Determination of Loading and Residual Stresses on Offshore Jacket Structures by X-ray Diffraction

Abstract

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.

An Overview on Structural Health Monitoring and Fault Diagnosis of Offshore Wind Turbine Support Structures

“…Under the action of long-term cyclic loads, it is necessary to monitor the fatigue behavior of the weld seams. The weld seam location is prone to stress concentration, which can cause local cracks [58]. Kolios et al (2019) proposed a method for calculating the stress concentration factors of weld seams by combining 3D laser scanning technology with finite element analysis cameras and critical points where fatigue cracks may occur [59].…”

Influence of Conventional Shot Peening Treatment on the Service Life Improvement of Bridge Steel Piles Subjected to Sea Wave Impact

“…To improve the fatigue life of metallic materials, scientists proved that the two factors of compressive residual stress on the surface and an ultrafine grain in the surface are very effective. This conclusion is due to the fact that the initial damages caused by cyclic loading always appear as surface cracks [17][18][19][20][21]. As a result, fatigue properties can be improved by increasing the surface strength.…”