buoyancy control for pipelines 5

[jharv]

I have been tasked with writing an engineering standard at my company that will dictate spacing requirements for set on pipeline weights. The pipeline weights I am writing the standard for are geotextile bags that can be filled with aggregate or sand.

Does anyone know if there is a good rule of thumb on how much negative buoyancy should be produced to overcome the buoyancy of a pipeline? In other words how much greater should the negative buoyancy of the geotextile bags be than that of the pipe. 20% 50% 100% more than the buoyancy of the pipeline??? Any thoughts?

 

[SNORGY]

We tend to settle out at 10% negative buoyancy for most cases / clients.

I strongly recommend purchasing the following book:

Pipeline Design And Construction - A Practical Approach - Third Edition (M. Mohitpour / H. Golshan / A. Murray)

The treatment of buoyancy control in Chapter 7 is quite good.

Irrespective of the fluid conveyed within the pipeline, it is my opinion that buoyancy control measures should be implemented assuming empty pipe in any areas where flotation is a risk. Nothing more irritating than trying to re-license an oil line for, say, gas at the same pressure and temperature and then, that spring, having 200 metres of it float up and pop out of the ground.

We (well, *I*, actually) developed calculations and a spreadsheet that establishes spacing based on whichever of three conditions govern:

(1) calculated bending stress as some percentage of allowable (thereby as some percentage of SMYS)
(2) calculated upwards deflection at mid span as some percentage of design burial depth
(3) calculated minimum number of weights to achieve target negative buoyancy

The numbers I currently suggest are:

(1) limit bending stress to 20% - 30% of SMYS
(2) limit upwards deflection to 10% of burial depth
(3) target 10% negative buoyancy of system

The "system" includes the mass and the contribution to buoyancy made by the weights themselves.

Bear in mind that I am a generalist who dabbles in a bit of everything; there are other folks on this site (BigInch / zdas04 come to mind) who can probably give you guidelines based on superior or at least more specialized expertise / experience.


Regards,

SNORGY.

 

[zdas04]

SNORGY,
I wasn't going to comment until I saw your last line, I think you covered the topic very well.

I've had a weighted pipeline float and it is way less fun than it sounds like. I went back to design documents and there were zero calculations, just a "feeling" that 3 bolt-on concrete river weights should be enough for a line running 1/2 mile in an ephemeral wash. I did the calcs (using a 25% negative bouyancy, I didn't want to ever deal with this again) and added 15 weights to their 3.

The advice to do the calcs with an empty pipe is excellent. I do the bending calcs too, but find that once I get enough weight on the line the bending stress has always worked out to insignificant.

David

 

[MTPipeliner]

For what it's worth, our company spec is 40% negative bouyancy. No idea where it came from, but that's what it is.

 

[SNORGY]

Thanks zdas04. From past posts on this and other threads, I figured you had lots of experience and insight. Forgive my name-dropping, though.

Regards,

SNORGY.

 

[jharv]

All, I appreciate the responses. It sounds like there is not a good rule of thumb for the negative buoyancy. I had in my mind to use 10-15%. I guess it comes down to ones best judgement.

Have your experiences been with installing the buoyancy control devices on pipeline in rivers/lakes or has it been in wetland areas?

 

[SNORGY]

Mine have been in wetland areas muskeg and swamps, and the calculations and design are, in fact, contingent upon a normal condition of having the pipeline at rest on a non-settling substrate at the bottom of the trench.

River crossings are completely different if the intent is for the pipeline to hang like a catenary across the body of water versus just sit on the bed, and the effects of current need to be accounted for, of course. Buoyancy control for those situations are discussed to some extent in the reference I cited earlier.

We typically directionally drill river crossings so you basically pull the pipe under the river bed through the "tunnel", so buoyancy considerations typically don't come into play.

Regards,

SNORGY.

 

[zdas04]

SNORGY,
The local Corps of Engineers office will not allow a pipeline to lie on the bottom of a river subject to currents. I don't know if other offices or regulators would allow it or not. I'm not sure I'd do that even if I could (it looks like a time bomb to me). Every river crossing I've ever designed has been a dam-and-cut with a bunch of river weights or a pipe bridge.


PatrickR,
I've seen other companies that wanted 50% or even 100% over weight. I think the bottom line is how much the authors of the company standards trust that we'll both do our sums and do them correctly. The more faith they have in Engineering, the smaller the number. Sounds like SNORGY's company trusts Engineers, mine just barely trusted them, and yours doesn't quite trust them.

David

 

[SNORGY]

zdas04:

Come to think of it, I don't think it would fly with our Regulators, either.

Growing up as a boy on Vancouver Island, I do know that fishing trawlers used to snag all kinds of utilities that were "sunk" across various bodies of salt water in Georgia and Juan de Fuca Straits. Most often they were cables, in retrospect.

I have seen some "Frontiers Of Construction" documentaries about how folks elsewhere in the world float pipelines into position using tugboats and then sink them once they are positioned and welded up. Presumably, they then sit on the seabed, undisturbed by fishing boats and oceanic / tidal currents.

I imagine that for a river, as you state, the only things that would be allowed would be an open-cut crossing, a directional drill or a pipe bridge.



Regards,

SNORGY.

 

[zdas04]

In the ocean there are a bunch of different rules (kind of hard to bore under the ocean I would expect). There are tens of thousands of miles of pipeline from platforms to onshore terminals. I've never worked in that field and it seems really mysterious to me how they do it, but not mysterous enough for me to have ever tried to work in that environment.

David

 

[pmover]

something else worthy of discussion ? . . .

any potential use for having the pipe coated with concrete? i recall 8 river crossings using concrete coated pipe (6-inch, 8-inch, 10-inch, 12-inch, 24-inch, and 30-inch) the entire river crossing length. from memory, <12-inch had 2-inches of concrete coating & the 24 & 30-inch had 6-inches (?). the pipe was buried will below the scour depth of the river. seems that if the concrete coated pipe is in place and if the scour depth ever reached (for ? disasters), then any flow would flow the contour of the pipe (not sure if + or - to pipe integrity). i think all we really determined was minimum bend radius of pipe with the conrete coating applied to ensure the concrete coating was not damaged. btw, the pipe was FBE coated before the application of concrete. sure makes for transportation and handling issues to address, but in the cases i'm aware of, the coating was applied at the work site using a portable concrete coating plant.

just a thought . . .

-pmover

 

[SNORGY]

I know the current trend is to move away from discrete concrete weights due to concerns over the environmental impact. Hence the rise in popularity of the Pipe Sak style weights referred to by jharv in the original post. Perhaps it's due to corrosion of the reinforcing steel that some such weights might use? I am not sure. In any case, provided it is environmentally harmless, continuous concrete coating is indeed a viable option. Probably a preferred one. It does create the issues with field handling that you describe.

Regards,

SNORGY.

 

[jharv]

My contractor has been using concrete coated pipe for 20-inch and 30-inch pipeline extensions this year. However concrete coated pipe proved to be problematic because the pipe had to be coated at the project contractor yards and then transported to the project right of way to be installed. They had to use larger construction equipment to handle the concrete coated pipe because the extra weight.

The use of the geotextile bags should eleviate the weight issue.

 

[zdas04]

I looked at concrete coated pipe for a 12-inch job once, but cost was 5 times the cost of bolt on weights and I couldn't make the cost/benefit work. The bolt on weights do have corrosion issues in a location that stays wet. I've always used them on "dry" washes that have the pipe in water a couple of weeks a year. The corrosion risk seemed manageable.

David

 

[BigInch]

Your SG target; is that with the line empty or full?

I would not use a mat for buoyancy control unless there is NO possibility for current washout. Make that "absolutely" no possibility. At first washout, your pipeline will be on its way to somewhere. I would put some kind of positive buoyancy control directly on the pipeline underneath that geotextile-rock bag. Rocks and mats are only for STABILIZATION of the bottom material designed to keep the currents from ever reaching your pipeline, or for mechanical protection to prevent ice from scraping your pipeline first, etc. Buoyancy control is another thing entirely, such as how will your pipeline stay underwater when the mat or bags are washed out. I wouldn't even hazard a guess as to how to calculate buoyancy of a pipeline with a mat on top or geotextile bags sitting on it. Geotextile IMO could be used to contain rocks for stability control only.

For buoyancy control, my rule and every other pipeline company I've worked for or with (and yes that is a LOT) is 20% negative buoyancy with no contents. To determine maximum span between concentrated weights, you must check pipe stress for the effect from upward distributed buoyancy loads between concentrated weights that adds bending stresses. Maximum spacing thus depends on your MAOP permitted under permanent environmental (buoyant) load, maximum installation stress residuals, hence also on operating pressure, temperature, pipe diameter, wall thickness and minimum specified material stress. Maximum spacing may even be limited in order to prevent upheaval buckling. It won't be convenient to make a standard addressing all possible combinations of loads, wall thicknesses, material stresses, etc, so just stick to defining a method of calculation to let you get your required MAOP.

Although it wasn't part of the question, in the discussion above, everybody forgot to mention screw anchors. IMO a much better alternative to weights, when current, soil conditions and installation method available makes sense.

Concrete coating makes sense when you have long lines and there is a coating plant nearby. Set-ons and bolt-ons don't work for offshore construction where the pipe must pass down a stinger. Tows or pulls may work, but the water depth must be very shallow to enable set-ons without expensive diver work for placement and subsequent inspections.

Never permanently suspend a pipeline in current unless you like catching boats, logs, rocks, ice chunks, or just having one of those 500 year floods that comes this year flood current rip out your pipeline directly. The first thing that happens when pipelines become exposed in large rivers is that they break.

A quote from Shell Oil spec.

3.7 STABILITY
All submerged pipelines, i.e. offshore pipelines and sections of onshore pipelines in
swamps, floodable areas, high water table areas, river crossings, etc., should be stable
under the combined action of hydrostatic and hydrodynamic forces. The on-bottom stability
can be achieved by increasing the pipe wall thickness, by the application of concrete weight
coating, by spaced anchor points, by trenching, or by burial.
Special considerations shall be given to pipelines installed in weak soils (e.g. peat), at dyke
crossings, etc. where differential settlements may lead to pipeline loss of integrity.
The one year return wave and steady state current conditions should be used for the
analysis of stability during the installation phase. The one hundred year environmental
return conditions should be used for the analysis during the operation phase. The negative
buoyancy should be sufficient to prevent unacceptable lateral pipeline displacements.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rconner]

I think there are many possible reasons various designers might want substantial safety factors vs. movement and/or damage, by significant burial and/or weighting of some pipelines etc. I think the most obvious is the fact that there have (as indicated in responses to this list) actually been installed or working pipelines that have been undermined, moved, come up out of the bottom, or even broken and in some cases maybe not long after the Owner pays the Piper for installation of such lines (an obvious admittedly knee-jerk, though maybe understandable response under the circumstances is to “add more weight” to the next one!) I believe there have probably also over the years been some occasional glitches in determination of the amount etc. of weight to add (that probably can be most significant with the most lightweight pipes). These have perhaps included:

1.Neglecting to consider that the weight volume added is itself counteracted by its own buoyancy once it is submerged. [E.g. a reinforced concrete with density say 145 # /ft3 (pcf) in air would only provide roughly 145-62 =~82 pcf, or only 57% of an assumed 145 pcf surcharge weighting capability to a submerged pipeline in fresh water (and even slightly less if the fluid is heavier including common saltwater, another slight error sometimes made!).
2.It is probably also reasonable to assume the weights might not be perfectly installed by the Contractor and/or, unless there are some quite fancy Engineering or contractor controls to prevent same, they might even slip axially at least some along a long, slick pipe surface. The net result of both would be of course installed axial spacing, effectiveness, and/or bending stress effects etc. on pipe barrel some different than what is assumed in the calculations.
3.Also, if the bottom soil in which the pipeline eventually resides is ever fluidized by whatever actions (or might essentially act as a act per Archimedes as a surrounding "liquid" somewhat heavier than water, with the assumed situation regarding bulk density of the pipe and/or contents at the same time), it might be argued that there is more buoyancy to counteract than if pipeline if the initial assumption was surrounded only in lighter pure water.
4.Lastly, and as maybe indicated already by others’ responses, in some cases it is not just upwards movement that may be of concern to pipelines, and extra weight e.g. as well as the profile and even shape of those weights may e.g. have bearing on whether or not the line ends up moving laterally in response to water current, external snag-dragging, or even sort of flowing, lateral soft soil movements. I have even seen much denser substances than concrete (e.g. cast iron) used in the past on probably rare occasion for bolt-on pipeline weights in very strong currents, I guess as they theoretically represent a lower profile to airfoil type drag in current actions etc.

I am not going to presume to know exactly how all of these factors or concerns should necessrily be addressed, as I don’t have any direct experience in this regard; however, I suspect (in the absence of some very good Engineering and construction inspection/control) a little extra weight and hell for stout pipe for at least critical crossings, may have in the long run knowingly or unknowingly helped to compensate for some of these variables, unknowns, or glitches.

 

[rneill]

I'm a generalist in this area but the guidelines I've applied for onshore pipelines are:

5% negative bouyancy in muskeg
10% negative bouyancy for rivers

Concrete coating prefered but bolt on weights acceptable with a maximum spacing of 4 m between weights to minimize shear forces.

There was not a bad paper on weights for muskeg areas published by Nova Corp a long time ago which provides a good overview of the calculation procedure. Info on this paper is:

http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6049034

 

[jharv]

rneill,

The file on the website is a enw. Is there a special program I need to open that kind of file.