wax simulation in olga

[MIANCH]

Hi,
I'm struck in simulation of crude oil pipeline in Olga software. I generated wax tab in multiflash successfully and when run software, Olga simulation aborted in 0 second, error in reading wax tab number of pressure & temperature points.
I tried with different pressure and temperature changes during wax tab generation in multiflash but Olga simulation aborting with same message.
Can anyone let to know where is the problem.
Thanks.

 

[1503-44]

I do not use Olga, but it sounds like you may simply be trying to feed it too many data points. Some simulators have limits on the number of data points you can have in a curves. How many data points do you have on your curve? 4076? Try feeding it just 16 points.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Wax is a bit of nightmare to handle in OLGA I think. What are you trying to do exactly?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MIANCH]

we have problem in our crude oil pipeline back pressure generating, pigging is not done for last 7-8 years, pipeline is 87 km long and 20" dia, at km 45 elevation changes to 167 m from 20m sea level. I want to simulate pipeline to check the most affected area.
1504-44; I used 100 points for pressure and temperature while generating wax tab file.

 

[1503-44]

Did 16 work?

What's your wax content, appearance temperature and operating temperature profile?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[MIANCH]

Hi, 1503-44
after reducing the pressure and temperature points to 20-25 simulation is running but not converge because of gas density is zero at 10C and 1 bar. wax appearance temperature in multiflash is 37C as per DSC method and other method is 58C.I checked in multiflash pressure changes have no effect on wax appearance temp. winter operating temperature is 75F and summer is 110F. pressure is 200 psig both cases.

 

[1503-44]

Wax appearance is almost all to do with temperature. The most will appear where the pipe temperature cools most rapidly. Usually towards the place where the operating temperature is highest. Wax deposition tends to get less and less as you move along the pipeline length of pipeline.

I think a WAT of 58°C is a bit low, indicating a relatively low wax content as far as waxy oil is concerned, but that is not easy to determine without actual test results. Since the pipeline apparently receives wax below the WAT, the great bulk of the wax should have already precipitated. It should be moving along with the flow, not precipitating on the walls, although precipitated wax. could be collecting at various places. I do not think OLGA would model that effect correctly.

This is also not to say that there is not a problem. Just not as severe as it could be. With no pigging for so many years, there is probably at least some potential for improvement.

Your pressure seems a bit low, as you will have around a 200 psig differential just from elevation change in pumping uphill, so I am guessing that the flow rate is relatively low, which may also facilitate accumulation of dirt and precipitated wax in the pipeline.

Rather than concentrating on wax deposition, let's just think for a minute about what the potential for improvement could be. What is your estimated pressure drop for your given flow rate in a clean 20D pipeline and what pressure drop do you actually have now?


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[MIANCH]

Elevation profile attached for your consideration and crude oil contains 7.45% wax. pipe was design for 45000 bpd but now we are pumping only 10000 bpd. termination pressure i checked in pipesime is coming for 250 psig for clean pipe. present pressure is not available. I will check with field staff for present pressure condition then will comeback.

 

[1503-44]

Yes. Try to get the current flow rate, inlet/outlet pressures and any temperatures ... if possible.
Is there any visible wax participates in the oil at the inlet or outlet?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[MIANCH]

1503-44, I ran simulation and found 1.5 m3/day wax deposition is there. this quantity is big.

 

[LittleInch]

The wax models are unreliable. It did sound though as though you have some serious wax deposition over time which is very difficult to remove.

As Mr44 says wax deposition occurs faster where the temperature difference between the crude and the steel temperature is highest. As the wax deposition continues, it starts to act as insulation and hence the wax deposition starts to move along the pipeline.

You need professional experienced help to remove this wax which could take a long period and many pig runs.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

It is not very much thickness wax if the wax is deposited uniformly along the 87km pipeline,
it is only 0.0172 mm3/m length of pipe.
Circumference of 20" is 1595mm.
Length is 1000mm.
Surface area = 1595000mm2
.0172/1595000 =
Wax Thickness = 1x10^-8 mm. /day
It takes 28,000 years to get a 1mm thickness of wax.

Where the wax forms could be important. If it is not uniform, it may concentrate at some points.
Does OGLA tell you where it is being deposited? Can you make a chart with wax thickness vs pipeline distance? I would want to verify with Schlumberger that OLGA can accurately model deposition of wax below the WAT on a pipe wall and how it does so. What assumptions are made, etc.. Please let us know what they say.

In any case, simulation is not going to clean the pipe. OLGA won't help there. If it's a problem, you had better get a pipeline cleaning operation started up.




--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Last time I asked that question the FA engineers wrote into their report that the accuracy was in the range 0 to 100% .

I joke not

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MIANCH]

Problematic area is before the hill as you can see from profile as well. I agree even we predict wax through olga but won't tells how to clean it. our production guys afraid of sending the pig because of long time pipeline is not pigged. if pig struck somewhere in the pipeline is the worry of our production guys. our crude is naphthenic nature and 45 API.

 

[pierreick]

Hi,
You should ask your colleagues from operation and maintenance where is their preventive plan maintenance to mitigate this risk " pipeline not pigged for a long time" and get support from your management to resolve the issue! Probably good also to get contact with a contractor familiar with the cleaning of pipeline.
Good luck
Pierre

 

[LittleInch]

They need to do a bit of research and look at the specialist wax cleaning pigs. They make a bunch that have spring loaded relief / bypass valves which jet the fluid past them and help keep the scraped off wax in suspension. so the pig ends up going about 2/3 of the velocity of the fluid.

Some wax can get very hard over time so you need very aggressive scrapers / ball bearing type things to get the wax off the wall.

e.g.

https://www.pigtek.com/products/advanced-bypass-pig/

Also start looking up wax inhibitors or start mixing the crude with something a bit lighter.

A bit of heat doesn't do you any harm either

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MIANCH]

Surely, we are in process and contacted with the contractor for cleaning of pipeline. we will do it, oil is flowing and pressure maintaining 210 psig before the hill.

 

[1503-44]

The choice boils down to do a cleaning or live with the pressure drop.
What flow rate do you get with that 210 psig at the hill?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

You are running this pipeline in right flow aren't you?

What is your pressure or head at the high point.

210 psig where exactly?

As mr 44 has asked what is the difference in inlet pressure between no wax and what you have now as you mentioned increased back pressure in the OP so someone must know. Still not sure what you are trying to simulate and why.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.