BLOWOUT PREVENTERS

A drilling mud should have sufficient density (mud weight) to prevent (hydrostatically) any gas, oil or saltwater from entering the wellbore uncontrolled. Sometimes however, these formation fluids do enter the wellbore under great pressure. When this happens, a well is said to “take a kick.” It is especially risky if the fluid is a gas or oil.

To guard against the dangers of such events, rigs are usually equipped with a stack of Blowout Preventers (BOPs). Depending on the well depth and other circumstances, there will be several BOP units bolted together and then to the surface casing flange. One or more of these BOPs can be engaged to seal off the wellbore if a kick occurs. Multiple BOPs in the stack provide flexibility and redundancy in case of a failure.

At the top of the BOP stack is a bag type preventer commonly referred to as a Hydril. This unit contains a steelribbed, elastomeric insert which can be expanded hydraulically to seal the annulus. Below the bag preventers are the ram-type preventers with hydraulically driven rams that close against the pipe or against themselves, thrusting in from opposite sides of the pipe. These preventers can be pipe, blind or shear rams. Pipe rams have heads with a concave shape to grip the pipe and form a seal around it; they accomplish the same function as the bag preventer but are rated at higher pressure. Blind rams come together over the hole to form a fluid-tight seal against one another in the event the pipe is not in the well or if it has parted and fallen down into the wellbore. Shear rams sever the pipe before sealing together.

Below the blowout preventers is the drilling spool. It has an opening in its side to allow drilling mud and the kick fluids to be pumped out. A high-pressure choke line connects to the spool with a special back-pressure valve (the choke) in the line. During well-control procedures, the choke is used to hold back-pressure on the annulus while heavier mud is pumped down the drillstring to kill the kick. If the invading fluid contains gas, the gas must be removed from the mud exiting the well. Gas-cut mud from the choke is sent to a mud-gas separator vessel. The gas is flared and the mud is returned to the pits for reconditioning.

Date: 2015-02-28; view: 1390