API 5CT Tubing

API tubing specifications contain several provisions that are optional for the purchaser and other stipulations that are by agreement between the purchaser and the manufacturer. Some of these added provisions may be critical to a particular application; therefore, familiarity with API/ISO tubing...

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API tubing specifications contain several provisions that are optional for the purchaser and other stipulations that are by agreement between the purchaser and the manufacturer. Some of these added provisions may be critical to a particular application; therefore, familiarity with API/ISO tubing specifications is needed.

The tubing purchaser should be aware of API requirements and testing procedures.  All tubing should meet API minimum requirements. In critical wells, the purchaser may want to receive and review the manufacturer’s test results. For tubing used in sour wells (wells with H2S content greater than 0.05 psi partial pressure), the specific sour service requirements should be reviewed.

API developed specifications for three different connectors for use as tubing joints:

External-upset tubing and coupling

Non-upset tubing and couplings

Integral-joint tubing

API Spec. 5CT includes an illustration of API tubing joint connections. All three connections have tapered and round thread forms with either 8 or 10 threads/in., depending on the size. When casing is used as tubing, long-thread coupling/short-thread coupling and buttress-thread coupling connections can be specified.

The API external-upset-end (EUE) tubing connection is widely used because it is a good, serviceable connection in most wells. The EUE joint has a designed joint strength in tension and pressure strength greater than that of the pipe body and, therefore, is considered a 100% joint efficient connection. For proper lubrication and sealing, the joint requires a good thread compound as outlined in API RP 5A3.To improve the seal performance of API EUE tubing in high-pressure service, a grooved coupling, which accepts nonmetallic seal rings, is sometimes used in the coupling (see API Spec. 5CT SR 13). To provide more clearance, API special clearance EUE couplings are available. API EUE joints come in OD sizes of 1.050 to 4.500 in.

API non-upset (NUE) tubing is used much less than EUE tubing. The cost of NUE is only slightly less than EUE, and the joint strength is substantially less. The coupling joint diameter of NUE is less than EUE, which offers some advantages when clearance is small. API NUE joints are available in sizes of 1.050 to 4.500 in.

API integral-joint tubing is available in OD sizes of 1.315 to 2.063 in. API integral-joint tubing has a 10-round form with a joint strength that is less than the body minimum yield, which restricts its use. The small OD of integral-joint tubing permits its use inside larger tubing strings or in wells as unloading or vent strings. The couplings should meet all the minimum requirements outlined in API Spec. 5CT.API Spec. 5B3 and API RP 5B14 cover threading, gauging, and thread inspection.

Several proprietary (non-API) connections are available. These joints are useful when greater leak resistance or more clearance is needed than that provided by the standard API joints. These specialty joints obtain their improved properties through the following:

Unique thread profiles

A torque shoulder

Metal-to-metal seals

Seal rings

Internal upsets

External upsets

Integral joints


Before ordering or using a specific proprietary tubing connection in a critical well, the suitability of such a connection for a particular application must be assessed by either a review of service history or a comprehensive connection test program such as ISO 13679 for guidelines on use of Coiled tubing.

API tubing grade guidelines

The following guidelines apply to the use of API tubing grades.

H40—Although an API grade, H40 is generally not used in tubing sizes because the yield strength is relatively low and the cost saving over J55 is minimal. Suppliers do not commonly stock this grade.

J55—A commonly used grade for most wells when it meets the design criteria. Some operators recommend it be full-length normalized or normalized and tempered after upsetting when used in carbon dioxide or sour service (ring-worm corrosion problems); however, such heat treatments increase costs. J55 has been the "standard" grade for tubing in most relatively shallow (< 9,000 ft) and low-pressure (< 4,000 psi) wells on land.

C75—No longer an official API grade and generally not available. It was developed as a higher-strength material for sour service but was replaced by L80 tubing.

N80—A relatively old grade with essentially open chemical requirements. It is susceptible to H2S-induced SSC (acronym). It is acceptable for sweet oil and gas wells when it meets design conditions. The quenched-and-tempered heat treatment is preferred. The N80 grade is normally less expensive than L80 grades.

L80—A restricted yield-tubing grade that is available in Type 1, 9 Cr, or 13 Cr. Type 1 is less expensive than 9 Cr and 13 Cr but more subject to weight-loss corrosion. L80 Type 1 is used commonly in many oil and gas fields because of higher strength than J55. L80 is satisfactory for SSC resistance in all conditions but may incur weight-loss corrosion. Though popular in the past for CO2- and mild H2S-contaminated wells, Type 9 Cr largely has been replaced by Type 13 Cr. L80 13 Cr tubing has gained popularity because it has good CO2 -induced weight-loss corrosion resistance properties; however, it is more costly. Type 13 Cr may not be suitable in sour service environments. Typically, the H2S partial pressure should be less than 1.5 psi for safe use of L80 Type 13 Cr. The user should consult National Assn. of Corrosion Engineers (NACE) MR-01-75.

C90—A relatively new API grade with two different chemical requirements: Type 1 and Type 2. Only Type 1 is recommended for use in sour service. Typically, this grade must be special ordered; its use has been generally supplanted by T95.

T95—A high-strength tubular grade that has different chemical requirements: Type 1 and Type 2. Only Type 1 is recommended for sour service. T95 is SSC resistant but not weight-loss resistant.

P110—The old P105 tubing grade, which allowed a normalized and tempered heat treatment, was discontinued, and the casing P110 grade, which is restricted to quench-and-tempered heat treatment, was adopted. This high-strength tubing typically is used in deep sweet oil and gas wells with high pressures. This grade is sensitive to SSC failures unless the temperatures are relatively high (> 175°F). The P110 grade is slightly more expensive than L80 Type 1 but usually less expensive than the C90 and T95 API restricted-yield grades.

Q125—Although not a specific API tubing grade, users can order Q125 API tubing. Type 1 chemistry is preferred.

Tubing range (length) and size tolerances

API acknowledges two tubing length ranges: Range 1 from 20 to 24 ft and Range 2 from 28 to 32 ft. Range 2 is normally used. Shorter tubing joints (pup joints) are available in 2-, 3-, 4-, 6-, 8-, 10-, and 12-ft lengths with a tolerance of ± 3 in. A complete set of tubing pups with the same connections as the tubing string typically is purchased for each well.

Stock API tubing available:
1. Steel grade.
   J55, K55, N80, L80, P110, etc.
2. Outdoors diameter.
   2-3/8″, 2-7/8″, 3-1/2″, 4″, 4-1/2”.
3. Wall thickness.
   2.4 mm – 16.0 mm.
4. Product type.
   Non-upset tubing (NU), External distressed tubing (EU), Integral joint tubing (IJ).
5. Length.
   R1( 6.10-7.32 m), R2( 8.53-9.75 m), R3( 11.58-12.80 m).
6. String type.
   Round thread( 8RD, 10RD).

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