API Specification 5CT N-80 Grade Casing May Burst or Part Unexpectedly if Supplementary Metallurgical Requirements are not Specified

摘要

N-80 grade casing is a preferred casing grade for production casing strings. Most often, N-80 casing is ordered to API Specification 5CT without specifying the appropriate supplementary requirements. Casing strings that meet the minimum strength requirements of API Specification 5CT have experienced sudden failures during normal workover and well stimulation operations. This paper presents a case study and lessons learned from the analysis of such a casing failure. The paper is based on the results of metallurgical analyses performed during the investigation. This paper details the standard manufacturing operations and heat treatment processes performed for N-80 casing ordered to API Specification 5CT, and the result of these operations on the casing's final material properties. By re-heat treating the failed casing with published supplementary requirements in mind, the casing material properties were improved substantially. If the casing initially had the properties of the re-heat treated steel, the failures would likely have been avoided. Factors that contributed to the N-80 casing failures are presented and the case is made for specifying supplementary tests for verification of fracture toughness.