Unsuccessful exposure to hydrogen sulfide containing production fluids occurred in API N-80 and P-110 grades in the early 1950's and prompted the development of metallurgically improved and strength restricted C-75, L-80 and C-90 grade tubulars for sour service. As deeper, higher pressure wells were drilled, the need for higher pressure wells were drilled, the need for higher performance grades escalated. For wells deeper than performance grades escalated. For wells deeper than about 17,500 feet, economics justified higher performing quality tubulars, yet C-100 or higher performing quality tubulars, yet C-100 or higher performing tubulars remained unavailable. performing tubulars remained unavailable. Following similar developments in other industries facing a need for higher performing, fracture resistant materials, Lone Star Steel Company has combined advanced metallurgical and processing techniques with ID and OD imperfection processing techniques with ID and OD imperfection removal and cold forming steps to upgrade both dimensional control and steel quality of tubulars. The result is a more conservative approach which conveys C-100 performance for the LSS-95 SSGS grade. A method for statistical analysis of dimensional properties is derived and its relationship to properties is derived and its relationship to manufacturing process design and product dimensional compliance is presented for plain-end OCTG casing.
Introduction
Although the brittle failure of steel under hydrogen sulfide conditions had long been a problem to the materials engineer, failures of API tubing, stainless steel wire lines and alloy steel fishing tools became a severe production problem in the early 1950's. The 1952 NACE symposium on sulfide stress corrosion outlined the basic engineering parameters which guided future metallurgical parameters which guided future metallurgical practice. In March, 1963, the NACE Calgary Area practice. In March, 1963, the NACE Calgary Area T1B Committee issued tentative guidelines for valves and tubular goods for sour service that evolved into the present API C-75 tubular and NACE MR-01-75 materials requirements specifications. The improved sulfide stress cracking resistance of quench and tempered tubular goods led to proprietary "Modified N-80" and "C-90" tubulars which then led to the adoption of API L-80 and work toward C-90 specification. Further processing refinements i.e., full martensitic transformation, high tempering practice, very clean, fine grained steels and low residual stresses have extended the useful minimum strength range of tubulars to 95,000 psi for ambient temperature exposure to wet, sour gas. No manufacturer or user has yet successfully proposed the use of a 100,000 psi or higher minimum yield strength tubular for severe sour gas service. This performance limitation remains a severe obstacle in the completion of deep, highly pressured, sour gas hydrocarbon reservoirs.
Following similar materials advances in other industries where improved toughness and imperfection size control were needed to increase performance, Lone Star Steel Company has taken advantage of both metallurgical and dimensional improvement avenues in developing a C-100 performance equivalent, sour gas tubular option. Imperfection size, wall thickness, diameter, ovality and inspectability have been improved beyond API Spec 5AX dimensional requirements so that the necessary improvement in burst and collapse properties may be realized. Steel cleanliness, heat treatment and SSC improvement are features of its new LSS-95 SSGS sour gas product. Combining these dimensional improvements with LSS-95 SSGS metallurgical quality offers a prudent option for achieving a C-100 performance equivalent grade in burst and collapse performance equivalent grade in burst and collapse resistance.
DIMENSIONAL QUALITY
The simplicity of the tubular shape allows its quality to be easily analyzed by statistical methods amenable to hand held calulators using relatively few measurements of its diameter and wall thickness. Detection and measurement of wall imperfections by nondestructive testing methods improves significantly with the surface quality of the pipe. The frequency and severity of wall imperfections are reduced when surface discountinvities introduced during primary manufacturing steps are removed prior to subsequent hot and cold metal forming operations.
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