Choke Manifold Function
If the hydrostatic head of the drilling fluid is insufficient to control subsurface pressure, formation fluids will flow into the well. To maintain well control, back pressure is applied by routing the returns through adjustable chokes until the well flow condition is corrected. The chokes are connected to the blowout preventer stack through an arrangement of valves, fittings, and lines which provide alternative flow routes or permit the flow to be halted entirely. This equipment assemblage is designated the "choke manifold."
Choke Manifold Parts
A choke manifold is an assembly of valves, through which the return flow from the well is routed when the blowout preventers are closed, with the purpose of applying calculated backpressure. Choke manifolds may be assembled in a variety of layouts but they will always include at least two adjustable chokes. In some cases, this may be one manual choke and one remote-controlled choke. The manifold provides alternative flow paths for the fluid so that if necessary chokes can be changed and valves repaired without stopping the flow. All the high-pressure parts of the manifold should have the same working pressure rating as the BOP stack.
The Main Components Of the Choke Manifold are:
Choke a device with an orifice installed in a line to restrict the flow of fluids. Chokes are also used to control the rate of flow of the drilling mud out of the hole when the well is closed in with the blowout preventer and a Kick is being circulated out of the hole.
Choke line: a pipe attached to the blowout preventer stack out of which kick fluids and mud can be pumped to the choke manifold when a blowout preventer is closed in on a kick.
HCR: The type 'HCR'pressure-operated gate valve is a flow line valve requiring relatively low operating pressures. This is a single BOP ram, hydraulic gate valve packed with elements similar to the old 'QRC' ram assembly. The closing ratio of well pressure to hydraulic operating pressure is approximately 8 to 1. Available sizes are 4-inch 3000 to 5000 psi working pressure, and 6-inch 3000 and 5000 psi working pressure with standard API flanges.
Chokes:
The choke is normally an adjustable orifice installed in the return line. It is used to restrict the flow area so that the pressure drop of the returns through this line can be regulated while a kick is circulated out. Three types of chokes may be encountered in choke manifolds:
The manual adjustable choke.
The replaceable fixed choke.
The remote controlled choke.
Manual Adjustable Choke
Typical needle valve type manual adjustable choke. The stem and seat area are of tungsten carbide to make them more wear resis tant; it must be understood that a choke is not meant to be used as a valve. The tool is designed to create a flow restric tion and not to provide a high-pressure seal. Washed-out sealing areas are also common. Therefore the choke must be used for initial closing in only and should immediately be backed up by closing the upstream valve. This type of choke should not be left "closed" for long periods of time. Temperature expansion of the needle can damage the seat and the needle may "freeze" in the seat.
Fixed Choke
Instead of using an adjustable spindle valve, the seat can be replaced by different sizes of "beans". Such chokes are used only if the well returns will have to be produced at a constant rate over a considerable period of time, such as is common during production tests. Fixed chokes are sometimes referred to as positive chokes.
The choke body in such a set-up is provided with a cap instead of a needle assembly
Remote Controlled Choke
Remote-controlled chokes are operated from a panel, usually on the rig floor. This operating panel should include:
- a drill pipe pressure gauge
- an annulus pressure gauge
- a pump stroke counter
- a choke selection switch
- a maximum allowable annulus pressure setting regulator (optional)
- a choke control lever
- and throttles for the pumps (optional)
There are different remote-controlled chokes, some of which have specific operating characteristics that may affect the well killing operation. It is therefore important to check the details of the unit installed.
Choke And Kill Line Outlet Valves
Owing to area and contractor specific requirements, it is not feasible to specify a standard layout, but the following minimum requirements should be adhered to:
The choke line must have a minimum ID of 76·2 mm (3″), the kill line may be as small as 50·8 mm (2″), albeit that this might restrain opera tional flexibility should immediate substitution of a choke line be required. During normal operation, the inner ( usually manual) choke and kill line valves should remain open and the outer (hydraulically operated) valves closed such as to prevent excessive solids build-up in these lines.
Wellhead outlets should, under normal operating conditions, not be used for a choke and kill line tie-in.
If the kill line is not meant to ultimately replace or augment the choke line, it is highly desirable to install a check valve upstream of the stack valves.
Choke manifold design should consider such factors as the anticipated formation and surface pressures, method of well control to be employed, surrounding environment, corrosivity, volume, toxicity, and abrasiveness of fluids.
As one of the professional suppliers of downhole equipment in the oil and gas industry, Vigor's Kill Manifold is designed and produced in strict accordance with API standards, and Vigor's team of quality engineers will also follow up the whole process to ensure that customers can get on-site feedback at any time during the production process. At the same time, we will also prepare the relevant product quality inspection certificates and formal reports in advance to ensure that our customers can access and call them at any time. If you are interested in Vigor's downhole drilling and completion tools, please do not hesitate to get in touch with us to get the most professional technical support and the most comprehensive quality service.
For more information, you can write to our mailbox info@vigorpetroleum.com & mail@vigorpetroleum.com
