At 7 megawatts (MW), Samsung Heavy Industries’ S7.0-171 is the world’s largest offshore wind turbine. The 196-metre tall structure is being installed 20 metres offshore in Fife, Scotland, with a connecting walkway to enable visitors to get up close to the structure.
FoundOcean has been contracted to secure the foundations for the turbine. Beaton Sutherland, Project Manager, explains how this project differs from the other offshore projects the company has worked on. “There are two main differences: Our equipment and personnel are based onshore at the dock side, with the grout lines running along a temporary platform to the piles, rather than being on a vessel. Secondly, this foundation has been designed to use pilot caissons around the piles. Everything else remains the same.”
And by the same token, Sutherland is referring to the factors that can influence how smoothly the project runs.
“The weather still affects all the contractors on site – if the wind is too high then the crane which moves hoses, people, and steel around the site cannot operate.”
The Samsung turbine is being housed on a four-legged steel jacket foundation. A number of characteristics are taken into consideration when designers recommend which foundation type is most appropriate, including turbine size, water depth, and soil properties.
In this case, although the water depth is negligible, the seabed is rocky and the turbine is very large – the nacelle alone weighs 500 tonnes.
A technique called rock socket grouting will be used for the project, ensuring a solid connection for the piles in the rock bed. When piles cannot be hammered, a socket is drilled in the seabed, resulting in a hole slightly larger than the pile that will be placed. This is to allow for fabrication tolerances and verticality of the socket. It also reduces noise, a key factor when so close to shore.
The installation of the jacket itself will happen when the substructure arrives onsite this autumn. Each of its four legs has a stab-in connection that sits inside the pile, a tried-and-tested design that has been used in other wind farms including Ormonde and Thornton Bank. Once the jacket is craned into position, grout will be injected into the stab-in leg connections and fill the pile annuli.
The walkway from the shore to the turbine foundation will be secured using the same procedure.
Innovation for Cost Saving
With any developing industry comes innovation, whether it is product-, service- or supply chain-related. The same can certainly be said for grouting operations for the offshore renewables sector, with the introduction of new materials, faster mixing equipment, and safer procedures.
The ultra-high strength (UHS) materials used in offshore wind, especially for monopile foundations, do not easily lend themselves to the high delivery rates of ordinary Portland cement (OPC) used in the majority of grouted connections, seen mostly in the oil & gas sector. This is due to the two different methods of mixing grout: batch-mixing for UHS materials and continuous, and silo-fed mixing for OPC-based materials.
Damien Murphy, Technical Director at FoundOcean, explained how the cost savings are achieved. “We got rid of the inefficiency of the one-step-at-a-time batch mixing grouting process. The new configuration means that we are able to perform simultaneously the three key functions of grouting: loading material, mixing it, and pumping grout to the structure.” He continues, “This means we can produce larger volumes of grout at a time and annuli can be filled quicker. Our clients get the benefit of much faster UHS grouting times, which ultimately leads to a reduction on the number of days offshore.”
Ultra-high strength material’s properties lend themselves well to the project requirements, including:
- Low heat of hydration, thus eliminating the risk of thermal cracking
- Rapid strength build-up even at low temperatures to support increased installation rates
- 28-day compressive strengths of 140 MPa
- Pumpability in temperatures as low as 0ºC, extending the available installation season
Finally, and arguably the most important attribute, autogenous shrinkage is a factor which, when present, is proven to cause cracking in high-strength concrete connections.
The Winds of Change
In the next five years, an estimated 4,000 new fixed structures are scheduled to be installed in the North Sea alone. This is mainly due to the upsurge in offshore wind activity associated with Round 2 and Round 3 projects.
Once consent is granted, all members of the supply chain will need to work together to make the installation process run efficiently: from tier two suppliers all the way to wind farm developers. This is happening to some extent already – but more joined-up project planning and collaboration across companies will be the real source of revolutionary innovation.
FoundOcean is already in talks with its suppliers to develop new high and ultra-high strength materials that are able to be mixed using a recirculating jet mixer-method, potentially quadrupling current grout output rates.
An interesting concept and one that the company would not expand on – yet.
Lead image: offshore wind farm via Shutterstock