Wellhead and Christmas Tree: Valve Opening and Closing Sequence, and Why

Educational content only: procedures and operating values vary between companies and wells. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.

Wellhead or christmas tree: what is the difference?

The two terms are often confused. The wellhead is the permanent surface equipment that suspends the casing and tubing strings, seals the annuli between them, and has side outlets to read each annulus pressure. It is used during both drilling and production. The christmas tree is an assembly of valves and fittings installed on top of the wellhead after completion; it controls production or injection flow and gives access to the well for slickline or coiled tubing intervention. Wellhead and tree equipment is manufactured to API Spec 6A.

The valves one by one

Above the swab sits the tree cap with a pressure gauge. With the swab closed this gauge should read zero; any pressure means a leak past the swab or trapped pressure.

The golden rule: gate valves are not for throttling

Most tree valves are gate valves: a gate sliding between two seats. They are designed to be fully open or fully closed. Partly open with flow continuing, fluid passes at high velocity through a narrow gap and erodes the gate and seats, especially with sand, until the valve can no longer seal. Closed against flow, its surfaces suffer the same erosion in the last moments before closure.

So the roles are split: only the choke handles flow and pressure changes, because it is designed and built from erosion-resistant materials for that purpose. Gate valves are opened and closed while flow is stopped somewhere else.

Opening order, and why

Before starting, confirm the approved procedure, check that the flowline and equipment downstream of the choke are ready to receive flow, and read and record wellhead and annulus pressures. Usual order:

1. Open the SCSSV: pressure up the control line until the valve opens. If pressure below is much higher than above, equalise first per the valve design and company procedure, because opening against a large differential can damage it.
2. Open the LMV with the valves above it closed, i.e. with no flow. Count turns to full open and record them.
3. Open the UMV/SSV hydraulically from the control panel, or manually depending on type, with flow still stopped.
4. Open the production wing with the choke closed, so there is still no flow.
5. Open the choke gradually: only now does flow start, under control, while watching pressures, rate and the equipment downstream.

In short: every gate valve is opened with pressure balanced or flow stopped, and flow starts at the choke built for it. If there is a large differential across a closed gate valve, equalise it before opening wherever possible: opening against a large differential needs more torque and strains the gate and seats.

Closing order, and why

Normal shut-in is the reverse of opening:

1. Close the choke, or bring it down gradually until flow stops.
2. Close the production wing.
3. Close the UMV/SSV.
4. Close the LMV when needed, for a long shut-in or as the procedure requires.

In an emergency shutdown (ESD) the actuated valves close in a similar order: the choke if actuated, then the wing, then the master. The SCSSV normally closes after the surface valves, so it is not slammed shut against full flow, and so the surface barriers remain the fastest. Exact timings are set by the facility's safety system design under practices such as API RP 14C; API RP 14B covers subsurface safety valve systems.

Why is the LMV closed last? Because it is the last surface barrier on the wellhead and is hard to replace: replacing it usually means killing the well or setting a back pressure valve in the tubing hanger. So we never use it to cut flow, and we open and close it with pressure balanced, keeping it intact for the moment we really need it. Day-to-day shut-ins use the wing and the SSV, which are easier to maintain and replace.

The swab valve and well intervention

For slickline or coiled tubing work, the lubricator and pressure control equipment are rigged up on the swab, after confirming zero pressure above it and safely removing the cap. After the equipment is pressure-tested, the swab is opened slowly, counting turns, to equalise, and the tools are run. At the end of the job the swab is closed only when the tools are inside the lubricator. Never close the swab on wire or CT except in an emergency per procedure: it may cut it and damage the valve. See the gauge cutter article.

The kill wing

Used to pump into the well: kill fluid during a workover, inhibitors and methanol against hydrates, or tests. It stays closed during production, and a blind cap or an extra valve is often fitted after it, depending on design.

Wellhead control panel and hydraulic valves

On many wells, especially offshore and high-pressure wells, not every valve is operated by hand. The wellhead control panel supplies hydraulic pressure to two main lines, the SCSSV line and the SSV line, sometimes also the wing. These valves are fail-safe closed: the valve stays open as long as hydraulic pressure is held, and an internal spring closes it the moment pressure is lost. So a control-line leak, a signal from the fire and gas system or an abnormal line pressure shuts the well in automatically.

In practice the technician opens the well from the panel in the same order: pressure the SCSSV line and confirm it is open, open the lower manual valves, pressure the SSV line, then the wing, then open the choke, watching each line pressure on the panel. A falling SCSSV line pressure, for example, signals a leak that must be reported at once.

Worked scenario: reopening a well shut in for several days

An oil well was shut in for maintenance on the wing and the SSV; the LMV and SCSSV are open. The task is to bring it back on line:

1. Read tubing pressure above the SSV if possible, read and record the A and B annulus pressures, and compare them with the last readings before shut-in. Any unexplained rise in annulus pressure must be reported before continuing.
2. Confirm the flowline and separator are ready and the valves downstream of the choke are correctly lined up.
3. Confirm the choke is closed or at its smallest opening.
4. Open the SSV from the panel, then open the wing.
5. Open the choke gradually in steps, watching wellhead pressure, downstream pressure and fluid temperature. On gas wells, watch for hydrates downstream of the choke.
6. Record the time, choke setting and pressures in the shift report.

Good practice on site

• Count and record turns: know the number of turns to full open for each manual valve. If a valve stops short of that number there is a problem: ice, hydrates, debris or damage.
• No cheater bars on handwheels unless the procedure allows it: excess torque can break the stem or hide a real problem.
• After full opening: some manufacturers recommend backing the handwheel off a fraction of a turn; others do not. Follow the valve instructions and your company procedure.
• Greasing and maintenance: gate valves need periodic greasing per the manufacturer and periodic leak (inflow) tests: close the valve, bleed off above it, and watch for pressure build-up.
• Trapped pressure: before removing any cap or breaking any connection, confirm zero pressure through a bleed point, and never stand in front of the bleed port.
• Hydrates and ice: in gas wells and cold weather, hydrates can form in valves or downstream of the choke as temperature falls with pressure. Inject methanol or inhibitor per the programme.

Common mistakes

• Opening the wing before the master with the choke open, so flow starts through a gate valve.
• Closing the LMV to stop a flowing well instead of the choke and wing.
• Leaving a gate valve partly open "to cut back production".
• Not reading the tree cap gauge and annulus pressures before and after the job.

Summary for the technician

Remember the order in one sentence: open from the bottom up and then the choke; close from the choke downward. That way no gate valve ever cuts flow, and the lower master and SCSSV stay intact for emergencies. Detailed procedures differ between companies and tree types (manual, hydraulic, subsea), so always refer to your site procedure. To go further, see the lesson Well anatomy.

Test your understanding

Glossary terms

• Christmas Tree
• Choke
• Tubing
• Lubricator
• Workover

Related lessons

• Well anatomy: from wellhead to bottomhole
• Inside the separator: how are gas, oil, and water separated?
• Workover operations: from well failure to production restart
• Slickline in detail: tools and fishing

Sources

1. Control Global (B. Liptak, 2015) - Wellheads and Christmas trees: valve functions and closing order
2. API - Standards catalogue (Spec 6A wellhead and tree equipment, RP 14B, RP 14C)
3. OSHA eTool - Special services (pressure release, bleed-off before breaking connections)