Engineering Critical Analysis (ECA) is a sophisticated approach employed to determine alternative acceptance criteria for weld defect lengths versus depths. As per the guidelines outlined in API 1104 – Option 2, ECA provides a framework to assess the structural integrity of pipeline girth welds by considering critical parameters such as fracture toughness, residual stresses, and combined axial stresses. This blog post delves into the detailed methodology for performing ECA in the context of Saudi Aramco’s Jafurah Gas Compression Plants and PWIS (Package-1) project. TCR has undertaken this work as a vendor for MANARAH AL-JUBAIL CONT. CO. LTD., with Saudi Aramco being the end client.
Project Scope
The scope of this analysis includes:
Assessment of girth welds:
Full circumferential welding of the pipeline using the same Welding Procedure Specification (WPS) intended for actual fabrication.
Specimen preparation from specified positions of the pipe—12’o clock, 6’o clock, and either 3’o clock or 9’o clock positions.
Laboratory Testing:
Fracture Toughness Tests: Conduct six Crack Tip Opening Displacement (CTOD) tests—3 from weld metal and 3 from Heat Affected Zone (HAZ).
Impact Tests: Perform six tests—3 each from the weld and HAZ.
Tensile Tests: Conduct tests with the weld centered in the specimen (up to 3 samples).
Finite Element Analysis (FEA):
Utilize advanced FEA tools to model stress distribution and calculate the combined axial stresses (σa) and related factors.
ECA Report Submission:
Develop a comprehensive ECA report aligned with API 1104 – Annex A, Option 2.
Detailed Methodology
A. Specimen Collection and Preparation
The customer is required to weld a full circumference of the pipe using the actual WPS for fabrication. From this, three cut pieces measuring 350mm x 350mm will be extracted from specific positions:
12’o clock: Top section.
6’o clock: Bottom section.
3’o clock or 9’o clock: Side sections.
These specimens will be sent to TCR’s laboratory for further analysis without additional cutting to preserve their structural integrity.
B. Testing Procedures
1. Fracture Toughness Testing
Crack Tip Opening Displacement (CTOD) tests will be performed on:
3 samples from the weld metal.
3 samples from the HAZ.
This evaluates the material’s toughness and its ability to withstand crack propagation at the Minimum Design Metal Temperature (MDMT).
2. Impact Testing
Impact energy absorption will be assessed on:
3 samples from the weld metal.
3 samples from the HAZ.
3. Tensile Testing
Tensile tests will be conducted to evaluate the weld joint’s overall strength under uniaxial loading.
C. Finite Element Analysis (FEA)
Using advanced software, TCR experts will perform FEA to:
Model stress distributions across the girth weld joint.
Calculate combined axial stresses (σa).
Simulate real-world loading conditions.
D. ECA Report Compilation
A detailed report will be prepared based on the following:
Results from CTOD, impact, and tensile tests.
Stress analysis findings from FEA.
Tabulated tolerable defect sizes based on percentage of wall thickness (e.g., 10%, 20%, 30%, etc.) and circumferential length.
API 1104 Compliance: Option 1 vs Option 2
Option 2: Comprehensive Analysis
This approach involves a detailed evaluation using multiple test specimens from different orientations, ensuring a robust assessment of weld integrity.
Option 1: Simplified Analysis
If a general ECA is required, the following will be performed:
One CTOD test.
One impact test.
One tensile test.
This approach provides a basic evaluation but may lack the depth of Option 2.
Key Considerations
Pipeline Wall Thickness: Each ECA assessment will address variations in pipeline wall thickness. The objective is to define tolerable defect sizes as a percentage of wall thickness and in absolute units (millimeters).
Residual Stresses: Residual stresses will be estimated following Annexure 9D of API 579/ASME FFS-1.
Material Restrictions: WPS must be limited to materials with carbon equivalents (CE-IIW) not exceeding the qualification coupon. This ensures adequate hardness and consistency with production conditions.
Conclusion
TCR’s expertise in performing advanced ECA ensures comprehensive evaluation of pipeline girth welds. With capabilities extending to CTOD assessments, FEA, and real-world damage analysis, we deliver actionable insights to ensure the structural integrity and safety of critical pipeline systems. Whether you require API 1104-compliant evaluations or customized solutions, TCR in India as well TCR Arabia in Saudi Arabia are your partner in achieving excellence in engineering assessments.