Speeding up wind installation

It takes less than half an hour to carry out the inward and outward folding, which activates IRIS, Trelleborg's innovative seal for offshore wind turbines. The first offshore activation of the seal took place without any issues this summer at a large wind park under development in the Americas.

Cutting offshore time
Trelleborg aimed to keep offshore installation time to a minimum when designing the seal.

"Offshore time is very expensive, because you have vessel crews operating, so everything is designed to be fitted onshore," explains Jurriaan van den Berg, the design engineering manager responsible for the project.

"The attraction of this solution is that you can activate it fairly quickly and you do not need to do a lot of offshore work."

The key innovation with IRIS is the elimination of the welded steel landing ring seals traditionally used to fix seals inside the transition pieces of monopile wind turbine foundations. The seal instead has pre-drilled holes and is fixed in place using bolts and clamping strips.

The outer lip of the seal is a flap which hangs vertically from the transition piece, which only creates an airtight seal when it is folded horizontally against the sides of the monopile wall in a short operation carried out offshore. This then prevents chlorides, hydrogen sulphide, and other gases produced by the anti-corrosion system in the transition piece from entering the turbine tower.

"The anti-corrosion system is very important to make sure that all the steel and paint remains intact," van den Berg explains. "The drawback is that it generates gases and you don't want those gases to be where people are working or where sensitive electrical equipment is."

Turbine designers have long sought a solution that does not need steel landing rings. The welds used to fix them in place tend to crack, allowing gases to escape into the tower. They are also expensive, heavy and time-consuming to install offshore.

The IRIS seal is only 15 millimeters thick and has a diameter of six meters , meaning it weighs just 300 kilograms . To make the outer lip flexible enough to carry out the inward-outward fold has required adjustments to the polymers and some compromises on durability and pressure capacity. Despite this , Trelleborg's extensive testing shows that even with the harshest weather and sea conditions faced by offshore turbines, the seal will remain effective throughout the 30-year expected operational life of the turbine.

Design to installation
Trelleborg's design team was first approached to develop an alternative solution in late 2021, initially working with a prominent wind industry design consultant. The consultant then took the design to their client, one of the major energy companies, who saw enough potential to invest in more detailed testing and in the end opted to include it in the final turbine design.

"In the industry, it's common to investigate your options early on and then perform a stage gate risk analysis at certain times," van den Berg explains. "If there's too much risk involved, the innovation goes out the window and they go back to a more conventional solution."

That the IRIS seal concept survived this process is testament to the design and testing work Trelleborg and its design partner performed right from the preliminary concept design.

"This gave the owner and the design company enough confidence that this was a realistic solution to investigate further, after which they subcontracted part of the seal design to us, which we carried out over a two-month window in the summer of 2023."

The owner then spent several months reviewing the results before commissioning full-scale testing of the seal, which took place a year after the design work was complete. Trelleborg's factory in Shanghai, China, then manufactured a prototype of the seal to a tight timeline and flew it to a constuction yard in Europe where Trelleborg and its partners tested it on a full-scale mock-up of a transition piece.

According to van den Berg, this testing process was used to assess how much pressure the seal can take, how easy it is to fold in and install, and the level of 'spring back'. The testing validated the calculations Trelleborg had made and convinced the project owner to keep the seal when installing the project.

Success through collaboration
The seal has now been successfully manufactured and installed in the 50 turbines that make up its first project and there is growing interest from other design consultants and energy companies, particularly after the seal was promoted at the WindEurope exhibition in Copenhagen in April.

For van den Berg, the key to this success has been Trelleborg's capacity to work internally across countries and divisions and externally with design houses and project owners.

"There's been a huge amount of teamwork within Trelleborg to get it designed and developed. The design house has been a massive support, and from the fabrication side, there's been the facility in Shanghai," he concludes.

"Through all these steps you're working in different sub teams and still making it a success together, which I think is the biggest achievement and something which now has real tangible results."

Writer: Richard Orange
Photo: Getty Images & Trelleborg