Technical Specification: API 5CT C90 API Tubing
1. Core Specification & Product Type
| Specification | Description |
|---|---|
| Governing Standard | API Specification 5CT - Casing and Tubing |
| Product Type | Tubing - For high-pressure, deep wells |
| Grade Designation | C90 - High-strength grade with sour service capability |
| Special Feature | Specifically engineered for sour service (H₂S environments) |
2. Material Properties & Mechanical Requirements
| Property | Requirement / Specification | Critical Notes |
|---|---|---|
| Yield Strength | 90,000 - 105,000 psi (621 - 724 MPa) | Narrow 15 ksi range - critical control |
| Tensile Strength | Min. 105,000 psi (724 MPa) | |
| Yield-to-Tensile Ratio | Maximum 0.85 (for all sizes) | Essential for SSC resistance |
| Hardness Limit | Maximum 25.4 HRC (Rockwell C) | Higher than L80 (23 HRC) |
| Elongation | Min. as per API formulas | Ensures ductility |
| Heat Treatment | Quenched & Tempered (Q&T) | Required for all C90 |
3. Chemical Composition Requirements
API 5CT imposes strict chemical control for C90 to balance strength and SSC resistance:
| Element | Maximum % | Rationale |
|---|---|---|
| Carbon (C) | 0.35% | Balanced for strength vs. SSC resistance |
| Manganese (Mn) | 1.40% | Lower than many carbon steel grades |
| Phosphorus (P) | 0.020% | Very low for improved toughness |
| Sulfur (S) | 0.010% | Ultra-low to minimize SSC initiation sites |
| Optional Alloying | Controlled Cr, Mo, Ni, V | For hardenability and tempering stability |
4. Sour Service Classification & Limitations
C90 has specific, controlled application in sour environments:
| Type | Designation | H₂S Partial Pressure Limit | Temperature Limit | Notes |
|---|---|---|---|---|
| C90 Type 1 | Sour Service | Up to 0.3 psi (2 kPa) | ≤ 175°F (80°C) | Standard sour service grade |
| C90 Type 2 | Non-Sour | Not permitted | N/A | Same chemistry/mechanics, no SSC testing |
| Key Restriction | Yield ≤ 95,000 psi in sour service | Often imposed | Additional safety margin | Common operator specification |
5. Comparison with Similar High-Strength Grades
| Grade | Yield Range (psi) | Max Hardness | Sour Service Capability | Primary Use Case |
|---|---|---|---|---|
| C90 Type 1 | 90,000 - 105,000 | 25.4 HRC | Yes - with restrictions | Deep sour wells needing 90 ksi |
| L80-1 | 80,000 - 95,000 | 23 HRC | Excellent (no pₕ₂ₛ limit at ≤23 HRC) | Standard sour service |
| T95 Type 1 | 95,000 - 110,000 | 25.4 HRC | Yes - more restrictive than C90 | Ultra-deep, higher strength needed |
| N80 | Min. 80,000 | 23 HRC | Conditional (if ≤23 HRC) | Non-critical sour service |
| P110 | Min. 110,000 | Not specified | Not permitted in sour service | Deep, non-sour wells only |
6. Manufacturing Process Control
Critical production aspects unique to C90:
Quenching & Tempering: Mandatory with strict temperature control
Yield Strength Targeting: Often produced to 90-95 ksi range for sour service, despite allowable up to 105 ksi
Hardness Uniformity: Full-body hardness surveys required
Traceability: Full heat/lot traceability with testing documentation
7. Applications & Design Considerations
| Application | Why C90 is Selected | Typical Well Parameters |
|---|---|---|
| Deep Sour Gas Wells | Higher strength than L80 for depth/pressure | 12,000-18,000 ft, H₂S present |
| High-Pressure Sour Wells | 90 ksi design allows thinner walls | > 8,000 psi wellhead pressure |
| Sour Service with CO₂ | Balanced corrosion/SSC resistance | H₂S + CO₂, moderate temperatures |
| Work Strings in Sour Wells | High strength for workover operations | Temporary exposure to H₂S |
Design Advantages:
Higher strength than L80 (90 vs. 80 ksi minimum)
Controlled hardness up to 25.4 HRC (vs. 23 for L80)
Validated SSC resistance when properly specified/manufactured
8. Limitations & Critical Risks
| Risk Factor | Details & Mitigation |
|---|---|
| Yield Strength Upper Limit | Risk: Yield > 95 ksi increases SSC susceptibility Mitigation: Specify "C90 with yield ≤ 95,000 psi" for sour service |
| Temperature in H₂S | Risk: SSC risk increases above 175°F (80°C) Mitigation: Temperature monitoring, possible downgrade to L80 |
| Chlorides with H₂S | Risk: Chlorides exacerbate SSC Mitigation: Control chloride content, inhibition |
| Welding/Repair | Risk: HAZ hardness > 25.4 HRC Mitigation: PWHT, qualified welding procedures |
| Misapplication | Risk: Using C90 Type 2 in sour service Mitigation: Clear procurement specifications |
9. Ordering & Quality Assurance
Essential to Specify for Sour Service:
Type: C90 Type 1 (explicitly for sour service)
Supplementary Requirement S: SSC testing per NACE TM0177 Method A
Yield Strength Restriction: Often "Yield strength not to exceed 95,000 psi"
Hardness Requirement: "Maximum 25.4 HRC, typical aim 23-24 HRC"
Certification: EN10204 3.2 with full traceability and test reports
Connection: Compatible with SSC requirements (premium connections common)
Testing Requirements:
Full SSC Test Suite: Method A (tensile), possibly Method D (DCB)
Hardness Surveys: Longitudinal and transverse
Charpy Impact: Often specified for low-temperature service
Full-body NDT: UT/MPI for defect detection
10. Industry Positioning & Cost Analysis
| Aspect | C90 vs. L80 | C90 vs. T95/P110 |
|---|---|---|
| Strength Advantage | +12.5% higher minimum yield | -10% vs. T95, -18% vs. P110 |
| Sour Service | More restrictive (0.3 psi H₂S max) | Similar restrictions to T95 |
| Cost Premium | 15-25% over L80 | 5-10% less than T95 |
| Availability | Widely available | Less common than L80 |
| Design Flexibility | Higher strength allows thinner walls | Less than higher grades |
Key Technical Insight:
C90 occupies a critical niche - it provides 10 ksi more strength than L80 while maintaining sour service capability, but with stricter environmental limits than L80. The higher allowable hardness (25.4 vs. 23 HRC) acknowledges that achieving 90 ksi yield typically requires slightly higher hardness, but this comes with the trade-off of reduced H₂S partial pressure tolerance (0.3 psi max).
The "90-95 ksi yield target" is industry best practice for sour service C90, despite the API allowance up to 105 ksi. This provides the strength benefit over L80 while maintaining adequate SSC resistance.
When to Specify C90:
When well design requires >80 ksi yield strength but ≤95 ksi is sufficient
When H₂S is present but limited (≤0.3 psi partial pressure)
When temperatures are moderate (≤175°F/80°C in H₂S)
When lifecycle cost favors higher-strength material over thicker L80 walls
Particularly valuable in deep, high-pressure sour gas wells where weight/cost savings from thinner walls are significant
Critical Reminder: C90 Type 2 exists - ensure Type 1 is specified for any sour service application, and consider imposing additional yield strength restrictions (max 95 ksi) for enhanced safety margins.
