API casing refers to steel pipes used for oil and gas industry applications, specifically for providing structural support to oil wells during drilling and production operations. They are a critical component that helps prevent contamination and ensure the stability of the well.
①Well Casing and Casing String
Well casing is made of high-quality steel, either seamless or welded tubing, with tapered threads machined at both ends. Most casings are connected using casing couplings to form a casing string. The casing string is used to seal off exposed rock formations in the wellbore. Oil well tubing has its specific standards, and each type should comply with these standards. The standards currently used in China are similar to those of the American Petroleum Institute (API).
API standards require 14 different nominal outside diameters ranging from 114.3 mm (4½ in) to 508 mm (20 in).
API casings come in various wall thicknesses. API standards specify wall thickness ranges from 5.21 to 16.13 mm for different types of casing. Generally, smaller diameter casings have thinner walls, while larger diameter casings have thicker walls.
In addition to standard steel grades and wall thicknesses, non-standard grades and thicknesses also exist, which are permitted by API standards.
②Casing connection threads are all tapered
API standard casing connection threads come in four types: Short Round Thread (STC), Long Round Thread (LTC), Buttress Thread (BTC), and Extreme Line Thread (XL). Non-standard threads also exist.
Round threads have a taper of 1:16 and a pitch of 3.175 mm (8 threads per inch). Buttress threads have a taper of 1:16 (for outside diameters <16 in) and 1:12 (for outside diameters ≥16 in), with a pitch of 5.08 mm (5 threads per inch).
A casing string is typically composed of casings with the same outside diameter but potentially different steel grades and wall thicknesses, connected using couplings. It should meet the requirements for strength and production needs.
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