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Trip and Riser Margin – Audit
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Trip & Riser margin, version 4, issued 29 March 2013. AUDIT version.
The current release of this worksheet exists in 2 versions. They are identical apart from the way they are formatted. The Work version shows inputs and outputs, hiding intermediate calcs. Audit version displays the intermediate calculations.
This worksheet calculates a minimum additional mud density over what is needed to balance formation pressures, to give a safe trip margin. For subsea wells, this is NOT added to the riser margin – the greater of trip or riser margin should be used.
User input
8.5 |
inmm |
Enter the hole diameter. L_dh =
5 |
inmm |
Enter the drillstring OD. L_Ddp =
13 |
lbf/100ft2Pa |
Enter the mud yield point. Pσyfluid =
1.354 |
psi/ftSG or kg/lppglb/ft3ppbkPa/m |
Mudweight to balance formation pressure. ρemwfm =
(Optional input for riser margin)
8.5 |
psi/ftSG or kg/lppglb/ft3ppbkPa/m |
Enter the local seawater density. ρsw =
2300 |
ftm |
Enter the water depth. L_Vwd =
25 |
ftm |
Enter the flowline height above sea level. L_Vfl =
5000 |
ftm |
Enter the well TVD. L_Vtd =
Click here when any values are modified to update the result.
Calculations
Calculate the trip margin to avoid swabbing under normal trip conditions.
and in ppg;
Calculate the mud density gradient to include trip margin.
and in ppg;
Assume that the mud in use actually just balances formation pressure. Calculate the riser margin, then the mud can be weighted up to account for both riser and trip margin (ie what is the minimum mudweight that will satisfy both issues?).
Calculate the pressure lost if the riser disconnects; this is the difference between mud and seawater hydrostatic pressure at the seabed.
Calculate the riser margin to account for riser disconnect. This is the amount to increase the mud density to give the required extra hydrostatic pressure in the mud column from seabed to TD (which in this case is ).
and in ppg;
Calculate the mud density gradient to include riser margin.
and in ppg;
Results
Mud density including trip margin
Mud density including riser margin
Note; in deep water, often the difference between pore and fracture pressure is too small to be able to carry the mudweight with full riser margin. In this case, emergency disconnect procedures are critical and must ensure that the BOP is closed (and if necessary the pipe sheared) before riser release.
Worksheet references
Original trip margin formulae (imperial units) from Shell (1979).
Version 1 of this worksheet was issued on 27 June 2008 by Steve Devereux.
Version 2 added riser margin and calculated resulting mud gradient and weight.
Version 3 accounted for sea level to flowline height and added ppg equivalents.
Version 4 added Specific Gravity to the results (kg/l)
