Engineering appraisal of pipeline floatation during backfilling
A case history of two 8-in. flowlines which were trenched and backfilled in the Northern North Sea is presented. The majority of the trench was cut in sands and soft clay overlying the uneven surface of a hard clay. The trenching operation was successful with the pipe set at the required depth, but the post-backfill survey revealed long sections of both lines exposed at the surface. The problems were resolved by retrenching, dumping a screed of rock and then repeating the backfilling operation. Post-installation engineering work was performed to establish the likely cause of the pipe uplift. A pipe flotation hypothesis was advanced and this paper concentrates on the work performed on this scenario. The conclusions reached were that during backfilling the mixing of the fine sand, silt, clay and shells produced a very high void ratio material that behaved as a slurry for long enough to allow the pipe to progressively uplift as the backfill plough passed. Soils to be trenched and backfilled should be investigated very carefully, not only for trenchability and in-service behavior, but also for short term behavior during backfilling and consolidation.
- OSTI ID:
- 423163
- Report Number(s):
- CONF-960525--
- Country of Publication:
- United States
- Language:
- English
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