A Christmas Tree comprises a series of valves, spools, a choke, and connection. It used for production or injection wells such as oil wells, gas wells, water injection wells, water disposal wells, etc. It provides a means of controlling the effluents, ensuring the safety of the facilities and giving measurement tools access to the well.
The Difference Between a Wellhead & Christmas Tree?
Many times, the words Christmas Tree and Wellhead are used interchangeably; however, a wellhead and Christmas tree are entirely separate pieces of well equipment. A wellhead must be present in order to utilize a Christmas tree and is used without a Christmas tree during drilling operations. Producing wells that require pumps, such as Sucher Rod Pumping Systems, frequently do not utilize any tree due to no pressure containment requirement.
In the oil and gas industry, depth in a well is the measurement, for any point in that well, of the distance between a reference point or elevation, and that point. It is the most common method of reference for locations in the well, and therefore, in oil industry speech, “depth” also refers to the location itself.
Because wells are not always drilled vertically, there may be two “depths” for every given point in a wellbore: the measured depth (MD) measured along the path of the borehole, and the true vertical depth (TVD), the absolute vertical distance between the datum and the point in the wellbore. In perfectly vertical wells, the TVD equals the MD; otherwise, the TVD is less than the MD measured from the same datum. Common datums used are ground level (GL), drilling rig floor (DF), rotary table (RT), kelly bushing (KB) and mean sea level (MSL). 
Terms and Abbreviations:
The common references used in operations include:
Kelly Bushing Height (KB):The height of the drilling floor above the ground level. Many wellbore depth measurements are taken from the Kelly Bushing. The Kelly bushing elevation is calculated by adding the ground level to the Kelly bushing height.
Rotary Table (RT): e.g. MDBRT or TVDBRT. MDBRT stands for Measured Depth Below Rotary Table (MDBRT),
Rig Floor (RF),
Driller’s Depth below rotary table (DDbrt): The depth of a well or features within the wellbore as measured while drilling. The measured length of each joint of drillpipe or tubing is added to provide a total depth or measurement to the point of interest. Drillers depth is the first depth measurement of a wellbore and is taken from the rotary table level on the rig floor. In most cases, subsequent depth measurements, such as those made during the well completion phase, are corrected to the wellhead datum that is based on drillers depth (reference: Schlumberger Oilfield Glossary).
Tubing is a relatively small-diameter pipe that is run into a well to serve as a conduit for the passage of oil and gas to the field surface facilities for processing. Tubing must be adequately strong to resist loads and deformations associated with production and workovers. Further, tubing must be sized to support the expected rates of production of oil and gas.
Tubing is specified by grade, outer diameter, weight, and connection. API tubing grades correspond to casing grades with the exception that P grade tubing has a tensile strength of 105,000 psi and is referred to as P105.
In this article, the last updated list of API & ISO standards for Tubulars (Casings, Tubings, Threads, Drill Pipe, Line Pipe) is presented. These standards can be considered as reference documents as well as a basis for training classes in the subject of Tubular Goods.
API RP 5A3/ISO 13678:2010 – Recommended Practice on Thread Compounds for Casing, Tubing, Line Pipe, and Drill Stem Elements (includes Errata 1 dated April 2011):
API RP 5A3 provides requirements, recommendations, and methods for the testing of thread compounds intended for use on threaded casing, tubing, and line pipe connections and for thread compounds intended for use on rotary shouldered connections. The tests outlined are used to evaluate the critical performance properties and physical and chemical characteristics of thread compounds under laboratory conditions.
Casing is the major structural component of a well. It is a tubular steel product used to line the wellbore (maintain borehole stability), prevent contamination of water sands, isolate water from producing formations, and control well pressures during drilling, production, and workover operations. Casing provides locations for the installation of blowout preventers, wellhead equipment, production packers, and production tubing.
The cost of casing is a major part of the overall well cost, so the selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration.
Since the well is normally drilled in segments, multiple concentric casing strings are usually installed in the well. There are six basic types of casing strings: