Arild Bolsø, an engineer who worked for Statkraft in Norway and Forewind in the UK, has developed a new technical concept for offshore wind foundation installation. According to Bolsø, the concept would reduce installation time by 30-50%, compared to current methods.
After Statkraft decided to stop new investments in offshore wind, Bolsø set up a new company – Arild Bolsø Engineering – and worked fulltime to develop the Hydrostatic Structural Connection Technology (HSCT), with the early phase of development financially supported by Innovation Norway.
The HSCT involves a suction cap that connects two or more submerged structures. The technology can be used on submerged structures with a tower extending to the surface such as a wind turbine substructure, on completely submerged structures such as in oil & gas applications and tidal turbines, or as a quick connection on a mooring line.
Bolsø told Offshore WIND that he decided to introduce the technology to the industry after filing a patent application in December.
A substructure with sealing elements is installed on top of a foundation, creating watertight compartments between the two structures. The pressure inside these compartments is quickly reduced to atmospheric level allowing for the water pressure from outside to act on the structures and keep them connected. A secondary simple mechanical locking device takes the loads in the case of a sealing failure.
To some extent, the principle is similar to that of the suction bucket technology, but instead of pushing a bucket into the seabed, the HSCT is connecting two or more submerged structures.
The HSCT suction caps consist of peripheral sealing and rubber cushions (fenders) inside.
Because of the very instant attachment of the suction cap, floating vessels can be used and integrated installation becomes possible, Bolsø pointed out, adding that one floating vessel can carry six suction bucket foundations or four complete turbines and substructures per trip.
How it works?
A suction cap is mounted at the lower end of the substructure, which is very similar to a three-legged jacket and goes between a tower and a foundation already installed at the seabed. The upper part of a suction cap is attached to a flat surface on the foundation.
From the suction cap up, the foundation can feature either a jacket structure or a monopile-shaped structure.
“Below the suction cap one can imagine several types of foundations. As long as the foundation has a flat horizontal area that matches the suction cap on the substructure, it can be either pin-piled to the ground, have suction buckets or be a heavy concrete slab. Also, floating structures can be used as long as they have the same horizontally surface at c. 35-meter water depth,” Arild Bolsø explained.
Standardization and industrialization of substructures for bottom fixed and floating wind turbines can lead to significant cost reduction for both alternatives, according to Bolsø, since one installation vessel can be used for the standard “topside”, which may also install most of the foundation types.
Offshore WIND Staff; Images: HSCT