API 5CT is standard technical specifications for steel casing and tubing pipes used for oil wells in petroleum and natural gas industries.Besides casing and tubing, it also includes pup joints, coupling stock, coupling material, and accessory materials, and establishes requirements for three product specification levels (PSL-1, PSL-2, and PSL-3). The requirements for PSL-1 are the basis of this standard.Besides casing and tubing, it also includes pup joints, coupling stock, coupling material, and accessory materials, and establishes requirements for three product specification levels (PSL-1, PSL-2, and PSL-3). The requirements for PSL-1 are the basis of this standard.