On the Calibration of Design Collapse Strengths for Quenched and Tempered Pip
- A.J. Adams (Amerada Hess) | P.W. Moore (Grant Prideco) | M.L. Payne (BP plc)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2003
- Document Type
- Journal Paper
- 214 - 227
- 2003. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 1.6 Drilling Operations, 1.14.1 Casing Design, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating
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The joint API/ISO Work Group 2b under Steering Committee 5 (SC5) for tubular goods is currently rewriting API Bull. 5C3/ISO 10400 for casing and tubing performance property equations. This paper describes the proposed new treatment for collapse of quenched and tempered (Q&T) pipe.
The work is still in progress, and the final text has yet to be agreed upon, but the present results are considered reasonably mature and have been circulated as a committee draft for international comment.
Committee work has shown that the current (sixth edition) rating equations give highly variable risk over the D o/t range for downhole well tubulars, whereas the proposed new equations give nearly uniform risk. Different ratings are proposed for cold- and hot-straightened pipe. Overall, they result in minor cost savings with respect to the sixth edition of API Bull. 5C3 collapse strengths.
Limitations of Existing Approach.
The technical basis of the current Bull. 5C3 1 was developed in the early 1960s. Various limitations have been identified since its first publication.
The collapse-test data,2 upon which the equations are based, are now rather old, and pipe production practices have improved considerably in the intervening period. Therefore, it is possible that the equations do not accurately represent the performance of modern tubulars.
The majority of the collapse tests were for short specimens (L/Do=2), which are now known to overestimate the collapse strength of real pipe. 3-6
The collapse-strength equations result in a widely varying margin between the ultimate and design collapse strengths over the Do/t range for well tubulars7 and, thus, also in predicted failure probability (pf).8
The mean value equations are relatively poor predictors of the ultimate collapse strength compared to modern formulations, which have been shown to be much more accurate.9
The same equations were used for both Q&T and non-Q&T pipe. However, recent work10,11 has shown that the two classes have different collapse behaviors and, therefore, need individual strength equations.
The collapse-test specimens were manufactured using a wide range of production methods [e.g., seamless and electric resistance welded (ERW), cold and hot rotary straightening (CRS and HRS, respectively)], and no attempt was made to determine the various effects on collapse strength. Subsequent work11 has demonstrated that both straightening and heat treatment have a significant effect on the collapse strength and, thus, should be included explicitly in any modern treatment.
The present formulation for plastic collapse strength is implicitly based on the assumption that collapse strength is proportional to the specified minimum, rather than the actual, yield stress. This is acceptable as long as the ratio of actual to specified minimum yield stress is constant for all grades. However, analysis of production quality data9 has shown that this ratio varies considerably by grade; thus, the current treatment can and should be improved.
Finally, the present approach cannot accommodate non-API grades, such as high-collapse (HC) pipe.
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