Seagreen - foundation installation for the world's deepest offshore wind farm
Huge buckets, silently sucked into the seabed with the opening down, keep the wind turbines on the Seagreen field off the east coast of Scotland in place on the seabed. Together with FRAMO, NGI had the task of installing the bucket foundations on the 114 jacket substructures for the wind turbines.
Saipem's giant crane barge S7000 was used for the transport and installation of the 114 foundation substructures at the Seagreen wind farm off the coast of Scotland ( SSE Renewables/Illustration)
The deepest jacket substructure was installed at a water depth of 58.6 meters, which is a new world record. The last foundations were installed in mid-April 2023.
The foundations, called "Suction bucket jackets", consist of a steel base with three legs anchored to the seabed with bucket foundations. The foundations are installed in the seabed by pumping out water from the inside after they have penetrated by their own weight. In this way, the upside-down buckets can be penetrated all the way down into the seabed.
Seagreen is a wind farm 27 kilometres east of Scotland with 114 wind turbines, in 2023 Scotland's largest wind farm. Once operational, it will generate 5,000 gigawatt-hours of green energy annually to supply more than 1.6 million households in England and Scotland.
Sketch map showing the area of the Seagreen field off the east coast of Scotland
NGI’s tasks
In the Seagreen project, NGI was a subcontractor to FRAMO, a company supplying pump systems for the marine industry, with headquarters in Bergen. NGI and FRAMO operated as subcontractors to Saipem, which had the contract to lift the 114 jacket frames into place on the seabed.
The main deliverable for NGI and FRAMO was to design, build and deliver four complete pump units with the necessary control and monitoring systems to manage the operations on the seabed. In addition, FRAMO and NGI were on board Saipem's giant crane barge S7000 during all installations to operate the installation equipment as well as to contribute with geotechnical expertise to ensure that all the foundations were installed correctly under partially demanding conditions.
In order to offer the best possible assistance and sharing of experience, data from the monitoring systems was also streamed to shore so that onshore experts could follow the operations and give advice if necessary. NGI was the first to offer facilities that enable real-time follow-up from shore for such operations.
Complex ground conditions resulted in gained experience
The ground conditions on the Seagreen field are complex and the installations of the 114 steel subframes each with three foundation buckets have contributed to increased knowledge and stretched the limits for which ground conditions where bucket foundations can be installed. NGI has gained a better basis, insight, and experience for calculating the forces needed to penetrate bucket foundations down into the seabed, as well as experience with various methods to reduce penetration resistance.
These experiences are very useful lessons for the upcoming development of offshore wind power. In addition, the installations at Seagreen show that the bucket concept is robust and that such foundations can be used in many offshore locations with varying ground conditions.
Technology developed for oil and gas is used in environmentally friendly installations
The bucket foundations were further developed and industrialized by Norwegian experts from Equinor (then Statoil) and NGI in the 1990s. This shows how technology that was developed for oil and gas can be used for environmentally friendly installations that supply the world with renewable energy and thereby contribute to the green shift.
This Norwegian niche technology is still an attractive foundation solution for installations in offshore areas around the world, now also for offshore wind turbines.
Seagreen suction bucket jackets on the dock at the shipyard and in transport for installation in the field
Withstands extreme loads
Bucket foundations were developed as an alternative to long piles. In deep water, and when the ground conditions make it difficult to install piles, there was a need for a solution that could be installed efficiently and without noise on the seabed. It also had to withstand heavy loads from waves and wind.
Illustration of the bucket foundations standing on the seabed (left) and after being pressed into the seabed with the pumping system of FRAMO with instrumented monitoring from NGI to control and verify the installation
When the bucket foundations hit the seabed, they first penetrate the seabed by self-weight. The water is then pumped out from inside the buckets. The bucket foundations are pushed further down into the seabed with a suction similar to huge hydraulic jacks.
After the installation is complete and the pump units are removed from the top of the buckets, they are closed. If you pull up the foundations, a suction is created on the inside, while the hydrostatic pressure on the outside is the same. In other words, it is the weight of the water column above the buckets that holds them in place and this holding capacity increases with increasing water depth.