Production drops during exports

[smartsii]

Hello All

I am a graduate petroleum engineer doing an investigation into production drops we are experiencing at our plant during export especially at our crude storage tanks.

Currently we have 3 Radars in use on each of our crude storages tanks (CSTs). There are 2 Rosemount 5300 guided wave radars and 1 Rosemount 5400 Two-wire radar level transmitter.

The guided wave radar is used for the Trip system while the two wire radar is used to obtain volume measurements.

In our tanks we store crude with a certain amount of condensate as well.

From our production reports we are experiencing drops of roughly 3000 barrels every time we export.

I would like to know if there are any similar cases out there that experience production drops during export and use the radar technology for volume measurement. I would like to discuss and share experiences and find solutions to issue.

Thank you

 

[TrustedDesire]

Hi Smart, it is really unusual, i haven't heard of such and it hasn't happen on our plant.

I don't know if you guys normally take the innage and ullage of your cargo tanks prior to the export, but if not, i will suggest you try this by taking the innage and ullage of the tanks not nominated for export before and after the export.

Or, it's also possible that the flowmeter upstream the distribution header to the tanks is faulty and needs calibration.

 

[LittleInch]

Not really enough information here to make am informed guess at to what might be happening.

Needs things like:
radar gives you level not volume - where / how is level converted to volume? have the tanks ever been properly "strapped"
How big are the volumes?
What is the difference in temperature?
Are you volumes all corrected to some common standard temp and pressure?
does the "condensate" flash off?
What are you comparing volumes to - just tank dips or some other meter?
please explain this sentence much better - "From our production reports we are experiencing drops of roughly 3000 barrels every time we export"



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[smartsii]

Thank you for the replies.

@ TrustedDesire
Yes we do take the innage and ullage of our cargo tanks prior to export. We also don’t have flowmeters in place but we have inventory meters.

@Little Inch to further clarify:
Firstly yes the radar gives a level reading which is obtained by the radar and this level is converted to a volume by the use of strapping charts. The calculation is done by a calculation block in the Distributed Control System (DCS).

For your question about volumes I assume that you are referring to tank capacity. The total vessel capacity of a storage tank is 110 000 barrels and the safe fill capacity is 104 000 barrels. However we never exceed 100 000 barrels when pumping crude into the tanks for safety reasons.

The volumes are corrected to standard temperature and pressure. The condensate entering the tanks is stabilised so we don’t experience any flash off.

Apart from tank dips we are comparing our volumes from the radars to our export meters when loading cargo onto the tankers. The discrepancy is that our tank volumes obtained via radar is less than what we actually export going into the tanker.
So on average we have a discrepancy of about 3000 barrels according to our daily production reports generated during export times. We export every 10 days so our production is done to meet shipping schedules.

I would like to find out if there are other companies using a similar set up as here and whether they have similar issues like this.

Please feel free to ask for further clarification. I am keen to get your feedback.

Thanks

 

[LittleInch]

Ok, Now we're getting somewhere.

So what I think you have here is a discrepancy of around 3% between tank dips and volumes and a fluid meter assuming you export 100,000 bbls at a time?

So the things to do is to check the uncertainty and calibration of each method.

Tanks - try and see if the DCS actually contains the proper tank strapping tables. It may have defaulted to a standard one during a software upgrade or just have the wrong data / wrong tank (each one is different). Try and see if the levels in the tank are continually out, i.e. for every say 1m fall in liquid level which equates to X bbls, what is your meter telling you or per hour what is the variance?

Meters - What type of meters are they?
when were they last calibrated / cleaned / removed for inspection?
When were the instruments last calibrated - Pressure and temperature

Is the same calculation being done in both systems to convert actual to standard?

You will never get two different volume measurements to be precisely the same, but you should be able to get the discrepancy down to less than 1% of volume.

If your tank volumes are less than the export volume then you should be quite pleased...

Let us know how you get on.


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way