Offshore Wind: Large-scale study delivers robust design guidance for grouted connections in pre-piled and lightweight jacket structures

In the dynamic offshore wind industry, the materials used are playing a major role in reducing costs and shortening construction and maintenance times. That is why a group of developers and operators of offshore wind farms – innogy SE, SPR/Iberdrola, SSE, EnBW, Ørsted, Statkraft andORE Catapult – joined forces in 2014 and launched a Joint Industry Partnership as part of theCarbon Trust’s Offshore Wind Accelerators (OWA) to investigate grout connections for offshore wind foundations. The main objective was to develop a comprehensive database of large scale test results to extend and validate design guidelines for grouted connections with geometries relevant for pre-piled jacket structures.

“Seawater mixed neat grout cement connections using Ordinary Portland Cement (OPC) have a long track record in offshore Oil and Gas industry as cost effective, reliable subsea connection type. In the last decade this technology has been transferred to offshore wind – with close to around 100 offshore jackets installed of this type so far. We have particularly emphasised the verification of the use of seawater mixed OPC as it significantly reduces the environmental footprint for these structures. In contrast high strength grouts require freshwater, which offshore adds significant to the cost”, explains Daniel Bartminn, Project Director GOAL (abbreviation for ‘Grouted OPC Connections with Annuli of Large Dimension’).

OPC grouted connections mixed with seawater technology has been used for existing offshore wind farms, for example Thornton Bank (Belgium), Ormonde (United Kingdom) and Alpha Ventus (Germany).

“Particular offshore wind jacket structures are requiring project specific certification, where applications exceeded the limits of existing standards. So there is an urgent need to fill the gap in the underlying test data and designs to establish and verify the use of OPC for large annuli offshore wind applications, because it brings clear cost advantages compared to the use of high strength grouts or grout mortar mixes. Especially because OPC is easier to install, with contractors having decades of experience with this material. Additional savings could arise from de-risking and avoidance of repair works, as well as ease of certification”, Daniel Bartminn adds.

The tests, carried out at KIT Karlsruhe, are the largest consistent tests series for large annuli in full and half scale adding extensive knowledge to further improve existing ISO guidelines. This comprehensive dataset will be analyzed in the upcoming month and summarized in a comprehensive study report, written by Arup and anticipated to be published during Q2 2018.

The research project is managed by innogy SE and was funded by the Carbon Trust Offshore Wind Accelerator, a world-leading collaborative research and development programme focused on reducing the costs of offshore wind energy.

Arup had the role as designer and lead consultant and were responsible for the design of the test programme and scientific evaluation, coordinating also the input from technical experts of partners and certification bodies (DNVGL, Lloyds Register, Prof Dr-Ing habil. Schmidt Prof Dr-Ing Fehling and Dr Colin Billington).