Flowlines vs Pipelines 2

[Yves22]

Hi,

I need to evaluate the risks of offshore damage/losses from flowline installation, at depths say 80m (Carnarvon Basin), compared to pipeline installation. How are they different in terms of installation, material, behaviour etc? Does flowlines face lesser risk of dry/wet buckling?

Grateful for your response. Pretty clueless.

Thanks.

 

[SJones]

Flowlines are pipelines - they just have a specific service application. The design code is exactly the same: you won't find a separate definition of flowline in DNV-OS-F101.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Yves22]

Perhaps a basic question. Am looking at the planned installation of 3" to 10" flowlines in 130-160m depth of water.

Is the D/t ratio dependent on whether it is an S-Lay or J-Lay?
I've read that a J-lay is generally for deepwater installation, however is it relative to pipe diameter as well?
Is a larger diameter flowline more robust against wet buckling?

Thanks.

 

[BigInch]

S or J lay is dependent on the tension that can be applied to the pipeline, which requires more and more tension as both water depth and weight of pipe increase. Maintaining the configuration of an unbuckled catanery S lay in deep water requires considerably more tension than does suspending the pipe vertically downward to make a simple bend at the bottom.

For those smaller diameters, you could do either.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

You would probably reel lay at those diameters and water depths if the spoolbase infrastructure is conveniently located.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

But that's really just a variation on an S-lay. Once it's off the stinger, its an S-shape.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

A very bendy variation!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

Back to the flowline/pipeline issue. Although there is essentially no physical difference in function, there is a distinction. The ASME pipeline design codes B31.4 or B31.8 do not cover flowlines, so it's a rather important distinction.

Flowlines can face a greater risk of buckling, since pressures can be much higher and temperatures much greater, as they often carry fluids directly from well stream conditions. Higher temperatures and pressures increase compressive buckling loads in restrained pipe segments, which is the main reason that "snake laying" flowlines has become a popular method of averting uncontrolled buckling.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

Since the Carnarvon Basin is offshore Australia, the OP is more or less bound to use DNV-OS-F101 which does not draw such a distinction. Very correctly, a flowline is a pipeline is a pipeline!! The designer makes the call about the buckling and other issues - all addressed by DNV.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

You're of course correct. Now I can ask, why is the OP trying to distinguish between them.

In this case, the hotter, higher pressure pipelines will tend to buckle before the lower pressure and lower temperature lines.

From "BigInch's Extremely simple theory of everything."

 

[BigInch]

So then that means the hotter, higher pressure pipelines will probably tend to buckle before the lower pressure or lower temperature pipelines.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

I would say the OP is trying to draw the distinction because their user name contains '(Structural)' and not '(Pipelines)'.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

You are probably again correct. Also contains the word "clueless" at that. OK, hopefully he's fixed up now.

From "BigInch's Extremely simple theory of everything."

 

[Yves22]

Thanks, guys. I used to be involved in offshore structure design, not pipelines, as Steve has pointed out. Am now involved in risk management and that gives me a broader picture, though it makes me feel uncomfortable at times.

I am looking at loss numbers from offshore pipe installation claims in general (but work scope mainly flowline-laying in the Carnarvon Basin), and have decided rather simplistically on 3 variables: 1) diameter, 2) depth water, 3)terrain. Regarding (3), I am assuming that the sea-bed would have been prepared with the allowable free-span etc. Assume pipe material would have been selected to enable specific service function e.g high temperature. Hence, to the designer, his/her concerns would be structural behaviour only. It then takes us to (1) & (2). Unless I am missing some special techniques during S or J-lays, I would expect higher losses resulting from small diametered pipes (flowlines) because it is generally slender, more hold-downs on the seabed(??) making it problematic. I mean a 3" is like a rope in these depths? Is it a hose of some sort?

Have I confused myself? Thanks.

 

[BigInch]

Buckling load is primarily dependent on pressure, temperature and x-sectional area of steel, not necessarily on diameter, although that does usually mean more steel, it doesn't have to all the time. Rougher terrain will also tend to increase the buckly frequency, due to the installation deviation from a straight line, but that drops out if the seabed is smoothed. Buckle resistance is primarily a function of friction between pipe and soil, if pipe is placed on the mud surface. If pipe is in a trench, or rock dump is placed on top, you may be able to count on friction/cohesion of soil to resist axial load and the weight of any soil or rock, etc. placed on top to provide an anti-buckling stabilization force.

It would be hard to draw general conclusions based on diameter; larger lines have more steel area (axial load), but higher radius of gyration to resist buckling and they also tend to grow axially rather than buckle, since gain in load with diameter is not resisted by the same ratio of a gain in soil friction. Smaller pipelines would not develop so much axial load, since x-sectional area is usually smaller, so can become fixed by soil friction, stopping axial growth, increasing compression load and setting up a buckling response. Water depth doesn't have much to do with the buckling problem. I think a function of operating temperature and pressure might go the farthest. Next correlate to diameter.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

BI,

A great discourse on operational issues, but does all that apply when the pipe is hanging off the barge? Yves says 'pipe installation claims'. It's not clear who is claiming from whom, and for what specifically, but I got the impression that it's damage before completion and handover.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

Thans Steve for pointing that out. You're right. Those concerns I discussed occur after installation. The question framed within the context of "Difference between flowlines and pipelines" has perhaps throughly skewed my perspective. Preparation of the seabed, terrain and spanning would also have little to do with buckling during installation.

During installation the concerns become more about contractor technical abilities and matching the equipment capabilities in lift to that of the weight of pipeline plus furnishing the proper amount of tension required to hold a smooth unbuckled pipeline profile from the end of the stinger over the distance to touchdown point on the bottom. Installation factors obviously are affected by diameter and depth, as much as those affect weight and span to touchdown, additionally by D/t ratio as that affects collapse pressure from water depth pressure, but all those factors are still easily managed in depths up to 80 m, at least as long as the equipment is right for the job and the contractor does not push the limits of operating margins during storms and high currents. I'd try correlating installation buckles to Pipe diameter: wall thickness ratio, D/t, and water depth H. 80 meters is a rough depth borderline for crushing collapses. Not usually a problem for lesser depths, but can quickly become a problem for higher D/t ratio pipe at 100 meters and more.

Being that 80 m shouldn't really be much of a technical problem in laying properly designed steel pipelines from a conventional S-lay barge, it leaves me wondering how many installation buckles have been experienced in that Carnarvon field to warrent a special evaluation... and why they occured? If D/t ratios are not at extreme values, pointing at the engineer, it might finger the contractor(s) as the problem.

From "BigInch's Extremely simple theory of everything."

 

[Yves22]

Thanks for your responses. As a rule of thumb, what sort of D/t would we be looking at for depths >100m? Say for depths 120-140m.

Because I am no longer in engineering, I do not have design standards for reference.

 

[BigInch]

Around 60 to 70 for 150m depth

From "BigInch's Extremely simple theory of everything."

 

[SJones]

DNV-OS-F101 is available at

https://exchange.dnv.com/Publishing/Codes/download.asp?url=2010-10/os-f101.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04