Wellheads and Christmas trees: Is there a difference?
Author: yisainuo : 2016-08-28
Both are vital to oil production, but they're not the same, and the 45-second valve close rule is there for a reason.
In oil and natural gas extraction, a Christmas tree, or "tree," is an assembly of valves, spools and fittings used for not only extraction, but also for water injection or disposal, gas injection and other operations. Wellheads and Christmas trees are different pieces of equipment (Figure 1). A wellhead is used during both drilling and production. During drilling, it is used without a Christmas tree while during production it is used in combination with the Christmas tree, to which the wellhead connects.
Subsea and surface trees can have a large variety of configurations of manual and/or automatically actuated (hydraulic or pneumatic) valves. Examples are identified in API specifications 6A and 17D. A typical surface tree will have four or five valves, normally arranged in a crucifix type pattern (hence the endurance of the term "Christmas tree"). Figure 2 shows a 5-valve Christmas tree. The five valves shown are described below, including their manual or automatic operation, but these methods of operation vary and each can either be manual or automatic.
1.Lower master valve. This is usually a manually operated gate valve which is not fully open during production.
2.Upper master valve. This is a fail-closed, automatically actuated (often hydraulic) gate valve, which is remotely actuated (ROV) and closes in case of an emergency or can be closed for maintenance.
3.Kill wing valve (KWV). It is a manually operated valve used for the 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).
4.Production wing valve. This right-hand valve is an automatically actuated, fail- closed valve (requiring positive hydraulic pressure to stay open), serving to shut the flow from the well under emergencies or to stop injection when water or gas are being injected. The "choke" is used to control the flow (volume) or to reduce the pressure.
5.Swab valve with cap and gauge. This is a manually operated valve used for well interventions, such as when wires or coils are lowered through the lubricator into the well. Some trees have a second swab valve to act as a second barrier against leaking. The gauge on the tree cap should read zero (0 psig) whenever the kill wing valve is open and the swab valve is closed.