Fatigue analysis of monopile weldments under service loading conditions
using a cyclic deformation modelling approach
Abstract
This paper investigates the cyclic deformation behaviour of S355 G10+M
steel which is predominantly used in offshore wind applications. The
thick weldments were identified as regions prone to fatigue crack
initiation due to stress concentration at weld toe as well as weld
residual stress fields. The monopile structure was modelled using a
global-local finite element (FE) method and the weld geometry was
adopted from circumferential weld joints used in offshore wind turbine
monopile foundations. Realistic service loads collected using SCADA and
wave buoy techniques were used in the FE model. A non-linear
isotropic-kinematic hardening model was calibrated using the strain
controlled cyclic deformation results obtained from base metal as well
as cross-weld specimen tests. The tests revealed that the S355 G10+M
base metal and weld metal undergo continuous cyclic stress relaxation.
Fatigue damage over a period of 20 years of operation was predicted
using the total elastic-plastic strain energy accumulated at the root of
the weldments as the life limiting criterion. This study helps in
quantifying the level of conservatism in the current monopile design
approaches and has implications towards making wind energy more
economic.