PSI - Issue 57

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Kashif Kamran Toor et al. / Procedia Structural Integrity 57 (2024) 772–784 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3. (a) Wave directions; (b) Wave distribution.

A target fatigue life is usually selected during the design phase of a structure and safety factors are applied to take into account various uncertainties. The selected substructure was designed for a minimum design life of 25 years and based on safe life design with high consequence level using the EN 1993-1-9 (2005). This is equivalent to a Design Fatigue Factor (DFF) of 3.3 based on an average slope of the bilinear S-N curve that is used. Design S-N curves are based on test results representing a through-crack in the plate material. For non-tubular joints a 25 mm reference thickness is used as basis for the design S-N curve in DNV-RP-C203 (2016) and IIW (2014). The fatigue calculation follows the following steps: 1) Select the weld detail and evaluate the level of complexity for assessment of analysis methodology. For this case the hot spot stress methodology is applied due to joint complexity. 2) Calculate the structural response from the applied environmental loads. The global analysis was performed using a conventional structural analysis tool by modeling the substructure in a 3D lattice using beam elements. Then the structural response was calculated for the environmental load spectrum. The response is in the form of nodal forces and moments in the structural elements. The fatigue load range was based on 16 wave steps applied to each wave height to capture the maximum and minimum response for adequate accuracy. This resulted into 3248 fatigue load cases to be run in the global analysis tool. 3) Calculate the resulted load effect at the selected stress singularity locations. In this case the hot spot stress was calculated in two steps, a. In the first step a local FE model of the join is prepared to calculate the influence factors (INF), which is a load effect at the singularity location from a unit load applied at each free end of the element of the joint. b. In the second step the hot spot stress is calculated by a linear summation of multiplied INF with actual load magnitudes calculated in step 2 from a global structural analysis. The following expressions are used to calculate the effective weld stress or hot spot stress: = ∑ ∑ , × , 6 =1 =1 (1) ℎ = × × . (2)