RIGSDOMRIGSDOM · Drilling glossary
ReferenceDrilling glossary
The terms that appear on every rig data sheet and drilling programme, explained the way a drilling engineer would explain them to a colleague — not a dictionary definition, but what the term means in practice and why it matters.
Annular BOP
A blowout preventer that seals around any shape in the hole, or on open hole, using a rubber packing element compressed hydraulically. Usually the top element of the stack. Governed by API Spec 16A.
API 4F
The API specification covering the design and manufacture of drilling and well servicing structures — masts, derricks and substructures. Inspection categories run I to IV, with Category IV the most thorough, involving disassembly and non-destructive testing of critical welds.
API 8C
The specification for drilling and production hoisting equipment: crown blocks, travelling blocks, hooks, elevators and links. Inspection intervals are set by API RP 8B.
BHA
Bottom hole assembly. The lower portion of the drill string, from the bit upward through motor, MWD, stabilisers, drill collars and jars. Its configuration determines whether the well builds, holds or drops inclination.
Buoyancy factor
The reduction in string weight caused by immersion in drilling fluid, calculated as one minus mud weight divided by steel density. Used to convert air weight to buoyed weight when computing hook load and weight on bit.
Casing shoe
The bottom of a casing string, fitted with a shoe to guide the string into the hole. Shoe depth defines the section boundary and, with the formation strength below it, sets the maximum allowable annular surface pressure.
Choke manifold
The arrangement of valves and chokes used to control flow and pressure from the well during a kick. Rated to the same working pressure as the BOP stack.
Dogleg severity
The rate of change of hole direction, expressed in degrees per 30 metres or 100 feet. High values accelerate tubular fatigue, increase torque and drag, and can prevent completions being run.
Drawworks
The hoisting machine on a rig. Its horsepower rating, together with line count, determines maximum hook load and how quickly the string can be tripped.
ECD
Equivalent circulating density. The effective pressure the formation sees while circulating, expressed as a mud density. Exceeding the fracture gradient causes losses; falling below pore pressure invites a kick.
H₂S
Hydrogen sulphide. A lethal gas found in sour formations that also attacks steel through sulphide stress cracking. Requires materials selected to NACE MR-01-75, fixed detection, breathing apparatus and a contingency plan.
Hook load
The total load carried by the hook, including drill string, travelling assembly and any overpull. The rating is stated at a given number of lines, and quoting it without the line count is meaningless.
IADC classification
The system for coding drill bits by cutting structure, bearing type and gauge protection, allowing bits from different manufacturers to be compared. Also used for dull grading after a run.
Kick tolerance
The largest influx a well can take at a given depth and be circulated out without fracturing the weakest formation, usually at the last casing shoe. It falls as the section deepens.
MAASP
Maximum allowable annular surface pressure. The highest pressure that can be held at surface before the formation at the casing shoe fractures. Recalculated whenever mud weight changes.
Mast
The vertical structure carrying the travelling assembly. Rated by height and gross nominal capacity, and certified to API Spec 4F. Telescopic masts are raised hydraulically, which is what makes fast moves possible.
Mud weight
The density of the drilling fluid, expressed in specific gravity or pounds per gallon. Set to hold formation pressure without fracturing the rock — the narrower that window, the harder the well.
NACE MR-01-75
The materials standard for equipment in sour service, now also published as ISO 15156. Sets hardness limits and material restrictions to prevent sulphide stress cracking.
Overpull
Load applied above the free-hanging string weight when working a stuck string. The margin of overpull is designed into the string, and exceeding it risks parting the pipe.
Rig move
The complete operation of rigging down, transporting and rigging up a rig at a new location. Measured in loads and days, and often the largest single cost item in a multi-well campaign.
Rotary table
The rotating machine in the drill floor that turns the drill string via the kelly. Its opening diameter limits the largest casing that can pass through, which is why table opening appears on every rig data sheet.
SCR
Silicon controlled rectifier. The system converting AC generator output to DC for traction motors on an electric rig. SCR rigs deliver smoother power control than mechanical rigs but need more electrical maintenance.
Slant rig
A rig whose mast is inclined rather than vertical, used to drill deviated wells from surface without a downhole motor. Common in shallow heavy oil and where surface access is constrained.
Standoff
The percentage by which casing is held off the borehole wall by centralisers. Low standoff leaves cement channels on the narrow side, which is the usual cause of poor zonal isolation.
Substructure
The steel structure supporting the mast and drill floor, providing height for the BOP stack beneath. Rated by casing load and setback load, and certified with the mast under API 4F.
Top drive
A motor suspended in the mast that rotates the drill string directly, replacing the kelly and rotary table. Allows drilling in stands rather than singles, and permits rotation while tripping.
Torque and drag
The rotational and axial resistance experienced by the drill string, driven by friction against the wellbore. Modelled before drilling to confirm the well can be reached and the casing run.
Weight on bit
The downward force applied to the bit, provided by the buoyed weight of the collars above it. Applying too much moves the neutral point into the drill pipe, causing buckling and fatigue.
WOC
Waiting on cement. The period after a cement job before drilling or testing resumes, set by the time for the slurry to reach a defined compressive strength at bottom hole temperature.
Workover rig
A unit for servicing existing wells rather than drilling new hole. Judged on move speed, footprint and pipe handling rather than hook load, and typically fitted with a power swivel rather than a rotary table.
Put the terms to work
The engineering suite applies all of this to your own well: casing design against burst and collapse, BHA with buoyancy and weight on bit, mud programme with ECD, well control with MAASP and kick tolerance by section.