What is API 5CT ?

Date:2024-03-22Tags:Casing and Tubing, casing pipe, pipe, API 5CT


Introduction: Why API 5CT Matters in Oil & Gas

API 5CT is a critical international standard developed by the American Petroleum Institute (API), specifically for casing and tubing used in oil and natural gas wells. It plays a central role in maintaining well integrity, ensuring production safety, and supporting efficient drilling operations across all major energy-producing regions.

As a widely adopted specification, API 5CT defines the technical delivery conditions, product grades, and mechanical requirements for casing pipes and tubing. Manufacturers and energy companies across the globe rely on it to ensure compliance, interoperability, and high safety standards.



What is API 5CT?

API 5CT is the “Specification for Casing and Tubing” published by the American Petroleum Institute. It governs the materials, sizes, connections, and performance properties of steel pipes used in oil and gas wells. These pipes are categorized as:
Casing: Structural support for the wellbore.
Tubing: Pathway for transporting oil or gas from the reservoir to the surface.



What is the Difference Between API 5CT Casing and Tubing?

API 5CT casing and tubing serve different but equally essential roles in oil and gas well operations. Casing is a larger-diameter steel pipe inserted into the wellbore during drilling to stabilize the well, prevent formation collapse, and isolate different pressure zones. It is permanently cemented into place and forms the structural backbone of the well. In contrast, tubing is a smaller-diameter pipe installed inside the casing after drilling is complete. Its primary function is to transport oil and gas from deep underground reservoirs to the surface. While casing must withstand external formation pressures, tubing is designed to endure internal pressure from produced fluids and is often replaced over time due to wear. In summary, casing ensures well integrity, while tubing ensures efficient fluid production.
Casing and Tubing


Scope of the API 5CT Standard

The API 5CT standard defines the technical delivery conditions for the following products:
Casing pipes (surface, intermediate, and production casing)
Tubing pipes
Pup joints (short segments of pipe)
Coupling blanks and coupling materials
Associated accessory materials

API 5CT establishes three Product Specification Levels (PSL):
PSL-1: Basic requirements (default standard)
PSL-2: Additional technical and testing requirements
PSL-3: Higher performance levels for critical applications
PSL-1 serves as the base requirement, while PSL-2 and PSL-3 introduce stricter controls on properties such as chemical composition, mechanical testing, and non-destructive inspection. These advanced levels apply to most steel grades, except for H40, L80 9Cr, and C110, which follow their specific provisions.

If mutually agreed upon by the buyer and the manufacturer, API 5CT can also be applied to plain-end pipes of other wall thicknesses or dimensions beyond the standard range.


API 5CT Steel Grades

API 5CT covers a variety of steel grades for different well conditions:


Grade Typical Application Yield Strength (psi)
H40 Mild depth wells 40,000
J55 Shallow to medium wells 55,000
K55 Similar to J55 but higher strength 55,000
N80 Sour service, moderate strength 80,000
L80 Deep wells and sour gas applications 80,000
C90 High collapse resistance 90,000
T95 Sour service with corrosion resistance 95,000
P110 Ultra-deep and high-pressure wells 110,000
Q125 Ultra-high-pressure applications 125,000

Each grade may have subtypes (e.g., L80 Type 1, Type 9Cr) with different performance in sour environments.



Thread Connections

API 5CT specifies standard thread types to ensure compatibility:
Thread Type Description
STC Short round thread coupling
LTC Long round thread coupling
BTC Buttress thread coupling
NU Non-upset tubing connection
EU External upset tubing connection
IJ Integral joint tubing (no separate coupling)


Test and Inspection of API 5CT Tubing and Casing

- Hydrostatic Test
This test verifies the pipe’s ability to withstand internal pressure. Water is pumped into the pipe at a specified pressure level to check for leaks. Pipes that fail are repaired and retested, ensuring safe fluid transport under pressure.

- Bend Test
A sample is bent to check the weld’s integrity. If cracks appear, the pipe is rejected. This test ensures weld strength and overall product quality.

- Flattening Test
This test evaluates the pipe’s resistance to deformation and cracking. A sample is compressed along its length, then inspected for defects, confirming the pipe’s structural integrity.

- Charpy Impact (CVN) Test

Used to assess toughness, this test measures how much energy the material absorbs when struck. Key areas tested include the pipe body, weld seam, and heat-affected zone, ensuring durability in harsh conditions.



What is API 5CT OCTG?

API 5CT OCTG refers to a specific standard under the American Petroleum Institute (API) that governs Oil Country Tubular Goods (OCTG)—a category of high-strength steel pipe products used extensively in the drilling, completion, and production of oil and natural gas wells.

OCTG includes casing, tubing, and drill pipe, with API 5CT specifically defining the manufacturing, mechanical properties, testing, and inspection standards for casing and tubing pipes. These components are vital to the integrity of a well, providing structural support and enabling safe fluid transportation under extreme downhole conditions.


API 5CT OCTG Chart: Common Grades & Applications

Steel Grade Application PSL Available H₂S Resistance Tensile Strength (min)
J55 Standard shallow wells PSL-1, PSL-2 No 517 MPa
N80 Medium-depth wells PSL-1, PSL-2 Limited 689 MPa
L80 Sour service environments PSL-1, PSL-2 Yes 655 MPa
P110 Deep, high-pressure wells PSL-1, PSL-2 No 758 MPa

API 5CT Casing & Tubing Size Chart for Oil and Gas Wells

DN O. D. Weight W. T. End Processing
Non-upset coupling-thread Upset coupling-thread Steel Grade
in mm lb./ft. lb./ft. in mm H40 J55 L80 N80 C90 T95 P110
2 3/8 2.38 60.3 4 – 0.167 4.24 PU PN PN PN PN PN –
4.6 4.7 0.19 4.83 PNU PNU PNU PNU PNU PNU PNU
5.8 5.95 0.254 6.45 – – PNU PNU PNU PNU PNU
6.6 – 0.295 7.49 – – P – P P –
7.35 7.45 0.336 8.53 – – PU – PU PU –
2 7/8 2.88 73 6.4 6.5 0.217 5.51 PNU PNU – – – – –
7.8 7.9 0.276 7.01 – – – – – – –
8.6 8.7 0.308 7.82 – – PLB PLB PLBE – PLB
9.35 9.45 0.34 8.64 – – PLB PLB PLBE – PLB
10.5 – 0.392 9.96 – – PLB PLB PLB – PLB
11.5 – 0.44 11.18 – – – – PLB –  
3 1/2 3.5 88.9 7.7 – 0.216 5.49 PN PN PN PN PN PN –
9.2 9.3 0.254 6.45 PNU PNU PNU PNU PNU PNU PNU
10.2 – 0.289 7.34 PN PN PN PN PN PN –
12.7 12.95 0.375 9.52 – – PNU PNU PNU PNU PNU
14.3 – 0.43 10.92 – – P – P P –
15.5 – 0.476 12.09 – – P – P P –
17 – 0.53 13.46 – – P – P P –
4 4 102 9.5 – 0.226 5.74 PN PN PN PN PN PN –
– 11 0.262 6.65 PU PU PU PU PU PU –
13.2 – 0.33 8.38 – – P – P P –
16.1 – 0.415 10.54 – – P – P P –
18.9 – 0.5 12.7 – – P – P P –
22.2 – 0.61 15.49 – – P – P P –
4 1/2 4.5 114 12.6 12.75 0.271 6.88 PNU PNU PNU PNU PNU PNU –
15.2 – 0.337 8.56 – – P – P P –
17 – 0.38 9.65 – – P – P P –
18.9 – 0.43 10.92 – – P – P P PLB
21.5 – 0.5 12.7 – – P – P P PLB
23.7 – 0.56 14.22 – – P – P P PLB
26.1 – 0.63 16 – – P – P P PLB
P——Plain；N—— Non-upset coupling-thread；U—— Upset coupling-thread； L——Integral

- Specification & Size of API 5CT Casing

DN O. D. Weight W. T.   End Machining Form
  Steel Grade
in mm lb/ft kg/m in mm H40 J55 L80 N80 C90 P110
K55 T95
4 1/2 4.500 114.3 9.5 14.14 0.205 5.21 PS PS – – – –
10.5 15.63 0.224 5.69 – PSB – – – –
11.6 17.26 0.250 6.35 – PSLB PLB PLB PLB PLB
13.5 20.09 0.290 7.37 – – PLB PLB PLB PLB
15.1 22.47 0.337 9.56 – – – – – PLB
5 5.000 127 11.5 17.11 0.220 5.59 – PS – – – –
13 19.35 0.253 6.43 – PSLB – – – –
15 22.32 0.296 7.52 – PSLB PLB PLB PLBE PLB
18 26.79 0.362 9.19 – – PLB PLB PLBE PLB
21.4 31.85 0.437 11.1 – – PLB PLB PLB PLB
23.2 34.53 0.478 12.14 – – – – PLB –
24.1 35.86 0.500 12.7 – – – – PLB –
5 1/2 5.500 139.7 14 20.83 0.244 6.2 PS PS – – – –
15.5 23.07 0.275 6.98 – PSLB – – – –
17 25.3 0.304 7.72 – PSLB PLB PLB PLBE PLB
20 29.76 0.361 9.17 – – PLB PLB PLBE PLB
23 34.23 0.415 10.54 – – PLB PLB PLBE PLB
26.8 39.88 0.500 12.7 – – – – – –
29.7 44.2 0.562 14.27 – – – – – –
32.6 48.51 0.625 15.88 – – – – – –
35.3 52.53 0.687 17.45 – – – – – –
38 56.55 0.750 19.05 – – – – – –
40.5 60.27 0.812 20.62 – – – – – –
43.1 64.14 0.875 22.22 – – – – – –
6 5/8 6.625 168.28 20 29.76 0.288 7.32 PS PSLB – – – –
24 35.72 0.352 8.94 – PSLB PLB PLB PLBE PLB
28 41.67 0.417 10.59 – – PLB PLB PLBE PLB
32 47.62 0.475 12.06 – – PLB PLB PLBE PLB
7 7.000 177.8 17 25.3 0.231 5.87 PS – – – – –
20 29.76 0.272 6.91 PS PS – – – –
23 34.23 0.317 8.05 – PSLB PLB PLB PLBE –
26 38.69 0.362 9.19 – PSLB PLB PLB PLBE PLB
29 43.16 0.408 10.36 – – PLB PLB PLBE PLB
32 47.62 0.453 11.51 – – PLB PLB PLBE PLB
35 52.09 0.498 12.65 – – PLB PLB PLBE PLB
38 56.55 0.540 13.72 – – PLB PLB PLBE PLB
42.7 63.54 0.625 15.88 – – – – – –
46.4 69.05 0.687 17.45 – – – – – –
50.1 74.56 0.750 19.05 – – – – – –
53.6 79.77 0.812 20.62 – – – – – –
57.1 84.97 0.875 22.22 – – – – – –
7 5/8 7.625 193.68 24 35.72 0.300 7.62 PS – – – – –
26.4 39.29 0.328 8.33 – PSLB PLB PLB PLBE PLB
29.7 44.2 0.375 9.52 – – PLB PLB PLBE PLB
33.7 50.15 0.430 10.92 – – PLB PLB PLBE PLB
39 58.05 0.500 12.7 – – PLB PLB PLBE PLB
42.8 63.69 0.562 14.27 – – PLB PLB PLB PLB
45.3 67.41 0.595 15.11 – – PLB PLB PLB PLB
47.1 70.09 0.625 15.88 – – PLB PLB PLB PLB
51.2 76.19 0.687 17.45 – – – – – –
55.3 80.3 0.750 19.05 – – – – – –
8 5/8 8.625 219.08 24 35.72 0.264 6.71 – PS – – – –
28 41.62 0.304 7.72 PS – – – – –
32 47.62 0.352 8.94 PS PSLB – – – –
36 53.57 0.400 10.16 – PSLB PLB PLB PLBE PLB
40 59.53 0.450 11.43 – – PLB PLB PLBE PLB
44 65.48 0.500 12.7 – – PLB PLB PLBE PLB
49 72.92 0.557 14.15 – – PLB PLB PLBE PLB
9 5/8 9.625 244.48 32.3 48.07 0.312 7.92 PS – – – – –
36 53.57 0.352 8.94 PS PSLB – – – –
40 59.53 0.395 10.03 – PSLB PLB PLB PLBE –
43.5 64.73 0.435 11.05 – – PLB PLB PLBE PLB
47 69.94 0.472 11.99 – – PLB PLB PLBE PLB
53.5 79.62 0.545 13.84 – – PLB PLB PLBE PLB
58.4 86.91 0.595 15.11 – – PLB PLB PLB PLB
59.4 88.4 0.609 15.47 – – – – – –
64.9 96.58 0.672 17.07 – – – – – –
70.3 104.62 0.734 18.64 – – – – – –
75.6 112.5 0.797 20.24 – – – – – –
10 3/4 10.750 273.05 32.75 48.74 0.279 7.09 PS – – – – –
40.5 60.27 0.350 8.89 PS PSB – – – –
15.5 67.71 0.400 10.16 – PSB – – – –
51 75.9 0.450 11.43 – PSB PSB PSB PSBE PSB
55.5 82.59 0.495 12.57 – – PSB PSB PSBE PSB
60.7 90.33 0.545 13.84 – – – – PSBE PSB
65.7 97.77 0.595 15.11 – – – – PSB PSB
73.2 108.93 0.672 17.07 – – – – – –
79.2 117.86 0.734 18.64 – – – – – –
85.3 126.94 0.797 20.24 – – – – – –
11 3/4 11.750   42 62.5 0.333 8.46 PS – – – – –
47 69.94 0.375 20.24 – – – – – –
54 80.36 0.435 8.46 – – – – – –
60 89.29 0.489 9.53 – – – – – –
65 96.73 0.534 11.05 – – – – – –
71 105.66 0.582 14.42 – – – – – –
13 3/8 13.375 339.73 48 71.43 0.330 8.38 PS – – – – –
54.5 81.1 0.380 9.65 – PSB – – – –
61 90.78 0.430 10.92 – PSB – – – –
68 101.19 0.480 12.19 – PSB PSB PSB PSB PSB
72 107.15 0.514 13.06 – – PSB PSB PSB PSB
16 16.000 406.4 65 96.73 0.375 9.53 PS – – – – –
75 111.61 0.438 11.13 – PSB – – – –
84 125.01 0.495 12.57 – PSB – – – –
109 162.21 0.656 16.66 – P P P – P
18 5/8 18.625 473.08 87.5 130.21 0.435 11.05 PS PSB – – – –
20 20.000 508 94 139.89 0.438 11.13 PSL PSLB – – – –
106.5 158.49 0.500 12.7 – PSLB – – – –
133 197.93 0.635 16.13 – PSLB – – – –
P——Plain；S——Short-thread；L——Long-thread；B——Buttress thread；E——Extreme thread


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