Back Analysis of Measured as-laid Pipeline Embedment in Soft Clay

Pipeline designs such as on-bottom stability, lateral buckling and axial walking analyses require reliable predictions of the as-laid pipeline embedment. The as-laid embedment is usually significantly larger than that predicted by its submerged self-weight alone. This is mainly attributed to the effects of the stress concentration in the touchdown zone and soil remolding caused by the pipeline dynamic motions during the laying process. Quantification of these effects is often made difficult by the lack of field observations. This study presents a detailed back-analysis of the observed pipeline as-laid embedment in soft clayey soils in the shallow waters of Bengal Bay. Statistical analysis is performed to characterize the inherent variability of the observed embedment. The observed dynamic embedment factor is found to be between 1.08 and 1.96. The degradation of the soil resistance to the pipeline during the dynamic laying process is back-analyzed by a sophisticated method in which the reduction of the stress concentration with the increased embedment of the pipeline is well considered. The remolded soil resistance to the pipeline is found to be 0.48~0.74 of the intact soil in the dynamic laying process. And the degree of soil resistance degradation is found to be smaller when the pipeline static embedment is larger.