Christmas tree (oil well)

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In petroleum and natural gas extraction, a christmas tree is an assembly of valves, spools and fittings for an oil well, named for its resemblance to a decorated tree. The term "Christmas Tree" has now become something of a misnomer as complexity has increased and they are more frequently manufactured from a single block of steel rather than made from multiple flanged joints. Especially in subsea applications the resemblance no longer exists given the frame and support systems into which the main valve block is integrated. It is also more usual now to determine either "Subsea Tree" or "Surface Tree".

The function of a christmas tree is to both prevent the release of oil or gas from an oil well into the environment and also to direct and control the flow of formation fluids from the well. When the well is ready to produce oil or gas, valves are opened and the release of the formation fluids is allowed through a pipeline leading to a refinery, or to a platform or to a storage vessel (known as a Floating Production Storage (and) Offloading Vessel or FPSO).

It may also be used to control the injection of gas or water injection application on a none-producing well in order to sustain "producer" volumes. On natural gas wells, the Christmas Tree may be outfitted with a lubricator/catch assembly to catch a mechanical plunger that returns to the surface to clear fluids from the tubing pipe.

On producing wells injection of chemicals or alcohols or oil distillates to solve production problems (such as blockages) may be used. Functionality may be extended further by using the control module on a subsea tree to monitor, measure and react to sensor inputs on the tree or even down the well bore. Crucial to safety the tree will probably also control the downhole safety valve.

Christmas trees are used on both subsea (current technical limits are up to around 3000 metres and working temperatures of -50 °f to 250°f with a pressure rating up to 15,000psi) and surface wellheads and both are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity or horizontal or vertical in their primary valve bore axis.

1 Christmas tree vs Xmas tree
2 Valves
3 Christmas tree images
4 Manufacturers
5 See also
6 External links

[edit] Christmas tree vs Xmas tree

It is a commonly recognised practise in many areas to refer spell the name as "Xmas tree", in line with the widespread (since the 16th Century or earlier) abbreviation for "Christmas".

[edit] Valves

A typical surface tree will have four valves, normally arranged in a crucifix type pattern (hence the endurance of the term "Christmas tree"). The two lower valves are called the master valves (upper and lower respectively) because they lie in the flow path, which well fluids must take to get to surface. The lower master valve will normally be manually operated, while the upper master valve is often hydraulically actuated, allowing it to be a primary means of well control from the control room. Hydraulic valves are also failsafe, meaning they require active hydraulic pressure to stay open.

The right hand valve is called the flow wing valve or the production wing valve, because it is nominally in the flowpath the hydrocarbons take to production (or the path water or gas will take from production to the well in the case of injection wells). It is hydraulically actuated too.

The left hand valve is called the kill wing valve. It is only used for injection of fluids such as corrosion inhibitors or methanol to prevent hydrate formation. In the North Sea, it is called the non-active side arm (NASA). It is typically manually operated.

The valve at the top is called the swab valve and lies in the path used for well interventions like wireline and coiled tubing. For such operations, a riser is rigged up onto the top of the tree and the wire or coil is lowered through the riser, past the swab valve and into the well. This valve is typically manually operated.

Some trees have a second swab valve, the two arranged one on top of the other. The intention is to allow rigging down equipment from the top of the tree with the well flowing while still preserving the Two barrier rule. With only a single swab valve, the upper master valve is usually closed to act as the second barrier, forcing the well to be shut in for a day during rig down operations. However, avoiding delaying production for a day is usually too small a gain to be worth the extra expense of a having a Xmas tree with a second swab valve.