Final Refusal Depth
The following procedure details the determination of final refusal depth.
A. When the driving depth of the conductor pipe is not specified in the Drilling Programme, the final depth of the driving is the ‘refusal depth’.
- The refusal value generally used is 1,000-1,100 blows/metre (300-330blows/ft)
- Local experience could dictate a different refusal value. The driving depth can be predetermined by conducting soil boring analysis.
- Examine offset well data for depths and potential problems in order to determine if the CP depth is adequate.
B. The driving depth of the conductor pipe which is specified in the Drilling Programme is established with the following formula:
Hi = [df x (E+H) - 103 x H]/[1.03 - df + 0.67 x (GOVhi - 1.03)]
Hi = Minimum driving depth (m) from seabed
E = Elevation (m) distance from bell nipple and sea level
H = Water depth (m)
df = Maximum mud weight (kg/l) to be used
GOVhi = integrated density of sediments (kg/dm3/10m)
If the refusal depth does not meet this value, internal washing may be required. CP internal washing might be necessary several times before reaching the planned depth.
C. It should be noted that if there is a high refusal value in very hard formations, the CP shoe could collapse.