- published: 18 Dec 2015
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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 and offloading vessel or FPSO.
A Christmas tree is a decorated tree, usually an evergreen conifer such as pine or fir, traditionally associated with the celebration of Christmas. An artificial Christmas tree is an object made to resemble such a tree, usually made from polyvinyl chloride (PVC).
The tree was traditionally decorated with edibles such as apples, nuts or dates. In the 18th century, it began to be illuminated by candles, which with electrification could also be replaced by Christmas lights. Today, there is a wide variety of traditional ornaments, such as garland, tinsel, and candy canes. An angel or star may be placed at the top of the tree, to represent the host of angels or the Star of Bethlehem from the Nativity.
The custom of the Christmas tree developed in early modern Germany with predecessors that can be traced to the 16th and possibly the 15th century. It acquired popularity beyond Germany during the second half of the 19th century. The Christmas tree has also been known as the "Yule-tree", especially in discussions of its folkloristic origins.