A common defect on a number of windfarms is failure of the seal between the j-tube and the monopile. This results in allowing oxygenated water to enter the internal chamber, which potentially increases the internal corrosion and therefore decreases the life of the monopiles.
In practice, many offshore wind turbine structures constructed have higher natural frequencies than the design intent. This is typically due to the soil providing greater stiffness than anticipated. A stiffer soil may benefit the design life of the turbines.
WT performed a study to consider what the design life of the structures would be, were oxygenated water able to circulate inside the monopiles. Since this windfarm was designed, significant advances have been made in design approaches. These mean that the foundations were most likely have significant safety margins especially with regards to fatigue loading.
Using the existing soil investigation but with a sharper interpretation, we reassessed the soil strength. Secondly, using the natural frequency measurements, the soil reaction curves were recalibrated. The new soil reaction curves allow a more realistic evaluation of the actual fatigue loads and in turn deliver a significantly greater design life.