Reference
Oilfield & drill string glossary
Clear, definition-first explanations of the drilling and rental terms our customers ask about most.
A
API 5CT
API 5CT is the American Petroleum Institute specification for casing and tubing, defining the grades, dimensions, mechanical properties and inspection requirements for the pipe used to case and complete oil and gas wells.
API 5DP
API 5DP is the American Petroleum Institute specification for the manufacture of steel drill pipe, defining dimensions, mechanical properties, grades and inspection requirements for new drill pipe tube and tool joints.
API Connection
An API connection is a rotary-shouldered thread standardized by the American Petroleum Institute — such as NC, Regular, Full Hole and Internal Flush — that provides interchangeable, published dimensions and make-up torques for drill string tubulars.
API RP 7G-2
API RP 7G-2 is the American Petroleum Institute recommended practice for the inspection and classification of used drill stem elements, defining inspection methods and acceptance criteria for drill pipe, tool joints and related components.
B
Back-Off
A back-off is the deliberate unscrewing of a drill string connection at or above the free point, usually with left-hand torque and a string shot, to separate the free pipe from a stuck portion so the free section can be recovered before fishing.
Bit Sub
A bit sub is a short crossover sub that connects the drill bit to the bottom of the drill collar or bottomhole assembly, adapting the bit's box connection to the collar's pin and often carrying a float valve or nozzle for the mud circuit.
Blowout Preventer
A blowout preventer is a large valve or stack of valves installed at the wellhead that can seal the annulus or the entire bore to shut in the well and contain formation pressure during a kick, forming the primary surface barrier for well control.
Bottomhole Assembly
A bottomhole assembly (BHA) is the lower portion of the drill string running from the bit up to the drill pipe, made up of the bit, drill collars, stabilizers, and specialized tools that provide weight on bit and control the direction and trajectory of the wellbore.
Box and Pin
Box and pin are the two mating halves of a threaded tubular connection — the pin is the externally threaded male end and the box is the internally threaded female end — that screw together to join drill pipe, collars and other downhole tubulars.
Bumper Sub
A bumper sub is a telescoping BHA or fishing tool that provides axial stroke, allowing the driller to deliver controlled downward jarring blows and to maintain constant weight while working a fish or drilling from a floating rig.
Buoyancy Factor
Buoyancy factor is the multiplier applied to a drill string's weight in air to find its buoyed weight when submerged in drilling fluid, calculated from the mud density and used to determine hook load and the neutral point.
C
Casing
Casing is the large-diameter steel pipe cemented into a wellbore to line and support the hole, isolate formations, and provide a pressure-tight conduit, manufactured to API Spec 5CT and run in successively smaller strings with depth.
Casing Running Tool
A casing running tool is a top-drive-mounted device that grips casing internally or externally to run, rotate, reciprocate and circulate through it, combining the functions of an elevator, spider, power tong and fill-up tool in one unit.
Class 2 Pipe
Class 2 is a used-drill-pipe classification under API RP 7G-2 and DS-1 in which the tube retains at least about 70 percent of its nominal wall thickness, giving lower load ratings than Premium Class but remaining serviceable for less demanding applications.
Coupling
A coupling is a short internally threaded sleeve used to join two lengths of externally threaded casing or tubing (a threaded-and-coupled connection), providing the box-box link between two pin ends of pipe.
Crossover Sub
A crossover sub is a short threaded sub that connects two tubulars with different connection types or sizes — a box of one thread on one end and a pin of another on the other — allowing otherwise incompatible components to be joined in the drill string.
D
Dart Valve
A dart valve is a dart-shaped check cartridge dropped or pumped down the drill string to seat in a landing sub, where it converts the string into a one-way, downward-flow barrier during well-control operations.
Die Collar
A die collar is an external-catch fishing tool with hardened, tapered internal threads that is screwed over the outside of a tubular fish, cutting its own thread onto the fish to grip it for pulling and jarring.
Differential Sticking
Differential sticking is a form of stuck pipe in which the drill string is pressed against a permeable formation and held there by the pressure difference between the overbalanced mud column and the lower formation pressure, embedding the pipe in the filter cake.
Directional Drilling
Directional drilling is the practice of steering a wellbore along a planned, non-vertical path to reach a target that is offset from the surface location, using tools such as mud motors, rotary steerable systems, and MWD to control inclination and azimuth.
Dogleg Severity
Dogleg severity (DLS) is the rate of change of a wellbore's direction, expressed in degrees per 100 feet (or 30 meters), combining changes in both inclination and azimuth to quantify how sharply the hole is curving.
Double-Shoulder Connection
A double-shoulder connection is a rotary-shouldered thread with two torque shoulders — the standard external shoulder plus a secondary internal shoulder — that share make-up load to give higher torsional capacity and better fatigue resistance than single-shoulder API connections.
Drift Diameter
Drift diameter is the guaranteed minimum internal diameter of a tubular, verified by passing a standard-sized drift mandrel through the full length, ensuring that tools, bits and inner strings of a specified size will pass freely.
Drill Bit
A drill bit is the cutting tool at the bottom of the drill string that mechanically breaks and removes rock to create the wellbore, driven by rotation from the surface or a downhole motor and loaded with weight on bit.
Drill Collar
A drill collar is a thick-walled, heavy steel tube run directly above the bit in the bottomhole assembly to provide weight-on-bit in compression and to keep the lower drill string stiff and under control.
Drill Pipe Fatigue
Drill pipe fatigue is the progressive cracking of drill pipe caused by repeated cyclic bending stress as the pipe rotates through doglegs and curved hole sections, and is a leading cause of drill string washouts and twist-offs.
Drill Pipe Grade
Drill pipe grade is the API classification of a drill pipe tube's steel by its minimum yield strength — E-75, X-95, G-105 and S-135 being the common grades — which sets how much tension and torque the pipe can safely carry.
Drill String
A drill string is the assembled column of drill pipe, tool joints, drill collars and downhole tools that transmits rotation, weight and drilling fluid from the surface rig to the drill bit at the bottom of the hole.
Drilling Jar
A drilling jar is a BHA tool that delivers a sudden, high-impact hammer blow up or down the drill string to free stuck pipe, activating when tension or compression is worked into the string.
Drop-In Check Valve
A drop-in check valve is a check valve dropped or pumped down the drill string to land in a preset sub, where it establishes one-way, downward-only flow so the string can be shut in against upward flow without breaking any connections.
DS-1
DS-1 is the widely used industry standard for drill stem inspection and specification, defining inspection procedures, acceptance criteria and dimensional classes for drill pipe, tool joints and bottomhole assembly components.
E
Electromagnetic Inspection
Electromagnetic inspection is a nondestructive method that scans the full length of a drill pipe or tubular tube body with magnetic flux and eddy-current sensors to detect wall loss, pitting, cracks and other defects as the pipe passes through the inspection unit.
Elevator
An elevator is a hinged, latching clamp suspended from the hook by bails that grips a tubular under its tool joint or coupling to raise and lower drill pipe, casing or tubing during tripping.
Equivalent Circulating Density
Equivalent circulating density is the effective mud density acting on the wellbore while fluid is circulating, equal to the static mud weight plus the added pressure from annular friction losses expressed as a density.
Extended Reach Drilling
Extended reach drilling (ERD) is the drilling of wells with an exceptionally high ratio of horizontal departure to true vertical depth, reaching distant targets from a single surface location while managing extreme torque, drag, and hole-cleaning challenges.
F
Fill-Up Tool
A fill-up tool is a device stabbed into the top of a casing string that fills it with mud as it is run and can circulate through it, maintaining hydrostatic pressure and avoiding the need to break connections to top up fluid.
Fish
A fish is any object left, lost, or stuck in a wellbore — such as a broken drill string, a dropped tool, or debris — that must be recovered or removed before drilling or completion operations can continue.
Fishing
Fishing is the downhole operation of recovering or removing lost, stuck, or broken equipment (a "fish") from a wellbore using specialized tools such as overshots, spears, and jars so drilling or completion can resume.
Fishing Magnet
A fishing magnet is a downhole tool with a powerful permanent magnet that retrieves small ferrous junk — such as bit cones, slips, hand tools, and metal debris — from the bottom of the wellbore.
Float Sub
A float sub is a short sub in the bottomhole assembly that houses a float (back-pressure check) valve, allowing drilling fluid to flow down the string while preventing backflow of mud and cuttings up through the bit.
Free Point
The free point is the deepest depth in a stuck drill string that is still free to move, determined with a free-point indicator tool so the operator knows where to back off or begin fishing the stuck portion.
Full Hole Connection
A Full Hole (FH) connection is an API rotary-shouldered connection designed with a large internal bore to maximize fluid flow through the connection, historically used on drill pipe and collars where hydraulic efficiency matters.
Full-Opening Safety Valve
A full-opening safety valve is a manually operated, full-bore ball valve stabbed into the top of the drill string to shut in the pipe bore during a kick, while its full-diameter opening allows tools to pass and circulation to resume once closed and reopened.
H
Hardbanding
Hardbanding is a wear-resistant weld overlay applied to the outside of a tool joint (and sometimes the tube center) that protects the drill pipe from abrasion and, with casing-friendly formulations, limits wear on the casing it rotates against.
Heavy-Weight Drill Pipe
Heavy-weight drill pipe (HWDP) is a thick-walled, extra-long-tool-joint pipe run as a transition between drill collars and standard drill pipe, adding weight to the bit while resisting the fatigue that occurs at that stiffness change.
Hole Cleaning
Hole cleaning is the process of carrying drilled cuttings out of the wellbore by circulating drilling fluid, essential to prevent cuttings accumulation, pack-off, high torque and drag, stuck pipe and elevated equivalent circulating density.
Horizontal Well
A horizontal well is a directional well drilled from vertical through a build section to an inclination near 90 degrees so that the wellbore runs horizontally through the reservoir, maximizing contact with the producing formation.
Hydraulic Jar
A hydraulic jar is a jarring tool that uses a metered hydraulic time delay to let the operator build tension or compression before it fires a high-energy impact blow to free stuck pipe.
Hydrostatic Pressure
Hydrostatic pressure is the pressure exerted by the column of drilling fluid in the wellbore due to its density and true vertical depth, and it forms the primary well-control barrier that holds back formation pressure.
I
Inside Blowout Preventer
An inside blowout preventer is a back-pressure check valve installed in the drill string that lets fluid pass downward but shuts off upward flow, stopping the well from flowing up through the pipe bore during a kick.
Internal Diameter
Internal diameter (ID) is the bore diameter of a tubular — the diameter of the fluid passage — which together with the outside diameter defines the wall thickness and governs fluid flow area and hydraulic pressure loss.
Internal Flush Connection
An Internal Flush (IF) connection is an API rotary-shouldered connection whose bore is nearly flush with the drill pipe tube's internal diameter, minimizing internal upset and turbulence for efficient fluid flow.
Iron Roughneck
An iron roughneck is a floor-mounted, hydraulically powered machine that combines a spinning wrench and a torque wrench to make up and break out drill pipe connections, removing crew members from the hazardous area between manual tongs.
J
Junk Basket
A junk basket is a fishing tool that catches and retains junk and debris from the wellbore, using a barrel and catcher fingers or reverse-circulation flow to trap fragments that cannot be recovered with a magnet or catching tool.
Junk Mill
A junk mill is a fishing tool dressed with hard cutting material such as tungsten carbide that grinds up junk, debris, or an unrecoverable fish downhole into small pieces that can be circulated out or caught in a junk basket.
K
Kelly
A kelly is a square or hexagonal length of pipe at the top of the drill string on a rotary-table rig that transmits rotation from the rotary table to the string while allowing it to slide down as the hole deepens.
Kelly Cock
A kelly cock is a manually operated valve installed at the kelly to shut in pressure inside the drill string, providing well-control isolation when a kick is taken while circulating through the kelly.
Key Seat
A key seat is a narrow groove worn into the wall of the borehole at a dogleg by the rotating drill pipe, into which a larger tool joint or BHA can wedge and stick while tripping out of the hole.
Kick
A kick is an unwanted influx of formation fluid — oil, gas or water — into the wellbore that occurs when the pressure in the well falls below the formation pressure, and if not controlled it can escalate into a blowout.
M
Magnetic Particle Inspection
Magnetic particle inspection is a nondestructive testing method that reveals surface and near-surface cracks in ferromagnetic components by magnetizing the part and applying fine magnetic particles that gather at flux leakage from a flaw.
Make-Up Torque
Make-up torque is the rotational torque applied to a threaded tubular connection when screwing it together, set within a manufacturer-specified range so the sealing shoulder is loaded correctly without over-stressing the threads.
Measurement While Drilling
Measurement while drilling (MWD) is a downhole system that measures wellbore inclination, azimuth, and toolface in real time and transmits the data to surface, allowing directional drillers to steer the well as it is drilled.
Mud Motor
A mud motor is a downhole tool that converts the hydraulic energy of circulating drilling fluid into rotation at the bit, allowing directional drilling and increased rate of penetration without turning the entire drill string from surface.
Mud Weight
Mud weight is the density of the drilling fluid, usually expressed in pounds per gallon or specific gravity, and it sets the hydrostatic pressure the mud column exerts to balance formation pressure and maintain well control.
N
NACE MR0175
NACE MR0175 (ISO 15156) is the standard governing materials selection and metallurgical requirements for equipment used in hydrogen sulfide–bearing (sour) oil and gas environments to prevent sulfide stress cracking and other forms of environmental cracking.
NC Connection
An NC (Number Connection) is a family of API rotary-shouldered connections identified by a two-digit number equal to the pin's pitch diameter in tenths of an inch, providing standardized, interchangeable threads for drill pipe and drill collars.
Neutral Point
The neutral point is the depth in a drill string where axial stress transitions from tension above to compression below, marking the boundary that separates the pipe in tension from the weighted components that supply weight-on-bit.
Nominal Weight
Nominal weight is the standardized weight per foot of a tubular used to identify and order it — for example 5 in 19.50 lb/ft drill pipe — serving as a size label that correlates with wall thickness and load capacity rather than the exact adjusted weight.
Non-Magnetic Drill Collar
A non-magnetic drill collar is a drill collar machined from non-magnetic alloy that houses MWD and directional survey instruments, isolating them from the drill string's magnetic interference so accurate azimuth measurements can be taken.
NORSOK
NORSOK is a body of standards developed by the Norwegian petroleum industry to define safety, quality, materials and operational requirements for offshore oil and gas activities, widely referenced for drilling, tubulars and material selection on the Norwegian Continental Shelf and beyond.
O
Outside Diameter
Outside diameter (OD) is the external diameter of a tubular's body, a primary sizing dimension that, with nominal weight and grade, identifies drill pipe and determines annular clearance in the wellbore.
Overshot
An overshot is an external-catch fishing tool that slips over the top of a fish and grips its outer surface with a spiral grapple, allowing the operator to pull, jar, and recover the fish, and to release it if it cannot be freed.
P
Power Tongs
Power tongs are hydraulically or pneumatically driven gripping jaws that make up and break out threaded tubular connections to a controlled torque, replacing manual rig tongs on drill pipe, casing and tubing.
Premium Class
Premium Class is a used-drill-pipe classification under API RP 7G-2 and DS-1 in which the tube retains at least about 80 percent of its nominal wall thickness with defined limits on cuts and damage, making it the standard grade for most drilling operations.
Premium Connection
A premium connection is a proprietary rotary-shouldered thread — usually double-shouldered — that delivers higher torsional capacity, better fatigue resistance and improved sealing than standard API connections, used in high-torque and extended-reach drilling.
Pup Joint
A pup joint is a short length of drill pipe, tubing or casing — typically 2 to 15 ft — used to adjust the overall length of a string for precise space-out when a full-length joint is too long.
R
Range Length
Range length is the API classification of a tubular joint by its overall length — Range 1, 2 or 3 — with Range 2 (about 27 to 30 ft) being the standard for drill pipe and Range 3 (about 38 to 45 ft) used to speed tripping.
Regular Connection
A Regular (REG) connection is an API rotary-shouldered connection with a relatively small internal bore and robust cross-section, commonly used on drill collars, bit subs and bits where strength matters more than flow area.
Releasing Spear
A releasing spear is an internal-catch fishing tool that is run inside a tubular fish and grips its inner wall with expanding slips, allowing the operator to pull and jar the fish and to release it if it cannot be recovered.
Remaining Body Wall
Remaining body wall is the percentage of the original nominal wall thickness still present in a used drill pipe tube after wear and corrosion, and it is the primary measurement used to classify pipe as Premium or Class 2.
Roller Reamer
A roller reamer is a BHA tool fitted with rotating, tungsten-carbide cutter rollers that ream the borehole to full gauge, stabilize the drill string, and reduce torque and drag in hard or abrasive formations.
Rotary Steerable System
A rotary steerable system (RSS) is a downhole steering tool that directs the bit while the entire drill string continues to rotate, enabling smoother, faster directional drilling and better hole quality than sliding with a mud motor.
Rotary Table
A rotary table is a rotating mechanical platform in the center of the rig floor that turns the kelly and, through it, the entire drill string and bit — the traditional means of transmitting rotation before top drives.
Rotary-Shouldered Connection
A rotary-shouldered connection is a coarse, tapered threaded joint used on drill string tubulars in which a machined shoulder — not the threads — carries the make-up torque and forms the pressure seal.
S
Safety Clamp
A safety clamp is a hinged, bolt-tightened collar of interlocking segments installed around a smooth-bodied drill collar or tubular as a backup that rests on the slips or rotary table to prevent the string from slipping and falling into the hole.
Saver Sub
A saver sub is a short, sacrificial threaded sub installed between the kelly or top drive and the drill string that absorbs the make-up and break-out wear of repeated connections, protecting the expensive threads above it.
Seamless Steel Tube
A seamless steel tube is a pipe formed by piercing and rolling a solid steel billet without any welded seam, giving the uniform, defect-resistant body required for drill pipe, drill collars and most oilfield tubulars.
Serialization
Serialization is the practice of assigning a unique identifier to each individual tubular or tool so its inspection history, dimensions, class and running record can be tracked throughout its service life.
Shock Sub
A shock sub is a BHA tool with a spring or elastomer damping element that absorbs axial vibration and bit bounce, protecting the bit, downhole tools, and drill string from destructive shock loads.
Sidetrack
A sidetrack is a new wellbore drilled off to the side of an existing hole, either to bypass an obstruction or junk that cannot be recovered or to redirect the well toward a new target, typically started with a whipstock or by drilling off a cement plug.
Slip Crushing
Slip crushing is the radial deformation or collapse of a pipe body caused by the gripping load of the rotary slips when a heavy string is suspended in them, a limiting concern when running very heavy strings such as deep casing on a landing string.
Slips
Slips are wedge-shaped, hinged tools with hardened, gripping dies that are set into the tapered bowl of the rotary table or a spider to suspend a string of drill pipe or casing when it is not hanging from the elevator.
Sour Service
Sour service refers to operations in wells containing hydrogen sulfide (H2S), where materials must be selected and controlled per NACE MR0175 / ISO 15156 to resist sulfide stress cracking and other forms of environmental embrittlement.
Spider
A spider is a floor-mounted device with power-set slips that grips and suspends a string of drill pipe, casing or tubing in the rotary opening while joints are added or removed, working as the ground-based counterpart to the elevator.
Stabbing Guide
A stabbing guide is a funnel-shaped tool placed over the box connection of a tubular to center the pin of the incoming joint during stabbing, protecting the threads and shoulder from damage as the connection is made up.
Stabilizer
A stabilizer is a BHA tool with raised blades that centralize the drill string in the borehole, controlling bit trajectory, reducing vibration, and maintaining hole gauge during drilling.
Standpipe Pressure
Standpipe pressure is the total circulating pressure measured at the standpipe as mud is pumped, equal to the sum of all friction pressure losses through the surface lines, drill string, bit and annulus, and it is a key indicator of downhole and well conditions.
Stuck Pipe
Stuck pipe is a condition in which the drill string can no longer be moved, rotated, or pulled free from the wellbore, caused by mechanisms such as differential sticking, pack-off, key seating, or wellbore instability.
T
Taper Tap
A taper tap is an internal-catch fishing tool with hardened, tapered threads that is screwed into the bore of a tubular fish to cut its own thread and grip the fish for pulling and jarring.
Tapered String
A tapered string is a drill or work string built from two or more sizes or grades of pipe — heavier sections up top and lighter sections below — so that each part of the string carries its loads efficiently in very deep or extended-reach wells.
Tensile Strength
Tensile strength is the maximum pulling stress a steel can withstand before it fractures, setting the ultimate limit on how much a drill pipe or connection can be stretched before failure — always higher than the yield strength.
Tool Joint
A tool joint is the heavy, threaded forged connection welded to each end of a drill pipe tube — a pin on one end, a box on the other — that lets joints be screwed together into a drill string and carries the make-up torque.
Tool Joint Upset
A tool joint upset is the thickened, forged transition at each end of a drill pipe tube where the wall is increased — internally, externally, or both — to provide the metal needed to weld on the tool joint and to reduce stress concentration.
Top Drive
A top drive is a motor-driven device suspended in the derrick that rotates the drill string from its top end and allows drilling and circulation in stand-length increments, replacing the kelly and rotary table for imparting rotation.
Torsional Yield Strength
Torsional yield strength is the amount of applied torque at which a drill pipe body or connection begins to deform permanently, setting the safe limit on rotational torque that can be applied while drilling or making up connections.
Tripping
Tripping is the process of running the drill string into the hole or pulling it out, one stand at a time, most often to change a worn drill bit or to run the bottomhole assembly to or from bottom.
Tubing
Tubing is the smaller-diameter pipe run inside casing to convey produced fluids to surface or to carry fluids for completion and workover operations, manufactured to API Spec 5CT with threaded and coupled connections.
U
Ultrasonic Inspection
Ultrasonic inspection is a nondestructive method that uses high-frequency sound waves to measure wall thickness and detect internal flaws in drill stem and tubulars, providing the precise wall data used to classify remaining body wall.
Underreamer
An underreamer is a downhole tool with expandable cutter arms that enlarge the borehole below a restriction such as existing casing, opening the hole to a diameter larger than the tool could pass through in its collapsed state.
W
Wall Thickness
Wall thickness is the radial thickness of a tubular's steel body — the difference between its outside and inside radius — and it governs the pipe's tensile, collapse and burst strength as well as its inspection class as it wears.
Weight on Bit
Weight on bit (WOB) is the downward axial force applied to the drill bit at the bottom of the hole, supplied by the weight of the drill collars and BHA, that drives the cutting structure into the formation to make hole.
Well Control
Well control is the practice of maintaining and restoring the balance between wellbore pressure and formation pressure to prevent the uncontrolled flow of formation fluids, using hydrostatic pressure as the primary barrier and the BOP stack as the secondary barrier.
Whipstock
A whipstock is a wedge-shaped deflection tool set in a wellbore that forces the bit or mill to one side, deflecting it into the wall to kick off a sidetrack or start a directional hole at a controlled toolface.
Wiper Trip
A wiper trip is a short round trip of the drill string through an open-hole interval to clean the wellbore of cuttings and mud cake, ream out tight spots and doglegs, and confirm the hole is in good condition before further operations.
Workstring
A workstring is a temporary string of pipe or tubing run into an existing well to perform completion, workover or intervention tasks — such as circulating, milling or setting tools — rather than to drill new hole.
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