Subsea pipeline - Pipe bedding and backfilling material

[EireChch]

Hi all,

We are undertaking a preliminary assessment of a subsea pipeline located in 6m depth of water. The pipe will be trenched into very weak rock.

I am looking for some references/codes to specify the pipe bedding and backfilling material for subsea pipes.

Client hasnt confirmed what the pipe material will be yet.

Thanks
Eire

 

[LittleInch]

If it's rock then just make sure it's concrete coated pipe....

You can prepare the bed, but it's much more difficult than onshore so mostly they don't bother unless it results in long spans.
Generally you excavate as best you can, survey it and remove any high points or fill in any significant low points.

Then if you want to back fill it rock dump it with 20-50mm stone initially then some larger stuff.

If this is in the sea, then the wave action will have a big impact on any dredged material dumps.

Generally they go down to the 5m below low water to trench it and back fill.

Some information here

http://atteris.com.au/wp-content/uploads/2014/11/Pipeline-Shore-Approach-Design-Case-Study.pdf

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

 

[chicopee]

For reference on underwater pipelines is "Tubular Steel Structures- theory and design" by Troitsky and supported by The James F. Lincoln Arc Welding Foundation. My 1990 second edition has a copyright of April 1982. Chapter 11 on Underwater Pipeline appears weak on the subject of bedding and backfilling in trenches under water. By contrast chapter 10 deals with underground pipeline and details trench loading which could be of help. It is possible that newer editions were published to further your research.

 

[EireChch]

Thanks Little Inch and Chicopee. I have a fair idea of what the bedding and backfilling should be. Similar to what you have indicated. Some smaller graded stuff first then increasing to larger diameter for protecting. I just need to find a standard that recommends it.

I have looked through many Marine Geotechnical book, subsea pipline books, Bureau of Shipping etc. Everything just discusses gravel dumping etc. Nothing on aggregate size. The pipe is 2.6m in diameter so i was hoping to find some recommendations for aggregate size which is dependent on pipe size.

Chic - I cant find that book on line (for free..) but thanks though. If you find anything on aggregate grading etc let me know and i may buy it.

Thanks for your help.

 

[LittleInch]

I actually don't think there is a"standard" per se, just custom and practice.

The aggregate size is related more to what you want it to do rather than pipe size.

Smaller size (20 -50mm) is used so that it fills in all the gaps around the pipe. Larger stuff then acts as protection and also stops the smaller bits getting swept away by wind and waves. Use 50 -100m then finally 100 -250 as your final armour layer

There's no real science about it....



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

 

[cvg]

suggest that you need to consider flow velocity (due to tides, currents or waves) and unit weight of the rock in order to size a protective rock layer that will be stable. 100 - 250 rock may or may not be large enough.

 

[dhengr]

EireChch:
There should be some research on backfill size and scour vs. water flow velocity, etc. Scour around bridge piers, piles, large footings, over filled trenches, etc. That’s were I’d start looking for something to hang my hat on. Also, what about buoyancy and the depth and size of fill material to prevent floatation?