Investigation on fatigue damage calibration factors in offshore structures

Abstract

Fatigue damage in marine structures is generally assessed by means of stress−number of cycles (S−N) curves. These curves are obtained from many experimental tests on structural components. Fracture mechanics (FM), using the crack propagation method is also a practical technique. In this approach, fatigue damage is estimated based on crack propagation from an initial crack size through the surface and depth of structural members. In this paper, S−N curves based on FM were compared with S−N curves obtained following two standards: DNVGL-RP-C203 and API-RP2A-WSD. The results showed that the curves did not agree well. Therefore, to obtain better fitting, calibration factors are needed. From S−N curves established for two stages, two calibration factors – α1 (for the first stage) and α2 (for the second stage) – are proposed for fitting S−N curves based on the FM approach with the DNV GL and the API S−N curves. The wall thickness, the initial crack depth and the critical crack depth were found to have a significant effect on determination of the calibration factors.

Numerical analysis and discussion on the hot-spot stress concept applied to welded tubular KT joints

“…Figs. [7][8][9][10][11][12] show the comparison between the stresses obtained from the Efthymiou's formulations proposed by DNVGL with the stresses obtained from the numerical simulations of the different meshes performed in this study. It is observed great conformity and compatibility between the numerical and the Efthymiou solutions in most points of the braces under analysis.…”

“…Codes and recommendations such as those from the International Institute of Welding -IIW [10] and DNVGL [11] provide global fatigue damage approaches, based on S-N curves, to be applied to welded joints, the latter being specific for offshore structures. Shabakhty et al [12] presented a discussion on the fatigue resistance modelling based on the fracture mechanics (FM) based-approach, where a comparison between the obtained S-N curves following two standards, DNVGL-RP-C203 [11] and API-RP2A-WSD [13], was done. These authors concluded that to obtain better fitting with the S-N curves, calibration factors are needed, and, therefore, these factors have been proposed.…”

A brief review of fatigue design criteria on offshore wind turbine support structures

“…Hot-spot is defined as the region in the joint where fatigue cracks are more susceptible to appear due to cyclic loading, meaning that hot-spot stress is the value of the stress in the surface of the hot-spot. Radaj [58] demonstrated that the hotspot stress corresponds to the sum of the membrane and bending stresses at the weld toe, which can be determined through extrapolation [57,59]. According to DNVGL recommendations, notch stresses must be considered in special cases in which is hard to evaluate with assurance the fatigue strength using other methods.…”