Q: I work as an instrumentation design engineer on an offshore platform and have some questions.
- According to API RP14C, the response time for closing all the valves in the Christmas tree should not exceed 45 sec. What is the basis of this requirement? Why should all valves close within 45 sec? What would happen if they did not?
- We have a noise level limit of 85 dBA for these valves. Is that limit for the noise level up- or downstream of the valves? What is the straight-run requirement for these valves, and does that relate to the noise limit?
A: 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.
- Lower master valve. This is usually a manually operated gate valve which is not fully open during production.
- 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.
- 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).
- 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.
- 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.
Your statement that "the response time for closing all the valves in the Christmas tree should not exceed 45 seconds" is incorrect, because the swab and the kill wing valves are always closed during production. If the other valves are automatic, they will close in the following order: choke, production wing, upper master and lower master. As to the reason for the fast-closure requirement (API RP14C: under 45 sec), it is because of well spilling. The more hydrocarbon is spilled, the more will accumulate, and therefore, both the probability and the size of the accidents will rise.
The operation of the Christmas trees is usually very primitive, often completely manual. This results not only in safety risks, but also in the loss of billions of dollars due to the leakage loss of valuable hydrocarbons. API-6A requires that the gas leak rate remain under 15 SCFM. Industry has lived with methane leaks (0.45 1012 ft3/yr), which at a residential price of $10/1000 cubic foot corresponds to $4.5 billion. Part of this waste is due to the strange and wasteful practice of powering pneumatic controls with natural gas.
Obviously, one could install a lot of automation for $4.5 billion/year, not even mentioning the environmental and safety benefits.
As to valve noise, refer to Chapter 6.14 in Volume 2 of my handbook.
A: The closing time requirement per API RP14C is 45 sec for the shutdown valves (SDV), providing the positive isolation to the well gas/oil stream) and the Xmas tree valves, except for the surface-controlled subsurface safety valve (SCSSV). This is the maximum/worst time, and generally all the valves meet this requirement.
The SCSSV on the well tube is the first valve which sees the reservoir pressure and has to close against the high pressure; hence, its closing time is always more than that of the other valves on the Xmas tree; i.e., a 2-min maximum. If the Christmas tree is not designed this way, the topside piping will see the reservoir pressure, which is not desirable.
The order in which valves close is 1) choke valve. 2) production wing valve, 3) upper master , 4) lower master. Swab and kill valves are always closed in the normal operation. In less critical applications, the double barrier is not needed, and there can be only one master valve. In subsea applications, all the valves are hydraulically actuated. For the surface tree, it all depends on the philosophy, ease of operation, access and the size of the production tubing /valve. Some of the surface trees have all the valves hydraulically actuated (via wellhead control panel), but can also be all manual or combination of hydraulic and manual.
The noise limit is applicable to both sides of the valve at 1 meter from the valve.
Generally control valves do not require straight pipe runs, however, for self-regulating control valves straight lengths are required for proper control.
Harvindar S Gambhir
A: API RP14C provides the answers to your questions, but the short answer is that one wants to: 1) minimize the possibility of spills in the area of the Christmas tree, and 2) minimize the possibility of fires or ignition of hydrocarbons caused by spills or by moving equipment in the area.
As to the noise levels, you need to perform a hydrodynamic noise calculation to define the thickness of the pipe and soundproofing you may require. Though in practical use, the valves on the Christmas tree are subjected to noise from the hydrocarbons flowing through the pipe more than the flow through the valves. Also the actual noise levels may be higher because of the design of the Christmas tree and its components, the design of which might not be within the scope of the instrument department.