By 2030, the European Union plans to build up the capacity of its offshore wind turbines to at least 60 gigawatts. Freudenberg Sealing Technologies (FST) is contributing to this vision with sealing solutions for their foundations. The goal is for new wind power facilities to be able to operate for 30 years or more despite the harsh conditions at sea.
The highest windspeed measured so far in the North Sea off the German coast was 191 kilometers(119 miles) per hour. There have certainly been wind turbines that have been shut down at much lower wind speeds. But they have to withstand the powerful forces of hurricanes in any case – over their entire life-spans, in fact. The foundations that anchor the towers in the seafloor play an important role in this. At the low water depths that prevail in the North and Baltic Seas, so-called “monopiles” are generally used for this. They are steel pillars that are rammed into the seafloor and whose upper end reaches above the surface. As a rule, the tower of a 10 megawatt facility, which climbs 150 meters into the air to the rotor hub, is not mounted directly on the pillars. Instead, it is supported by a connecting piece – a flange. It is attached with several dozen bolts, which continue to be inaccessible to maintenance after the tower is assembled. That means moisture must be kept away from the screw connection so it does not corrode in the harsh conditions. Seals from FST already handle this important task in more than 800 offshore wind turbines.
A ring seal at the upper end of the pillar has the most important function. It encloses the interior portion of the connecting piece that carries the screw connection. Known by the abbreviation MP-TP (for monopole transition piece), this flange seal may have a diameter of 8 to 10 meters. FST has also been offering a double version of the seal for several years. The advantage of the two-row configuration is mainly redundancy, as Freudenberg expert Manuel Hille explains. “If there is slight damage to one of the two seals during transport or assembly, their functioning is assured. This also makes it possible to combine various designs and materials in the best possible way.”
In a conventional design, there is a gap between the lower end of the connecting piece and the pillar. One or two ring seals with air chambers ensure that no seawater can penetrate the space. The seal serves as an elastic wave breaker. If the gap is filled with cement, a ring seal can hold the cement in position during hardening and protect it from saltwater later. Maintenance platforms are often attached within the monopiles above the water’s surface and can be protected with a specially designed seal to keep gases out. Gases may form due to biological processes in the seawater.
Designed for a Long Lifespan
The expectations for the lifespan of components in sea-based wind turbines have risen sharply in recent years. While 25 years was considered a good lifespan at first, the standard is now as high as 35 years. These long lifespans cannot be guaranteed with test stand trials alone. That’s why Freudenberg extrapolates the results of intensive stress tests carried out over a period of six weeks with the help of an Arrhenius algorithm, named for the Swedish chemist and Nobel Prize laureate Svante August Arrhenius. Over the last few years, Freudenberg experts have systematically taken the approach further, significantly improving the operating life model by coupling chemical and physical effects with the structural-mechanical behavior of the material.
“Monopile technology is the basis for 80 percent of all projected offshore wind parks,” said Marcel Schreiner, who is in charge of FST’s sales in the energy sector. “This is an important growth market for us.” So is the trend to ever-larger installations – for 2030, rated outputs of 20 megawatts are under discussion. Schreiner sees that as a good reason to pay special attention to longevity in the selection of components.
