Piggability of a 16" pipeline on elevated supports

[brpillai]

I am working on a pipeline project where the off shore pipeline ( DNV F 101) is reaching shore and further goes to the on shore with a pig reciever 1500 m running on elevated supports. The onshore pipeline section consists of bends for expansion loops, turnings & road crossings.

The offshore portion of the pipeline will be done by another contractor, while the on-shore will be done my company . Company isists of doing the pigging on this pipeline together by the offshore contractor from the platform.

The key pipeline parameters are as follows and also teh constraints

Design Code - ASME B31.3
Length of on shore pipeline 1500 m
Length of offshore pipeline 50 kms.
Pipeline OD - 16” (406.4mm),
Wall Thickness - 25.4mm
Material - API 5L Gr.X-60 (seamless)
Service - Crude Oil

Constraints
Number of bends in onshore pipeline section - 3D Hot bends about 90 nos due to space constarints.
Few of the bends do not have minimum tangent lengths of 500mm and at few places there is space for the staright pipe piece between bends.
Shore pipeline is on elevated supports as stated above.
wall thickness of mther pipes for hot bends varies from 12.7 mm (offshore) to 25.4 mm and 30.2 mm for hot bends.
My queries to teh pipeline experts are below:
1. Whether is it possible for doing pigging with so many bends, considering pig getting stuck up due to many bends with out even the tangent lengths and staright pipe in between bends at few places.

2. Could the pigging issue solved if ID based pipe (matching the ID for all thickness)is selected.

4. Is it adviceable to do pigging of the pipeline on elevated supports, considering the high pigging forces at bends and pipeline falling off the supports?

3. Also would like to know how to calculate the pigging forces, specifically at the bends, due to movement of pigs that is required to be considered in the design of elevated supports, this being a mostly above ground pipeline section.

 

[bimr]

There are no significant forces created from pigging operations at the velocities used in liquids applications. Pigs normally travel at 10 ft/sec or less and smart pigs run at about half that.

Elbows and pipe bends installed in the pipeline should have a minimum radius of three times the main-line pipe diameter—3D bends. Intelligent pigs may require greater radius to diameter elbows and bends because of the longer length of the pigs.

When operating at their minimum bend radius, smart pigs may require a distance of straight pipe equivalent to the tool length between adjacent pipeline bends. A rule of thumb is that there should be a length of straight pipe between bends of between 3D and 5D.

Contact TDW:

http://www.tdwilliamson.com/Pages/Pigging-Products.aspx

 

[zdas04]

First you have to ask yourself "why am I pigging". There are several possible answers to this and they all lead to different answers to your basic questions:
[ul]
[li]Descaling[/li]
[li]Evaluation[/li]
[li]Product Separation[/li]
[/ul]
Descaling would most likely be done with brush pigs (either turbo or poly). Some people still use mandrel pigs for descaling, but that is becoming increasingly rare (the pigs are too heavy, too expensive, to delicate, and not really as effective as a turbo pig). Mandrel pigs would have a major problem with your 3D bends and the weight of the pigs would make any force distribution issues worse. Let's assume you aren't using them. Turbo brush pigs and poly brush pigs weigh about the same and either can negotiate fittings and 3D hot bends without problems.

Evaluation (i.e. "Smart") pig have been getting progressively more effective at traversing fittings and bends, but they still do a crap job of dealing with major ID changes (and 0.5 inch to 1.0 inch is pretty major). I would be a lot more concerned about the ID change than the bends.

Product separation pigs are rarely run from platforms to onshore, but when they are they are typically spheres and those can traverse any bend.

Doing an API 14E calculation (assuming 0.8 SG crude) gives me a maximum velocity of 14.15 ft/s (4.3 m/s) so your maximum pipeline capacity is around 235,000 bbl/day (37,000 m³/day). Running a brush pig at those velocities is a pretty low force distribution since the assumption is that the line is full and you don't have to deal with slugging. If you have a lot of gas in the line, then the velocities will be significantly higher and less constant (i.e., the liquid load in front of the pig will be preceded by a gas slug, that transition is very high force distribution).

To me the big question that I would want answered way before any of your list of questions is "how stable is the crude oil stream?". If it is dead oil, then this is a cake walk. If it is nearly raw wellhead crude that will exhibit phase changes with cooling and gas evolving out of solution with pressure drop then pigging would be my last concern, you would need to be doing slugging calcs not pigging calcs.

For an elevated gas line, I get really concerned about filling the elevated pipe with liquid and collapsing the supports, but for an oil line it is always assumed to be full of liquid and if the supports are not adequate for a 0.5 SG pig interspersed in the oil then something major is wrong. Same with lateral stresses, if the line is liquid full then the pigs add approximately zero lateral stress, if the line is not running full then be more concerned about slugs than pigs.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

br pillai -

for curiosity I have to ask a few questions.

Why is this being done in B 31.3. This is not a Pipeline code and as such often causes more problems than it solves. This could easily explain why your wall thickness doubles, which at 16" is not normally a good idea for pigging. B 31.4 is a much more suitable code. You often find plant designers just don't think this through properly and insist on 31.3 even when it is the wrong choice, which is what it seems to be here.

Address the key design issue first then address the rest later.

Why is this above ground? It is after all a pipeline which is much happier when it is buried.

Bends back to back is not a great idea, but can be done so long as it is in the same plane.

You can model pig forces as a simple momentum issue, i.e. a mass of X, minus the mass of the fluid in the same volume, travelling at X m/s then turning 90 degrees will require a force to do so with a vector at about 45 degrees. Can seem quite big, but to a pipe, it takes a few hundred tonnes to start to become a problem. If your pipe is restrained within guides then you shouldn't have an issue, but simple supports it might jump a little.

Dave, I'm disappointed in you using API 14E to work out a max velocity - that figure is virtually worthless as any sort of guide, however I suppose the point is that the max velocity for a crude line over that sort of distance is more likely < 3 m/ sec to be economic and at that sort of velocity you're not going to get a problem.

So just to close the loop to answer your q's
1) Yes, but the more you have in close succession the more chance of getting something stuck
2) It could, but 1500m of special pipe is very expensive. consider using thinner wall thickness by using a pipeline design code, not a piping code. The number of 3D bend sis till an issue
4) You can pig pipelines on supports, but it is advisable to fit guides and vertical restraints to prevent too much movement
3) See above - momentum change of a moving mass is the best idea I've ever used then plug it into a piping stress model

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

 

[zdas04]

LittleInch,
I use API 14E to define the upper design limit for crude oil lines. It always works out as significantly higher than any design max velocity that anyone would actually use.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

I don't believe in design max velocity limits worked out by equation or diktat, but each to his own. It doesn't take away from the points made so lets agree to disagree....

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

 

[zdas04]

I don't like them either ("believe" is a different concept and I'm not going there), but every one of my clients has design maximum velocities for gas and liquid pipelines, and all of them are as arbitrary as anything could be. I've learned to live with it.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[brpillai]

First of all I would like to tank all for the prompt reply.

Reply to some of the points raised above are as below:
1. The fluid is stabilised crure coming from the paltform after seperation of gas. It is single phase flow of oil and bout 10% water, this eliminates the fear of fillig the elevated sections and damaging the pipe/supports.specific garvity of the fluid is 0.8, flow rate 6oo cu m /hr.

2. I agree that dealing with major ID changes (and 0.5 inch to 1.0 inch is pretty major)is an issue, but not a major issue, as most of the pipelines have number of sections having diffrent thicknesses, at station approach , road crossings, populated areas, HDD ;locations etc. And these pipelines are also pigged successfully. I have built and operated ( including pigging)few of major pipelines as long as 1500 kms.
To Litlle inch,
3. I agree with you that the right design code is 31.3 and the pipeline should not be on supports. But teh client's FEED is based on this concept and are adamant not to change. I also told tehm that the pig reciever could be moved near the shore point and the inspection of this small 1500 m section not necessary to be pigged as alternate methods for in plant piping is possible. They do not want to change.
4. Thanks for comments that the number of 3D bends is an issue. and proper restaints are required for the supports to conatin the pigging forces.
5. I am still not fully convinced that the ID based pipe( even it is expensive and difficult to get within schedule) will allow the pig to pass through with all the bend configurations.

Thanks to all once again.

 

[LittleInch]

Ah yes, the joys of a client that won't listen to sense. Just make sure you bring it up at all relevant locations like the HAZID, the HAZOP and any other suitable location to cover yourself when the inevitable happens.

I assume you meant 31.4 in your reply above.

Without seeing the bend configuration it is difficult to say whether there are too many bends or not. Could you sketch the worst case set of bends and then we might be able to give some better guidance.

Getting constant ID pipe would definetly make things better.Yes, you get changes, but when it is such a high percentage (many long distance will change D but only by a few mm in a large number. 25mm decrease in ID in 380 (16" ID at 13mm wt) is a much bigger reduction than 25mm in 1000. Also 1500m with multiple bends is not the same as 50m of a straight road crossing.

The only other thing to try on them is whether you can make the bends 5D??

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

 

[brpillai]

little Inch
Thanks for prompt reply. I meant 31.4 as you ponyed out.
Bend details are as below:
Total Number of 3D bends in 1600 m of pipe - 84 Nos.

Straight pipe between two bends 5M to 10M length - 6 Nos.

Straight pipe between two bends 3M to 5M length - 7 Nos.

Straight pipe between two bends up to 3M length - 10 Nos.

Without Straight pipe between two bends (Tangent to Tangent) - 12 Nos.


Without Straight pipe & Tangent length between two bends - 3 Nos.

Deatils of bends are attached. Request your further review and comments.

As rightly pointed out, HAZID and HAZOPs are the right forums to take up this further.

Thanks

 

[LittleInch]

That's a lot of bends in a relatively short distance. Most of those are probably OK, but the key ones are those 3 bend to bends without straight sections. Keeping them in the same plane will be key.

This makes the bend pull through test vital to perform to make sure you're getting a round pipe and not a squashed oval pipe.

A single flexible scraper type pig will probably make it without issues, but a multi pig Intelligent pig??? Would be a fairly brave person to say it was low risk IMHO.

All you can probably do is get a lot of people present with either a magnetic detector as the pig goes past or simply that they hear / feel it. At least then you'll know where to cut it out.

The fact this is happening at the end of the run will make it worse as the cps could be damaged, worn a little or the pig sitting nose or tail down if not balanced properly.

Good luck and be sure to come back and tell us if you managed to get any changes or it worked in the end.

LI

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

 

[zdas04]

It is a shame you couldn't have started your road bore outside the cable duct so you didn't have to go over it.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[brpillai]

My plan of action is as below:
1.trying to convince the company to agree for shifting the pig receiver before all these bends.
2. advocating to for improving the route.
3. trying to improve the bend configuration
4. Trying to change the design code to 31.4 so that the thickness could be reduced.
5. Talking to as many Intelligent pig vendors and to take their opinion and speak to company with that.
6. Trying to convince company that on shore portion of the pipeline could be inspected by methods other than pigging.

I will keep the forum informed on the out come.

Thanks to every body.

 

[rconner]

s others have aptly noted there can be a good bit involved in "pigging", per practicioners depending on many factors/variables e.g.

http://www.slideshare.net/TD_Williamson/tdw-guide-to-pigging

(and perhaps some aspects are non-obvious?)
I think also while some "pigs" are quite light, others for other purposes (and/or maybe also stuff carried along with same) may now also have pretty good heft (or weight), and perhaps greater than product bulk density/weights etc some aboveground pipelines or supports of same have been designed to carry - I guess may not hurt to also check any stress and/or deformation effects therefrom. Have a good weekend.