Can offshore wind turbine foundations that are bolted together withstand the extreme environment found offshore? FORCE Technology has developed tests that show whether bolted flange joints corrode offshore.
Traditionally, monopiles and jacket foundations are used – large, welded steel structures installed on the seabed to support the wind turbine towers. However, the jacket foundations, which are suitable for deeper sea areas, present logistical challenges and require time-consuming inspections. This makes them expensive and complicated to produce.
Bolted joints as a solution
To reduce costs and strengthen the green transition, there is significant potential in using bolted flange joints for offshore wind turbine foundations. This can enable prefabrication of foundations that can be assembled offshore, making the process both cheaper and faster. Siemens Gamesa has had success with bolted foundations on land, but the question is whether these can also be used in the challenging offshore environment.
There are currently no standardised tests for bolted flange joints offshore, and there are only limited guidelines for them. FORCE Technology has contributed knowledge about the potential damage and challenges, including cathodic protection and the risk of hydrogen embrittlement.
Specially designed tests for durability
To find solutions to these problems, FORCE Technology has developed tests that investigate the effectiveness of cathodic protection under water as well as protection in the splash zone – the area where seawater constantly splashes up and creates varying conditions. The first test results show that it is possible to protect both flange joints and bolts against corrosion.
Surprising results from the splash-zone test
As there is no standardised test of bolted joints in the splash zone, FORCE Technology has developed one. Since it is not possible to use cathodic protection in the splash zone, we wanted to investigate whether it is possible to use relatively low-alloy stainless steel bolts of the type EN 1.4435.
In a test of bolted joints in the splash zone, relatively low-alloy stainless steel bolts (EN 1.4435) were used instead of the more costly high-alloy steel materials. The test showed no signs of corrosion attack and the projections indicate that bolted joints can achieve the desired service life of 25 years.
First step taken towards a standard for testing jacket offshore wind turbine foundations
The results have been presented at the European corrosion conference EUROCORR 2021. The tests carried out point towards further work to develop a proper standard for testing bolted flange joints in the splash zone. The work must include tests in warmer water, as offshore wind farms will also be located in tropical and subtropical areas. This places additional demands on the materials used, because when the temperature rises, conditions for corrosion become more favourable.
