FEA Fitness-For-Service Level 3 Crack Analysis at Joint 27 - Tag PL-12A
Figure 1: Emergency Pipe and Joint 27 Location
How advanced Elastic-Plastic FEA and API 579 Failure Assessment Diagram (FAD) validated continued safe operation of a cracked high-pressure pipe without shutdown or repair.
2. Project Overview
3. FEA Methodology
Elastic-Plastic analysis with multilinear hardening using SOLID186 20-node hex elements. J-integral and SIF extracted at each load step for FAD construction per API 579 Level 3.
Figure 2: Crack modelling from PAUT data – Crack 1 at CS-102
Stress Results
Figure 3: Von Mises Stress at Full Design Load
FAD Assessment
Kr = 0.000380, Lr = 0.9999
ACCEPTABLE – Crack lies within the safe zone on the FAD curve
Figure 4: Failure Assessment Diagram
Model Validation
Hand Calc: 58.14 MPa | FEA: 58.115 MPa | Error: < 0.05%
4. Lessons Learned
Key Finding 1: Elastic Analysis Alone Is Not Enough
For a cracked component, traditional stress linearization cannot capture the fracture mechanics response. Only a full elastic-plastic analysis with J-integral extraction can properly evaluate crack stability per API 579 Level 3.
Key Finding 2: Crack Is Stable and Non-Growing
Despite operating at near-design pressure (154.7 out of 162.4 kg/cm²), the crack at Joint 27 lies well within the acceptable zone on the FAD curve, confirming it is non-propagating under current service conditions.
Key Finding 3: Material Plasticity Was Properly Captured
Using a multilinear stress-strain curve for SA-335 Gr. P5 at 285°C was essential. A simple bilinear model would have over-estimated crack-tip yielding and produced non-conservative K_r values.
5. What Could Have Gone Wrong
Risk 1: Unplanned Plant Shutdown
Without a Level 3 FFS assessment, the crack detection at Joint 27 would have forced an immediate shutdown and costly weld repair. FEA proved continued operation is safe, saving significant downtime costs.
Risk 2: Incorrect Crack Characterization
Using Level 1 or Level 2 assessments may have incorrectly flagged the crack as critical, or conversely, Level 1 screening alone might have been insufficiently conservative for this high-pressure application.
Risk 3: Catastrophic Pipe Rupture
At 162.4 kg/cm² design pressure, an undetected growing crack could lead to brittle fracture or leak-before-break failure. The FAD analysis confirms this crack is non-growing, preventing potential catastrophic failure.
6. Recommendations
- Continue periodic NDT inspections at Joint 27 to monitor crack dimensions over time.
- Establish a baseline crack measurement log for trend analysis during future turnarounds.
- If any crack growth is detected, immediately re-evaluate using updated FFS Level 3 analysis.
- Maintain operating pressure within documented limits (≤ 154.7 kg/cm²).
- Avoid pressure transients or thermal shocks that could alter crack-tip stress state.
- Document the FFS assessment results for regulatory and insurance compliance.
Download the Full Technical Case Study
The full technical report includes:
- API 579 Level 3 crack assessment with J-integral and FAD evaluation
- Stress intensity factor (SIF) and crack stability analysis
- Elastic-plastic FEA methodology with multilinear material modeling
- Load step-based analysis and crack modelling approach
- Stress contour plots and crack region results
- Validation including hand calculation comparison and accuracy checks
- Code references including API 579-1 / ASME FFS-1 and ASME Section VIII