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Slug Factor for 3 phase horizontal separator sizing.Helpful Member!(3) 

10815L (Chemical) (OP)
26 Aug 12 6:41
Hi,
Senior engineer told me to keep 20% of vessel size for slug margin but SHELL DEP is saying 0.05D or minimum 0.3m. I would like to understand which method to be used for slug margin during three phase horizontal separator design.
Thanks
TD2K (Chemical)
26 Aug 12 19:49
Why didn't you ask your senior engineer?

Predicting slug volumes isn't a hard and fast science, there is a lot of uncertainty in your value. 20% to me isn't a large margin, 0.05D is very little. Given that slug catchers are normally fairly large vessels 0.3m isn't much of a margin either.

How confident do you feel with your predicted slug volume? What happens if the slug is larger than you predict?
10815L (Chemical) (OP)
27 Aug 12 2:13
TD2K,
No slug catcher before separator, therefore, 20% of vessel volume is keeping as slug margin but i agree with you if more slug is coming then separation problem will be there.
Is this better to keep a big margin in separator sizing or put slug catcher, field is getting old and more sediments coming with production. Keeping 20-30% slug margin will increase the vessel size and cost will also go up, now comparing the cost of vessel and puting the slug catcher which is best option?
MortenA (Petroleum)
30 Aug 12 6:47
If you work for SHELL DEP is the bible - and you dont question the bible winky smile

Maybe you should consider reference?

Is the margin form normal level to HH

or from H to HH

or from H to overfilled??

Best regards

Morten
10815L (Chemical) (OP)
30 Aug 12 9:12
Hi Morten,
My question is to seek the advise from members and I'm not considering Shell DEP is not a bible. Shell DEP is just for reference but your judgment is the final and best fit as per situation.
The best solution for this type of services is desanding option instead of increasing the vessel size.
Thanks for your valuable comments.
10815L
Helpful Member!  MortenA (Petroleum)
2 Sep 12 4:40
Well then Shell has changed or maybe customs varies! As a consultant i have tried many times to be encouraged to come up with "solutions" just to see them discarded because they did not comply with the DEP.

Anyway, what i was trying to say with regards to your margin for slugs is that you could consider the margin from NL or from HH and your rationale "would depend". So are your certain that the Shell DEP margin and your "senior engineer" has used the same reference?

You say that the " best solution for this type of services is desanding option instead of increasing the vessel size" - how does this solve a slug issue?

There are a number of issues wrt slugs:

-Quality of separation
-Level control (overfilling)
-Gas compressor control (large variation in gas flow)
-?

The sizing should consider your upstream design: Is it a separator connected directly to wells, or are there (long) sub sea flow lines, risers? Your margin should be justified based on these and not just a number (in a manual or otherwise). Should your separator be able to handle a slug when at HH or only when (near) NL? If you in addition to your estimated slug size and frequency ALSO want a margin then you maybe a manual will help. But in many cases adding "fat on fat" creates other problems (aside from the obvious cost issue)

Best regards

Morten

SNORGY (Mechanical)
2 Sep 12 8:57
Yes, the Shell DEPs and Specs and what have you...

Every Shell project I have ever worked on, Shell have encouraged the EPC contractor(s) to recommend "industry standard" and "fit for purpose" optimizations to their voluminous documents. Every time, the effort is carried on - with their encouragement - right up until the first shop inspection meeting with various fabricators. At that point in time, we are dragged into a conference room, declared either incompetent or stupid or both, admonished for having the audacity to challenge the DEPs / Specifications / Standards, told to discard the optimized documents, and revert back to the originals.

That appears to be the "Shell" way.
MortenA (Petroleum)
2 Sep 12 9:21
@SNORGY - been there and also been called stupid and incompetent for suggesting something that not as the DEP specifies it smile But i guess that either the world has moved on or everbody just have to learn by their own mistakes. Im not a consultant anymore but if i became again and took a job for SHELL i would tray from the DEPs.

Best regards

Morten
MortenA (Petroleum)
2 Sep 12 9:33
sorry i meant WOULD NOT stray from DEPs
SNORGY (Mechanical)
2 Sep 12 9:39
MortenA,

Yes, I agree. It is pointless to deviate from a Shell DEP when, in the end, the DEP will be upheld.
Latexman (Chemical)
2 Sep 12 9:50
Shell is not the only narcissistic company out there that lives by the "Designed Our Way" mentality.

Good luck,
Latexman

maddocks (Petroleum)
15 Sep 12 15:19
Our recent experience is that some of the TA's are allowing deviations and derogations from the DEP's when you can show that your design is solid, based on fact, and presents a strong operating case and some capex/opex cost savings. However, there are a lot of DEP's that we get locked into that have little basis in reality..... Seems to depend on TA expereince. Too many producer companies do not reward tech experts with the ability to make decisions - in other words, why deviate if the only reward to me is increased risk?
10815L (Chemical) (OP)
19 Sep 12 9:09
Hi Maddocks,
What is the meaning of TA's,and please elaborate more in detail of your quoting,"Too many producer companies do not reward tech experts with the ability to make decisions - in other words, why deviate if the only reward to me is increased risk?"

Thanks
Helpful Member!  zdas04 (Mechanical)
19 Sep 12 10:08
"TA" is generally "Technical Authority". When I had that title at BP I had a ton of latitude to approve very wide deviations from our standards. Since I retired the people who followed me were progressively less willing to test the boundaries and today BP is doing plant-style PSM ("Process Safety Management") analysis for low pressure onshore gas wells because no one is willing to sign off on a simpler evaluation. I've never done a project for Shell, but I can promise that their adherence to the DEC (i.e., some folks saying that they are guidelines and others saying that they are inviolate) is far from unique.

As to slug capacity, that should be absolutely dependent on upstream conditions and the impact of overrunning your separator on downstream processes. If all you have downstream is more pipe, then you get away with a higher probability of overtopping the separator. If you have a piggable gas line upstream of the vessel and a compressor with no suction scrubber downstream I'd put a VERY large factor for slugs (and a HH ESD, etc.). I did this "analysis" for a project in 1993 and turned a 30" X 12' vertical separator into a 90" X 25' horizontal vessel (that still slugged the compressor once in the intervening 19 years).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Helpful Member!  maddocks (Petroleum)
19 Sep 12 16:31
Thanks David - your assessment is competely correct. To the OP, follow the DEP's and use your senior engineer for advice. Document your work carefully and make sure everyone know that you're using XX BBLS for slug sizing criteria. Remember, nobody ever says, "hey, that separator is too big". But they will remind you for years if you make it too small.
10815L (Chemical) (OP)
20 Sep 12 2:20
Thanks David and maddocks,
one more explanation, can I ask the vendor for fly change over the compartments after one or two year of operation, if Increase of water cut and water & oil compartment can be readjusted with out cutting and welding. And this easy for operation and save lot of time and cost. is this good option for integrity of the vessel or not.
Thanks

zdas04 (Mechanical)
20 Sep 12 7:01
I've never seen a field vessel for mechanical separation that was too big. If you had an adjustable separator I don't know of a situation where you wouldn't start out with it adjusted to the biggest possible and never change the adjustment. The only reason you don't go bigger on every vessel is cost.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

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