The main functions of the offshore rotary drilling rig are as follows:
- Penetrating operations. The drill bit breaks down rock at the bottom-hole by the rotation under the weight. The rotating force of the rotary table is transmitted through the drill string to the bit. Some portion of the weight of drill collars is applied to the bit as the bit weight to push the bit against the rock.
- Hoisting operations. The drill string with the bit is lowered and lifted by the hoisting system, eg. Drawworks. The casing is also handled by the hoisting system.
- Conditioning and circulating the drilling fluids by the circulation system, eg. the mud pumps providing the high pressure mud to the well bore and return of the mud is being processed, filtered and clean through the shakers. The drill cuttings are contained into containers and shipped back to shore.
- Preventing the formation fluids from entering into the wellbore and controlling them to prevent collapse of the well. More serious consequence, after the collapse of well, will be kicks and subsequently well blowout. The highly potential gas contained in the mud return will likely causes ignition of the gas on board and creating serious fire during the blowout.
The drilling fluids, conventionally known as muds, have lots of important functions in the offshore rig drilling. Main functions are as follows:
- Removal of cuttings from the bottom of the hole to the surface. Cuttings are separated from the mud at the shale shaker. The cuttings and samples of the mud may be further analyzed to study geological properties of the rocks penetrated, and to find out the indication of oil and gas in the formations.
- Controlling hydraulic pressure in the hole by adjusting the density of the mud to prevent collapse of the wall of the borehole, and to contain formation fluids in the formations.
- Cooling and lubricating the bit and the drill stem.
In the conventional system of the offshore drilling, the rotary table rotates the drill stem, but the down-hole mud motor and the top drive device are applied to rotate the bit in the directional and horizontal well drilling, or to improve operations in the vertical well drilling. The technical advancement of the measurement-while-drilling tools (MWD) and the logging-while-drilling tools (LWD) has contributed to the almost real-time acquisition of the down-hole information. Owing to these tools it has become easy to drill directional and horizontal wells.
Terminology :-
Casing: Steel pipe lowered into a hole drilled and bonded to formation by cement to keep the well safe.
Christmas tree: An assembly of valves installed at the top of a well to control the flow of oil and gas after the well has been completion.
Drill stem: A drilling assembly of tubulars, to rotate a bit at the bottom of the hole from the surface, which comprises of the kelly,the drill pipe, and drill collars.
Riser: Any pipe with the fluid flow upward in it. In offshore drilling a marine riser system is used to establish a connection between the rig and the seabed. In offshore petroleum production, production riser systems extend from the seabed to the deck of the production platform.
Well completion: A series of work to make a well ready for production after it has been drilled and tested. Although there are wide variations, it typically involves installing the production (deepest) casing, perforating the casing and installing tubing and the Christmas tree. A subsea completion or subsea-completed well is a well that sits entirely, that is, up to its Christmas tree, on the seabed.
