API 5CT Casing Pipe

Casing Grades and How They Are Chosen

Casing grade is selected from a calculated string design covering burst, collapse, tensile and triaxial load cases, not from well depth alone. The lower grades J55 and K55 share the same minimum yield strength of 379 to 552 MPa and the same chemistry; they differ only in minimum tensile strength, 517 MPa for J55 and 655 MPa for K55. They are the cheapest option and are used in shallow, low pressure wells and in surface strings. N80 in both Type 1 and Type Q (quenched and tempered) is the standard workhorse for intermediate and production casing at medium depth. L80 is specified where the well produces wet H2S, because its chemistry and hardness limit are controlled to resist sulfide stress cracking. C90, C95, T95 and R95 extend into deeper, higher pressure and more severe sour service. P110 is the standard high strength grade for deep, high pressure wells, and Q125 is reserved for ultra-deep, high pressure high temperature applications.

Connection Types: STC, LTC, BTC and Premium

The connection is often the limiting element in a casing string, so it should be selected with the same care as the pipe body. STC (short round thread coupling) has the shortest thread engagement and gives the lowest joint strength; it is used for shallow wells and low pressure service. LTC (long round thread coupling) has a longer thread, distributing load over more turns and giving higher joint efficiency, and suits medium depth wells. BTC (buttress thread coupling) uses a trapezoidal thread form with a steeper load flank, which gives substantially higher tensile efficiency and better resistance to jump-out under axial load, and is the standard choice for deep and high pressure wells. Premium connections use a metal-to-metal seal plus a torque shoulder instead of relying on thread compound for sealing. They provide gas-tight performance under combined load and are specified for HPHT, gas and offshore wells. Premium connections are proprietary, so confirm compatibility if you are mixing with an existing string.

Length Ranges R1, R2 and R3

API 5CT defines three length ranges for casing and tubing. R1 covers 4.88 to 7.62 m, R2 covers 7.62 to 10.36 m, and R3 covers 10.36 to 14.63 m. R3, sometimes called range 3, corresponds roughly to 40 feet and is the most commonly ordered in modern operations because fewer connections per string means faster running and fewer potential leak paths. R2 is still widely used where handling equipment limits joint length. When ordering, state the length range explicitly, because mills may otherwise ship a mix and the resulting string tally will be more complex on the rig floor.