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:

Riser Margin - equation

Where;

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.

Riser Margin

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

Riser Margin - equation 2

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

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3 Responses to Riser Margin – One of Important Concepts For Deep Water Drilling

  1. I read your latest, “riser margin,” with great amusement. Surely you should know that calculating it is relative easy, BUT seldom used. The notion of adding a 1.2ppg to zones that probably will not withstand such an increase is irresponsible. And, as you get deeper and have to increase mud weight, the greater the danger of beginning a possible underground blowout.

    I cannot think of any oil company or drilling contractor that use a riser margin. If you know, please let me know who/what/where this practice is used.

    • This will give you an idea. Like you said, adding riser margin can cause formation breakdown. Therefore, oil companies and drilling contractors will have emergency procedures to deal with this issue.

  2. Omojefe Felix Egboro says:

    Never thought of the Riser Margin, but it’s a great idea to point to it as an additional precautionary measure in deep water drilling. I think the trip margin is simply sufficient as the necessary safety factor and where you’re faced with a very delicate drilling window, the additional riser margin is a direct invitation for underground blowout. I still would love to know which oil companies have used the riser margin and the advantages it provided for their pore pressure regime relative to the mud gradient.

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