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Video: Ulstein Unveils Novel Monopile Installer With a Notched Stern

monopile installation vessel in operation
Ulstein's U-Stern in action (Illustrations courtesy Ulstein)

Published Apr 24, 2023 3:40 PM by The Maritime Executive

Norwegian vessel designer Ulstein has borrowed a page from Allseas' playbook and created a new class of vessel with a notch for heavy offshore lifting. Unlike Allseas' Pioneering Spirit, however, the new Ulstein design is purpose-built for installing offshore wind monopiles, the most common form of ground-fixed wind turbine foundations. 

Ulstein's "U-Stern" has a deep notch at the stern and a tiltable rack. To deploy each ultra-heavy foundation, the crew skids the pile into the rack, then uses the ship's crane to pick up the top end of the pile and tilt it into the water. At no point is the monopile hanging freely in the air, supported solely by the crane, as is standard practice for current installation vessel designs. 

This has several advantages, according to Ulstein. The whole installation evolution happens on the ship's centerline, so the vessel can point right into the waves during operation without turning to form a lee. This reduces power demand in DP mode by 50 percent and cuts vessel motion by 25 percent. The carefully engineered notch - or "moonbay" - still shields the operation from wave action when the pile goes over the stern. 

Ulstein spent two years studying and developing the vessel concept, with particular attention to the notch. “The shape and location of the recess are finely tuned with CFD (Computational Fluid Dynamics) and motion analysis to make it work in harmony with the crane and mission equipment," says Ko Stroo, lead naval architect with Ulstein Design & Solutions BV.

Ulstein believes that the U-Stern will be a particularly attractive solution as wind turbines get taller and monopiles get heavier. 

“The idea of longitudinal monopile installation originates in our vast experience in developing heavy lift pipelay vessels with a centerline firing line,” says Edwin van Leeuwen, Managing Director at Ulstein Design & Solutions BV. “The U-Stern is however not a ‘one-trick pony’, as the vessel is also suitable for jacket installation and other heavy lift installation work. In the past months, we have presented the U-Stern to major contractors and operators, many recognizing this as the most viable solution for installing the next-generation monopile foundations.”