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design of trench backfill for subsea pipelines

design of trench backfill for subsea pipelines

(OP)
thread198-90581: Trench backfill

I am undertaking a design very similar to that described in the above post. Does anyone have any design recommendations?

Thanks  

RE: design of trench backfill for subsea pipelines

A detailed assessment with some calculations is likely to be beneficial, so probably worth getting an appropriately experienced consultant onboard. Anyway some high level thoughts;

Some of the following issues may be of importance in designing the backfill, or verify the suitability of local backfill. The importance or relevance of these points will depend on the specifics of your project. The method of excavation or trenching is also important.

•    Loss of backfill due to sediment transport
•    Protection provided to the pipeline e.g. from dropped objects, fishing,  anchoring.
•    Impact of backfilling on the pipeline coating
•    Thermal insulation (for hot lines you need to keep warm)
•    Resistance to upheaval buckling (for hot lines that may buckle)
•    Liquefaction risk (wave induced, and possibly seismic)
•    Other shallow water geohazards

These points apply to the backfill material, but in some cases also the adjacent seabed.

The sediment transport (scour) aspect is likely to be relevant in most cases. In conjunction with some Metocean data (wave and current) at an appropriate return period you can review the particle size distribution data for the backfill  and undertake some calculations to confirm it will not be washed away in the first storm. If things can't be made to work then mitigation with some sort of scour protection may be needed.

With respect to further reading the BRE rock manual is worth along with less specific books on sediment transport. However, no substitute for getting some involved with experience in this area.
http://www.brebookshop.com/details.jsp?id=287078
 

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