Offshore gas flow meter sizing/ranges

[DrillEng]

All,

Really just after peoples thoughts and views, and their experiences to give me some ideas.

The problem - We are building a new very minimal facility offshore platform that will only be manned once every 2-3 yrs for brief maintenance trips. The gas wells that the meters will be attached to are new and their flow rate will drop at least half in the first 6 months and then go into a very gradual decline thereafter for the next 10 + yrs.
The problem is that if you scale the plates or venturis for the initial flow, the differential pressure across the plates/nozzle is very low when you are in middle field life, and you get erratic readings.
Has anyone got any suggestions given the lack of human intervention that this project will have ??
Thoughts etc all very welcome. Idon't think there is a magic bullet and some comprimise will have to be made.

Thanks.

 

[abeltio]

this is very similar to the gas fuel flow metering in gas turbines... where you have low load flow and high load flow... one [Δ]p transducer cannot manage the full range.

the solution adopted is:
1. ONE [Δ]p[sub]1[/sub] transducer for the high flow
2. ONE [Δ]p[sub]2[/sub] transducer for the low flow

the low flow (small [Δ]p) transducer will be saturated (max reading) during the initial phase of the process.
During the mature phase the high flow will read zero, while the low flow will be happily reading in it's useful range.

the transducers are selected so that there is some overlap in the ranges to avoid having blind spots.
the good thing is that if properly designed the stand-by transducer can be changed out while the system is in operation to adapt to unforeseen flow conditions.

as the facility will be un-manned... may be you would like to consider triple redundant transducers... with a median select block for the actual reading and error detection, if any transducer falls beyond a certain deadband an alarm will drop.

the orifice plate can be calculated to minimize the error in the desired condition, i.e. initial or long term.

with this arrangement i've seen flows measured with a ratio of 20:1 between high and low flow and acceptable accuracy for performance calculations.

i have never seen orifice plates in paralell (need blocking devices) or 2 orifice plates in series (1 for low and 1 for high flow).

saludos.
a.

 

[TD2K]

What is the use for these meters? Are these custody transfer meters? Is there any reason you aren't looking at other technologies with better turndown like ultrasonics?

 

[jsummerfield]

If electronic, the range of smart transmitters is very easily changed. Start with a relatively high pressure differential. For gas some people might consider the initial pressure differential in inches water equal to the inlet pressure in psig. This is a maximum rule of thumb. I would limit this at 300 inches water max. As the field plays out, drop the differential in half each time the range becomes an issue.

John

 

[DrillEng]

All,

Thanks for the replies. To answer a couple of pts;

Class two meters - NOT fiscal metering. That takes place on another platform at great cost and is labour intensive. Remember this is an unmanned platform with limited visits and changing out the actual metering like John suggests is easy but the meters must have the dP's to work.

Our current method is to have a venturi fabricated and then inserted(welded) into the flow line with a meter connected to the up/downstream gas flow.

Wet gas metering which is a difficult area to acheive good results cheaply.

I think the robust solution would be to have the twin meters like abeltio suggests but with possibly orifice plates, that would allow a greater pressure drop to be seen at lower gas flows. ie someone could go out and change the plates when the dP's get too low and things don't work anymore.

Or to have the venturi pipework flanged for easier removal.

Thanks to you all for the input. It is much appreciated and it is good to hear your ideas and stimulate debate.

Thank you again.