Riser Margin – One of Important Concepts For Deep Water Drilling

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:

Where;

ρ_rm is riser margin, ppg

ρ_df is drilling fluid density equivalent to formation pressure, ppg

ρ_sw 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

ρ_rm = 1.2 ppg

Riser margin for this case is 1.2 ppg.

Reference book: Well Control Books