Project Background
In early 2025, a national oil company in the United Arab Emirates launched a high-pressure sour gas development program targeting the Khuff formation at depths exceeding 15,000 feet. The operator required a complete wellhead package — Christmas tree, casing head, tubing head spool, and blowout preventer stack — rated to 15,000 PSI with full NACE MR0175 compliance for H₂S service exceeding 8% molar concentration.
Operational Challenges
- Sour Environment: Reservoir fluid contained 8–12% H₂S and 4–6% CO₂, demanding materials compliant with NACE MR0175/ISO 15156 Region 3.
- HP/HT Conditions: Bottomhole pressure exceeded 13,800 PSI with flowing temperatures up to 320°F (160°C), requiring PSL 3G and PR 1 verification.
- Delivery Deadline: A 90-day delivery window from PO issuance to avoid rig standby costs exceeding $85,000/day.
- Heavy-Lift Logistics: Equipment weighing over 18 metric tons per unit required specialized port handling at ADNOC-designated facilities.
Technical Specifications
| Parameter | Specification |
|---|
| Design Standard | API 6A (21st Ed.), API 16A |
| Product Spec Level | PSL 3G (gas-tested) |
| Performance Req. | PR 1 |
| Working Pressure | 15,000 PSI |
| Temperature Class | P-U (−20°F to 250°F) |
| Material Class | EE (NACE MR0175 sour service) |
| Body Material | AISI 4130 / F22 alloy steel |
| Trim Material | Inconel 718 / 17-4 PH stainless |
Equipment Selection
The wellhead package included: 18-3/4" * 13-5/8" * 9-5/8" casing head housing with slip-and-seal suspension, 13-5/8" 15K tubing head spool with dual-seal mandrel hanger, 7-1/16" 15K dual-bore Christmas tree with fail-safe gate valves, and an 18-3/4" 15K double ram BOP with shear/blind ram configuration.
Our integrated design approach — engineering all components as a single system rather than assembling from disparate suppliers — eliminated interface leakage risks that had caused issues in two previous tenders. All pressure-containing forgings were sourced from one ISO-certified foundry with full chemical traceability. Factory hydrostatic testing was performed to 22,500 PSI (1.5* MWP) with 15-minute hold per API 6A Appendix F, plus inert gas testing for PSL 3G validation.
Manufacturing & Quality Assurance
- Forging: Multi-directional forged AISI 4130 bodies with Charpy V-notch impact testing at −20°F per ASTM A370.
- NDT Suite: 100% volumetric UT (ASME Section V), wet fluorescent MPI on all accessible surfaces, hardness testing (ASTM E18) across weld overlay zones.
- PMI: Positive Material Identification via XRF on every pressure-containing component.
- FAT: Full-body hydrostatic test at 1.5* MWP, gas shell test at MWP, seat leakage verification with nitrogen, functional cycling of all gate valves (3 cycles).
- Third-Party Witness: All FAT procedures witnessed by Bureau Veritas per operator requirements.
| Test | Standard | Criteria |
|---|
| Hydrostatic Body Test | API 6A 10.3.3 | 22,500 PSI, 15 min, zero leak |
| Gas Shell Test | API 6A 10.3.5 | 15,000 PSI N₂, zero bubble |
| Seat Leakage | API 6A 10.3.7 | ≤10 cm³/min liquid |
| UT + MPI | ASME V / ASTM E709 | No rejectable indications |
Packaging, Shipment & Installation
Each assembly was nitrogen-purged, sealed in VCI film, and crated in ISPM-15 certified timber with tilt sensors and shock indicators. Shipment executed via breakbulk vessel with 40-day transit from Shanghai to Jebel Ali Port, including customs pre-clearance. Our field service engineer mobilized within 72 hours of equipment arrival for installation supervision, flange alignment via cross-dial laser method, and bolt tensioning per ASME PCC-1 to 50% of bolt yield with calibrated hydraulic torque tools.
Field Performance & Client Feedback
After 14 months of continuous production, all wellhead components maintain zero unplanned downtime. Gate valve cycle count exceeds 240 operations with no detectable seat wear.
Client Testimonial: "The technical documentation package exceeded expectations. Complete material certification with heat-by-heat traceability gave our QA team full confidence during the vendor audit. Delivery was on schedule despite the compressed timeline." — Senior Drilling Engineer, Operator
Procurement Lessons Learned
- Single-Source Integration: Procuring the complete wellhead package from one manufacturer eliminates interface risks and reduces project management overhead by an estimated 30%.
- Material Traceability: Insist on EN 10204 Type 3.1 or 3.2 certification for all pressure-containing components — body, studs, nuts, ring gaskets, and seat rings alike.
- Witness Testing: The $12,000–15,000 cost of third-party FAT witness is insignificant compared to a failed installation. Always budget for it in critical sour service projects.
- Spares Strategy: Pre-position 2 ring gaskets, 1 full stud/nut set, and 1 spare gate/seat kit per wellhead assembly to minimize deferred production risk.
FAQ
PSL 3 vs. PSL 3G? PSL 3G adds mandatory gas testing per API 6A Table 19. For gas-producing wells with H₂S, PSL 3G provides critical assurance against micro-leak paths that liquid-only testing may miss.
AA vs. DD vs. EE Material Classes? AA is general service. DD requires NACE MR0175 compliance with HRC 22 max for carbon/low-alloy steels. EE adds stricter chemistry controls and requires Inconel or equivalent CRA for trim — the prudent minimum for H₂S > 5% partial pressure.
Which flange type for 15,000 PSI? API 6A Type 6BX flanges with metal-to-metal ring-groove sealing featuring BX-type ring gaskets. Bolt tensioning per ASME PCC-1 Appendix J target torque methodology is mandatory.
Conclusion
A well-executed API 6A wellhead solution — combining metallurgically appropriate material selection, rigorous NDT protocols, and disciplined project management — consistently delivers reliable field performance in the most demanding sour gas environments. EPC contractors and operators evaluating wellhead OEMs should prioritize demonstrated sour-service track record, in-house NDT capability, and supply chain transparency as key procurement criteria.
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