What Is an API Oil Casing
API oil casing is the steel pipe that lines an oil or gas well after drilling. It is run into the borehole and cemented in place to support the well wall, isolate formations and contain wellbore pressure. The product, the dimensions and the steel grades are controlled by API Specification 5CT, and the sizes most commonly ordered are 4-1/2 inch, 5-1/2 inch, 7 inch, 8-5/8 inch, 9-5/8 inch and 13-3/8 inch outside diameter.
This specification sheet summarizes the dimensional and mechanical data a purchaser needs to check when buying casing: outside diameter, wall thickness, theoretical weight, inner diameter, coupling dimensions, thread type, steel grade and length range. Data in the tables follow API 5CT published values, with the obvious transcription errors of the original table corrected.
Casing Steel Grades and What They Mean
The grade letter and number in API 5CT indicate the minimum yield strength in thousands of pounds per square inch. The three grades in the standard casing program are J55, N80 and P110:
| Grade | Min yield, MPa (ksi) | Min tensile, MPa (ksi) | Typical use |
|---|---|---|---|
| J55 | 379 (55) | 517 (75) | Shallow wells, low-pressure service, low cost |
| N80 | 552 (80) | 689 (100) | Medium-depth wells, the most common general grade |
| P110 | 758 (110) | 862 (125) | Deep wells, high pressure, higher collapse and burst rating |
Yield strength is the design basis for collapse and burst resistance. At the same wall thickness, P110 resists roughly twice the internal pressure of J55, which is why the grade choice has a direct effect on both safety and cost.
Casing Dimension and Weight Table
The table lists the standard API 5CT casing sizes with nominal wall thickness, theoretical plain-end weight and inner diameter. Weights are shown in kg/m with lb/ft in brackets; inner diameters are shown in mm with inches in brackets. Coupling outside diameters are for standard round and buttress thread couplings.
| OD, mm (in) | Wall, mm (in) | Weight, kg/m (lb/ft) | ID, mm (in) | Coupling OD, mm (in) | Thread | Grades |
|---|---|---|---|---|---|---|
| 139.7 (5-1/2) | 6.20 (0.244) | 23.09 (15.50) | 127.3 (5.012) | 153.7 (6.050) | STC, LTC, BTC | J55, N80, P110 |
| 6.98 (0.275) | 25.32 (17.00) | 125.7 (4.950) | ||||
| 7.72 (0.304) | 29.79 (20.00) | 124.3 (4.892) | ||||
| 9.17 (0.361) | 34.26 (23.00) | 121.4 (4.778) | ||||
| 10.54 (0.415) | 38.73 (26.00) | 118.6 (4.670) | ||||
| 177.8 (7) | 5.87 (0.231) | 25.30 (17.00) | 166.1 (6.539) | 194.5 (7.656) | STC, LTC, BTC | J55, N80, P110 |
| 6.91 (0.272) | 29.76 (20.00) | 164.0 (6.456) | ||||
| 8.05 (0.317) | 34.23 (23.00) | 161.7 (6.366) | ||||
| 9.19 (0.362) | 38.69 (26.00) | 159.4 (6.276) | ||||
| 10.36 (0.408) | 43.16 (29.00) | 157.1 (6.180) | ||||
| 11.51 (0.453) | 47.62 (32.00) | 154.8 (6.090) | ||||
| 12.65 (0.498) | 52.09 (35.00) | 152.5 (6.004) | ||||
| 13.72 (0.540) | 56.55 (38.00) | 150.4 (5.920) | ||||
| 219.1 (8-5/8) | 6.71 (0.264) | 35.72 (24.00) | 205.7 (8.098) | 244.5 (9.625) | STC, LTC, BTC | J55, N80, P110 |
| 7.72 (0.304) | 41.68 (28.00) | 203.7 (8.020) | ||||
| 8.94 (0.352) | 47.62 (32.00) | 201.2 (7.927) | ||||
| 10.16 (0.400) | 53.58 (36.00) | 198.8 (7.827) | ||||
| 11.43 (0.450) | 59.53 (40.00) | 196.2 (7.724) | ||||
| 244.5 (9-5/8) | 7.92 (0.312) | 48.06 (32.30) | 228.7 (9.004) | 269.6 (10.625) | STC, LTC, BTC | J55, N80, P110 |
| 8.94 (0.352) | 53.52 (36.00) | 226.6 (8.921) | ||||
| 10.03 (0.395) | 59.52 (40.00) | 224.4 (8.835) | ||||
| 11.05 (0.435) | 64.73 (43.50) | 222.4 (8.756) | ||||
| 11.99 (0.472) | 69.94 (47.00) | 220.5 (8.681) | ||||
| 13.84 (0.545) | 79.62 (53.50) | 216.8 (8.535) |
The full API 5CT casing program extends from 4-1/2 inch to 20 inch outside diameter, and tubing from 1.050 inch to 4-1/2 inch, in wall thicknesses beyond those listed here. Always confirm the exact weight and wall combination against the API 5CT tables issued with the material certificate, because the plain-end and coupling-end weights differ slightly.
Thread Types: Round, Buttress and Premium
Round thread (STC, LTC). Short and long round threads are the standard casing connection. They seal by thread compound and are economical, but they are not gas-tight under all conditions.
Buttress thread (BTC). The offset or buttress thread has a square load flank that carries high axial loads, making it the standard choice for heavy casing and for pipe that must be pulled or set in tension.
Premium connections. Proprietary metal-to-metal seal connections are used for high-pressure, high-temperature and gas wells where a fully gas-tight joint is required. Each premium connection is rated for its own pressure and tension envelope and is supplied with its own running procedure.
Length Ranges and Inspection
API 5CT defines three length ranges: R1 from 6.10 to 7.32 m, R2 from 8.53 to 9.75 m and R3 from 11.58 to 12.80 m. Common mill supply is 8 to 12 m, covering R2 and R3. Every joint is inspected for surface defects, measured for out-of-roundness and wall variation, and hydrostatically tested. On the order, state whether the pipe must also be full-length ultrasonically tested and whether third-party inspection is required.
Purchasing Checklist
Standard and specification: API 5CT, latest edition, with the exact grade, J55, N80 or P110
Size: OD and weight per foot, for example 139.7 mm x 23.09 kg/m
Connection: STC, LTC, BTC or a named premium connection, with thread compound
Length range: R2, R3, or random with minimum average
Testing: hydrostatic, full-length UT, third-party inspection, and the required certificates
FAQ
Q1: What does the grade number in J55, N80 and P110 mean?
The number is the minimum yield strength in thousands of psi: 55, 80 and 110 ksi, equal to 379, 552 and 758 MPa. Higher numbers mean higher collapse and burst resistance.
Q2: What is the difference between casing and tubing?
Casing lines the wellbore and is cemented in place; tubing is the smaller pipe run inside the casing to carry produced fluids. Both are covered by API 5CT but have different size ranges and connection designs.
Q3: Which wall thickness should I choose?
Select the wall that gives the required collapse and burst rating for the well depth and pressure, then verify the weight against the API 5CT table. Do not mix wall and weight values from different rows of a supplier table.
Q4: Are N80 and N80Q the same?
Both have the same 80 ksi minimum yield, but N80Q is quenched and tempered, which gives better toughness and is often required for sour service. The purchase order must state which heat treatment is required.
Q5: What length range is standard?
API 5CT ranges are R1, R2 and R3, corresponding to 6.10 to 7.32 m, 8.53 to 9.75 m and 11.58 to 12.80 m. Mill supply of 8 to 12 m covers R2 and R3.
Q6: What tests are required on casing?
Each joint is hydrostatically tested; additional full-length ultrasonic, eddy current or magnetic flux leakage testing can be specified. Third-party inspection is usually agreed on the order.
