The foundation can be connected to the tower underwater, enabling a more balanced weight distribution between monopile and transition piece, the marine engineers claimed.
Van Oord said the solution could be used with the next generation of offshore wind turbines but still be produced in existing manufacturing facilities and installed using the current fleet of vessels.
A slip-joint is an alternative connection between a monopile and transition piece. It typically consists of a pile with a slight outer coning top section, with a tower section with an internal matching coning surface above.
The strength of the connection is based on friction, with the joint’s weight ensuring the link is firm and stable, Van Oord explained.
Installation is carried out by sliding the transition piece over the monopile removing the use of grouts or bolts, which can only be used above water.
The more balanced weight distribution aids installations in deeper waters or soft soils. These project sites require longer monopiles that extend above water to allow for grouting and bolts.
But the longer monopiles are becoming increasingly heavy and will soon be beyond the capabilities of installation vessels.
The slip-joint design means monopiles can remain shorter and lighter as offshore wind projects move into deeper waters.
Van Oord’s pilot slip joint was installed at the 19MW Borssele V demonstration site, which features two MHI Vestas V164 9.5MW turbines. It is surrounded by the Borssele I-IV commercial projects awarded through the Netherlands’ offshore wind tenders and which are now under construction.
The test site was developed by the Two Towers consortium comprising Van Oord, project development services provider Investri Offshore, and financial advisors Green Giraffe.
Borssele V will host several other offshore wind innovation demonstrations including thermally-sprayed aluminium coatings and impressed current cathodic protection to protect steel from corrosion.
Van Oord was also set to trial a new noise mitigation system during the installation of the monopiles.
Meanwhile, rocks are set to be installed around the base of each turbine as a form of scour protection but will also form a habitat for oysters.