A rig is the machine used to drill a wellbore. In onshore operations, the rig includes virtually everything except living quarters. Major components of the rig include the mud tanks, the mud pumps, the derrick or mast, the drawworks, the rotary table or topdrive, the drillstring, the power generation equipment and auxiliary equipment. Offshore, the rig includes the same components as onshore, but not those of the vessel or drilling platform itself. The rig is sometimes referred to as the drilling package, particularly offshore.
Offshore rigs can be of various type depending on the water depth :
- Swamp barge can drill up to 5-7 m of water
- Jack-up are raised on 3 to 5 legs and can drill from 10 to 90 m of water
- Barge with a platform : the drilling rig is just on kind of skids on top of a platform and a barge is moored to the platform.
- Semi sub are floating on the water with a system to compensate the heave. They can drill between 100 and 2000 m of water. They can be moored or dynamically positionned with small engines maintaining their position of the vessel
- Drill ship : these are ship with 1 or 2 masts for drilling. They are mainly dynamically positionned.
The amount of people working on a rig is variable. People work in shift, 2 x 12 h shifts or 3 x 8 hours. There are around 10 to 50 people depending on the size.
There is nothing like a rig 'pumping the oil', this is done on a later phase of exploitation.
For drilling, one will usually use what is called rotary drilling. There is a bit made of abrasive material (tungstene, diamond, carbide) which will break the rock. This bit is screwed at the end of a string. The string itself consists in drill pipes and the bottom hole assembly. The bottom hole assembly (BHA) is made of very heavy type of pipe while the drill pipe are more or less like a kind of thread (at this scale). The whole is rotating either from the surface, either at the bottom with a turbine.
By gently dropping the bit on the bottom, the driller will control the weight applied on the rocks by the bit which will be between 0 and the total weight of the BHA. This can go up to 30-40 tons ! When I say, gently, I say plus minus 1-2 tons... Once the rock is grounded, the weight will drop and the driller will drop the bit on the bottom again and so one.
In addition to that, to cool down, lubricate, hold the borehole side and take all the small cuttings away, a muddy mixture of water and clay or oil and clay is pumped through the pipe to the bit and then it is moving through the annulus to the surface.
This is the basic.
A lot of additionnal operations are performed during drilling including logging (measurement versus depth or time, or both, of one or more physical quantities in or around a well. ), testing, completion etc...
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