Q&A No.717 - Equalising v Non-Equalising Tubing Retrievable Safety Valves on Subsea Oil Projects

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A while ago, a colleague had requested some advice in help for choosing between equalising (E) or non-equalising (NE) Tubing retrievable surface controlled subsurface safety valve (TRSCSSVs) for new subsea gas wells. This was back in 2002. I have read the 2002 COIN Q&A for the respective question, and looking at any updates on the same matter but also addressing not just gas wells but subsea oil wells.

We are planning on 6 x horizontal subsea oil production wells and 6 x water injection wells. The plan will be to run 5.5" TRSCSSV's with 7"x 5.5" tubing. The predicted production rates will range from 10-20bopd and typical THP 1900-3000psi. Life of the development is about 20years.

We would like to learn from other operators experience with either NE or E TRSCSSVs in similar subsea wells. Proposal is to run E in the oil and water developments to improve operability - but the concern is still there about reliability and survivability of E vs NE. Key to the decision is the long term survivability of the valve to help maximise the up time of the wells. Plan is for no intervention.

Questions:

1. What TRSCSSV's do you prefer to install (NE vs E) in subsea GAS and OIL wells. What is the reasoning behind this decision? Ours is an oil development.

2. What experiences are you have with either NE or E valves in similar, or otherwise relvant situations.

3. *Our wells are predominantly well for this development, so some thoughts on oil developments would be appreciated.

Thanks again,

Brijesh
asked Oct 3, 2016 in COIN by Brijesh Dutta (120 points)
edited Oct 4, 2016 by Brijesh Dutta

5 Answers

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Brijesh,

If it’s a gas field or if the well will have a gas cap when shut in then you may have methanol injection to prevent hydrate formation on start-up. If you have methanol injection this can be used to equalise the SSSV.

If you don’t have methanol injection above the SSSV then you should seriously consider an equalising valve.

The difference in reliability between equalising and non-equalising SSSV’s  is minimal.

Important that well start-up logic is correct if an equalising valve is used. SSSV has to be functioned and THP stabilised before production wing valves is opened. Otherwise you will be attempting to flow the well through the equalising feature in the SSSV. This will result in either plugging or premature failure of the equalising feature.

Regards

Fraser Martin,

NOJV Drilling & Completions Engineer, Chevron
answered Oct 3, 2016 by Fraser Martin (280 points)
0 votes
Brijesh,

From Statoil’s side, we have not used the self-equalizing valves in any fields. This is a decision based on reliability, and it is an old decision (20+ years).

Lars Vinje

Statoil
answered Oct 3, 2016 by Lars Vinje (140 points)
Thanks Lars, are you indicating that the decision may need to be looked into again? Do you utilise external databases for reliability? (Thoughts on oil wells?)
0 votes
Hi Brijesh,

We prefer to install non-equalizing TRSVs, primarily because the simpler design will have a statisticlly lower failure probability.  We have adequate methanol injection supply pressure from our FPSO for equalizing valves, so we can manage without self-equalizing valves.  However, we have in the past installed a couple SE valves in gas wells when we needed a rush order and they were the quickest available.

 

Regards,

Dan

Husky Energy
answered Oct 3, 2016 by Dan Schoonhoven (370 points)
0 votes
Hi Brijesh,

Non-equalising TRSCSSVs are by far the most common choice for oil and gas wells in Norway. It is driven by simplicity and reliability as Lars says, - and it has worked out well. So history become a factor as well... Obviously you need some method of equalising the DHSV, - if this is difficult then you can be driven towards self equalising valves.

The Exprosoft WellMaster database is excellent for DHSV reliability, but unfortunately it does not resolve NE versus E DHSV properly. Personally, I am very much in favour of simplicity when it comes to such a critical component, in true Skunkworks spirit. KIS is your friend... Quite a few years back there was a case here in Norway where the equalising mechanism failed catastrophically, - with full communication through the closed valve.

There is one mature NCS subsea development with close to 30 wells that use equalising DHSVs. On one well the equalising mechanism does not work, one 3 wells the equalising is super slow (feature plugged or scaling up), and on the rest (about 25 wells) it works as intended.

Finally you should also be aware of the hydrate risk for this issue. The DHSV is probably the place in the well where you are most at risk for hydrates, unless placing it deep below the hydrate region. When the valve is equalised, glycol is used (MEG), thereby mitigating the hydrate risk. For a self-equalising DHSV, you have JT cooling through the equalising valve, and the hydrate risk increases and needs to be mitigated.

Anyway, - good luck with your selection!

Best regards,

Lasse Hermansson, SCE

Ridge AS
answered Oct 5, 2016 by Lasse Hermansson (180 points)
0 votes
Good Day Brijesh,

We have used Equalizing TRSCSSVs in our subsea fields here in the UK North Sea. We have used them in oil ans Gas wells... injection and producers and we have not had any issues in over 50 wells in at least 3 fields - all oil developments 24-40API with GOR ranging from 250 to 1200scf/bbl. We have had good reliability from these valves also. We have certainly not had to replace any in recent times.

I also think they are simple to operate as you dont have to pump MeOH or the like to equalize to open up. they are not often shut anyway except there is a high level trip or for routine testing. Also helps reduce MeOH fouling too depending on your product route/export.

Agreed you have to look at yor hydrate risk and let that guide your placement but we place our valves normally ~500-700ft below seabed.

Hope this helps

cheers

epoH
answered Oct 6, 2016 by hope.okhuoya (240 points)
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