Basic Rig Functions The components of a rotary drilling rig can be grouped into five major systems: • Hoisting • Rotating • Circulating • Well control • Power and auxilliary Picture shows the principal components of an onshore drilling rig, each of which is an element of one of these five main systems. The mast, or derrick, supports the crown block and traveling block, which are operated via the drawworks and its drilling line — these are part of the hoisting system. The kelly, swivel and rotary table assembly (or top-drive unit) are connected to the drillstring and are suspended from the hook beneath the traveling block, allowing the drillstring to be rotated. A drilling fluid circulation system pumps mud from the pits through stand-pipe, hose, swivel, and drillstem, returning the mud and cuttings up the annulus and back to the pits. The blowout preventer (BOP) stack, accumulator unit and associated equipment enable the well to be shut in quickly in the event of an emergency. And the driller's console serves as a control center during well operations.
Hoisting and Rotating Systems Hoisting The mast (derrick) and the substructure upon which it sits support the weight of the drillstring and allow it to be moved up or down. The substructure also supports the rig floor equipment and provides workspace for the drilling crew. When the drill string has to be removed from the well, it is the height of the mast that determines what length of pipe can be disconnected and stacked to one side of the rig floor. A joint of drillpipe is about 30 ft [9. 1 m] long; the required height for a mast that will allow pipe to be pulled from the well and stacked in three-joint sections [90 ft or 27. 4 m] is about 140 ft [42. 7 m]. The drawworks is a spool or drum around which a heavy steel cable, or drilling line, is wrapped. From the drawworks, the line is threaded through the crown block at the top of the mast and then through the traveling block, which hangs suspended from the crown block. By reeling in or letting out drill line from the drawworks drum, the traveling block and suspended drill string can be raised or lowered. In order to safely manage the movement of such a heavy load with precision, the driller relies on an electrical or hydraulic brake system to control the speed of the traveling block and a mechanical brake to bring it to a complete stop. The drawworks also features an auxiliary axle, or "catshaft", with rotating spools on each end called "catheads". One spinning cathead is used to provide power to tighten the drillpipe joints via a cable from the cathead to the rotary tongs. The other cathead is for "breaking out" or loosening the pipe joints when the pipe is being withdrawn in sections. The drilling line undergoes a certain degree of wear as the traveling block is raised and lowered. For this reason the line is routinely "slipped" (moved onto the drawworks drum) and replaced with a new section from the continuous spool on which it is stored. The line is clamped at the storage spool end by a deadline anchor. The hook is attached to the traveling block and is used to pick up the drill string via the swivel and kelly when drilling, or with elevators (a type of hinged clamp that can be closed around the pipe) swhen tripping into or out of the hole.
Rotating In a conventional kelly-rotary table rotating system, the swivel allows the drill string to rotate while supporting the drill string's weight and providing a pressure-tight connection for the circulation of drilling fluid. The drilling fluid enters the swivel by way of the gooseneck, a curved pipe connected to a high pressure hose. Connected to the swivel is a (12. 2 m) length of hollow steel known as the kelly. The flat sided-kelly fits through a corresponding opening in the kelly drive bushing, which in turn fits into the master bushing set into the rotary table. The rotary table is turned by the rig's power source, the table turns the bushings, the kelly bushing turns the kelly, the kelly turns the drillpipe, and so on. . . down to the bit. The kelly cock is a special valve on the end of the kelly nearest the swivel, which can be closed to shut in the drillstem. A lower kelly cock is also available on the bottom end of the kelly to perform the same function when the upper kellycock is not accessible. In place of this traditional rotating system, many modern rigs employ an electric or hydraulically driven power swivel and top-drive assembly (Pic: Top drive unit, taken aboard the deep sea scientific drilling vessel D V Chikyu. Image provided courtesy of JAMSTEC. All rights reserved). In this type of arrangement, the top-drive is suspended from the traveling block or hook, so the rotary mechanism is free to travel up and down the derrick. The unit can be dolly-mounted to a vertical track or rails attached to the derrick structure. The rails stabilize the top drive and absorb the torque from the motor. In a top-drive with integral swivel, drilling fluid enters the drill string through the top-drive unit.
Circulating System The mud pumps, which are powered by the rig's prime power source, are the heart of the circulating system. Mud pumps are positive displacement pumps that push a volume of drilling mud through the system with each stroke of their pistons. The output of a mud pump can be determined from the piston and cylinder sizes, the number of strokes per minute, and type of piston arrangement. Depending on the capacity and pressure requirements of the circulating system and the redundancy of its design, the number of mud pumps on a rig may vary. A land rig designed for shallow drilling typically has two mud pumps — one for normal operations and one in reserve — while a deepwater offshore rig may have three or four. The mud pump moves the drilling fluid from the mud pits (storage tanks), up the standpipe, to a point on the derrick where the rotary hose connects the standpipe to the swivel. This flexible, high-pressure hose allows the traveling block to move up and down in the derrick while maintaining a pressure-tight system. The circulating drilling mud moves through the swivel, kelly, drillpipe, and drill collars, exiting through nozzles or watercourses in the drill bit at the bottom of the hole. The mud then moves up the annular space between the drill string and the hole (or casing), carrying the drilled rock cuttings in suspension. At the surface, the mud leaves the hole through the return line and falls over a vibrating screen called the shale shaker. This device screens out the cuttings and dumps some of them into a sample trap and the rest into the reserve pit. Once cleaned of large cuttings, the mud is returned to a mud tank, from which it can be once again pumped down the hole. Fine particles are removed by centrifugal force by flowing the mud through desanders, desilters, or a centrifuge. A degasser is used to remove small amounts of gas picked up in the mud from the subsurface formations.
Well Control System A primary goal of any drilling operation is to control subsurface pressures. The first line of defense in this regard is the drilling mud in the wellbore, which exerts a hydrostatic pressure that counteracts the pore pressure of fluids in permeable formations. (Pic: Hydrostatic pressure of drilling mud column just counteracts formation pressure; pressure differential between wellbore and formation = 0).
There are circumstances, however, where the pore pressure can exceed the pressure of the mud column, causing formation fluids to flow into the wellbore. This condition is known as a kick. Left uncontrolled, a kick can lead to a blowout, where formation fluids forcefully erupt from the well, possibly igniting, and endangering the crew, the rig, and the environment. To prevent this uncontrolled influx of formation fluids, drilling rigs are equipped with a second line of defense, namely the blowout preventers and their associated equipment. The blowout preventers are a series of powerful sealing elements designed to close off the annular space between the pipe and hole where the mud is normally returning to the surface. By closing off this route, the well can be "shut-in" and the mud and/or formation fluids forced to flow through a controllable choke, or adjustable valve. This choke allows the drilling crew to control the pressure that reaches the surface and to follow the necessary steps for "killing" the well and restoring a balanced system. Pic. shows a typical set of blowout preventers, including the cylindrical-shaped annular preventer (top), which has a rubber sealing element that is hydraulically squeezed to conform tightly to the drillpipe in the hole.
Power Generation/Transmission System Hoisting, rotating, and circulating equipment is supplied with power from a prime power source, usually diesel engines. Power may be transmitted to the rig either mechanically or electrically. Mechanical drive rigs have a combination of belts, sprockets, clutches, and pulleys, which transfer power from the diesel engines to the drawworks, pumps, and rotary table. Diesel-electric rigs use their engines to drive generators that produce electricity. This electricity is sent through cables to a switch and control house from which point it is relayed to power the electric motors of each end user. On a small land rig, engine capacity on the order of 500 HP may be adequate to provide power for the main drilling systems. Floating offshore rigs, on the other hand, have much higher pwer requirements — the main power systems on floating rigs must provide electricity to run the drilling systems, all utilities and accommodations and, if the rig is dynamically positioned, the computer-controlled thrusters that enable it to stay on location. The installed power aboard a deepwater, dynamically positioned offshore drilling vessel can range from 25, 000 to over 60, 000 HP, with many rigs topping the 40, 000 hp mark.
Check yourself 1) Briefly describe the parts that the following rig components play in drilling operations. • mud pump • kelly • drawworks • blowout preventers Possible answers: Mud pumps: The mud pumps play an integral part in the circulating function. They provide the pumping action necessary to displace the drilling fluid from the mud pits or tanks up the standpipe, through the rotary hose, through the swivel, kelly, and drillstring, out the bit, and back up the annulus to the return line. • Kelly: The kelly allows the rotational motion of the rotary table to be transfer red to the drillstem, and thus is part of the rotating function. Because the kelly is a pipe with flat outside surfaces, perhaps square or hexagonal, it fits into the kelly bushing, which in turn fits into the rotary table. The kelly is also strong enough to allow the entire drillstring to hang from it as it in turn hangs from the hook. • Drawworks: As a component of the system performing the hoisting function, the drawworks powers the blocks in the derrick by reeling in or reeling out heavy steel cable. The drawworks also powers the tongs used to tighten and untighten tool joints. • Blowout preventers: The blowout preventers help perform the pressure controlling function of the drilling system. The annular preventer and the ram-type preventers squeeze tightly around the drillpipe and close off the space between the drillpipe and the hole. This function prevents the mud in the annulus from flowing out of the return line.
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