Fatigue assessment of offshore tubular joints: stress concentration factor vs. detailed finite element method using stress influence matrix

In this work, two hotspot stress estimation methods were compared in prediction of the fatigue damage level of tubular joints of jacket-supported offshore wind turbines (OWT). The first method is the stress concentration factor (SCF) method, which uses empirical functions to determine eight hotspot stresses (HSSs) along the brace-chord weld interface of each tubular joint. The second method is the stress influence matrix (SIM) method, which is based on a single realization of detailed finite element simulation of tubular joints, determining HSSs through matrix multiplication and linear stress extrapolation. After determined HSSs, fatigue damage is computed using the rainflow counting and S-N curve. Two complete fatigue damage analysis processes for the tubular joints (TJs) of OWT were proposed. Furthermore, an implementation study involving TJ1-TJ5 of the reference NREL 5 MW OWT was presented. The results through comparison indicate that the SIM-incorporated fatigue assessment procedure that considers an extended number of hotspots on the inner or outer wall of the joint member could lead to a more precise fatigue analysis. The SIM-incorporated fatigue assessment procedure provides an optimal and less conservative solution for FLS design in view of the fact that the discrepancy between the SCF- and SIM-incorporated fatigue procedures increases with the joint elevation level.