Review of the Standard Method for Assessing the Fatigue Life of Bolts in Flanged Connections of Offshore Wind Turbine Structures
TWI Industrial Member Report 1172-2023 [pdf / 819 KB]
By John Wintle
The primary structure of most offshore wind turbines can have three or more bolted flange connections throughout its length. The offshore wind energy sector needs to be able to estimate the fatigue lives of the bolts in flanged connections and how changes in bolt pre-tension can affect them. The Schmidt-Neuper method is recommended in standards and used by designers to determine the fatigue life of the bolts, but it makes a number of simplifications and conservative assumptions.
A better understanding of the method could lead to improvements that would enable the sector to justify longer design fatigue lives of the bolts and avoid costly premature replacement.
This report reviews relevant standards relating to the design and fatigue life of bolted flange connections of offshore wind turbines. It explains the Schmidt-Neuper method used in the standards to determine the safe working fatigue life of the bolts and to highlight assumptions and conservatisms in the method.
A case study is used to estimate the fatigue life of bolts in a bolted flange connection of a representative offshore wind turbine using the Schmidt-Neuper method, in order to demonstrate the sensitivity of estimated fatigue life to different level of bolt pre-tension.
It concludes with a review relevant research on the fatigue life of bolts in flanged connections of offshore wind turbines.
- All the evidence from practical observations and theoretical arguments indicates that the Schmidt Neuper approach for determining the stress in the bolts from external loading from the TP (or tower shell) overestimates the actual stress in practice.
- The Schmidt Neuper approach is a simple 2D analysis of a single bolt subtended by an arc of flange subject to the force corresponding to the maximum axial stress around the circumference of the TP. Several important effects are not considered, and these are discussed in the report.
- The fatigue life of the bolts predicted with the method depends critically on the gradients of Schmidt Neuper tri-linear transfer function. Without knowledge of the form of the transfer function in practice, the approach may have resulted in bolts being replaced when there are many years of safe fatigue life remaining, use of the approach could be appropriate or even non-conservative for the calculation of fatigue life in some situations.
The use of this simplified method may have resulted in bolts being replaced when there are many years of safe fatigue life remaining. Measurements and further analysis is needed to refine the method, and produce more realistic fatigue life predictions for bolts in flanges in wind turbine structures.