The influence of residual stresses on structural integrity of renewable energy marine structures.

Abstract

Offshore wind turbines operate in harsh environments and are subjected to severe cyclic loading conditions which result in corrosion and fatigue damage particularly in the support structures which are predominantly made of monopile type foundations. A reliable assessment for the fatigue life of offshore wind turbine monopile foundations is significantly dependent on the level of locked-in welding residual stresses at circumferential weld regions. In this work, fatigue crack growth tests have been conducted in air and seawater on S355 G10+M structural steel which is widely used in the fabrication of offshore wind turbine foundations. Fracture mechanics tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. All tests were performed at room temperature and the obtained results are compared with the literature data available on a range of offshore structural steels and also the recommended trends in BS7910 using the 2-stage law and simplified law. Moreover, the specimen orientation, with respect to the weld geometry, has been examined and discussed in this work. This study presents, for the first time, residual stress characterisation in compact tension specimens extracted from monopile weldments using three different methods; 1- Neutron diffraction, as a non-destructive technique which is widely used to measure lattice spacing from which residual strains and subsequently residual stresses can be calculated, 2 Neutron imaging, which is a relatively new non-destructive technique that enables residual stresses to be measured through strain mapping of the area of interest, 3- Contour method, as a destructive technique which can be used to measure residual stresses in engineering components and structures. Neutron diffraction and neutron imaging are two complementary techniques which have been employed in this work by performing measurements on the Engin-X and newly developed IMAT instruments, respectively, at the Rutherford Appleton Laboratory. Neutron diffraction residual strain measurements were conducted along all three directions (i.e. transverse, longitudinal and normal with respect to the weld geometry) on compact tension specimens with the crack tip located in the heat affected zone whilst neutron imaging technique was used to measure residual strains in the longitudinal direction. A comparison of the obtained results from neutron diffraction and neutron imaging techniques has shown that neutron imaging can provide an acceptable measure of residual strains, and subsequently residual stresses, if an accurate value of strain-free lattice spacing, dₒ, is employed in data analysis. Moreover, it has been shown that the contour residual stress measurement results are in good agreement with the neutron diffraction results. The residual stress measurement results have been discussed in terms of the possible sources of error encountered in each technique and the accuracy of each method against the others. The residual stress measurement results showed notably significant remnant residual stresses in compact tension specimens that could have an impact on subsequent fracture and fatigue test results. In addition, the measured 2D map of transverse residual stresses, acting normal to the crack plane, exhibited variations in through thickness direction. This implies that the residual stresses in small laboratory samples extracted from large scale weldments should be carefully characterised and taken into account in interpretation of the structural integrity test results. In order to examine the specimen size effects on welding residual stress profiles, the contour method has been applied to a compact tension specimen as well as a large welded mock-up, typical of the weldment used in fabrication of offshore wind monopiles. The measurement results on the welded mock-up showed that the level of damaging tensile residual stress in large-scale mock-ups, hence real size structural welded monopiles, is considerably larger than residual stresses in extracted laboratory samples. This means that the welding residual stresses play an even bigger role in structural integrity assessment of full scale offshore wind turbine monopiles. Finally, a numerical model has been developed to implement the residual stress profile in ABAQUS finite element simulations and calculate the effective stress intensity factor range in the presence of residual stresses for accurate characterisation of fatigue crack growth behaviour, particularly in the near threshold region. The results have shown that when the transverse residual stresses ahead of the crack tip are predominantly compressive, the values of effective stress intensity factor range in compact tension specimens are less than the applied stress intensity factor range by around 10 MPa√m. This indicates that residual stresses play a key role in the fatigue life of welded structures, especially in the near threshold region.