The buoyancy of a pipeline depends upon the weight of the pipe, the weight of the volume of water displaced by the pipe, the weight of the liquid load carried by the pipe and the weight of the backfill. As a conservative analytical practice, consider the pipeline empty for two reasons; so the weight of the liquid will be considered as an additional safety factor and the possibility of the pipeline not being in use during a period of time.
If you would like, I did a calculation a while back based on a few common parameters around concrete coating pipe or using anchor blocks for bouyancy control. I pulled ALL the API 5L pipe grades and weights into the table and made it bulletproof.
It's the only time since college I used the quadratic equation. See, when you increase concrete coating thickness, you exponentially displace more water, and you MUST calculate for it. One of our vendors hadn't and... well, the pipe floated across a bay where it should have sunk. Then I got a hold of the raw data and "mathed" it. Told them they "forgot to carry the naught".
Please see the attached calculation. As a new member, I wasn't aware I could attach documents to the threads (Thanks for the email David!... Now it seems painfully obvious). The document is locked and formulas hidden to keep people from breaking them. To unlock, just unlock it... no password. (I truly believe in openly sharing information and knowledge (that's why I joined this group). May someone else benefit from this. Any questions or pointers, just ask.
Notes:
1) Each cell with a little red flag has comments I put to explain each cell & it's purpose (attempt to engineer proof it, and provide explanations for each process).
2) Don't forget to scroll.
The question of how and what to calculate is all about the 'swamp' condition and what you want to ensure. I imagine the main question is to secure the main pipeline staying buried/sunk at a certain depth and contrary not be overstretched.
Too light pipeline combined with too liquid swamp, the pipe will float up.(Empty pipeline)
Too heavy pipeline, combined with too liqiud and 'bottomless' swamp, and too long stretch of pipeline without support will give a stretch of the pipeline larger than the pipeline is constructed for. (Full, weighted and long)
Given theese 'worst cases' data's, it should not be too complicated to find a practical solution. The upper and lower weigt/buyancy allowance limits for the pipeline is probably fairly wide.
mjmeyers2, great spreadsheet and I agree on sharing information (instead of saying "hire an expert").
To the OP, I might suggest that swamp water is a mixture and not a solution, meaning that while the SG might be higher than unity, this higher SG will not contribute to the buoyancy of the pipe. The undesolved particles will simply move around the pipe letting it sink or float. Contrast this with seawater which is a solution and the SG does contribute to buoyancy.
I agree "swamp water" is basically water, but a lot of "swamps" actually have some level of shear and weight in the back fill which can add to the weight of the pipe.
How much is a difficult thing to judge, but should not be ignored.
Hence why "swamp" needs to be defined rather better than it is at the moment...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
"mjmyers2" Thanks for the spreadsheet is very interesting and directly applicable to my case, however the client asks me to deliver a buoyancy study and with those parameters indicate the calculation, the Pantano crossing is done with Pipesak, the swamp has sections dry and other longer sections are liquid, the question I have now is: What parameters or variables should I request in the buoyancy study to cover the calculations? Maybe mud weight of the swamp, how much mud and water mix in the swamp? mud density? I would greatly appreciate your comments on this.
You'd need a geotechnical survey. That calculation only considers water at 62.4 lbs per cubic foot. Once you have the correct figure, you can simply enter it in cell F6 (Specific weight of water). Now thinking about your question, I should labeled this Specific Weight of Medium, and clarified what Freshwater is, Saltwater, or the need for a geotechnical survey in the comments on that cell. You use the specific weight of the medium your in, and THEN add the bouyancy factor to be more conservative (Meaning industry standard is 1.2 for 20% MORE weighted than needs to be, so it will "sink").
The issue is not always one of buoyancy, i.e. stopping the pipeline from floating, but also whether there is sufficient bearing pressure in the "swamp" to prevent the pipe from sinking and resulting in the pipe essentially spanning and having additional axial stress between two hard points.
If your pipe is carrying liquid this gets even worse because the worst case for floating is when it's empty or just laid so you need additional weight, but then you fill it full of liquid and increase the weight to the point where it starts to slowly sink.
Hence why you need a good and comprehensive geotechnical investigation to see how deep this swamp is and what the ground bearing pressure is.
Swamps and similar very poor ground are not good place for pipelines and a simple buoyancy calculation is but the start of the issue.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
