Wellhead Equipment And Christmas Trees: The “Throat” Of Oil & Gas Production
Sep 13, 2025
In the entire oil and gas production chain, one type of equipment can be called the "throat of the wellhead." It must bear the weight of the downhole tubulars, precisely control production, and act as the last line of defense in emergencies. That equipment is the wellhead system and the Christmas tree.
This article explains API 6A wellhead equipment in plain language. Whether you're a newcomer or an experienced professional, you'll find useful insights here.
1. First Things First: What Are Wellhead Systems and Christmas Trees?
Simply put, the wellhead system is a family of equipment, consisting of three main components:
Christmas Tree – the "faucet" that controls oil and gas production.
Tubing Head – the "bridge" connecting tubing and casing.
Casing Head – the "foundation" that anchors the casing and seals the annulus.
Together with valves, flanges, and other accessories, they perform four basic functions: suspension, support, control, and sealing. These functions directly determine whether a well can operate safely and efficiently.
2. Standards Come First: 5 Key Specifications to Remember
Whether for design, manufacturing, or selection, wellhead equipment must comply with recognized standards to avoid "non-standard" risks. The five most common are:
| Standard | Application | Key Points |
|---|---|---|
| API Spec 6A (2018, 21st Edition) | Global wellhead/Christmas tree design | Defines PSL1–PSL4 levels; sets pressure/temperature requirements |
| ISO 10423:2022 | International standard | Harmonized with API 6A; better suited for global supply chains |
| GB/T 22513-2023 | Chinese national standard | Optimized for local geological conditions |
| API Std 6AR (2019, 2nd Edition) | Repair & remanufacturing | Sets quality baseline for refurbished equipment |
| SY/T 7694-2023 | China repair standard | Details localized repair processes |
⚠️ Note: These standards do not apply to field test tools or installation aids (such as wear bushings or test plugs).
3. Key Terms Explained: 13 Common "Buzzwords"
The oilfield is full of jargon. Here are 13 of the most common terms, explained in plain English:
Wellhead System – all permanent equipment from the top of the surface casing to the tubing head; the "skeleton" of the well.
Casing-Head Housing – mounted on the top of surface casing, used to suspend casing strings.
Tubing-Head Spool – connects the casing head to the Christmas tree, suspends tubing, and seals the annulus.
Mandrel-Type Hanger – a solid-bar style hanger to support casing.
Slip-Type Hanger – wedge slips grip the pipe like a clamp, quick and versatile.
Gate Valve – the most common wellhead valve, using a rising/falling gate to shut in flow.
Surface Safety Valve (SSV) – automatically closes when power supply (electric/hydraulic) is lost, preventing leaks.
Choke – controls flow and pressure, like the "throttle" of the well.
Back-Pressure Valve (BPV) – one- or two-way check valve, prevents well fluids from escaping during maintenance.
Blind Flange – a solid flange used to seal unused outlets.
VR Plug (Valve-Removal Plug) – threaded plug that allows gate valve removal under pressure.
Top Connector – the uppermost connection of the tree, linking the tree bore to equipment above.
Tubing-Head Adapter – matches the tubing head outlet to the Christmas tree inlet when sizes differ.
4. Core Functions: How the Wellhead Works
Despite its complexity, the wellhead system serves five essential purposes:
Suspension & Load Bearing – supports casing, tubing, and downhole tools, while sealing annuli to prevent crossflow.
Production Control – chokes regulate flow and pressure to meet production needs safely.
Operational Support – provides access for well kill, acidizing, fracturing, or tubing operations without equipment removal.
Data Monitoring – pressure gauges provide real-time monitoring for safe operation.
Safety Protection – safety valves and check valves activate automatically during abnormal conditions.
5. Equipment Selection: Materials & PSL Levels
(1) Material Selection: Match to Well Conditions
| Material Class | Application | Body/Pressure Parts | Control Parts (stems, hangers) |
|---|---|---|---|
| AA/BB/CC | Conventional wells | Carbon/low-alloy steel (CC uses stainless) | Carbon/low-alloy steel (BB/CC use stainless) |
| DD/EE/FF | Sour service | Carbon/low-alloy steel (FF uses stainless) | Carbon/low-alloy steel (EE/FF use stainless) |
| HH | Severe sour & high-pressure | Corrosion-resistant alloys (CRA) | Corrosion-resistant alloys (CRA) |
(2) PSL Levels: Match to Risk
PSL1 – Low-pressure, conventional wells
PSL2 – Medium-pressure wells
PSL3/3G – High-pressure gas wells, sour service
PSL4 – Ultra-high-pressure, high-risk wells
6. Installation in 6 Steps: Precision Required
Wellhead installation must follow strict procedures, typically in six steps:
Conductor Casing – drill → run conductor → cement, forming the first foundation.
Surface Casing – drill → run casing → install casing head → cement.
Technical Casing – drill → run casing → install casing-head spool → pressure test.
Production Casing – drill → run production casing → install mandrel hanger → pressure test.
Tubing-Head Spool – run tubing → install hanger & wear bushing → pressure test.
Christmas Tree – install back-pressure valve → land tree → full pressure test → remove BPV, completing wellhead assembly.
