Steel grades for oil country tubular goods (OCTG) casing include H40, J55, K55, N80, L80, C90, T95, P110, Q125, V150, etc.; end connection types include short round thread (STC), long round thread (LTC), buttress thread (BTC), and special connections.
Weight Calculation
[(Outer Diameter - Wall Thickness) × Wall Thickness] × 0.02466 = kg/m (weight per meter)
Chemical Composition
(1) In accordance with SY/T 6194-96. Casing and its couplings are made of the same steel grade. Sulfur content < 0.045%, phosphorus content < 0.045%;
(2) Chemical analysis samples are taken in accordance with GB/T 222-84. Chemical analysis is conducted according to the relevant sections of GB 223;
(3) In accordance with API SPEC 5CT (1988, 1st Edition). Sample preparation for chemical analysis follows the latest version of ASTM E59, and analysis follows the latest version of ASTM E350.
Physical Properties
(1) In accordance with SY/T 6194-96. Flattening tests (GB 246-97), tensile tests (GB 228-2010), and hydrostatic tests are performed;
(2) In accordance with API SPEC 5CT (1988, 1st Edition): hydrostatic tests, flattening tests, sulfide stress corrosion cracking tests, hardness tests (per the latest version of ASTM E18 or E10), tensile tests, transverse impact tests (per the latest versions of ASTM A370, ASTM E23, and relevant standards), and grain size determination (per the latest version of ASTM E112 or other methods).
Collapse resistance is a critical performance indicator for oil casing; research utilizes prediction formulas such as the K-T model, considering influencing factors such as manufacturing quality (e.g., ovality and residual stress). Resistance to CO₂/H₂S corrosion is achieved through cost-effective designs, such as low chromium content; corrosion mechanisms and analysis techniques involve SEM, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).
The development of high-grade materials-ranging from 125 ksi to 165 ksi-meets the demands of ultra-deep wells; the performance metrics of these high-strength, high-toughness oil casings represent an improvement of over 50% compared to conventional technologies.
Thread inspection utilizes non-contact, in-line techniques-such as area-scan CCD and digital image processing-to enhance accuracy and efficiency while reducing failure incidents.
