Damage to a road tanker doesn’t always mean it’s the end of the road. A Fitness-for-Service (FFS) Level 3 assessment can determine if the equipment is still safe to operate or if repairs are necessary.
In this case, an LPG tanker sustained a dent, raising concerns about its structural integrity. Instead of immediately replacing the damaged section, an elastic-plastic finite element analysis (FEA) was performed in accordance with API 579-1/ASME FFS-1 to assess whether the tanker could remain in service.
How was the assessment performed?
- Damage Measurement – The dent’s dimensions were captured using a grid method, ensuring accurate input for analysis.
- 3D Modeling – A detailed surface model was built, incorporating the dent and internal stiffeners. Second-order shell elements were used to improve accuracy.
- Elastic-Plastic FEA – The actual design loads were applied to simulate real operating conditions and assess structural response of the tanker wall.
- Failure Evaluation – The results were assessed against plastic collapse and local failure criteria, per API 579 guidelines.
Results and Findings
- The tanker passed the plastic collapse criteria, meaning it could withstand expected loads without catastrophic failure.
- However, it failed the local failure evaluation, indicating excessive plastic strain in the dented area.
- Conclusion: The damaged section must be replaced to restore the tanker’s structural integrity.
A Level 3 FFS assessment provides a more realistic evaluation compared to conservative hand calculations. By leveraging elastic-plastic FEA, we can avoid unnecessary repairs when a vessel is still fit for service—or justify repairs when safety is compromised.