What is Closing Ratio in Blow Out Preventor (BOP)?

People asked me about what the closing ratio is and what it tells us. Closing ratio is defined as the cross sectional area of the ram piston (cylinder) divided by the cross sectional area of the ram shaft. The closing ratio is used to determine Ram closing pressure which will overcome wellbore pressure acting to Ram body.

Closing Ratio = Ram Piston Area ÷ Ram Shaft Area

Before going into the detailed calculation, we would like to show you where the cylinder and the ram shaft are in BOP. In Figure 1, the yellow shaded parts demonstrate these two areas which will be used to calculate the closing ratio.

 Figure 1 - Shaffer SL-Ram BOP

Figure 1 – Shaffer SL-Ram BOP

Example: Ram has a piston cylinder of 12 inch and 4” of ram shaft (see Figure 2).

Ram piston area = (π x 122 ) ÷ 4 = 113.1 square inch

Ram shaft area = (π x 42 ) ÷ 4 = 12.6 square inch

Closing Ratio = 113.1 ÷ 12.6 = 9.0

Figure 2 - Basic Diagram of Rams

Figure 2 – Basic Diagram of Rams


How To Use Closing Ratio To Determine Minimum Operating Pressure

When you know the closing pressure of the BOP ram, you can use the figure to determine the minimum operating pressure. The following equation is used to determine the minimum operating pressure from the accumulator unit (koomey).

Minimum Operating Pressure = Working Pressure ÷ Closing Ratio

 Example: What is the minimum operating pressure would be needed to close the ram against 10,000 psi maximum anticipated pressure on BOP? Please use the ram details from the example above.

Minimum Operating Pressure = 10,000 ÷ 9 = 1,111 psi

With operating pressure of 1,111 psi, hydraulic force will equal to force acting from the wellbore in this case (see Figure 3).

 Figure 3 - Force Acting at Ram Shaft and Force At Piston

 Figure 3 – Force Acting at Ram Shaft and Force At Piston

In this case, a standard accumulator (3,000 psi system) with minimum operating pressure of 1,200 psi is good enough to shut the well in with 10,000 psi surface pressure.

Reference book: Well Control Books

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5 Responses to What is Closing Ratio in Blow Out Preventor (BOP)?

  1. warren ovie precious says:

    Want to know more about drilling and its calculations

  2. NM says:

    From the equation the pressure from wellbore is acting against only ram shaft area.
    It means that the pressure also affects from backside of ram not only in front of ram.
    Is there no seal around ram and bop body? (I belive there is a seal on upper side of ram at least)

  3. OPSINGH says:

    What is the closing time for rams and annular on a 18 3/4″ BOP with a operating pressure of 1500 psi and where can i find the API recommendation.

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