Practice for Pipeline Design in Ice Scoured Environments: Application to the Kashagan Project

摘要

The Kashagan project in the North Caspian Sea, when fully developed, could include as much as 1000 km of buried pipelines, umbilicals and power cables. Since the North Caspian freezes each winter, these structures are subject to loads from ice scouring and stamukha formation associated with moving ice. The high cost of deeper burial to reduce risk of damage to pipelines means that considerable effort has been put into the data collection and design methods for pipeline during the various stages of the project. An approach to pipeline design for ice loads has evolved during ten years of data collection, studies and engineering design for the Kashagan Project. This paper describes the criteria developed for Phase Ⅱ of the project. The design process starts with determining the probability of exceedence appropriate for the design load cases. However, the loads are never measured, but have to be determined based on observed effects of ice scouring and stamukha on the seabed. Then the mechanism by which the ice interacts with a buried pipeline is considered so that the design scour width and depths can be translated into soil displacements which are applied to the pipeline (in a pipe - spring model). The interactions between stamukha pits and a pipeline are different from scours. If the pit depth is greater than the burial depth, direct contact between the ice and pipeline can occur and ice crushing pressures may need to be considered as well as soil displacements. Additional complications to the design include the geometry of the pipeline trench and the difference in soil strength within the trench and the surrounding seabed. Combinations of scouring events at the same location, as well as a single extreme event also need to be considered in terms of accumulated plastic strains in the pipeline. Upheaval buckling could also be triggered at a scour or pit location.