Conductor string is a string which is installed to cover unconsolidated formations, to seal off shallow water sands and to provide protection against shallow gas flows. The string is cemented to the surface or sea bed if possible, and is always the first string onto which one or more BOPs are installed.
- prevent poorly consolidated formations from sloughing into the hole
- provide a full mud-circulation system,
- protect fresh water sands from contamination by the drilling mud
- provide protection against shallow hydrocarbons.
This string is usually cemented to surface or seabed and is always the first casing on which one or more BOPs are mounted.
For onshore wells the conductor string usually supports the wellhead, the BOP, the Xmas tree and subsequent casing strings.
For offshore wells with a surface BOP, the conductor string also usually supports the wellhead, the BOP, the Xmas tree and subsequent casing strings. Compressional loads are therefore often the most critical design parameters for this casing. Above the top cement, the conductor must be centralised to prevent column buckling. The annulus between the marine conductor and conductor string is usually left uncemented above the mudline, in order to minimise load transfer from the environment and hence bending stresses in the conductor string.
For offshore wells with a subsea BOP, the conductor string is landed on the foundation pile, and stays in tension.