semi submerged pipeline 1

[Yahyaoui Amin]

Im working on a 4 km submerged 12 inch carbon steel X60 pipeline that will drive CH4 gas from the offshore unit to the onshore unit.(as shown in the attached pictures)

The problem is how to size marine accessories such as buoys, cables and anchor blocks so that the pipeline is stable at a depth of 20 m. moreover how to calculate the number of buoys and the spacing between them?

Thank you very much.

 

[SandCounter]

Sounds like a simple free-body diagram. Maybe start with finding out what kind of buoys are available and then calculate the span they can support. Divide your full length by that span to determine the quantity of buoys. Check that your span does not exceed the the pipe’s structural limit. I would use a beam calc for this.

Your buoy’s buoyancy should be greater than the sum of cables + the weight of the length of pipe but don’t forget to subtract the buoyancy of the gas-filled pipe from the pipe weight. The anchors should overcome the buoyancy of the entire system. Your cable lengths should be appropriate for the distances between the pipe and the buoy (20 m?) and the pipe and the sea floor. For stability I might suggest at least two anchors with cables at each attachement point making a triangle between the anchors and the pipe when looking down the axis for reduce transverse drift.



I used to count sand. Now I don't count at all.

 

[LittleInch]

Yahyaoui,

what you are proposing is something very strange, that's why there are no established "rules" or procedures for this.

A 12" pipe suspended 20m below the sea surface will be subject to many different and extensive forces, both static and variable leading to stress and fatigue issues that mean there is a reason why you don't see this in practice.

The pipeline will never be stable in this type of design and will move sideways due to current and wind and vertically due to wave action or have vertical forces. The level of redundancy so that one or two or three floats can fail and not break the pipe? a 12" pipe will simply break either due to direct stresses or fatigue as this long piece of string moves about in a randon snake manner.

I don't understand why the more conventional approach where the pipe is laid on the sea bed and you then have a riser up to your platform won't work for you.

Nice idea, but once you start to get into any sort of details, doesn't work, at least economically.




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