On Fatigue Damage Assessment for Offshore Support Structures with Tubular Joints

The hot spot stress approach is commonly used for fatigue lifetime estimation of tubular joints. The standard approach consists of the linear superposition of stress components from axial, in-plane, and out of plane action, leading to 8 hot spot stresses equally divided along the circumference of each tubular joint. The fatigue lifetime for a joint is calculated by accumulating the fatigue damage over several load cases. The fatigue damage used per load case is commonly the maximum fatigue damage out of the 8 hot spots. In this study, another approach has been evaluated. Fatigue lifetime is estimated by accumulation of fatigue damage over load cases for each hot spot individually, instead of taking the maximum out of the 8 hot spots. The proposed approach is compared with the commonly used approach using a generic lattice type support structure for offshore wind turbines. In addition, the number of hot spots along the circumference of the joint is increased to 32 points in order to study the influence on the fatigue lifetime estimation. Results show a difference in the fatigue lifetime estimation for individual joints up to 26% when using the proposed approach for estimating the fatigue lifetime. Furthermore, it is shown that the consideration of 32 points along the circumference of tubular joints lead to more precise fatigue damage. Differences up to 11% were detected