Definition of Terms Associated With Datums
This section defines datum terms. They are grouped by the Properties dialog box in which they are found.
Project Properties
System Datum
The System Datum is set in the Project Properties/General dialog box and represents absolute zero. It is the surface depth datum from which all well depths are measured, and all well depths are stored in the database relative to this datum. Usually, the System Datum is mean sea level, mean ground level, or lowest astronomical tide, but it can also be the wellhead, rigfloor, RKB, and so on.
Elevation
The Elevation is set in the Project Properties/General dialog box, and represents the elevation above mean sea level. (If Mean Sea Level is selected as the System Datum, Elevation is grayed out.)
Well Properties
Depth Reference Datum
The Depth Reference Datum represents zero MD. It is sometimes known as the local datum, and it is measured as an elevation from the System Datum. You can define one or more Depth Reference Datums for a well in the Depth Reference tab (Well Properties dialog box). For each Depth Reference Datum, you must specify the elevation above or below the System Datum.
The selected default Depth Reference datum in the list box is the viewing datum in all applications (the viewing datum can be changed on the fly only in the OpenWells and COMPASS applications).
Chapter 2: The Engineer’s Data Model (EDM™) Database
You cannot delete or change the elevation of a Depth Reference datum after it is referenced by a Design.
Offshore Check Box
Select the Offshore check box to indicate an offshore well or leave it unselected to indicate a land well.
Subsea Check Box (Offshore Well)
Select the Subsea check box to indicate this offshore well is subsea.
Ground Elevation (Land Well)
This value is the elevation of the ground above the System Datum. It is set in the Depth Reference tab (Well Properties dialog box).
Water Depth (Offshore Well)
This value is the total depth of the column of water (MSL to mudline).
It is referenced to Mean Sea Level.
Mudline Depth (Only For Offshore Subsea Well)
This value is the depth below system datum (such as MSL/LAT) of the wellhead flange.
Wellhead Depth (Subsea Well)
This value is the distance from the wellhead to the system datum. It is used in some calculations as the hanging depth for casing leads when they are set. To determine wellhead depth:
Wellhead Depth (to rig floor) = Depth Reference Datum + Wellhead Depth Wellhead Depth (set in the Well Properties/General dialog box) is positive for offshore subsea and negative for wellheads above MSL (that is, onshore or offshore platform). So, it does not matter in the preceding calculation whether it is offshore or subsea. Depth Reference Datum is always positive. Both wellhead depth and wellhead elevation are distances from the system datum to the flange.
COMPASS™ Software Release 5000.1.7 Training Manual 2-23 Chapter 2: The Engineer’s Data Model (EDM™) Database
Wellhead Elevation (Platform and Land Wells)
Wellhead Elevation is the height above system datum (MSL/LAT) of the wellhead flange (surface casing). It may happen that for some land wells using ground level as the system datum that the user may have to enter a negative value because the wellhead “cellar” is often below the ground.
Air Gap (Calculated)
Air Gap is the distance from the system datum to the rig floor. It is used in some calculations for hydrostatic head. This value is always positive.
To calculate air gap, the application uses:
• Air Gap (offshore wells) = Depth Reference Datum – Elevation
• Air Gap (land wells) = Depth Reference Datum – Ground Level Elevation is set in the Project Properties/General dialog box. Ground Level is set in the Well Properties/ Depth Reference dialog box.
Design Properties
Depth Reference Information
From the pull-down list of defined Depth Reference datums, select the datum you want to reference for this design. After you select a datum, the Datum Elevation, Air Gap, current System Datum, Mudline Depth, and Mudline TVD are all updated/calculated and displayed adjacent to the rig elevation drawing on the Design Properties dialog box.
Setting Up Datums for Your Design
1. In the General dialog box (Project Properties > General), select the system datum you want to use.
2. In the General dialog box’s Elevation field, enter the value the system datum is above mean sea level. If your system datum is below mean sea level, this number is negative. If your system datum is mean sea level, Elevation is grayed out.
Chapter 2: The Engineer’s Data Model (EDM™) Database
3. In the Depth Reference dialog box (Well Properties > Depth Reference), if the well is offshore:
a) Select Offshore and enter the water depth below the system datum.
b) If the well is subsea, select Subsea and enter the wellhead depth below the system datum.
4. In the Well Properties dialog box in the Depth Reference tab, if the well is a land well, make sure the Offshore check box is not selected and enter the ground level elevation above the system datum.
5. In the Well Properties dialog box in the Depth Reference tab, define the depth reference datums you want to use, such as RKB or Rigfloor. Type the elevation above the system datum in the Elevation field, and specify the effective date for the datum.
6. Import or create a design for this well.
7. In the Design Properties dialog box in the General tab, select the depth reference datum you want to use for this design from the pull-down list of datums that you defined in step 5.
COMPASS™ Software Release 5000.1.7 Training Manual 3-1