– Bela Liptak
When a "kick" is detected, the pressure balance must be reestablished by doing the following (in sequence):
- Increase the nitrogen (N2) blanket pressure on the mud tank (Figure 2).
- If that does not stop the "kick", gradually close the variable bore ram (Figure 1) in the BOP and if "soft" closure is desired (no sudden rise in pressure), throttle the choke valve (C in Figure 2) while doing it.
- If the "kick pressure" is still rising, first close the casing ram in the BOP and then (if needed) the blind shear ram (Figure 1).
During the production phase, the pipe that in Phase 1 was taking the drilling fluid down into the well serves to carry the oil or gas up from the formation to the rig. They rise under their own pressures, so normally no pump is needed to provide the driving force. During this phase, the flow direction is reversed, the fluid properties are changed (pressures, flows, densities, conductivity), but the process is similar.
The laws of hydraulics through a vertical U-pipe still describe the process. Therefore, some of the same sensors that were used in Phase 1 can also be used, although they require recalibration and range change.
The oil/gas is collected in storage tanks and is transported by barges or through pipelines under the ocean to the shore. During this phase the required safety controls to protect against "kicks" are similar to those described in Phase 1. The fact that the product is flammable requires additional protection to guarantee safety. If the BOPs failed and the presence of flammables is detected (Chapter 7.8 of Volume 2 of the Instrument Engineers’ Handbook), the immediate response should be to turn off all ignition sources on the rig or start nitrogen purging them. If, in spite of these steps, fire is detected and can not be extinguished, the rig should be disconnected from the well and moved away.
Phase 3--"Killing" the Well
When, for any reason, the well is to be closed ("killed"), killing fluid (usually a concrete mixture) is pumped into the well through the kill line shown in the upper right of Figure 1. As this mix is heavier than the oil, it displaces the oil and plugs the well. In this phase too, until the concrete sets, the maintaining the pressure balance is critical and can be controlled by keeping the mix density high enough to provide the required hydrostatic pressure to prevent blowout. If the concrete is too dilute (low density), a methane kick can blow it out, as occurred at the BP accident.
The key to improving safety during all three phases of the described process is to have reliable (redundant) sensors and fully automatic response to unsafe conditions so that time is not wasted by the need for obtaining management approvals, and mistakes are not made because operators are untrained or panicked. It is also important to keep the operators fully informed of the conditions on the rig. This requires using "smart annunciators," which not only inform them about the existence of unsafe conditions, but also gives them instructions on what to do about them.