A back-pressure valve is a type of check valve, typically installed in the tubing hanger, to isolate the production tubing. The back-pressure valve is designed to hold pressure from below yet enable fluids to be pumped from above, as may be required for well-control purposes. Thus, it reduces downtime and operating cost by allowing for repairs without killing the well.
BPV is commonly used during the nipple down and up of the drilling BOP stack, nipple up or down of Christmass trees, test the Christmas tree (2-way check valve), and during the replacement of the master valve.
When selecting steel type for pipes and connections it is important to consider the corrosive environment that the steel will be subjected to. There are several parameters in the well that affect the corrosion, like temperature, chloride ion concentration, partial pressure of CO2 and H2S, pH and presence or absence of Sulphur [Craig et al. 2011].
When selecting a material there are certain aspects that have to be taken into consideration [NORSOK M-001 2004]:
Failure possibility and the consequences related to failure;
Resistance to brittle fracture;
API tubing steel grades are identified by letters and numbers which dictate various characteristics of the steel. For each grade, the number designates the minimum yield strength. Thus L-80 grade steel has a minimum yield strength of 80,000 psi. In other words, it can support a stress of 80,000 psi with an elongation of less than 0.5%. The letter in conjunction with the number designates parameters such as the maximum yield strength and the minimum ultimate strength which for L-80 pipe are 95,000 psi.
The following table shows the yield values for various API tubing grades:
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.