The exploration of various methods to enhance the Finite Element Analysis (FEA) of a 6-bed polybed vessel focuses on addressing potential fatigue issues. A Finite Element (FE) model is developed using the NozzlePRO and FEPipe software packages to perform repeated fatigue analyses on comparable vessel configurations. Special attention is given to the analysis of weld details at the head-to-skirt connection of the vessel.
Analysis and Approach
The limitations of detailed modelling of the head-to-skirt weld using NozzlePRO, due to constraints in geometry and load definitions, necessitate exploring alternative improvement strategies. The approach involves:
- Analysis of the As-Received Model: Evaluating the initial model provided by the client in NozzlePRO.
- Model Enhancement: Improving the NozzlePRO model with several modifications to increase the accuracy of stress analysis.
- Detailed FE/Pipe Model Development: Creating and analyzing a detailed FE/Pipe model to accurately represent weld curvature as per the drawings.
Additionally, an alternative fatigue assessment approach, less reliant on detailed modelling, is examined. This secondary method employs stress linearisation and is often used to adhere to the ASME VIII B&PV Code Div. 2 Chapter 5. Both the enhanced NozzlePRO model and the detailed FE/Pipe model are subjected to these approaches.
Results
The analysis yields the following insights:
- Model Improvements: NozzlePRO is effective for refining the as-received model concerning vessel length, absorber bed weight, nozzle loads, and vessel weight.
- Detailed Weld Modelling: For accurate representation of the head-to-skirt weld curvature, a detailed FE/Pipe model is recommended alongside NozzlePRO.
- Weld Modelling: A circular inside head-to-skirt weld can be modelled for any desired radius by designing the elements appropriately. Detailed guidelines were provided to the client.
- Alternative Fatigue Assessment: An alternative approach following ASME B&PV Code Section VIII Div. 2 Chapter 5, involving the use of a Stress Concentration Factor (SCF), is presented.
- Recommended Approach: Dynaflow Research Group advises using stress linearisation in conjunction with an SCF for fatigue analysis. This method is particularly useful when assessing the accuracy of FEA model details.
- Conservative Results: The stress assessment approach using linearisation and SCF generally results in more conservative outcomes.
Furthermore, a training session was conducted at the client’s site. During this session, model improvements were reviewed with the client’s engineering team, various modelling options were demonstrated, and several fatigue analysis exercises were performed to enhance understanding of the different approaches accepted by the ASME VIII B&PV Div. 2 code.
