logistical assistance throughout the project, which facilitated the project's smooth execution. Geoquip Marine and Skyborn Renewables committed to shared objectives that ensured timely decision-making and efficient coordination, enabling Geoquip Marine to deliver high-quality data and results.
The project’s success was also bolstered by strong supplier relationships. Robertson GEO provided P-S logging equipment and operators. Subcontractors WRS and BSM contributed consistent trained personnel with BSM also offering marine expertise, ensuring seamless field operations.
Onshore, Geoquip worked closely with the Skyborn Renewable’s geotechnical expert Cathie for an 808-test campaign, covering advanced strength, dynamic and rock testing. By aligning on testing schedules and parameters in advance, the team avoided delays from test cancellations caused by gravel in glacial till samples. Backup samples were pre-emptively dispatched, ensuring continuous progress and minimizing transport emissions.
Conclusion
The Gennaker offshore wind site investigation for Skyborn Renewables demonstrated how integrating innovative coring methods, managing UXO risks, and expediting field and laboratory work can drive efficiency and cost savings across offshore wind projects.
Geoquip conducted more than 1,797 tests and submitted the final geotechnical data report 47 days ahead of contractual deadlines, allowing the project owner to meet crucial design deadlines.
