PSI - Issue 57

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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1. Introduction Finite Element Analysis (FEA) is widely used to perform fatigue calculations at geometric singularities at welded components. The analysis methods are described in various design codes and recommended practices such as DNV, IIW and Eurocodes. The hot spot stress methodology is adopted when nominal stress cannot be calculated for a given weld detail. The local joint geometry is modelled in a finite element tool to capture the effect of the geometry. Generally, a mid-surface shell modelling technique is adopted due to its robustness and without requirement to including the weld profile. The hot spot stress is then calculated by reading the stress at read out points located at pre defined distances from the intersection and eventually extrapolated to the intersection line representing of the weld toe. The hot spot stress methodology is depending the singularity location which is either located at the plate surface or at the plate edge as indicated in Fig. 1. In the present study the focus is on the weld toe located at the plate edge and this type of hot spot is generally known as type ‘b’ hot spot in recommended practices and standards, Fig. 1.

Fig. 1. Hot spot types based on weld toe location.

The methodology recommended for the type ‘b’ hot spot has been found too conservative to be applied to the cope hole type welded details. In the absence of local weld stiffness a high hot spot stress is predicted. This effect has been investigated in detail by Aygul et al. (2011). DNV also recommends including the weld if the local geometry is complex and if there is a significant stress gradient which is typically the case for type ‘b’ hot spots. The results from full-scale test specimen by Lotsberg et al. (2005) also showed that the shell modelling approach applied to cope hole details may over-simplify the structural detail resulting in a high hot spot stress. Niemi et al. (1995), Eriksson et al. (2003) and IIW (2014) also suggest different modelling approaches to include the weld geometry and stiffness in shell type FE models. The literature and general engineering practice encourage to include the weld geometry and stiffness into the analysis model for similar types of weld details. If it is difficult to select the appropriate modelling approach, then the effective notch stress method can be applied. In the current study the significance of including the weld (geometry and stiffness) in the finite element model has been verified by performing a fatigue check on an example cope hole weld detail by applying different FE modeling approaches. The considered hot spot is in a pile sleeve connection in a jacket substructure installed using a piled foundation. The total weight of the jacket is approx. 1000 tons with a topside of 2200 tons. The substructure and the local pile sleeve connection is shown in Fig. 2. The primary connections of the jacket were designed using steel grade S355ML/NL under execution class ‘EXE3’ and fabrication tolerances representing the quality class B in EN1090. The pile sleeve connection is a conventional shear key grouted connection with a high strength grout material.