Wellhead & Christmas tree

The wellhead is the stack of spools and heads at surface: the casing head hangs the production casing and seals the B-annulus, the tubing head hangs the tubing and seals the A-annulus. On top sits the Christmas tree, an assembly of valves that controls and isolates the flow from the well.

The figure is a front elevation in the proportions of API 6A 5,000 psi equipment (valve and flange dimensions from manufacturer tables), with a quarter section through the heads so you can see the hangers, seals and annuli. Numbers and times are illustrative and compressed for learning.

How to use the simulator

• Valve: Tap a valve on the figure or in the valve list to open or close it: the handwheel turns and the gate travels gradually.
• Choke: The slider sets the choke bean in 1/64 in; rate, THP and FLP follow the illustrative Gilbert (1954) relation.
• Mode: Free play or guided sequences: start-up (LMV, SSV, wing, then the choke step by step), shut-in in reverse, or wireline access through the swab valve after the lubricator is rigged up and tested.
• Fault: Create a fault: A or B annulus SCP, or a passing swab valve, then use the bleeds to diagnose it.
• ESD: Closes the hydraulic SSV at once; to restart, reset the ESD and reopen the SSV with the choke closed.
• Zoom: Use + and − or pinch; tap any part to read its function, pressure class, typical failures and inspection.

Safety note: these valves sit on a live, high-pressure well. Never open the swab valve without a tested lubricator, do not close the masters on a flowing well except in an emergency, and bleed an annulus only under the approved procedure with gas monitoring. Always follow your company procedures and permit to work.

What each part does

• Casing head (A-section)Lowest head, welded or threaded onto the surface casing; lands the casing hanger and packoff, with a side outlet to the B-annulus.
• Tubing head (B-section)Lands the tubing hanger, carries the A-annulus valves and seals between tubing and production casing.
• Tubing head adapterJoins the tubing head to the bottom of the tree and passes the SCSSV control line where fitted.
• Lower master valve (LMV)Manual isolation gate valve: opened first, closed last, never used to shut in a flowing well.
• Upper master valve / SSVSurface safety valve with a hydraulic actuator (API 6A, API 14H); closes on loss of control pressure or on ESD.
• Flow crossFour-way block that splits the bore to the production wing, the kill wing and the swab valve.
• Production / kill wing valvesThe production wing sends the well to the choke and flowline; the kill wing is the inlet for pumping kill fluid.
• Adjustable chokeSets the rate through a bean read in 1/64 in; the choke throttles, the gate valves do not.
• Swab valve + tree capTop valve, opened only for vertical access (wireline or coiled tubing) after the lubricator is rigged up and tested.
• A / B annulus valvesAnnulus valves and gauges; pressures are logged daily, and a build-up after bleed-down points to SCP.

Simplified teaching model: dimensions come from API 6A 5K equipment, but pressures, rates and times are illustrative only (SITP 3,000 psi, flowline 250 psi). Not a design tool or an operating procedure.

Sources

1. API Specification 6A / ISO 10423: Wellhead and Tree Equipment https://www.api.org/products-and-services/standards
2. API Specification 14H / API RP 14B: Surface safety valves and safety systems https://www.api.org/products-and-services/standards
3. API RP 90-1 / RP 90-2: Annular Casing Pressure Management (offshore / onshore wells) https://www.api.org/products-and-services/standards
4. NORSOK D-010: Well integrity in drilling and well operations
5. SLB Energy Glossary: “Christmas tree”, “wellhead”, “master valve”, “swab valve”, “choke” https://glossary.slb.com/
6. Wikipedia: Christmas tree (oil well) https://en.wikipedia.org/wiki/Christmas_tree_(oil_well)
7. Bonney Forge and Mento API 6A gate valve dimension catalogues; API 6B flange dimension tables (MSM)
8. Gilbert, W. E. (1954). Flowing and gas-lift well performance. API Drilling and Production Practice
9. NACE MR0175 / ISO 15156: Materials for use in H2S-containing environments