Content – Oil & Gas
A drilling rig contain 5 main systems:
- Hoisting system and Draw works
- Rotary system
- Circulation system
- Power system
- Safety and blow out prevention system
Hoisting system and Draw works
The main purpose of the lifting system and the draw worsk is lifting and lowering the drill string and carrying the weight of the drill string and the casing.
- The derrick
- The lifting equipment and machinery
The main purpose of the derrick is to support the travelling blocks and the drill line.
The derrick also carries the weight of the drill string and the casing.
Two main types:
- Four legged derrick
- Mast type
Oil and gas drilling derricks are manufactured in various heights spanning from 25 m – 60 m (80 – 190 ft)
Derricks in the lower range, 25 m – 35 m (80 – 110 ft) are primarily used for well workover activities.
A drilling derrick have to withstand two types of load; compression load and wind load, If the derrick is located on an offshore floating drilling rig it also woll have to withstand dynamic loads caused by wave motion.
The main function of the draw works is to transfer power to the drill line, the sand line and the “cat heads”. The machinery for an offshore rig would be around 2300 KW (3000hp).
Main components of the draw work:
- Hoisting drum
The draw works have two independent brake systems.
The main purpose of the hoisting system is to lift and lower equipment into or out of the well and to lift the drill string during drilling.
For offshore drilling the cerified lifting load would be in the range of 700 – 900 tons.
The travelling block is connecting the hook and the drill line. The travelling block normally typically has 3 – 7 sheaves with a diameter of 5 – 11 inches.
The crown block is the stationary end of the block and tackle, typically having 7 -10 sheaves.
Heave compensator (offshore drilling)
For ogffshore drilling from a floating drilling rig it is also necessary to have heave compensation to eliminate vertical forces from wabe movement.
The guide dolly, a frame with rollers fixed to the travelling block and rails fixed to the derrick eliminates horizontal movement of the travelling block.
Elevators are a hinged device with handles that are used to wrap around the tool joint of drill pipe, casing or lift nipples (for collars) to facilitate the lifting or lowering of them singly or of the drill string as a whole.
Drill line is a multi thread metal cable threaded through the traveling block and the crown to raise and lower the drill string. The purpose is to carry load between the crown block and the travelling block.
Rotary system and drill floor components
Drill floor components and terminology
- Rotary table – Rotates, along with its constituent parts, the Kelly and Kelly bushing, the drill string and the attached tools and bit. Unless the rotation is provided by a top drive.
- Top drive – A mechanical device that provides clockwise torque to the drill. It is an alternative to providing torque by the rotary table. It is located at the swivel place on top of the drill string, hanging from the travelling block and allows a vertical movement up and down the derrick.
Several different kinds of top drives exist, and are usually classified based on the “Safe Working Load” (SWL) of the tool, and the size and type of motor used to rotate the drillpipe. For offshore and heavy duty use, a 1000 short ton, top drive would be used, where a smaller land rig may only require a 500.99 Motors are available in all sizes, and come in Hydraulically powered, AC, or DC motors.
- Kelly – Kelly drive and Kelly bushing – A square, hexagonal or octagonal shaped tubing that is inserted through and is an integral part of the rotary table that moves freely vertically while the rotary table turns it.
- Swivel – The top end of the kelly that allows the rotation of the drill string without twisting the block, unless a top drive is mounted then there is no swivel.
- Draw works – The mechanical section that contains the spool, whose main function is to reel in/out the drill line to raise/lower the traveling block
- Drillers console – Drilling control panels
- Mouse hole – A hole beside the rotary table where drill pipe is located before being attached to the rest of the drill string
- Rat hole – A hole where the Kelly is located during trips
- Dog house – Drillers office and storage
- Pipe ramp – A slide-like ramp used to pick up and lay down tools to and from the catwalk/pipe rack area. Also used to pick up drill pipe, drill collars or any other heavy equipment that could not be done safely because of the height of the substructure and close proximity to the blowout preventers, electric and hydraulic lines, and other equipment in the area.
- Cat walk – A long, rectangular platform 1 m ( 3 ft ) high, usually made of steel and located perpendicular to the V-door at the bottom of the pipe ramp This platform is used as a staging area for rig and drillstring tools, components that are about to be picked up and run, or components that have been run and are being laid down.
An advantage of a top drive is that it allows the drilling rig to drill longer sections of a stand of drill pipe. A rotary table type rig can only drill 9.1 m (30-ft) sections of drill pipe while a top drive can drill 18–27 m (60–90ft) stands.
Having longer sections of drill pipe allows the drilling rigs to drill deeper sections of the wellbore, thus making fewer connections of drill pipe. Another advantage of top drive systems is time efficiency. When the bit progresses under a Kelly, the entire string must be withdrawn from the well bore for the length of the Kelly drive. With a top drive, the draw works only has to pick a new stand from the rack and make two joints.
The savings in time reduces the risk of a stuck string from annulus clogging.
Top drive is the most common device tfor rotating the drill string on newer rigs.
A mud motor is a progressive cavity positive displacement pump (PCPD) placed in the drill string to provide additional or directional power to the bit while drilling or to provide all the power to the drill bit during coild tubing drilling.
The pump is driven by drilling fluid creating eccentric motion in the power section of the motor, which is transferred as concentric power to the drill bit.
- Manual tongs – Large-capacity, self-locking wrenches used to grip drillstring components to spin or to apply torque. The tourque tongs must be used in opposing pairs. As a matter of efficiency, one set of tongs is essentially tied off with a cable or chain to the derrick, and the other is actively pulled with mechanical catheads. The breakout tongs are the active tongs during breakout (or loosening) operations. The makeup tongs are active during makeup (or tightening) operations.
- Hydraulic tongs – Same as manual tongs but hydraulic operations.
- Iron roughneck – An assembly of 2 hydraulic make up and brake up tongs and one spinner tong that may be wheeled into position such that no or limited manual work is reuired. The use of an iron roughneck significantly improves the safety on the drill floor.
- Slips/Power slips – Is used to secure the drill string at the rotary table when it is not secured by the travelling block. Power slips are usually pneumatically operated such that the safety hazards connected to manual operation to a large degree may be avoided.
- Stabbing arm – A hydraulic arm used to guide and manipulate drill pipe such that the need for manual handling of the pipe is reduced.
- Pipe racking system – 2 hyraulic arms used to move and guide the stand between the Rotary table and the set back area
- Retracaable block – The travelling block may be retracted from the center line of the derrick and still be able to run up and down
- Finger board – A working platform halfway up the derrick where drillpipe and drill collars are stored during trips out of the hole. The platform has several steel fingers with slots between them that keep the tops of the drillpipe in place.
The drill string consists of drill pipe, tool joints (welded to the drill pipe), drill collars, stabilizers, reamers, subs.
Drill pipe – Tubular steel pipe with threaded ends, the tool joints. The drillpipe transfer torque and weight to the drill bit and connects the bottom hole assemply with the rig equipment such that drilling fluids may be transferred. Standard lengths are (excluding the threaded part):
|API Range||Length (ft)|
|I||18 – 22|
|II||27 – 30|
|III||38 – 45|
The most commonly used drill pipe length is RII – 27 – 30 ft.
Drill Collars – Drill collars are components of the drillstring that provides weight on the bit for drilling. Drill collars are thick-walled tubular pieces machined from solid bars of steel.
The main functions of the drill collars are:
- Put weight on the drill bit
- Control the drill string compressive and torsional loads
- Bottom hole assembly directional control
- Hole size integrity
- Drill string clearance
Stabilizers – Stabilizers are a part of the bottom hole assembly and do come in various types.
The purpose is to mechanically stabilizes the BHA in the borehole in order to avoid unintentional sidetracking, vibrations, and ensure the quality of the hole being drilled.
Reamers – The main function of the reamers during drilling is to maintain the diameter of the borehole. The drill bit it self, will for various reasons, such as wear and lateral shifting not always be able provide a continious diameter of the hole. The reamer is normally a shorter piece of pipe with cutting devices attached.
Subs (crossover subs) – Subs are short pieces of drill pipe used as adapters to enable connection of different connection types in the same drill string.
Drill bits primarily comes in two main types:
- Rolling cutter bits works by fracturing or crushing the formation with “tooth” shaped cutting elements typically on three cone-shaped elements
- Fixed cutter works by scraping or grinding using a set of blades with very hard cutting elements, typically natural or synthetic diamond.
The rolling cutter falls into two categories:
- Steel-tooth bits have cones that have wedge-shaped teeth milled directly in in the cone steel itself with tungsten carbide material welded to the surfaces of the teeth to improve durability.
- TCI bits have tungsten carbide teeth of sintered tungsten carbide press-fit into drilled holes in the cones.
The fixed cutter bits are mechanically simpler than rolling cutter bits. The cutting elements do not move relative to the bit and there is no need for bearings or lubrication as it is for the rolling cutter. The most common cutting element used is polycrystalline diamond.
BOP (Blow out preventer)
The blowout preventer is a device (or stack of devices) used to seal, control and monitor oil and gas wells and in particular to withstand and to control potential formation kicks control. The blow out preventer is intended to provide a “fail safe” protection.
A BOP stack typically combines ram preventers and annualar preventers.
The BOP shall:
- Confine the well fluids to the wellbore.
- Allow fluids to be added to the wellbore
- Allow fluids to be returned from the wellbore.
In addition, if required (eg. any extraordinary situation that might escalate beyond the controllable) the BOP shall be able to:
- Seal the annulus (shut in the well).
- Confine the well fluids to the wellbore
- Prevent the flow of formation fluid from the reservoir into the wellbore
- Seal the wellhead (close off the wellbore);
- Cut the casing and drill pipe.
Ram preventer – Uses a pair of opposite steel plunger rams that extend towards the centre of the well bore to restrict flow. The rams have elastomer packers that seal against the well bore, each other or around any tubing running through the well bore.
- Pipe rams close around a drill pipe, restricting flow in the annulus between the outside of the drill pipe and the wellbore.
- Blind rams or sealing rams will close and seal the well if no drill string or tubing runs through.
- Shear rams cut through the drill string or casing. Requires that there is a pipe ram below to seal the well bore and to hang off the drill string.
- Blind shear rams or seal rams, cut the drill string and seals off the well bore.
Annual preventer – The annular blowout preventer has rubber seals and may close against a drill string, casing or another any other pipe or tool that might be running through the well bore as well as an open hole. The annualar preventer provide an efficient seal around the drill pipe even when the pipe is rotating.
A basic BOP stack would as a minimum have the following main components:
- A diverter system to enable sealing with the drill in place
- One or more annual preventers
- One ore more shear rams
- Two or more pipe rams
During onshore drilling or in very shallow waters the BOPs are normally activated by hydraulic pressure from a remote accumulator. Control stations will be mounted around the rig. Manual operation may also be possible.
The largest power consumers on a rotary drilling rig are the hoisting and the circulation system; these components determine mainly the total power requirements.
On most rigs the power is generated using diesel engines, turbines or is provided from available power grids including in some cases power from shore.
Offshore drilling from MODU (Mobile offshore drilling unit) – Sub sea and heave compensation
Drilling wells from an offshore floating rig are in principle to a large extent similar to drilling onshore wells. However there are a few additional factors related the connection between the well and the rig that needs attention. Those are:
- Well head located at the sea bed
- BOP located on the sub sea wellhead and is connected to the rig through the marine riser and is controlled hydraulically or electrically from the rig.
- Ball joints and slip joints are necessary at the marine riser connections to cather for the movements of the rig.
- Kill and choke lines (Blow out preventing devices) run on the outside of the marine riser.
At commencement of a drilling operation the temporary guide base is positioned at the seabed by means of a piece of drill string.
The temporary guide frame has a hole in the centre, which has a tapered inlet at the top. The four guidelines and a number of steel pins are embedded in the seabed such that displacement is prevented and the pipe string is disconnected.
Next step is drilling a hole such that the foundation pile (the first casing) may be set at a depth of 30 – 50 metres. An ordinary drill string guided inside the temporary guide base by the use of a frame holds the drill string in place.
The drill fluid used at this stage is water, the cuttings are disposed at seabed and no circulation of cuttings takes place.
The permanent guide frame with four columns is fixed at the end (top) of the foundation pile.
The four columns with the guide wires running through on the permanent guide frame is used to guide the BOP into position. The permanent structure contains the housing for the wellhead where the casing will be anchored.
Before any further drilling can take place the foundation pile will be cemented.
Drilling then continues inside the foundation pile to set the conductor pipe (the second section of casing) wsith subsequent cementing.
Sub sea BOP (Blow out preventer)
The sub sea BOPs is prior to installation stacked (BOP Stack) together into a cage structure, which will fit onto the columns of the permentant guide frame.
The BOP stack is fastened to the wellhead by means of a hydraulically controlled connection, to secure sealing.
During offshore drilling from mobile units with the BOP located close to the seabed the BOPs normally are activated by hydraulic pressure either directly or by electro hydraulic means-
In case an emergency situationwhen time is critical the shear rams may be activated, the drill string cut, the marine riser disconnected and the MODU abandon the well and leave the site.
Under other circumstances it is possible to perform a controlled disconnection (disconnecting the drill string and the marine risers with tubing at the BOP level) and if necessary abandoning the site.
The rig is connected to the well via the top of the BOP stack by a ball joint connector. The riser similar to casing is a heavy-duty steel pipe.
The riser is used to guide tools and to transport drilling fluid up to the rig and is hydraulically operated to enable fast disconnection.
The choke and kill lines and service lines are clamped to the marine riser. The riser is connected to the rig by a telescopic joint that accommodate vertical movement of the rig.
For deep-water applications the riser may have flotation elements attached to compensate for the heavy weight of the riser.
The riser system include:
- Diverter assembly
- Slip joint (The telescopic joint)
- Riser joints (This riser pipe itself)
- Balljoint connector
Diverter assembly – Is connected between the inner cylinder of the slip joint and the rig. The diverter valve is a annular valve that may be used to close the well and to direct formation fluids away from the rig.
Slip joint – Two sliding cylinders, The outer cyslinder is fixed to the riser while the inner cylinder is fixed to the diverter valve by a ball joint.
Riser joints – A riser section is an unvended pipe section with mechanical connectors at each end. Kill and chocke lines are attached to the outside of the riser.
The heave compensator decreases the influence of the heave of a drilling vessel on the drill bit, drill string and the guide wires.
Main types of compensation systems:
- Psssive systems used as tension devices for primarily for the guide wires ansd the marine riser. The sytems are normally pneumatic
- Active systems are hydraulic and operate by pumping fluid from one location to another. External power is required.
- Bumper subs are shock absorbing telescopic devices used inside the drill string to absobe vertical movement.
The drill string heave compensation system is on most rigs a combination of passive and active devices.
The main purpose of the heave cpmpensator is to provide a constant tention under dynamically changing conditions.
The Mud circulation system
The circulation system can be divided into to main parts:
- The pressure system consisting of components such as pumps, manifolds, pipes and hoses, stand pipe, Kelly hose, svivle, drill string, drill bit and annulus.
- The surface system consisting of components such as return lines, shale shakers sand traps, mud tanks and agitators
Mud pump – Is a piston or plunger reciprocating device designed to circulate drilling fluid under high pressure (up to 7,500 psi or 52,000 kPa) down the drill string and back up the annulus.
Pulsation dampers – The Pulsation Dampener is used to dampen pressure surges (or water hammer effect), when a piston pushes through a volume of mud. The pulsation damper typically consists of an upper section holding a rubber bladder filled with Nitrogen. The lower section is filled with drilling fluids. With each stroke of the pump, the bladder deforms to absorb the pressure surge. Between each piston stroke the charged bladder will return the surge.
Kelly hose (rotary hose or mud hose) – The Kelly hose is a flexible, steel reinforced, high-pressure hose that connects the standpipe to the gooseneck on the swivel above the Kelly and allows free vertical movement of the Kelly while facilitating the flow of drilling fluid through the system and down the drill string.
Standpipe – The Standpipe is a solid metal pipe attached to the side of the derrick. It is used to conduct drilling fluid from the mud pumps to the Kelly hose.
Shale shaker – Shale shakers are used to remove large solids (cuttings) from the drilling fluid by vibration.
The Shale shaker is the primary solids separation tool on a rig. After returning to the surface of the well the used drilling fluid flows either to a dump tank to remove the larger solids or directly to the shale shakers where it is being processed. From the shale shakers the mud is deposited into the mud tanks where other solid control equipment removes finer solids. The solids removed by the shale shaker are discharged into a separate holding tank where they await further treatment or disposal.
Hydroclones and centrifuges – Are devices based on utilizing centrifugal forces to separate solids from liquids. The Hydroclones are primarily used to separate clay, sand and silt from the mud.
Degasser – The degasser is typically a tank connected to a pump providing “vacuum” to remove air and other gases from the drilling fluids.
Agitator – Agitators consisting of jets and/or impellers are used to maintain movement of the mud to enable removal of solids and to avoid settlement.
Mixing hoppers – These funnel shaped devices allowing various dry compoments to be mixed into the mud are utilizing “vacuum” created by high pressure mud, forced through a nozzle to create high speed and thereby “vacuum”.
Drilling mud (drilling fluid) – The drilling fluids are water-, oil- or synthetic-based.
The basic functions of the drilling fluid or drilling mud are:
- Remove cuttings from drill hole or well
- Suspbend and release cuttings
- Pressure management by hydrostatic pressure. Balances or overcomes formation pressures in the wellbore to minimize the risk of well-control issues.
- Prevent or reduce damage to the producing formation.
- Maintain wellbore stability. Supports and stabilizes the walls of the wellbore until casing can be set and cemented.
- Minimizing formation damage
- Cool, lubricate, and support the bit and drilling assembly.
- Transmit hydraulic energy to tools and bit.
- Enables information gathering about the formation by cuttings analysis,