Pigging Time calculation on Crude line

[sathishsusa]

Dear Experts,
I'm currently working on an upstream oil and gas project where we are conducting a BIDI pigging operation on a 16" MOL line that spans 50 km. I need help creating an Excel sheet to calculate the estimated time of arrival (ETA) for the pigging operation based on the following details:

Pipe diameter: 16" OD (406.4 mm)
Wall thickness: Minimum 15.9 mm, Maximum 30.2 mm
Length: 50 km
Gas-Oil Ratio (GOR): 353
Sealine Pressure: 34.32 BAR
Arrival Pressure: 26 BAR
Flow Rate: 19500 BBL/DAY
Pig Launched Time: 02-08-2024
Pig Arrival Time: ??

Could you help me with this challenge?

​

 

[1503-44]

Oil Flow = 19500 BBL/Day
= 4562 ft3/h

The gas/oil ratio (GOR) is the ratio of the volume of gas ("scf") that comes out of solution to the volume of oil — at standard conditions.
You say it is 353, so Gas Flow would be
353 bbls of gas per one bbl of oil
Please check that. You basically have a gas well with some oil.

If so, Your Gas Flow at 30 Bar average pressure
would be 353 x 4562 ft3/h x 1/31 =52,000 ft3/h of gas
Total Flow = 56,562 ft3/h
Oil Flow is 4562 ft3/h (about 10% of your gas flow rate AT PIPELINE PRESSURE)

For a 14" I'd pipe, area is 1.07 ft2
Which would be a velocity of 52,861 ft/h 14.7 ft/sec (around 10 mph)

Distance is 50km x 1000 × 3.083 = 154,150 ft
154150/14.7 = 10,486 seconds or 2.9 hrs



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

 

[LittleInch]

Mr 44, you can't have forgotten GOR is scf/sbbl...

So 353 is quite low, but isn't barrels and needs to be converted to actual volume.

So you have about 2 barrels gas to one barrel of oil at 30 bar average pressure, kiquid volume fraction of about 0.3

A pig will go more or less at the speed of the gas, creating a large surge of fluid in front of it....


So work on about 40000 bbls/ day (actual volume) of gas.

You do the maths, I have a hangover.

But its about 19 to 20 hours I reckon

Two phase pigging is not a precise thing and profile will make a difference as well.

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

 

[1503-44]

I didn't forget, Problem is that GOR is not well defined. Wiki says scf of gas released from a volume of oil, that volume isn't defined. And it should also be defined at before, or after gas release. GOR as a dimensionless ratio is preferred in reservoir analysis, so just stating a number can be confusing.

The GOR is a dimensionless ratio (volume per volume) in metric units, but in field units, it is usually quoted in cubic feet of gas (at standard conditions: 0°C, 100 kPa) per barrel of oil or condensate, scf/bbl.

[On the other hand]

In the states of Texas and Pennsylvania, the statutory definition of a gas well is one where the GOR is greater than 100,000 ft3/bbl or 100 Kcf/bbl. The state of New Mexico also designates a gas well as having over 100 MCFG per barrel.[1]

Petrowiki does not give a definition, but does mention units of scf of gas/bbl, which should or could be, but there it is not dimensionless.

Since GOR, R means ratio, if his GOR has units, he should state them, but OP did not say for certain, so I asked him to check it. In the meantime I assumed it was a direct dimensionless ratio. Makes the calc easier. Then I squeezed that std gas volume down to the higher 30 barg pipeline pressure.

To do the problem, you only have to realize how to adjust your std volume of gas to pipeline pressure and that velocity equals actual flow rate /Pipe Area. The rest is just simple unit conversion. He will only have to correct his gas volume according to how his GOR is defined, if it is not a direct ratio, then do the unit conversions himself.

I find taking an antihistamine with an aspirin helps.

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

 

[sathishsusa]

Hello experts,

Thank you for your response. I wanted to clarify that the GOR unit states that GOR = 353 SCFT/BBL.

My oil producer wells are producing more oil and a little gas, around 2 MMSCFD/day. However, the pig has not reached and it's not possible to reach in 2.9 hours, it should take more than 39 hours. Please review and let me know the solution.

 

[LittleInch]

So why are you asking if you know how long it takes??

The issue is that the pig speed is far too low. At anything much less that 1m/sec, the pig will tend to "hop" and hence it passes a lot more gas than it should or it gets hung up on things. Also forgot about the large difference in wall thickness. That won't help with sealing on the thin part or added friction in the thick part.

Also your GOR is defined usually as gas per stock tank barrel. The liquid in the pipeline at 30 odd bar is not stock tank liquid. You need to work out or calculate actual volume fraction at your pressure and temperature

You really need to run this through a multiphase simulator and run a pigging scenario. Any hand calculation will be very approximate for a two phase system. I estimated about 20 hours but clearly not matched by reality.

Maybe the pig is stuck somewhere??

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

 

[georgeverghese]

This sealine is downstream of the Main Oil Line pumps on this offshore production station, so there shouldnt be any gas in the line, unless line pressure drop is very high and line pressure drops below the pressure of the production separator feeding the MOL pumps.
Am no pigging expert, so here goes:
Solids scraped out will tend to accumulate ahead of the pig, so the pig will tend to decelerate as the pig moves along the line, given MOL pump discharge pressure is constant. So has pumping flow been constant at 19.5kbd all through so far ?
If flow is constant, then simple arithmetic tells me travel time = 39hours. If it hasnt arrived in this time, say +/ - 10%, the pig may have disintegrated.

 

[LittleInch]

Ah, good point.

The GOR bit threw me off and I thought it was a multi phase line. What is it satish?

If it's just oil then yes 44hrs plus a bit for slippage.

But it is too slow so that might be 20% instead of the normal 2 to 5 %.

George - if the line has a lot of dirt in it, the pig does tend to generate a bit more DP, but much more than 1 bar and the cups deflect letting the fluid past so you don't tend to see a reduction in flowrate. If it has a lot of dirt it can just stop dead... Then you pump backwards, hence the bi directional pig - and try again.

But the flowrate in oil or oil and gas is far too low for a good piggin operation. You want 1 to 2.5 m/sec for a good smooth run

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

 

[1503-44]

Offshore oil/gas is sometimes separated for water extraction and measurement, then recombined for 2phase transport.

If they remained separated, there is no need to mention GOR.

IF THEY REMAIN SEPARATED, then oil goes in the oil line, gas in the gas line.

If we are talking about only an oil line, no gas, then travel time is 38 some hrs.

So, what exactly are we talking about here Sataish?



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

 

[sathishsusa]

Hey everyone, thanks for the feedback and additional information.

I need a precise calculation for future pigging operations as the rough estimation took a long time. The sea line has multiple sections with varying wall thickness, which affects the pig's speed in reaching the arrival area.


16" PIPELINE
OD = 406.4mm

WT =
Pipeline:
0 to 6 km - 15.7 mm
6 km to 32.6 km - 12.6 mm
32.6 km to 69.4 km - 9.5 mm
69.4 km to 70 km - 12.5 mm
Spool & Risers - 19mm
3D Bend - 25mm

ID =
Pipeline:
0 to 6 km - 375mm
6 km to 32.6 km - 381.2mm
32.6 km to 69.4 km - 387.4mm
69.4 km to 70 km - 381.4mm
Spool & Risers - 368.4mm
3D Bends - 356.4mm
5D Bends - 368.2m


GeorgeVerghese, mentioned that due to infrequent pigging, solid accumulations ahead of the pig can slow down its progress. The pipeline is for oil with minimal gas production and relies on natural flow from production wells as there are no discharge pumps.

I need help calculating the pigging time in an Excel sheet based on the provided data. Pigging operations will occur every 15 days. I also need assistance factoring in the Gas-Oil Ratio (GOR) and upstream and downstream pressure of the sea line with pipeline data.

Thanks!

 

[sathishsusa]

We flow the production of Oil & gas without being separated from the discharge wellhead towers and reach to 60 km destination area which will be separated by 3 phase separator. if not considered the GOR ? are you sure why should not consider it both a Crude oil mixture of oil & gas. Pressure role is important on the pigging ETA right so need to know which is the best suitable formula and calculation needs to use for ETA.

 

[LittleInch]

If you want a "precise calculation" then you need to model this line in something like OLGA or another transient simulation tool which will calculate the volume fraction in the pipeline at any one time and they normally allow for a pigging operation to generate this data. To do it well they need a complete breakdown of the components in the gas and liquid fractions. Plus this will tell you what the surge of oil is when pigging occurs. Trying to do this in a simple spread sheet way probably has an accuracy of about 50%.

Your numbers though are all over the place and you need to recognise the difference between stock tank and standard conditions and actual ones in the pipeline.

Where you measure the flow also makes a massive difference.

So you say 19,500 bbls and 2MMSCFD. Where are these measured?
If it's the output of the separation plant, it tells you very little about the condition in the pipeline. And 2MMSCFD for 19,500 bbls oil is a GOR of 102, not 353.

So you're on average somewhere around a volume fraction of about 0.7. That's quite high.
If you want less time, try lowering the arrival pressure and the gas will expand and sweep the pig and the liquid out faster - you porb need something like 5 to 8 bar arrival pressure to make it work a lot better. IMHO.

But I repeat. your flow rate in any event is too low for good pigging activities and your risk of a stuck pig or "lost" pig is much higher.
I suspect your line, if this is a subsea line to shore has a large liquid holdup and could be subject to large surges of liquid.

Why do you suddenly want to go to pigging every 15 days?

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

 

[sathishsusa]

Hi LittleInch,

I need a software to help me with the exact calculation of pigging data. I want to be able to check in an Excel sheet using some relative data and formulas. Accuracy isn't crucial, but I need to be able to provide my supervisor with an approximate timing of the pigging receiver end so we can mobilize the people at the end.

The Gas-Oil Ratio (GOR) is obtained from the Multi Phase Flow Meter (MPFM) which calculates the oil and gas, and the average GOR shows 353 SCF/BBL. The liquid is in units of BBL/day, and gas is in mmscf/std. Pigging occurs every 15 days, as instructed by the Integrity team due to the high volume of water staying in the pipeline. At the end, we receive more oil and very little water. In reality, there is more water in the production wells upstream of the pipeline.

It would be helpful if I could get at least 50% accuracy on the pig arrival to know the estimated time of arrival (ETA) for the pig. How to say before 44 hrs its almost matching of pig arrived time. can you send me on the excel sheet.

 

[LittleInch]

You never mentioned water!

So is the 19,500 bopd crude oil or total liquid?
I really don't know how a MFM works out GOR as this normally needs some idea of the composition, not just actual gas and oil and water volumes.

What's the average water flow?

There is no Excel sheet, just some basic flow calculations.
Trying to calculate this is a fools game given the huge variation in actual flow versus standard flow, the way the pipeline will take more pressure to flow as the pipeline rises from its low point with more and more water and oil in front of the pig and the very low pig speed.

If you're pigging to reduce water hold up in the pipeline then OK, but you really need to lower the arrival pressure to something like 5 or max 10 bar to get the gas flowing faster. Then you could get the transit time down to 10 to 12 hours or so. Aim for 5km/hr (walking speed).

Just send a pig off, try and monitor its progress down the line and once you've sent 10 down you'll have a much better idea of how long it takes....




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

 

[1503-44]

This is called a flowline.

25mm ... on the elbow.
That's the difference between 38 and 44 hrs.

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

 

[LittleInch]

Who cares what it's called, it's too big is the main point...

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

 

[sathishsusa]

This is my readings from flow meter shows upstream of pipeline:

Combined Flow reading MPFM:
Liquid flow rate - 7214 STB/D
Oil flow rate - 6480 STB/D
Water flow rate - 735 STB/D
Gas Flow rate - 2 mmscfd/D
Water cut - 10.2%
GOR - 354 SCFT/BBL

 

[georgeverghese]

If I make approx corrections for new ID info / new liquid flow of 7214bpd, travel time = 114hours, assuming little or no vapor breakout in the sealine
If this fluid leaves at bubble point conditions, and there is significant vapor breakout in the sealine, things get complicated.
You first said this is an MOL line, but now this line is operating without pumps - can you explain ? I may be getting obsolete - multiphase flow meters cannot be relied on for just about anything you need flow readouts for - there's 2mmscfd gas flow in this recombined line.

 

[LittleInch]

sathish,

First you need to be able to define your input data.

So far we've had a pipeline length of 50km (OP), 60 km and now 70 km (5th Aug 10.27) There's a 40% increase in time right there!

Flow was 19,500 bopd, now its 7200 total liquid with 6500 oil and 700 water. And we have 2MMscfd of gas as free gas.

Working on the basis that this is what is usually termed a flowline, i.e. transporting raw well fluids from a well to a processing plant and hence 3 phase flow (oil, water and gas).

Something not mentioned is the pipeline profile which is critical to getting an understanding of how this line works, where liquid accumulates, surges etc. For a sealine this is normally some sort of upwards slope to the shore? but you don't say. It will make a difference.

Anyway, in such lines, the gas layer normally flows faster than the liquid phase due to lower friction and lower density.

The issue when you stick a pig in is that changes the dynamics and prevent the gas from flowing over the top of the liquid phase. SO after a short while, the pig is going faster than the normal liquid velocity and sweeping all the liquid in front of it. So you key parameter to consider is actual gas velocity assuming it is at least 95% of the available volume in the pipe behind the pig. There is also an issue here with the very slow speed of the pig and as the pig moves down the line, the forces on the pig increase as liquid builds up in front of it. This results in the gas phase increasing in pressure until the pig starts moving again. But over 70km, this should more or less even out.

So very simplistically, the gas flowrate is 2MMscfd. I'm going to go metric as its easier for me. So that's 57,000sm3/day = 0.66sm3/sec. At an average of 30 bar that's 0.022m3/sec (actual volume). A 16" pipe at average wt of 15mm is 0.111 m3/m. so that's 0.2m/sec. Far too slow, but hey you only have what you have.

So 70,000m takes 97 hours. Add a bit / a lot of slippage and you're looking at anywhere from 95 to 110 hours for this run. I think.

But three phase flow is a complex beast hence why to be able to use the profile correctly and allow for phase changes, slopes, liquid hold up, gas flow etc etc, you really need to run this through a transient simulator.

If you want it to go faster, you really need to drop the arrival pressure to about 5 barg. All IMHO.


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

 

[1503-44]

Average Gas velocity is less than liquid velocity. Therefore I don't think that it can take longer than the time needed needed for the 20,200 BPD of pure liquid to arrive + slip. Assume no gas in the flowline, 70km x 16" x 15mm wt and that's 58h + slip.

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