In petroleum and natural gas extraction, a Christmas tree, or "tree" (not "wellhead" as sometimes incorrectly referred to), is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells. It was named for its crude resemblance to a decorated tree.
Christmas trees are used on both surface and subsea wells. It is common to identify the type of tree as either "subsea tree" or "surface tree". Each of these classifications has a number of variations. Examples of subsea include conventional, dual bore, mono bore, TFL (through flow line), horizontal, mudline, mudline horizontal, side valve, and TBT (through-bore tree) trees. The deepest installed subsea tree is in the Gulf of Mexico at approximately 9,000 feet (2,700 m). (Current technical limits are up to around 3000 metres and working temperatures of -50°F to 350°F with a pressure of up to 15,000 psi.)
The primary function of a tree is to control the flow, usually oil or gas, out of the well. (A tree may also be used to control the injection of gas or water into a non-producing well in order to enhance production rates of oil from other wells.) When the well and facilities are ready to produce and receive oil or gas, tree valves are opened and the formation fluids are allowed to go through a flow line. This leads to a processing facility, storage depot and/or other pipeline eventually leading to a refinery or distribution center (for gas). Flow lines on subsea wells usually lead to a fixed or floating production platform or to a storage ship or barge, known as a floating storage offloading vessel (FSO), or floating processing unit (FPU), or floating production, storage and offloading vessel(FPSO).
A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means, monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback), and connection points for devices such as down hole pressure and temperature transducers (DHPT). On producing wells, chemicals or alcohols or oil distillates may be injected to preclude production problems (such as blockages).
Functionality may be extended further by using the control system on a subsea tree to monitor, measure, and react to sensor outputs on the tree or even down the well bore. The control system attached to the tree controls the downhole safety valve (SCSSV, DHSV, SSSV) while the tree acts as an attachment and conduit means of the control system to the downhole safety valve.
Tree complexity has increased over the last few decades. They are frequently manufactured from blocks of steel containing multiple valves rather than being assembled from individual flanged components. This is especially true in subsea applications where the resemblance to Christmas trees no longer exists given the frame and support systems into which the main valve block is integrated.
Note that a tree and wellhead are separate pieces of equipment not to be mistaken as the same piece. The Christmas tree is installed on top of the wellhead. A wellhead is used without a Christmas tree during drilling operations, and also for riser tie-back situations that later would have a tree installed at riser top. Wells being produced with rod pumps (pump jacks, nodding donkeys, and so on) frequently do not utilize any tree owing to no pressure-containment requirement.
ELSA HD (High debris, Deep Water)
Elsa HD was specifically developed to operate in applications with highly aggressive erosive solids and deep water environments, offering exceptional reliability for all horizontal / vertical subsea xmas tree well completion and intervention operations.
The ELSA HD comprises of a full suite of valve assemblies. The lubricator valve, retainer valve and subsea test tree provides a full range of well intervention, pressure control functions and disconnection capabilities for harsh completion installation, workover or intervention operations.
ELSA DH (Direct Hydraulic)
These landing string assembles are designed for direct hydraulic completion and intervention applications and interface with all mono-bore completions when used in conjunction with horizontal and conventional subsea production trees.
The design incorporates control ports enabling tubing hangar running tool and other downhole functions such as subsurface safety valve (SCSSV) and chemical injection sleeves to be controlled monitored. In addition electric cabling can be run through the system to monitor downhole gauges or electric submersible pumps.
ELSA HP (High Pressure)
The ELSA-HP has been developed to service the high pressure horizontal tree completion and intervention market. With systems designed and qualified up to 15,000 psi, 250 degF and 10,000 ft water depth, this landing string assembly provides our clients the safety and reliability required to develop fields with these challenging conditions. Expro used the highly successful and field proven ELSA-HD design as the basis for the development of the ELSA-HP products.
ELSA LBEH (Large Bore)
These landing string assembles are designed for electro hydraulic completion and intervention applications and interface with all mono-bore completions when used in conjunction with horizontal and conventional subsea production trees.
The design incorporates control ports enabling tubing hangar running tool and other downhole functions such as subsurface safety valve (SCSSV) and chemical injection sleeves to be controlled monitored. In addition electric cabling can be run through the system to monitor downhole gauges or electric submersible pumps.
ELSA CLV (Cutting Lubricator valves)
Cutting lubricator valves are used when coiled tubing and / or wireline cutting capability is required in addition to the more traditional requirements for Expro’s lubricator valves.
To facilitate the introduction of through tubing tools (i.e. coiled tubing and wireline) into production strings longer than those acceptable in a customary derrick installed lubricator assembly and to provide a means of pressure testing the surface equipment and lubricator sections once the wireline tool string has been installed.
ELSA CLV provides a method of isolating surface equipment from production flow and a pressure tight barrier between the well bore and BOP stack and/or marine riser.
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