Riser margin in the mud weight increase below mud line to compensate bottom hole pressure in case of an accident disconnect or a failure marine riser close to the BOP stack at sea bed.
The riser margin is described by a following equation:
ρrm is riser margin, ppg
ρdf is drilling fluid density equivalent to formation pressure, ppg
ρsw is sea water density, ppg
L is riser length from sub sea BOP to rig floor, ft
D is true vertical depth of the well, ft
DW is water depth, ft
Note: When you consider adding the riser margin, you need to make sure that formation strength is sufficient. Additionally, you must not add trip margin and riser margin to the system because it is way over safety factor. If trip margin is higher than the riser margin, you must use the trip margin.
Example: Determine riser margin for this case.
Drilling fluid weight equivalent to formation pressure = 9.2 ppg.
Sea water weight = 8.6 ppg
Length of riser to the rig = 8000 ft
Well TVD = 13,000 ft
Water depth = 8,000 ft
ρrm = 1.2 ppg
Riser margin for this case is 1.2 ppg.
As you can see, riser margin is normally impractical because it can fracture formation. Normally, oil companies and drilling contractors will have emergency disconnect procedures because they cannot depend on hydrostatic of seawater for well control
Reference book: Well Control Books