API 5CT C110 solves the problem that no other standard API grade can: it delivers 110 ksi minimum yield strength — the same floor as P110 — while carrying full qualification for severe sour service under NACE MR0175 / ISO 15156. Where P110 is excluded from any H2S environment and T95's 95 ksi yield cannot provide sufficient pressure containment for deep high-pressure sour wells, C110 is the designated engineering solution. It is a specialty grade: fewer mills produce it to full specification, supply lead times are longer, and procurement requires more careful verification than standard P110 or T95.
ZC Steel Pipe supplies API 5CT C110 casing to PSL-2 with full NACE hardness qualification, Charpy impact testing, and EN 10204 3.2 MTC documentation. We supply to operators and EPC contractors on deep sour and HPHT sour projects in Africa, the Middle East, and South America. This guide covers C110 specifications, the metallurgical basis for its sour service capability, comparison against T95 and P110, and complete procurement guidance for deep sour wells.
What Is API 5CT C110?
C110 is defined in API Specification 5CT / ISO 11960 as a casing and tubing grade with a minimum yield strength of 758 MPa (110,000 psi). Its position in the grade ladder is defined by one achievement that no other standard API grade matches: combining 110 ksi yield with NACE MR0175 sour service qualification.
Three characteristics define C110's engineering position:
110 ksi minimum yield with sour service compatibility — C110 achieves the same pressure containment as P110 while maintaining a controlled maximum hardness of 30 HRC that qualifies it for severe H2S service under NACE MR0175. This combination does not exist in any other standard API 5CT grade.
Controlled alloy chemistry — C110 requires specific alloying elements (typically Cr, Mo, Ni combinations) to achieve high yield alongside sulphide stress cracking resistance. The chemistry is significantly more controlled than P110, with tight S and P limits essential for sour service fracture resistance.
Mandatory Q+T with hardness ceiling — like T95, C110 requires quench and temper heat treatment and a maximum hardness limit (30 HRC). Unlike T95's 22–25.4 HRC range, C110's 30 HRC ceiling reflects its different alloy system, which achieves SCC resistance at higher hardness than conventional carbon steel.
Mechanical Properties
| Property | Value |
|---|---|
| Minimum yield strength | 758 MPa (110,000 psi) |
| Maximum yield strength | 965 MPa (140,000 psi) |
| Minimum tensile strength | 793 MPa (115,000 psi) |
| Maximum hardness (API 5CT) | 30 HRC |
| Maximum hardness (NACE MR0175) | 30 HRC — C110 is explicitly listed |
| Heat treatment | Quench and temper — mandatory |
| Min elongation | Per API 5CT formula |
| Charpy impact (PSL-2) | Per API 5CT Table C.36 or SR2 at project temperature |
The 30 HRC maximum is both the API limit and the NACE limit for C110 — unlike T95 where the API limit (25.4 HRC Type 1) is higher than the NACE limit (22 HRC). For C110, API and NACE are aligned. However, hardness must still be verified by individual survey on the MTR — not inferred from yield strength.
Chemical Composition
C110 chemistry is more tightly controlled than P110 and requires alloying beyond what API 5CT specifies as minimums for most grades.
| Element | API 5CT Limit | Notes |
|---|---|---|
| Carbon (C) | ≤ 0.35% | Lower C preferred — typically 0.25–0.30% in production heats |
| Manganese (Mn) | ≤ 1.90% | Controlled for hardenability |
| Silicon (Si) | ≤ 0.45% | Deoxidiser |
| Phosphorus (P) | ≤ 0.020% | Tight limit — P is a sour service embrittler |
| Sulphur (S) | ≤ 0.005% | Very tight — sulphide inclusions initiate HIC |
| Chromium (Cr) | Mill-controlled | Key alloying for SSC resistance |
| Molybdenum (Mo) | Mill-controlled | Hardenability and SCC resistance |
| Nickel (Ni) | ≤ 0.99% | Toughness contribution with limits |
The 0.005% sulphur limit is the tightest in the API 5CT grade ladder — tighter than T95 (0.010%) and significantly tighter than P110 and N80 (0.030%). This reflects the fundamental role of sulphide inclusions as hydrogen-induced cracking initiation sites in sour service steels. C110 supply from mills with ladle refining and desulphurisation capability is essential.
Standard Sizes
| OD (inches) | OD (mm) | Common Weights (lb/ft) | Typical Application |
|---|---|---|---|
| 4½ | 114.3 | 9.50–13.50 | Production tubing, small casing |
| 5 | 127.0 | 11.50–18.00 | Production casing, deep sour wells |
| 5½ | 139.7 | 14.00–23.00 | Production casing — most common C110 size |
| 7 | 177.8 | 17.00–38.00 | Intermediate casing, deep sour |
| 7⅝ | 193.7 | 24.00–39.00 | Intermediate casing |
| 9⅝ | 244.5 | 32.30–47.00 | Intermediate casing |
C110 is most commonly ordered in 5½" and 7" — the typical production and intermediate casing sizes for deep sour wells in formations where T95 does not provide sufficient collapse resistance.
C110 vs T95 vs P110 — Grade Selection
The three grades most compared when a well has both high pressure and H2S:
| Property | T95 Type 2 | C110 | P110 |
|---|---|---|---|
| Min yield strength | 655 MPa (95 ksi) | 758 MPa (110 ksi) | 758 MPa (110 ksi) |
| Max yield strength | 758 MPa (110 ksi) | 965 MPa (140 ksi) | 965 MPa (140 ksi) |
| Max hardness | 22 HRC | 30 HRC | Not specified |
| H2S / Sour service | Moderate sour | Severe sour | Not permitted |
| NACE MR0175 | Yes | Yes | No |
| Heat treatment | Q+T | Q+T | Q+T |
| Collapse resistance | Moderate | High | High |
| Supply availability | Good | Limited — specialist mills | Wide |
| Relative cost | Moderate premium | Significant premium over P110 | Baseline |
Choose T95 when H2S is present at moderate levels and T95's 95 ksi yield provides adequate pressure containment for the casing design.
Choose C110 when the well has significant H2S and the collapse or burst loads require 110 ksi yield that T95 cannot provide. This is the precise engineering case C110 was designed to fill.
Choose P110 when the well is sweet — no H2S — and maximum pressure containment at minimum cost is required.
Why C110 Is Not a Simple P110 Substitute in Sour Wells
The question arises frequently: why not just order C110 whenever P110 is needed in a potentially sour well as a precaution? Three reasons why this logic is wrong in practice:
Availability and lead time — C110 is a specialty production grade. Mills that routinely produce high-volume P110 do not necessarily have C110 production capability or recent production records. Sourcing C110 without confirming mill capability risks receiving non-conforming material or extended delays.
Verification burden — C110 requires more extensive MTR verification than P110: hardness survey by joint or heat, Charpy impact at project temperature, tight sulphur chemistry confirmation, and full-length NDE. The inspection scope for C110 on a NACE-qualified order is substantially heavier than P110 PSL-2.
Cost — C110 carries a significant cost premium over P110 — typically 20–35% on the pipe body alone, before connections. For sweet wells with no H2S risk, the premium provides no engineering benefit.
Specify C110 when the well engineering requires it. Do not use it as a convenience substitute for P110 in wells where sour service qualification has not been confirmed by reservoir data.
PSL-1 vs PSL-2 for C110
PSL-1 is not appropriate for C110 in sour service applications. All C110 for H2S environments should be specified PSL-2 as a minimum.
| Requirement | C110 PSL-1 | C110 PSL-2 |
|---|---|---|
| NDE of pipe body | Not mandatory | Mandatory — full length UT or EMI |
| NDE of pipe ends | Not mandatory | Mandatory |
| Dimensional tolerances | Standard | Tighter |
| Heat and pipe traceability | Heat number | Full heat + pipe number per joint |
| Charpy impact testing | Not mandatory | Mandatory |
| Hardness survey | Not mandatory | Strongly recommended — effectively required for NACE |
| NACE hardness qualification | Cannot confirm | Confirmable per heat or per pipe |
HPHT Sour Service Considerations
C110 in HPHT sour service — wellhead pressure above 690 bar and bottomhole temperature above 150°C with H2S present — represents the most demanding OCTG specification combination in routine oilfield practice. Key additional considerations:
Thermal de-rating: C110 yield strength decreases at elevated temperature — typically 5–7% at 150°C vs ambient. Casing designs must apply temperature de-rating to the nominal 758 MPa minimum yield.
Charpy testing temperature: PSL-2 Charpy requirements are set at standard temperatures. HPHT sour wells with low bottomhole temperature during shut-in — particularly deepwater wells — may require additional Charpy testing at lower temperatures per SR2. Confirm test temperature against the minimum operating temperature in the casing design.
Premium connections are mandatory: No standard API thread — STC, LTC, BTC — is suitable for HPHT sour C110 applications. Metal-to-metal seal premium connections qualified for the full C110 yield body strength and rated for the combined pressure, axial, and thermal load envelope are required.
Cement behind pipe: C110's collapse resistance in HPHT conditions is realised only with competent cement. Unsupported pipe in cement voids at the critical collapse depth will still fail regardless of grade.
Connection Types for C110
| Connection | Suitability | Notes |
|---|---|---|
| STC | Not suitable | Inadequate for C110 string loads |
| LTC | Not suitable | Inadequate for deep or HPHT C110 |
| BTC | Limited — mild conditions only | Acceptable only for shallow, moderate-pressure sour wells |
| Premium | Required | Metal-to-metal seal, rated to C110 body yield, gas-tight |
ZC Steel Pipe supplies C110 with premium connections qualified to API 5C5 CAL IV, rated to full C110 body yield strength.
What to Check on a C110 MTR
| MTR Item | What to Verify | Why It Matters |
|---|---|---|
| Yield strength | 758–965 MPa — both limits | Confirm yield band — over-yield above 965 MPa is non-conforming |
| Tensile strength | Min 793 MPa (115 ksi) | Confirms correct Q+T microstructure |
| Hardness | ≤ 30 HRC — per joint or heat survey | 30 HRC is both API and NACE limit — must be confirmed per heat |
| Heat treatment | Q+T confirmed | Only Q+T is permitted for C110 |
| Sulphur | ≤ 0.005% | Critical — confirm actual value, not just compliance statement |
| Phosphorus | ≤ 0.020% | Embrittlement risk at higher P |
| Alloy chemistry | Cr, Mo, Ni values recorded | Confirm mill chemistry supports SSC resistance at 30 HRC |
| NDE records (PSL-2) | Full-length UT or EMI body scan | Absence = PSL-1 regardless of labelling |
| Charpy impact | Values, temperature, specimen size | Confirm adequacy at project operating temperature |
| NACE qualification statement | Explicit NACE MR0175 / ISO 15156 compliance per heat | Required for sour service acceptance |
How to Specify C110 on a Purchase Order
A complete C110 sour service purchase order must include:
- Standard — API 5CT or ISO 11960
- Grade — C110 (single grade, no subtypes)
- OD and nominal weight — e.g. 5½ inch × 20.00 lb/ft
- Connection type — premium connection designation (mandatory for HPHT/severe sour)
- Range — R1, R2, or R3 (most strings are R3)
- PSL level — PSL-2 mandatory
- Supplementary requirements — SR2 (Charpy at project temperature), SR13 (additional hardness survey), NACE MR0175 / ISO 15156 qualification per heat
- Chemistry requirements — confirm S ≤ 0.005% and alloy system with mill before order
- Quantity — in joints or metric tonnes
- Delivery port — C110 lead times 90–120+ days; plan accordingly
- MTC level — EN 10204 3.2 (third-party witnessed) — 3.1 is not sufficient for most severe sour projects
- Third-party inspection scope — mill visit, witness heat treatment, witness mechanical testing, witness NDE, dimensional inspection
References
- API Specification 5CT — Specification for Casing and Tubing (American Petroleum Institute)
- ISO 11960 — Petroleum and Natural Gas Industries: Steel Pipes for Use as Casing or Tubing
- NACE MR0175 / ISO 15156 — Materials for Use in H2S-Containing Environments in Oil and Gas Production
- API TR 5C3 — Technical Report on Equations and Calculations for Casing, Tubing, and Line Pipe