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Sunday, September 2, 2012

Rig systems

No matter what the transport or carrying mode, a rotary drilling rig has five main systems — hoisting system, rotary system, circulation system, power system and blowout prevention/safety system.
The hoisting system consists of a winch (draw-works) on the rig floor. A wire rope (drilling line) is spooled around the winch drum and is run up to the top of the steel derrick, over the crown block and down to an attachment on the travelling block. This latter has a hook for attaching the drill string via a rotary swivel. The draw-works are controlled from the rig floor and are used to raise and lower drill pipe, casing and tubing, or any other equipment run into the well. The exceptions may be logging tools. These are often run on a separate winch as a separate operation to drilling, although in modern systems they may actually be mounted in the drill string just behind the bit to enable a continuous logging record to be kept while drilling.
The rotary system has three main components. First is the rotary swivel for suspension of the drill string to the travelling block. Second is the rotary table located in the rig fl oor and turned mechanically. Its speed and direction is controlled by the driller. The third item is the kelly, a hexagonal or square hollow pipe about 15 metres long which is attached to the rotary swivel at the top and to the drill pipe at the bottom by tapered screw threads. A piece called the kelly bushing fi ts into the rotary table so the rotary motion can be transferred from the table to the drill pipe via the kelly. The kelly bushing runs freely up and down the kelly, but cannot rotate independently of the kelly. Another method of rotating the drill string is a top drive system. This
involves hanging a motor from the hook and connecting it directly onto the drill pipe from above. It imparts the rotation without the need for a kelly or rotary table. The method promotes faster drilling and is particularly advantageous during directional drilling programs.
An additional item in offshore drilling is the marine riser which is a large diameter tubular connection between the rig and the blowout preventer on the sea fl oor. It is a conduit for the circulation of drilling fl uids as well as a guide for running drill pipe and casing. It is fi tted with a giant shock absorber called a telescopic joint to allow for the vessel’s movement on the sea surface and it can be quickly disconnected if sea states become too rough for drilling to continue.

The circulation system pumps drilling fl uid down the well and consists of mud pumps, suction and storage tanks for the mud itself, a stand pipe which runs up the derrick, a kelly hose connecting the stand pipe to the swivel and a return mud line below the rotary table which returns mud from the well to the shale shakers. The latter item removes the drill cuttings before the mud is sent to the mud tanks for further cleaning by de-sanders and de-pitters to remove the fi ner debris before recirculation.

The power system to operate the rig is either a diesel motor via a direct drive compound system, or (particularly offshore) a direct current electric drive.

The blowout prevention system consists of a series of hydraulically operated valves and pipe rams which are open to allow the mud to circulate during drilling, but which can be quickly closed around the pipe if excessive pressure (a kick) enters the well and threatens to circulate during drilling. If a kick occurs (i.e. excessive pressure from the formation being drilled suddenly entering the well), the pipe rams are closed to prevent this overpressure reaching the surface out of control. The last line of defence in such an emergency are the shear rams which, if necessary, cut right through the drill string and seal the well completely.

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