Determination of loading and residual stresses on offshore jacket structures by X-ray diffraction

As basements of offshore wind turbines (OWTs) in deep water (>50 m), jacket structures are an economic alternative to monopiles. For this reason, the structural durability of jackets has become more important. In such structures, welded tubular joints are weak points for fatigue design. The harmful effect of tensile residual stresses in welding joints is well known. For these reasons, the residual stresses and the loading stresses of offshore jacket structures were determined by X-ray diffraction (XRD) using a mobile diffractometer. This allows us to directly determine the load stress at the fatigue-critical locations, namely at the weld toe at the testing rig. High tensile residual stresses up to 250 MPa were determined in a welded (and unloaded) condition. At a loaded structure (10,000 load cycles), a lower residual stress level was determined. During loading, a local increase in the stress at the welded joint that is between 1.4 and 4 times higher than the applied nominal stress was determined. Furthermore, it is shown that additional treatment (grinding and clean blasting) influences the local stress state significantly.