The paper explains issues associated with pipeline uplift during backfilling and floatation, specifies the high risk conditions and presents analytical work and full scale laboratory testing carred out to enhance the current state-of-the-art. Pipelines are buried to provide on-bottom stability, physical protection and thermal insulation. If pipeline uplift during backfilling or floatation occurs, the beneficial effects of soil cover can be partially or completely negated. Quantifying and designing to mitigate against these risks is therefore essential In most cases this can be achieved by ensuring the pipeline has sufficient weight to resist any uplift or buoyancy forces. However, this may not always be possible If a pipeline is deemed too light to be backfilled, additional remedial measures are required. Since the installation of remedial measures is a high cost operation, optimisation is essential. Research has been pioneered into the field of pipeline movements both during and after backfilling A finite element modelling study has been carrd out to assess the forces experienced by pipelines during backfilling using computational fluid dynamics. Full scale testing has also been carried out to assess pipeline floatation potential. The paper concludes with a discussion of new analysis tools which can be used to predict potential floatation or uplift during backfilling in the future. Upward movement of pipelines can represent a high risk to the assumptions made for pipeline design Therefore, it is imperative that phenomenon such as floatation and uplift during backfilling are understood to avoid significant increases m project costs.
As smaller hydrocarbon reserves are progressively developed, particularly in the North Sea, small diameter pipelines with thick coating systems are required to provide flow assurance. This not only makes for lighter pipelines but invariably introduces the need to trench and backfill them (OTH 1999), increasing the importance of intelligent geotechnical design.
Pipelines used m sub sea environments are trenched and backfilled to provide stability, physical protection, thermal insulation and download to prevent upheaval buckling. One or any combination of these may be required for a given pipeline, but all are interrelated and have a significant effect on the pipeline design process (Finch et al., 2000) In any pipeline design the method of trenching must therefore be given due consideration.
There are three mam types of trenching (Fisher 1998):
Cutter trenchers / Cable ploughs
Each method carries a degree of risk which if not addressed could cause upward movement of the pipeline. If the pipeline does move up from the base of the trench the beneficial effects of the soil cover could be partially or completely negated. It is therefore imperative that phenomenon such as uplift during backfilling and floatation are understood and accounted for in pipeline design to avoid significant increases in project costs from remedial works (Finch et. a1 2000).