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The two barrier rule is a policy of safety for work on piping carrying high pressure and/or high temperature fluids. It is commonly used by oil companies such as BP. The rule simply states that for any work that must be done on a section of piping, which involves breaking containment, there must be at least two barriers separating the fluids from the broken area. The purpose behind specifying two barriers is to ensure redundancy. The risk of a total loss of containment is reduced if there are two independent isolations as it is highly improbable they would both fail at the same time.
For example, if a section of pipe from a production line leading from a Xmas tree to the process plant, were to need replacement, this area of the piping would need isolation before the offending section was removed or oil would be able to flow freely out of the piping into the environment, causing environmental damage, a health and safety hazard and waste of a precious resource. Under the two barrier rule, there must be two separate isolations between the oil flowing from the well to this area of the piping. The logical barriers in this case would be the production wing valve and the upper master valve.
Uncontrolled release of pressure has been the cause of many accidents in the oilfield and so the two barrier rule is considered very important for increasing safety.
Tree complexity has increased over the last few decades. They are frequently manufactured from blocks of steel containing multiple valves rather than made from multiple flanged valves. 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.
It is common to identify the type of tree as either "subsea tree" or "surface tree". Each of these classifications has a number or varieties within them. 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 primary function of a tree is to control the flow into or out of the well, usually oil or gas. A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means (such as annulus vent), tree and well monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback, connection points for devices such as down hole pressure and temperature transducer (DHPT).
Subsea and Surface Trees have a large variety of valve configurations and combinations of manual and/or actuated (hydraulic or pneumatic) valves. Examples are identified in API Specifications 6A and 17D.
A basic surface tree consists of two or three manual valves (usually gate valves because of their strength).
A typical sophisticated surface tree will have at least four or five 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 means of well control while an actuated wing valve is normally the primary well remotely (from control room or control panel) controlled valve. Hydraulic tree wing valves are usually built to be fail safe closed, meaning they require active hydraulic pressure to stay open.
The right hand valve is often called the flow wing valve or the production wing valve, because it is in the flowpath the hydrocarbons take to production facilities (or the path water or gas will take from production to the well in the case of injection wells).
The left hand valve is often called the kill wing valve. It is primarily 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 lubricator is rigged up onto the top of the tree and the wire or coil is lowered through the lubricator, 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.
Subsea trees are available in either vertical or horizontal configurations with further speciality available such as dual bore, monobore, concentric, drill-through, mudline, guidlineless or guideline. Subsea trees may range in size and weight from a few tons to approximately 70 tons for high pressure, deepwater (>3000 feet) guidelineless applications. Subsea trees contain many additional valves and accessories compared to Surface trees. Typically a subsea tree would have a choke (permits control of flow), a floline connection interface (hub, flange or other connection), subsea control interface (direct hydraulic, electro hydraulic, or electric) and sensors for gathering data such as pressure, temperature, sand flow, erosion, multiPhase flow, single phase flow such as water or gas.
A blowout preventer (BOP) is a large valve that can seal off at the surface wellhead a well being drilled or worked over. During drilling or well interventions, the valve may be closed if overpressure from an underground zone causes formation fluids such as oil or natural gas to enter the wellbore and threaten the rig. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew can prevent explosive pressure release, thus regaining control of the downhole pressure. Once this is accomplished, often the drilling mud density within the hole can be increased until adequate fluid pressure is being placed on the influx zone, and the BOP can be opened for operations to resume. The invention and use of BOPs were instrumental in the end of oil gushers, which were dangerous and costly.