11 replies to this topic

[linamus]

Recently, I've simulated a case of gaslift landing at a remote platform. The landing pressure is 1300 psig. Gaslift is mostly used for wellhead but some is used for the instrument gas (IG) system. for IG, the operating pressure for scrubber is at 200 psig and filter separator, downstream of scrubber at 120 psig. due to huge pressure difference, hydrate formation bounds to happen. however, with hysys, i've tried to do few stages of pressure letdown and heating up gaslift in between. at the moment, i'm using production fluid as the heating medium (please comment if this is feasible) to increase the temperature, to prevent hydrate. the scheme does work. No hydrate forms. However, after reducing the pressure to 120 psig, even though hydrate still does not form but ice forms first. what does it mean by this statement? i've tried heating up the temperature for 120 psig up to 212 °F (equivalent to 100°C) but still, ice forms first.

Please advise.

[JoeWong]

Before proceed, have you consider Instrument air (IA) instead of IG ?

Can you advise the composition of gas ?

[Zauberberg]

It's always better to dehydrate the gas, instead of considering complex heat exchange which may fluctuate over the time, and make your life miserable. The simplest process solutions are always the best.

In addition, if Lift Gas contains certain sour components (H2S/CO2), dehydrating the gas will provide you with an opportunity to relax material specification for Gas Lift network: instead of having expensive CRA (wet sour systems), you could probably end up with NACE carbon steel (dry sour systems), and that is a lot of money to be saved. The cost difference between these two cases will more than justify the cost of TEG or Solid Adsorption units installed for the purpose of Lift Gas dehydration.

Best regards,

[joerd]

The easiest way to answer the question is if you could post your Hysys file here. If you have confidential information in there, could you at least post the IG composition and temperature?

[linamus]

All,
thanks for ur support. Attached is the composition of the gaslift. From pressure of 1300 psig, it is further reduced to 1140 psig. Gaslift is heated up by means of production fluid to 111 °F and further letdown to 200 psig. This is where the gaslift (now known as IG) enters scrubber. IG then enters filter separator at 120 psig after undergoing another PCV.

All this info is in file attached here. As you can see, at pressure of 120 psig, ice forms first. I've tried to heat up just the IG after the scrubber to eliminate possibility of ice forms first but to no avail. Something tells me that I shud just eliminate the water at the initial state, but my senior does not agree with this.

Please advise.

Attached Files

•  gaslift_compo_2.xls   103KB   115 downloads

[Zauberberg]

0.1 mole% water is quite high, and if I am at your place I would definitely choose to dehydrate the gas - especially if you are planning to use it as Instrument Gas. The possibility of plugging the impulse lines could make a chaos in your plant, with very serious safety issues.

In my opinion, hydrate formation - like any other potential threat in process operations - is something which should be prevented, rather than cured - whenever possible. Using production stream for heating purposes may eliminate the problem (at least on the paper it will look like this), but have you considered the possibility that things will not go so smooth as you predict them? There might be insufficient heat available to make the gas warmer, or water content of gas could call for even higher heating temperature than the one you envisage at the moment. In addition, connecting production system with instrument gas system will make things more complicated and inter-dependent, resulting in reduction of the overall plant stability.

I haven't checked your simulation outputs, because I think 0.1% water content is sufficient for raising the alert. High level of water-saturation will also call for corrosion-resistant alloys (CRA) in Instrument Gas system (1.15% CO2 in the Lift Gas); simply dehydrating the gas would make things so much simple and easy for operation.

Good luck,

[joerd]

Thanks for posting. Please take Zauberberg's advice.
Re. your original question, "Ice forms first" means that the predicted hydrate temperature is lower than 32 F, so instead of a hydrate you would get ice when the temperature drops below freezing.

[linamus]

Thanx Zauberberg and joerd...

But I must emphasize here that the one I'm dealing with now is a remote platform. It means, this is unmanned platform with limited power supply and space. I've thought of glycol dehydration system but it's a no-no for this small platform. Maybe silica gel is OK but i've never heard it being used at a platform before. And this is for continous operation. Maybe someone could suggest something low maintenance and use litle space for gas dehydration?


Please advise....

[Zauberberg]

TEG units are small packages, and maybe they could do the work for you. Power consumers are regenerator reboiler (usually few hundreds of kilowatts) and TEG circulation pumps.

If you are looking for something without moving parts, contact Twister BV in The Netherlands. Twister is quite small piece of equipment, acting as simultaneous dehydration and dew-pointing unit, and it is frequently used in offshore platforms. It will occupy significantly less footprint space than solid-bed system or TEG package, but the trade-offs are high pressure loss (~30%) and high sensitivity of Twister performance on inlet gas properties/composition. There might be a requirement for hydrate inhibitor injection as well. You are definitely between the rock and a hard place, but Lift Gas dehydration in your case is, I would say, 100% mandatory.

http://twisterbv.com/

[JoeWong]

I have similar experience more than 10 years ago...i understand your situation...

Some comments in attached file...

Check out if you can dry the gas at mother platform... check out possibility of adsorption bed (smaller size) but more expensive...

If you in Asia pacific region, check out the performance of Twister in B11 (hope this is correct) platform own by SH*LL inj similar application...

 gaslift_compo_2_rev1.xls   148KB   92 downloads

[linamus]

Thanx All...

I do agree with both of u on gas dehydration. As Joe Wong proposed, maybe gas dehydration is better done at mother platform. Also, my comment on twister, actually I was involved in a conceptual phase project to install Twister at a platform in Malaysia last year. I did gather some information from B11. Unfortunately, I didn't get good review from the users. They are complaining that twister is hypersensitive, hence, operators are having a hard time monitoring the process. I guess, maybe because it's still new? But I sure hope twister will be able to replace the current glycol system.

Back to Joe Wong comments ( I do appreciate you taking ur time to answer my questions, I feel overwhelmed):

1) Margin is low on DT: I know that good DT between hydrate temperature and operating temperature is at least 5°C, but it's almost impossible to achieve it in this case. I need heater to achive that, and as u know, we don't have that. The only source of heat is from FWS (Fluid WellStream/Production Fluid). I think I need to mention this, the current TEG (Thermo Electric Generator) can only accomodate up to 1kW of energy.

2) MST: From DBM, it is mentioned between 18 to 21°C.

3) Hot Well Trip: For heat exchanger, it is designed for half of the total FWS. Currently, we have 6, so it means, if 3 of the wells trip, it will shut down IGS. However, quantity of wells does not justify the IGS shutdown, it is the total FWS. We are currently in contact with a HEx vendor on this, and they have not come out with any proposal yet.

4) HI: At the moment, we have not suggested any HI yet. But this will come in the near future discussion. I'm suggesting Methanol. But I've yet to figure out the installation and such.

5) Production Trip: Yes. But since this is remote platform, the shutdown of well will eventually lead to shut down of IGS. I guess it is OK but am I missing a point here?

6) Black start-up: For black start-up, since we don't have any FWS at that time, Hex will be bypassed. Hydrate formation will happen but we try to prevent it by injecting HI while waiting for FWS to online.

I hope I answer you correctly.

If you have any doubts, please advise.

[JoeWong]

linamus,

I am glad that you have the feedback on B11 platform. I have similar feedback on this platform 2 years back...

Your problem and configuration is exactly same as a project about 10 years ago by ES*O, the Satellite Field Project with unmanned minimum facilities. Try to get some information in these platform. The instrument gas system is exactly same as what you have proposed now...

1) Increase heating and heaters
2) when you mentioned DBM, lookslike a ES*O project. Check out the information in Satellite Field Project. I hope you have consider minimum arrival temperature.
3) What i worry is in case of 3 hot well trip will lead to IGS trip. Then total platform blowdown. Then you may need operator to goes to platform for restart the IGS. Remote start-up could be difficult or an issue with IH injection (check out with operation team)
4) You need IH for black start and restart. Check out the power is sufficient for restart.
5) read my point 3
6) refer point 4.

The IGS is much smaller than gaslift flow. I guess it is not justify to install a drying package for gaslift and IGS at mother platform...