A Comprehensive Update in the Evaluation of Pipeline Weld Defects
Girth weld defect acceptance criteria are set and enforced in all pipeline constructions in the U.S. per federal regulations (CFR 49 Parts 192 and 195). With the increased use of mechanized welding and AUT (Automated Ultrasonic Testing) in new pipeline constructions, alternative defect acceptance criteria based on ECA (Engineering Critical Assessment) principles are frequently used in lieu of the traditional workmanship criteria. The alternative defect acceptance criteria in the current Appendix A of API 1104 have remained largely unchanged since its introduction in the early 1980’s. In the meantime, the characteristics of the linepipe materials, welding processes, and construction practice have evolved since the adoption of the code. The recent surge in the use of mechanized welding/AUT/ECA created a mismatch between the new materials/welding processes and the outdated alternative defect acceptance criteria. Looking ahead, the trend in pipeline construction is moving towards larger diameter and higher strength linepipes, such as X80, X100, and even X120. The characteristic of these ultra-high strength materials and their welding processes make the use of the current Appendix A highly questionable.
This report presents the girth weld defect assessment procedures for stressbased design. The major components of this report are (1) technical basis for the development of the revised girth weld defect acceptance criteria, (2) validation of the acceptance criteria against experimental test data, and (3) recommended structure for the revision of API 1104 Appendix A. The main body of the report is written in such a way that it can be easily turned into code language. The supporting data, both analytical and experimental, are given in the appendices. Examples are given to show the use of the new assessment procedures. Comparisons in defect acceptance criteria are made between the new procedures and the current API 1104 Appendix A.
The new proposed procedures have two options. Option 1 is given as an easy to use graphical approach, whereby allowable flaw dimensions can be determined on the basis of a somewhat more restrictive minimum toughness level. Option 2 provides more flexibility and generally allows larger flaws, at the expense of more complicated calculations.
In comparison to the current API 1104 Appendix A, the major advantages of the newly proposed procedures are:
- Consistent level of conservatism
- Inclusion of both plastic collapse and fracture criteria. The current API 1104 Appendix A includes only fracture criterion.
- The acceptance criteria are easier to use for the most frequently occurring defects in modern pipeline construction.
- Reduced minimum CTOD toughness requirements, accompanied by tighter defect tolerance, allows wider application of the alternative acceptance criteria.