API 5CT K55 Casing And Tubing Pipe
API 5CT K55 is a medium-strength OCTG (Oil Country Tubular Goods) grade widely used for casing and tubing in oil and gas wells. It belongs to Group 1 of the API 5CT specification and is commonly selected for shallow to medium-depth wells requiring higher tensile strength than J55 while maintaining economical cost.
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API 5CT K55 Casing and Tubing Pipe is a medium-strength Oil Country Tubular Goods (OCTG) product manufactured in accordance with the API 5CT specification for oil and gas well drilling and production operations.
K55 casing is used to support the wellbore, prevent formation collapse, and isolate different geological zones, while K55 tubing is used to transport oil, gas, or injected fluids between the reservoir and the surface. The grade is characterized by a minimum yield strength of 55 ksi (379 MPa), higher tensile strength than J55, good weldability, reliable mechanical performance, and cost-effectiveness.
API 5CT K55 casing and tubing are widely used in the oil and gas industry, including onshore and offshore oil wells, natural gas wells, water injection wells, geothermal wells, and thermal recovery projects where moderate strength and dependable well integrity are required.
API 5CT K55 Casing and Tubing Pipe Specification
| Item | Specification |
|---|---|
| Standard | API Spec 5CT |
| Grade | K55 |
| Product Type | Casing Pipe & Tubing Pipe |
| Grade Group | Group 1 |
| Product Specification Level | PSL-1, PSL-2 |
| Manufacturing Method | Seamless (SMLS), Electric Welded (ERW/EW) |
| Application | Oil Wells, Gas Wells, Water Injection Wells, Geothermal Wells |
| Service Environment | Non-sour service, low to moderate pressure wells |
| Thread Types | STC, LTC, BTC, Premium Connections |
| Length Range | R1, R2, R3 |
| End Finish | Plain End (PE), Threaded & Coupled (T&C) |
API 5CT K55 Casing and Tubing Pipe Mechanical Properties
| Property | API 5CT K55 |
|---|---|
| Minimum Yield Strength | 379 MPa (55 ksi) |
| Maximum Yield Strength | 552 MPa (80 ksi) |
| Minimum Tensile Strength | 655 MPa (95 ksi) |
| Minimum Elongation | 15% (API Formula Based) |
| Hardness Requirement | No Mandatory Hardness Limit |
| Collapse Resistance | Moderate |
| Burst Resistance | Moderate |
| Thermal Fatigue Resistance | Better than J55 |
| Crack Resistance | Better than J55 |
API 5CT K55 Casing and Tubing Pipe Chemical Composition
API 5CT does not prescribe a fixed chemistry for K55, but GNEE typically control the composition within the following ranges:
| Element | Typical Value (%) |
|---|---|
| Carbon (C) | ≤ 0.28 |
| Manganese (Mn) | ≤ 1.20 |
| Silicon (Si) | ≤ 0.55 |
| Phosphorus (P) | ≤ 0.030 |
| Sulfur (S) | ≤ 0.030 |
Heat Treatment Requirements of API 5CT K55 Casing and Tubing Pipe
| PSL Level | Heat Treatment Requirement |
|---|---|
| PSL-1 | As Rolled / Hot Rolled |
| PSL-2 | Full-Length Normalized or Normalized & Tempered |
| ERW Weld Seam | Mandatory Weld Seam Heat Treatment |
| Thickened Ends | Heat treatment before upsetting |
API 5CT K55 Casing Sizes Chart
| OD (in.) | OD (mm) |
|---|---|
| 4 1/2 | 114.30 |
| 5 | 127.00 |
| 5 1/2 | 139.70 |
| 6 5/8 | 168.28 |
| 7 | 177.80 |
| 7 5/8 | 193.68 |
| 8 5/8 | 219.08 |
| 9 5/8 | 244.48 |
| 10 3/4 | 273.05 |
| 11 3/4 | 298.45 |
| 13 3/8 | 339.72 |
| 16 | 406.40 |
| 18 5/8 | 473.08 |
| 20 | 508.00 |
API 5CT casing sizes generally range from 4 1/2 in. to 20 in. OD.
API 5CT K55 Tubing Sizes Table
| OD (in.) | OD (mm) |
|---|---|
| 1.050 | 26.67 |
| 1.315 | 33.40 |
| 1.660 | 42.16 |
| 1.900 | 48.26 |
| 2 3/8 | 60.32 |
| 2 7/8 | 73.02 |
| 3 1/2 | 88.90 |
| 4 | 101.60 |
| 4 1/2 | 114.30 |
API tubing sizes generally range from 1.050 in. to 4 1/2 in. OD.
Outside Diameter (OD) Tolerance
| Nominal Outside Diameter | Tolerance |
|---|---|
| < 4-1/2 in. (114.3 mm) | ±0.79 mm (±0.031 in.) |
| ≥ 4-1/2 in. (114.3 mm) | +1.0% OD / -0.5% OD |
Upset Tubing OD Tolerance
Applicable to EUE (External Upset End) tubing.
| Tubing Size | Tolerance Behind Upset |
|---|---|
| ≤ 3-1/2" | +2.38 mm / -0.79 mm |
| > 3-1/2" to 5" | +2.78 mm / -0.75% OD |
| > 5" to 8-5/8" | +3.18 mm / -0.75% OD |
| > 8-5/8" | +3.97 mm / -0.75% OD |
API 5CT K55 Casing and tubing Length Range
| Range | Length |
|---|---|
| R1 | 6.10 – 7.32 m (20 – 24 ft) |
| R2 | 8.53 – 9.75 m (28 – 32 ft) |
| R3 | 11.58 – 14.63 m (38 – 48 ft) |
API 5CT K55 Casing and tubing Length Tolerance
| Range | Length |
|---|---|
| R1 | 6.10 – 7.32 m (20 – 24 ft) |
| R2 | 8.53 – 9.75 m (28 – 32 ft) |
| R3 | 11.58 – 14.63 m (38 – 48 ft) |
API Color Identification of API 5CT K55 OCTG Pipe
| Grade | API Color Code |
|---|---|
| K55 Tubing | Two Bright Green Bands |
| K55 Casing | Two Bright Green Bands |
| Coupling | Bright Green |
API 5CT K55 Casing Thread Connection Types
| Connection Type | Full Name | Thread Family | Tensile Efficiency | Sealing Performance | Typical Application |
|---|---|---|---|---|---|
| STC | Short Thread Coupling | Round Thread | Low to Medium | Moderate | Surface casing, shallow wells |
| LTC | Long Thread Coupling | Round Thread | Medium to High | Better than STC | Intermediate casing, medium-depth wells |
| BTC | Buttress Thread Coupling | Buttress Thread | High (up to pipe body strength) | Better mechanical strength | Deep wells, heavy casing strings |
| Premium Connection | Proprietary Premium Thread | Premium | Very High | Gas-tight seal | HPHT wells, offshore wells, sour service |
API 5CT K55 Tubing Thread Connection Types
| Connection Type | Full Name | Upset Type | Tensile Capacity | Sealing Performance | Typical Application |
|---|---|---|---|---|---|
| NUE | Non-Upset End | Non-Upset | Standard | Moderate | General production tubing |
| EUE | External Upset End | External Upset | Higher than NUE | Better than NUE | Production tubing, injection wells |
| Premium Connection | Proprietary Premium Thread | Various | Very High | Gas-tight | High-pressure gas wells, offshore wells |
Thread Selection Guide for API 5CT K55
| Well Condition | Recommended Connection |
|---|---|
| Water wells | STC / NUE |
| Shallow oil wells (<2,000 m) | STC |
| Medium-depth wells (2,000–3,500 m) | LTC |
| Heavy casing strings | BTC |
| High-pressure production wells | EUE |
| Gas wells requiring leak resistance | Premium Connection |
| Offshore wells | Premium Connection |
| HPHT wells | Premium Connection |
Inspection and Testing Requirements of API 5CT K55 Casing and tubing
| Test Item | Requirement |
|---|---|
| Visual Inspection | Required |
| Dimensional Inspection | Required |
| Chemical Analysis | Required |
| Tensile Test | Required |
| Hydrostatic Test | Required |
| Drift Test | Required |
| Thread Inspection | Required |
| Ultrasonic Testing (UT) | PSL-2 / Optional |
| Electromagnetic Inspection | Optional |
| Magnetic Particle Inspection (MPI) | Optional |
| Charpy Impact Test | Project Requirement / PSL-2 |
| Hardness Test | Optional |
API 5CT K55 Heat Treatment Requirements
PSL 1 Requirements
For traditional API 5CT PSL 1 K55 casing and tubing:
| Item | Requirement |
|---|---|
| Delivery Condition | Hot Rolled (As Rolled) |
| Full-Length Normalizing | Not Mandatory |
| Normalizing & Tempering | Optional |
| Quenching & Tempering | Not Required |
| ERW Weld Seam Heat Treatment | Mandatory |
Under PSL 1, K55 can be supplied in the hot-rolled condition without full-body heat treatment, making it one of the most economical OCTG grades available.
PSL 2 Requirements
For API 5CT PSL 2 K55 casing and tubing:
| Item | Requirement |
|---|---|
| Full-Length Normalizing | Required |
| Normalizing & Tempering | Permitted |
| Quenching & Tempering | Not Required |
| Upset Ends | Heat treatment after upsetting |
| ERW Weld Seam Heat Treatment | Mandatory |
PSL 2 products require a more controlled heat-treatment process to achieve improved consistency in mechanical properties and toughness.
Why Heat Treatment Matters for API 5CT K55
Heat treatment directly influences the internal microstructure of the steel.
Before Heat Treatment
Potential issues include:
Coarse grain structure
Residual rolling stress
Uneven mechanical properties
Reduced toughness
After Normalizing
Benefits include:
| Property | Improvement |
|---|---|
| Grain Size | Finer |
| Mechanical Uniformity | Better |
| Toughness | Higher |
| Ductility | Improved |
| Residual Stress | Reduced |
| Service Reliability | Increased |
Normalizing helps K55 casing withstand the complex loading conditions encountered during drilling, cementing, completion, and production operations.
API 5CT K55 Impact Testing Requirements
PSL 1 Impact Testing
| Item | Requirement |
|---|---|
| Charpy V-Notch Test | Not Mandatory |
| Toughness Verification | Generally Not Required |
| Acceptance Criteria | Not Specified |
For standard PSL 1 K55 products, impact testing is normally not required unless requested by the purchaser.
PSL 2 Impact Testing
For PSL 2 K55 products, Charpy V-Notch testing is required.
Pipe Body Requirements
| Test Item | Requirement |
|---|---|
| Test Method | Charpy V-Notch |
| Specimen Orientation | Longitudinal (Horizontal) |
| Minimum Single Specimen Energy | 20 J |
| Minimum Average Energy | 27 J |
Coupling Requirements
| Test Item | Requirement |
|---|---|
| Test Method | Charpy V-Notch |
| Specimen Orientation | Transverse / Longitudinal |
| Minimum Single Specimen Energy | 20 J |
| Minimum Average Energy | 27 J |
These requirements verify that the casing or tubing has adequate toughness and can absorb impact energy without sudden brittle failure.
Why Impact Testing Matters for API 5CT K55
Impact testing evaluates the material's ability to absorb sudden energy before fracturing.
In oilfield service, casing and tubing may experience:
Running impacts during installation
Tool collisions
Pressure fluctuations
Temperature cycling
Vibrational loading
Emergency shut-in conditions
Impact testing confirms that the material can tolerate these dynamic loads without catastrophic failure.
Applications of API 5CT K55 Casing and Tubing
Intermediate Casing for Medium-Depth Wells
This is the most representative application of K55 casing.
Compared with J55:
The yield strength is the same.
The tensile strength is significantly higher.
As a result, K55 is frequently selected for:
Oil wells between 2,000 and 3,500 meters (6,500–11,500 ft)
Medium-depth natural gas wells
Wells with long casing strings
In these applications, engineers often prefer K55 because it offers additional tensile capacity without a substantial increase in cost.
Long Casing String Wells
As well depth increases, the casing string must support a greater amount of its own weight, resulting in higher axial tensile loads.
K55 is particularly well suited for these conditions because it provides:
Higher tensile strength than J55
Lower cost than N80
This combination makes K55 a practical and economical choice for conventional oil and gas wells with longer casing strings.
Water Injection Wells
Water injection is widely used in secondary oil recovery operations to maintain reservoir pressure and improve production.
Typical applications include:
Water injection tubing
Water injection casing
Waterflood projects
Because these wells generally do not require the corrosion resistance of premium grades, K55 is often selected for its ability to meet strength requirements while keeping project costs under control.
Thermal Recovery Wells
K55 is commonly used in moderate-temperature thermal recovery operations, including:
Steam Injection Wells
Cyclic Steam Stimulation (CSS)
Heavy Oil Recovery Projects
Although K55 is not specifically designed as a thermal-service grade, its higher tensile strength compared with J55 makes it a popular choice for thermal applications where operating conditions are not excessively severe.
Geothermal Wells
K55 is also widely used in geothermal projects, such as:
Medium-temperature geothermal wells
Geothermal reinjection wells
Geothermal water production wells
For many geothermal applications, K55 offers a cost-effective combination of strength, durability, and availability.
API 5CT K55 casing and tubing are not intended for a specific niche service like L80 or P110. Instead, their most characteristic applications include intermediate casing in medium-depth wells, long casing string installations, water injection wells, conventional natural gas wells, geothermal wells, and moderate thermal recovery projects. K55 occupies the ideal position between J55 and N80, delivering an excellent balance of strength, reliability, and cost-effectiveness, which makes it one of the most practical and widely used OCTG grades in the global oil and gas industry.
API 5CT K55 Casing and Tubing Pipe Factory
API 5CT K55 OCTG Pipe test equipment
API 5CT K55 Casing and Tubing pipe certificate
FAQ
What is the difference between K55 and J55 casing?
The main difference is tensile strength.
Both grades have the same yield strength range of 379–552 MPa (55–80 ksi), but K55 has a significantly higher minimum tensile strength of 655 MPa (95 ksi), compared with 517 MPa (75 ksi) for J55.
As a result, K55 is generally preferred for medium-depth wells and applications requiring higher tensile capacity.
Can K55 replace J55?
Yes.
In most cases, K55 can be used as a direct substitute for J55 because it offers higher tensile strength while maintaining similar dimensions, thread compatibility, and operating characteristics.
However, the final selection should always follow the well design requirements.
Is K55 suitable for sour service (H₂S environments)?
Generally, standard K55 is not recommended for severe sour-service applications.
For wells containing significant hydrogen sulfide (H₂S), operators typically select specially qualified sour-service grades such as L80 Type 1 or other NACE-compliant materials.
What factors affect the price of K55 casing and tubing?
The main pricing factors include:
Steel raw material costs
Seamless vs. ERW manufacturing
Pipe size and wall thickness
Thread connection type
PSL level
Inspection requirements
Coating requirements
Order quantity
Shipping destination
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