Choke Manifold and Kill Manifold are special well-control manifolds composed of Choke Valve, various valves, manifold piping and pressure gauges. They are used to control annular-fluid discharge and control well back pressure. By pumping clean water, kill fluid, cement slurry and other fluids into the annulus, they support well killing, water control, fire extinguishing and related operations. They are indispensable equipment for well-control operations, especially well-kill operations.
Figure 1 shows the installation layout of a Choke Manifold and Kill Manifold with maximum working pressure of 35 MPa.
Figure 1. Installation layout of Choke Manifold and Kill Manifold with maximum working pressure of 35 MPa
1. Connect to flowback line or drilling pump; 2. Slab Gate Valve; 3. Check Valve; 4. connect to high-pressure pump; 5. Choke Valve; 6. dual Slab Gate Valve; 7. hydraulic Slab Gate Valve; 8. five-way cross; 9. to drilling-fluid gas separation; 10. buffer pipe; 11. discharge; 12. flow tube; 13. flowback valve
1. Functions of Choke Manifold and Kill Manifold
The functions of the Choke Manifold are as follows:
(1) Carry out well killing through the throttling action of the Choke Valve, replacing contaminated kill fluid in the well while controlling wellhead casing pressure and standpipe pressure, restoring bottomhole pressure control by the kill-fluid column and stopping overflow.
(2) Reduce wellhead pressure through the pressure-relief action of the Choke Valve, realizing "soft shut-in".
(3) Reduce wellhead casing pressure through the large-flow discharge action of the flowback valve, protecting the wellhead BOP stack.
The functions of the Kill Manifold are:
(1) When the wellhead is fully closed with blind rams, heavy kill fluid is forcibly pumped into the wellbore through the Kill Manifold to carry out well killing.
(2) When blowout has already occurred, clean water can be pumped forcefully into the wellhead through the Kill Manifold to prevent ignition.
(3) When the well has blown out and caught fire, fire-extinguishing agent can be pumped forcefully into the wellbore through the Kill Manifold to assist fire extinguishing.
2. Classification of Choke Manifold
Choke Manifold is usually divided into manual Choke Manifold and hydraulic Choke Manifold. In a manual Choke Manifold, both the service and standby Choke Valve are manual Choke Valve units, and the five-way cross is equipped with a casing-pressure gauge and a standpipe-pressure gauge, with the standpipe-pressure gauge line led from the standpipe. In a hydraulic Choke Manifold, the service Choke Valve is a hydraulic Choke Valve and its opening is controlled by a dedicated hydraulic control console, while the standby Choke Valve remains a manual Choke Valve.
The composition of a hydraulic Choke Manifold is shown in Figure 2.
Figure 2. Hydraulic Choke Manifold and hydraulic control console
1. Oil-pressure gauge; 2. standpipe-pressure gauge; 3. valve-opening indicator; 4. casing-pressure gauge; 5. air-source pressure gauge; 6. three-position four-way reversing valve; 7. speed-control valve; 8. standpipe-pressure transmitter; 9. casing-pressure transmitter; 10. valve-position transmitter; 11. hydraulic Choke Valve
It is installed on both sides of the wellhead spool. Its rated pressure is consistent with the rated pressure of the BOP stack. The general bore is not less than 50 mm, or 2 in., and the flowback manifold is not less than 76 mm, or 3 in. At present, domestic Choke Manifold and Kill Manifold working pressures are divided into five classes: 14 MPa, 21 MPa, 35 MPa, 70 MPa and 105 MPa.
The operator only needs to observe pressure changes on the standpipe-pressure gauge and casing-pressure gauge and the indication of the valve-opening indicator at the console. By operating the handle of the three-position four-way reversing valve with one hand and adjusting the speed-control valve handwheel with the other, the kill operation can be carried out. The hydraulic Choke Manifold has centralized and simple operation, which is very beneficial for well-control work.
A standpipe-pressure gauge switch knob is installed below the standpipe-pressure gauge on the hydraulic control console. During normal operation, the Choke Manifold is not in service, and the knob should be turned to the closed position to prevent damage to the standpipe-pressure gauge caused by excessive fluctuation of standpipe kill-fluid pressure. Once the Choke Manifold is put into kill service, the knob should immediately be turned to the open position.
3. Model Designation of Choke Manifold and Kill Manifold
The Choke Manifold model designation is as follows:
1) Pressure class, MPa.
2) Number of Choke Valve units with different control modes.
3) Choke Valve control mode: S for manual control, Y for hydraulic control, Q for pneumatic control.
4) Choke Manifold.
The Kill Manifold model designation is as follows:
2) Kill Manifold.
Example: for a Choke Manifold and Kill Manifold rated 21 MPa and equipped with manual Choke Valve, the models are JG/S2-21 and YG-21 respectively.
4. Main Valves of Choke Manifold and Kill Manifold
1. Slab Gate Valve
Slab Gate Valve
is divided into manual Slab Gate Valve and hydraulic Slab Gate Valve. The manual Slab Gate Valve is a floating structure, as shown in Figures 3 and 4.
Figure 3. Manual Slab Gate Valve structure
Figure 4. Hydraulic Slab Gate Valve structure
The operating points for opening a Slab Gate Valve are: rotate the handwheel counterclockwise, raise the gate fully, then turn the handwheel back 1/4 to 1/2 turn.
The operating points for closing a Slab Gate Valve are: rotate the handwheel clockwise, lower the gate fully, then turn the handwheel back 1/4 to 1/2 turn.
A Slab Gate Valve can only be fully open or fully closed. It is not allowed to be half open or half closed, otherwise high-velocity well-fluid erosion will cause premature damage. Therefore, a Slab Gate Valve can only be used for flow-through or shutoff service and must not be used as a Choke Valve. Opening a Slab Gate Valve slightly for pressure relief is strictly prohibited.
2. Choke Valve
The Choke Valve is the key valve on the Choke Manifold. Choke Valve units include manual Choke Valve and hydraulic Choke Valve. By shape, they include needle Choke Valve and cylindrical-disc Choke Valve. Cylindrical-disc Choke Valve units are widely used in the field. The structure of a manual cylindrical-disc Choke Valve is shown in Figure 5.
Figure 5. Manual cylindrical-disc Choke Valve
The valve disc is cylindrical. There is an annular clearance between disc and seat, and inlet and outlet are always connected. Therefore, this valve does not seal when closed. Both disc and seat are made of wear-resistant material, and the disc can be reversed and reused after wear. Compared with a needle Choke Valve, this Choke Valve is more erosion resistant, has larger flow capacity and produces less vibration during throttling.
When operating the Choke Valve, clockwise rotation of the handwheel reduces the opening and tends toward closing; counterclockwise rotation increases the opening. The Choke Valve opening can be judged by observing the position of the valve-stem end through the slot in the shield. Normally the Choke Valve on the manifold should be in a half-open state.
A hydraulic cylindrical-disc Choke Valve replaces the handwheel mechanism with cylinder and piston; the rest is the same as the manual cylindrical-disc Choke Valve. The required hydraulic control oil pressure is not high; 1 MPa oil pressure is sufficient. Hydraulic control oil is supplied by the hydraulic control console.
3. Check Valve
A Check Valve is installed on the Kill Manifold, as shown in Figure 6.
Figure 6. Check Valve
When the high-pressure pump injects kill fluid into the wellbore, the kill fluid enters from the low port of the Check Valve and is discharged from the high port. When the pump stops, the kill fluid does not flow backward. In normal conditions or during blowout, high-pressure wellhead fluid will not flow out along the Check Valve. This valve has good self-sealing effect, long service life and convenient field maintenance.
Choke Manifold model designation
Kill Manifold model designation
