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Ram preventers
06 November 2019
Dear Colleagues,

I would like to gain a better understanding of the meaning and implications of the closing ratio of ram preventers.  My understanding is lacking and I am supposed to be teaching this.

IWCF questions used to ask a candidate to calculate a new minimum bottle pressure required to close a ram on RWP.  For example, a 15,000 Ram with a closing ratio of 7:1 would require a minimum hydraulic pressure of 2,143 psi (15,000 / 7).  The question would then go on to ask something like how many gallons of useable fluid is now available from a bottle.  The simplified Gas Law would be used to calculate V2 Nitrogen, and oil volume at minimum pressure would be inferred from the standard bottle volume of 10 gallons. 

API Standard 53 defines closing ratio as that between the surface area of the closing piston and cross-sectional area of the ram shaft exposed to well-bore pressure.  It makes sense until you consider friction is not mentioned.  To my mind, if you are closing a ram on RWP (highly unlikely, but possible) you might get the piston moving at a lower hydraulic pressure but as the ram block approached the pipe and the flow area past the block decreased, the friction needing to be overcome would rise because the top seal is increasingly being forced onto the seal face.  Assumes well not already closed in above.

My questions:
  • Is Standard 53 correct?
  • Is friction accounted for by the manufacturer in the closing ratio?
  • Why would any surface ram, apart from shears, need more than standard manifold pressure to be closed successfully?
  • Is closing ratio only relevant in the extreme case of closing on RWP concurrent with loss of primary and secondary charging pumps?
Thanks in advance,
Chris


4 answer(s)
cmcandrew
SPREAD Associates
Total Posts: 32
Join Date: 16/08/10
A couple of points to bear in mind.

  • From that instant when a hydraulic seal is established, then wellbore pressure increases sealing force on rams.
  • Before that instant, wellbore pressure must always be less than the final SICP - shutting on a flowing well ( infinitesimally less immediately prior to establishing seal but still less.)
There is no static case- unless you have top rams closed above ! 

harshmind
Drilling Adviser
SPREAD Associates
Total Posts: 7
Join Date: 14/11/15
Thanks, Scott and Bruce, for your responses.

I agree a ram front packer may not survive the journey from the open position to closed, in what is essentially a full blow out and MASP situation which should never happen.  Conditions are more likely to be static, as Bruce says.

My misunderstanding may have stemmed from the old IWCF questions I mentioned, where a new minimum bottle pressure had to be calculated, using only closing ratio with no mention of friction.

In the static case I can see there would be a peak closing pressure to initiate ram movement, becoming less once the ram shaft started to move and ending at the lower value calculated from SICP and closing ratio when friction is no longer a consideration.

Best regards,
Chris


BruceRichardson
Drilling Specialist
SPREAD Associates
Total Posts: 12
Join Date: 27/08/09
Chris,

Here are my thoughts on the subject;

1. Yes. Std53 defines closing ratio in this manner. (I don't have the latest version at hand to show the reference)
2. No. By definition its a dimensionless number being the ratios of areas. I think it's a question of static well and operating pressures rather than the dynamic case during closing. (I doubt whether a ram seal could sustain integrity for long with any fluid flowing past at a pressure of 15ksi while closing)
3. Every ram has a closing ratio, most lower rated rams can close against their RWP at operating pressures lower than 1200psi. Only when the RWP is higher, like 15M and above, is a higher pressure required because of the operating cylinder geometry constraints.  A larger operating piston (lowering the closing ratio) would require more volume, more volume means more bottles and bigger hydraulic lines to ensure closure within the maximum allowable time.
4. On a standard koomey, the manifold pressure is regulated to 1500psi. If it operated this particular ram, it could only be closed (theoretically) on a 10.5ksi (1500x7) well pressure. So closing ratio counts.

It would be an interesting test to see what pressure initiate ram movement as a measure of the system friction.  Perhaps the manufacturers have done this test for science?
Scott_McNeil
Consultant
SPREAD Associates
Total Posts: 140
Join Date: 05/03/08
Hi Chris,

I don't have an answer for you, but intuitively what you say about increasing friction sounds correct.

However, this is the sort of question that SPREAD is great for - it's something I'd never thought about before and I'm hoping someone (from a BOP manufacturer) does have the answer!

All the best.

Scott
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