This article describes the various depth references used for drilling onshore and offshore wells: Reference Datum, Chart Datum (CD), Mean Sea Level (MSL), High Tide (HT), Low Tide (LT), Water Level (WL), Lowest Astronomical Tide (LAT), Depth Subsea (SS)         

Reference Datum

For operations involving a rig, either onshore or offshore, all depths (either along hole or true vertical) are referred to the drill floor of the rig which initially drilled the well (original drill floor).

These depths below drill floor (BDF) measured from the rotary table kelly bushings (RTKB) are in turn referred to a universal datum, the Chart Datum (CD), for the local area.

Note:   For onshore operations, the reference point for depth is the mean sea level (MSL) and the rotary table kelly bushings (RTKB).

Chart Datum (CD)

CD is a constant reference datum which has the following features:

It is a fixed datum

The actual level of the water (A) will not normally fall below it. The height of the sea surface above CD at a particular time of the day is published in Tide Tables for the local area.

It is the reference for water depth on a chart (B).

The height of Mean Sea Level (MSL) above CD is usually known (C)

Mean Sea Level (MSL)

The overall average level of the sea calculated over a period of time for the local area. The height of MSL is fixed distance above CD.

High Tide (HT), Low Tide (LT)

Daily extreme water levels during High Tide and Low Tide respectively. Heights and times of HT and LT vary daily.

Water Level (WL)

The Level of the sea surface at any instant.

Lowest Astronomical Tide (LAT)

The lowest level which can be predicted to occur under average meteorological conditions under any combination of astronomical conditions.  It can only be obtained properly by studying tidal predictions covering , ideally, 19 years.

Depth Subsea (SS)

The vertical depth, usually to a specific formation or target of a deviated well, measured from MSL.

Tide Tables

For offshore operations the Drilling Supervisor shall ensure that the correct Tide Tables are available on the rig. Tide Tables are predictions and may not be accurate because of local environmental conditions e.g. changes in atmospheric pressure and surges caused by conditions outside the immediate area. Marine department may be consulted if additional guidance is required.

Well Depth References

On a new well, depths shall be recorded in feet Below Drill Floor (BDF) measured from rotary table kelly bushings (RTKB). The datum shall be mean sea level (MSL).

On a side track or workover well depths shall be recorded in feet Below Drill Floor (BDF) but shall also be referenced to the "Original" rig drill floor elevation.

Reservoir Depth Reporting

For reservoir engineering purposes the depths shall be reported below MSL or subsea (SS).

Offshore Existing Structures

In the case of an existing structure, the elevation of a marker e.g. the top of the skid beam, above MSL shall be obtained from the Engineering Department before the rig arrives on location and shall be noted in the Drilling Programme.  The distance to the actual water level (D) plus the tide correction to MSL shall be added to (C)  to obtain RTKB above CD.

RTKB above CD = D + tide correction + C

Offshore Single Well (Open Location)

The RTKB height above CD shall be established by measuring the distance from the Drill Floor (DF) to the actual water level (D) and adding including the tide correction. The air gap (D) shall be determined by the Drilling Contractor Toolpusher and verified by the Drilling Supervisor.

RTKB above MSL = D + tide correction

During the rig move a Marine representative will be on board to establish the exact location. He shall be advised of the agreed DFE prior to his departure from the rig.

Land Locations

The land elevation above MSL shall be provided by Petroleum Engineering and shall be noted in the drilling programme.

For individual wells the depths are referenced to RTKB.