Abstract
AbstractThick weldments used in offshore structures frequently act as fatigue crack initiation sites due to stress concentration at weld toe as well as weld residual stress fields. This paper investigates the cyclic deformation behavior of S355 G10+M steel, which is predominantly used in offshore wind applications. Owing to the vast size difference of monopile structure and weld cross‐section, a global–local finite element (FE) method was used, and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using supervisory control and data acquisition (SCADA) and wave buoy techniques were used in the FE model. A nonlinear isotropic–kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal (BM) as well as cross‐weld specimen tests. The tests revealed that the S355 G10+M BM and weld metal (WM) undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the local stress 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.
Highlights
Over the last 15 years, constant efforts have been directed towards promoting renewable energy technologies, and the deployment of new offshore wind farms has rapidly accelerated around the world, in Europe
Mehmanparast et al.[19] studied the effect of environment and microstructure (BM and heat affected zone (HAZ)) on the fatigue crack growth in S355 G8 +M and reported that in free corrosion condition, the crack growth rate was increased by a factor of 2 as compared to tests conducted in air
The circumferential weldments joining the thick monopile sections lead to material property variations at the weld metal (WM)-HAZ-base metal (BM) interface, which turns into a favorable site for fatigue crack initiation
Summary
Over the last 15 years, constant efforts have been directed towards promoting renewable energy technologies, and the deployment of new offshore wind farms has rapidly accelerated around the world, in Europe. Most of the installed OWTs consist of monopile foundations which are built by stacking 3 to 7 m diameter cylindrical sections of 30 to 125 mm thickness and can cost up to 35% of the total setup cost of an OWT.[25,26,27] Besides, the circumferential weldments joining the thick monopile sections lead to material property variations at the WM-HAZ-BM interface, which turns into a favorable site for fatigue crack initiation This is due to the difference in microstructure and chemical segregation as a result of rapid heating and cooling associated with the submerged arc welding process. It is worth mentioning that this measurement indicates that the sea waves at the sea level would have maximum contribution towards wave loads; the present study uses the height of sea waves at the surface to compute the wave loads on the monopile foundation
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