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Fatigue analysis of monopile weldments under service loading conditions using a cyclic deformation modelling approach
  • Romali Biswal,
  • Abdullah Mamun,
  • Ali Mehmanparast
Romali Biswal
Cranfield University

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Abdullah Mamun
University of Bristol
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Ali Mehmanparast
Cranfield University
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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.