Kelly Bushing: Definition, Depth Datum, and KB Elevation

What Is a Kelly Bushing?

The kelly bushing fits inside the master bushing of the rotary table and grips the kelly to transmit rotation from the table to the drill string, while simultaneously serving as the universal elevation datum from which all depth measurements on the well are calculated and reported to regulators, reservoir engineers, and measurement-while-drilling service companies worldwide.

Key Takeaways

The kelly bushing serves two distinct functions: mechanically, it drives the kelly; as a depth reference, it is the point from which all measured depth (MD), true vertical depth (TVD), and log depths are reported in well files and regulatory submissions.
Kelly bushing elevation (KB elevation or RKB) is measured from mean sea level and is reported to the nearest 0.1 m (0.33 ft); onshore KB heights typically range from 0.6 m to 1.8 m (2 to 6 ft) above ground level, while offshore platforms may place the KB 20-40 m (65-130 ft) above sea level.
Petroleum engineers, geologists, directional drillers, log analysts, and regulators all use KB elevation to convert measured depth to true vertical depth subsea (TVDSS) for formation mapping, pressure analysis, and cross-well correlation.
Regulatory bodies requiring KB elevation on well submissions include the AER (Alberta), BSEE (US Gulf of Mexico), Sodir (Norwegian Continental Shelf), and the National Petroleum Agency of Australia (NOPSEMA).
Incorrect KB elevation entry in the well header corrupts all depth-converted log data and directional surveys, leading to reservoir mapping errors that can cost millions of dollars to correct during field development.

How the Kelly Bushing Works

The kelly bushing is a precisely machined steel sleeve that seats inside the master bushing's tapered or pinned recess. It contains a central opening machined to match the square or hexagonal profile of the kelly, sized to grip the flat faces of the kelly and transmit torque while allowing free vertical sliding motion. On a square kelly system, the drive bushing opening has four flat faces that bear directly against the four kelly faces. On a hex kelly, six faces provide the driving contact. The drive force transmitted through these faces can exceed 40,000 ft-lbs (54,233 Nm) on a deep high-torque well, requiring the bushing faces to be case-hardened to prevent galling and wear.

When the rotary table spins, it drives the master bushing through pin connections or a taper fit, the master bushing drives the kelly bushing through a square or pin interface, and the kelly bushing drives the kelly through its face contacts. This chain of drive elements is designed so that worn components can be replaced individually rather than replacing the entire table or drive system. Kelly bushings are manufactured in matched sets with their master bushings to ensure dimensional compatibility and are replaced when wear on the driving faces exceeds API Spec 7K allowances. A worn kelly bushing allows lateral movement of the kelly, causing drill string vibration and hole deviation in soft formations.

As a depth datum, the KB elevation is measured precisely during rig setup by a licensed surveyor on land wells or by an offshore position team on floating rigs. The surveyor uses differential GPS and a total station to measure the vertical distance from the rotary kelly bushing flange face to mean sea level (MSL) datum. This single elevation number, reported to regulators in the well license application before spudding, anchors all subsequent depth measurements to a common reference framework that allows direct comparison between wells drilled years apart on different rigs. Without a common datum, a formation top called at 2,500 m (8,202 ft) on one well cannot be reliably correlated to a formation top called at 2,500 m on an adjacent well drilled on a rig with a different floor height.

Kelly Bushing Across International Jurisdictions

In Alberta, the AER requires KB elevation in the General Administration Data System (GADS) license application before any drilling commences. The AER's Directive 059 specifies that all depth measurements in drilling reports use RKB (rotary kelly bushing) datum and that final well reports include certified KB elevation survey data. Schlumberger (now SLB) and Halliburton log headers for all Canadian wells list KB elevation, ground level (GL), and water table depth as mandatory header fields. The KB elevation at a Montney horizontal well pad in northeast Alberta where the terrain is relatively flat typically ranges from 0.8 m to 1.2 m (2.6 to 3.9 ft) above ground level, representing only the rig floor height above the ground.

BSEE requires KB elevation in all well permit applications under 30 CFR Part 250. On fixed-platform wells in the Gulf of Mexico, KB elevation above MSL can range from 15 m to 45 m (49 to 148 ft) depending on platform deck height above water. On jackup rigs, the KB elevation changes with leg penetration and preload, requiring a re-survey after final rig positioning and before spud. Floating rigs (semisubmersibles and drillships) use a reference datum called the rotary table elevation (RTE) or air gap reference, which must account for tidal variation and vessel heave compensation when converting depths to TVDSS.

On Norway's Continental Shelf, the Norwegian Oil Directorate (Sodir) requires KB elevation in the well completion report, expressed in meters above MSL. NORSOK standard D-010 specifies that all well barrier elements including the BOP must be depth-referenced to RKB to ensure that pressure calculations for well control procedures correctly account for hydrostatic head. The Johan Sverdrup jackup wells drilled by Equinor list KB elevations of approximately 30-35 m (98-115 ft) above MSL due to the platform cantilever height above the shallow North Sea floor.

In Saudi Arabia, Saudi Aramco's GeoScience and Petroleum Engineering standards require KB elevation in the Well Data Sheet submitted before drilling. The flat desert terrain at Ghawar means KB elevations there are typically 0.7-1.0 m (2.3-3.3 ft) above ground, making the GL and KB depths nearly identical. In Abu Dhabi, ADNOC's operations in the Arabian Gulf use offshore platform KB elevations of 15-25 m (49-82 ft) above MSL, critical for accurately converting reservoir depths across a field where multiple wells drilled from different platform heights must be depth-correlated for reserve calculations.

Fast Facts

During the Macondo blowout investigation in the Gulf of Mexico in 2010, investigators used the certified KB elevation of the Deepwater Horizon's rotary table (approximately 75 m / 246 ft above mean sea level) to reconstruct the exact hydrostatic pressure conditions at every depth in the wellbore at the time of the blowout, demonstrating how a single elevation datum anchors an entire well's pressure history for post-incident forensic analysis.

Kelly Bushing Technical Specifications and Depth Calculations

The relationship between KB, ground level, sea level, and reservoir depths forms the foundation of all subsurface mapping. The chain of depth conversions used in every producing field worldwide is: Measured Depth (MD from KB) minus KB Elevation above MSL equals True Vertical Depth Sub Sea (TVDSS). For a vertical well, MD and TVD are identical; for a horizontal well, the true vertical depth is calculated from the directional survey using the minimum curvature method, with every depth point still referenced to KB. This standardization allows a geologist to map a formation top across 200 wells drilled over 50 years on dozens of different rigs, as long as each well's KB elevation was accurately recorded at the time of drilling.

KB elevation surveys use three primary methods. On land wells, a licensed land surveyor establishes KB height using a total station instrument tied to a government geodetic benchmark. Typical measurement uncertainty is +/- 0.05 m (+/- 0.16 ft). On fixed offshore platforms, the KB height above MSL is measured during platform installation and updated after any structural settlement. On floating rigs, the KB height above MSL varies with vessel draft and must be corrected for tide and heave; SLB and Halliburton wireline crews record the rig heave compensator set point and tidal offset in the log header to account for this variation.

RKB (Rotary Kelly Bushing) is the most common depth datum designation on North American wells. Other designations in use internationally include: DF (drill floor) elevation, used in older UK North Sea well files; RT (rotary table) elevation, used interchangeably with RKB on many international wells; and MSL (mean sea level) reference, sometimes used as the final reported datum after all KB-to-MSL conversions have been applied. The difference between RKB and RT is negligible on most rigs (the master bushing flange is essentially at the rotary table level) but must be documented in the well file if they differ by more than 0.1 m (0.33 ft).

The drive mechanism of the kelly bushing uses either a square-drive interface (the master bushing has a square internal recess and the kelly bushing has a matching square shoulder) or a pin-drive interface (the master bushing has four drive pins that engage recesses in the kelly bushing). Pin-drive systems transmit torque through shear loading on the pins, while square-drive systems use face contact. Square-drive bushings are preferred for higher torque applications and are more common on modern rigs. The kelly bushing must be removed and stowed when running casing, as the casing collar OD is larger than the kelly bushing bore and cannot pass through it.

Tip: Always confirm the KB elevation listed in a well completion report against the original licensed survey before using log depths for reservoir correlation in a new area. Transcription errors in KB elevation, even as small as 1 m (3.3 ft), will shift all formation tops by that amount and can cause costly errors in perforation placement, pressure gradient calculations, and field development well targeting. Investors reviewing field development plans should verify that KB elevation data quality is audited as part of the geoscience review process.

KB: The standard abbreviation used in all drilling and geological documents; appears on log headers, well completion reports, and directional survey printouts.
RKB: Rotary Kelly Bushing, the more specific designation distinguishing the kelly bushing reference from the drill floor reference; preferred in regulatory submissions.
Drive bushing: The mechanical engineering term for the kelly bushing, emphasizing its torque-transmission function rather than its use as a depth datum.
Master bushing: The outer sleeve in the rotary table into which the kelly bushing seats; the master bushing is the platform, the kelly bushing is the drive element that fits inside it.
DF elevation: Drill floor elevation, used in UK North Sea well files as the equivalent of RKB; may differ from RKB by the height of the master bushing flange above the drill floor surface.

Related terms: kelly, rotary table, mud weight, spud, directional drilling, MWD