Oil country tubular goods (OCTG) are the pipes used to drill, line and complete oil and gas wells, and API 5CT Gr.N80-1 Casing Pipe is one of the grades in this family. This article explains the grade, the API 5CT standard that governs it, the mechanical properties that matter downhole, and how casing and tubing are selected for a well design.
API 5CT (ISO 11960) governs casing, tubing and their connections.
The number in the grade name is the minimum yield strength in ksi.
Sour-service grades such as L80 control hardness against sulfide stress cracking.
Threading and make-up torque are as critical as the pipe body.
Well design decides grade, weight, connection and PSL.
What Are Casing and Tubing?
Casing is the large-diameter pipe that lines the borehole and gives the well structural integrity; tubing is the smaller pipe inside the casing that carries produced fluids to the surface. Both are covered by API 5CT, the specification for casing and tubing, together with the couplings and threading used to join them. Grades are designated by a letter and a number, such as N80, L80 or T95, where the number is the minimum yield strength in ksi.
Governing Standard: API 5CT
API 5CT covers seamless and welded casing and tubing in a range of grades and three product specification levels. It defines the chemistry, mechanical properties, dimensions, weights, threading, inspection and testing requirements for each grade. ISO 11960 is the international equivalent standard, and a pipe certified to either may satisfy the same order.
Grade Properties and Heat Treatment
The mechanical properties of OCTG grades come from chemistry plus heat treatment. N80 is available in two classes (N80-1 and N80-Q), L80 is a low-carbon, chromium-containing grade that resists sour service, and T95 is a quenched and tempered grade for deeper, higher-pressure wells. Yield strength, tensile strength, hardness and impact toughness are verified by testing per heat, and hardness control is critical because hard zones are vulnerable to sulfide stress cracking.
Threading and Couplings
Casing and tubing are joined with threaded couplings. The most common connections are the API round thread (STC/LTC) and buttress thread for casing, and the non-upset (NU) and external-upset (EU) threads for tubing. Premium connections add a metal-to-metal seal for high-pressure and high-temperature wells. Thread protection, handling and make-up torque follow the connection manufacturer instructions, because most downhole failures start at connections.
Sour Service and Special Requirements
For wells containing hydrogen sulfide, grades such as L80 and C90 with controlled hardness and toughness are specified to resist sulfide stress cracking per NACE MR0175/ISO 15156. The order must state the service class, test temperature and any additional testing such as through-wall hardness surveys or impact tests, so the mill can supply the right product.
Selection and Ordering
Selecting the OCTG grade starts from the well design: casing depth, collapse and burst pressures, tension loads, temperature and the corrosivity of the produced fluids. Each grade has published pipe body and connection performance properties. The order should state grade, size, weight, connection type, PSL, and the edition of API 5CT, and the delivery should include mill certificates with full traceability.
Frequently Asked Questions
It is the minimum yield strength in ksi. For example, N80 means a minimum yield strength of 80 ksi, L80 means 80 ksi with a low-carbon sour-service chemistry, and T95 means 95 ksi quenched and tempered.
Casing lines the borehole and supports the well structure; tubing is installed inside the casing to carry produced fluids. They have different diameter ranges and are both covered by API 5CT.
API 5CT (with ISO 11960 as the international equivalent) covers casing and tubing, their couplings, threading, inspection and testing.
Service where the well fluids contain hydrogen sulfide, which can crack hard steels. Sour-service grades such as L80 and C90 have controlled hardness and are selected per NACE MR0175/ISO 15156.
Hard zones in casing and tubing are vulnerable to sulfide stress cracking and other hydrogen-related failures, so API 5CT sets hardness limits and requires hardness testing per heat.
Grade, size, weight per unit length, connection type, product specification level, edition of API 5CT, service class (sweet or sour) and the required documentation.
