If the oil and gas in the well is encountered during the drilling process, the next step after closing the BOP is to remove the oil and gas (Influx) entering the well and the BOP; increase the density of the drilling fluid and re-establish a higher static Pressure to balance the pressure in the formation; choke & kill lines are specifically designed for this purpose.
The bottom of the ram blowout preventer has a hole and a control valve and a pipe are installed; when the control valve is opened, the liquid in the well can be returned from the pipe to the platform; and the returned liquid is under controlled conditions. Slowly release the previous pressure difference; this pipe is called Choke line. If the drill pipe has been cut, the upper blind seal ram blowout preventer has been closed, the wellhead is completely sealed, and the aggravated mud cannot enter the well from the drill pipe normally; it is necessary to use another interface on the side of the BOP and install the valve and pipe. And call it the Killline killing tube; it has the same configuration as the choke line.
On deepwater drilling units, two choke & kill lines are required for the following reasons:
Safety relationship, if one fails (blocking, leaking, etc.), the other one can be used as a backup;
The pressure in the BOP can be monitored by the pressure on a static throttle killing pipe to compensate for the resistance loss on the throttled killing pipe;
When the cut drill pipe is suspended on the closing ram blowout preventer, the mud can be circulated;
It is possible to recycle the oil and gas trapped in the closed BOP;
Two throttling killing tubes can be used simultaneously to reduce the loss of resistance along the path;
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