DUWIND, a research institute at Delft technical university in the Netherlands, released a poster presentation around five years ago with a comparison between three state-of-the-art offshore foundation alternatives for 30 to 50 metre water depths.
These included a steel monopile, a steel tripod foundation and a four-legged steel jacket. Jackets and tripods incorporate smaller diameter corner piles for anchoring the foundations to the seabed. The project further included a 5.5MW reference turbine with global specifications. The research’s main focus was on foundation mass in relation to total manufacturing costs. The study was aimed at future offshore projects in the Dutch section of the North Sea.
At the time, a seven-metre pile diameter was considered large and the development of the hydro-hammers had to keep pace with monopile size increment.
A key conclusion was that, despite high steel prices, these foundations should not be written off for deep-water North Sea locations. A second conclusion was that the many steel joints in tripod and jacket foundations are a key cost driver.
It was pointed out that labour-intensive fabrication processes tend to require substantial investments in industrial capacity.
One of the researchers later confirmed that the comparison did not include transport logistics and installation costs. This fact is not without importance as the big and heavy monopile for the ramming process requires a costly hydro-hammer, which represents a major cost driver.