Design of Buried Pipelines in Soft Clay: A Case Study

Abstract

Abstract High pressure high temperature offshore pipelines are generally buried for protection. For buried pipelines, upheaval buckling can be triggered in the presence of vertical imperfections when the pipeline compressive state is increased. One of the main uncertainties in the design calculation is the soil resistance to upward and downward movement and the displacement at which the peak resistance is mobilised. For clays the soil response will usually be undrained, certainly for most loading events, however it may also be necessary to consider the drained response (particularly if there is any long term applied uplift). In addition, local flow-round failure mechanism may occur, generally for deeply buried pipelines. If local failure dominates the design, it may not be possible to bury the pipeline. This could have significant cost implications. An experimental programme was carried out to determine the uplift resistance of buried pipelines in soft clay soils. The results of scale model testing and computational analysis (finite element limit analysis) showed that the local behaviour was not applicable. The implications of these results are illustrated in a case study upheaval buckling (UHB) analysis. The overall result was a significant saving on rockdump quantities. The implications for the case study are presented in this paper alongside a description of other backfill requirements and practical considerations for a UHB design.