drilling · directional
True Vertical Depth Subsea (TVDSS)
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Inputs
ft
ft
Description
Converts true vertical depth (TVD), measured from the rig's rotary kelly bushing, to true vertical depth subsea (TVDSS) — depth referenced to mean sea level. TVDSS is the common datum used to correlate depths and compare formation tops across wells drilled from rigs with different RKB elevations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| TVDSS | ft | TVD Subsea |
| TVD | ft | True vertical depth measured from the rig's rotary kelly bushing (RKB) reference datum. |
| E_RKB | ft | Elevation of the rotary kelly bushing above mean sea level (MSL) — the height of the drilling reference datum itself, not the ground elevation. |
Assumptions
- RKB elevation is referenced to mean sea level (MSL), the standard subsea datum used across the industry
- TVD input is already correctly computed from survey data (e.g. via min_curvature_survey) and referenced to the same RKB as the elevation input
Limitations
- Some operators reference elevation to ground level or a different local datum instead of MSL — confirm the elevation reference before using this calc's output to correlate across wells
- Does not itself correct for RKB elevation changes during the well's life (e.g. rig substructure changes) — use the RKB elevation in effect when the TVD was surveyed
Use Cases
- → Cross-well formation top correlation: Convert TVD-based formation tops from wells drilled off rigs with different RKB elevations onto a common subsea datum for structural correlation.
- → Regional structure mapping input: Supply TVDSS values as the depth input for structural contour mapping and cross-section work spanning multiple wells.
- → Offshore/onshore elevation reconciliation: Reconcile depths between offshore platform wells (RKB elevation referenced to a marine datum) and onshore pad wells (RKB elevation referenced to ground survey).
Related Calculations
Region Notes
Global
Always confirm the RKB elevation reference (MSL vs. a local/ground datum) before comparing TVDSS values across wells from different operators or basins — datum mismatches are a common source of mis-tied formation tops.
References
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