Stamukha Pits - Input Characteristics for Design of Pipelines in the Caspian Sea

摘要

Stamukhi form when moving sea ice piles up on top of grounded ice ridges. Pits in the seabed occur under heavily grounded stamukhi, due to the associated loads. In designing subsea pipelines in the northeast Caspian Sea, where stamukhi up to 15 m freeboard have been observed, potential loads on the pipe associated with development of such pits need consideration. The present paper describes the methods to determine the frequency of pit forming in the vicinity of a pipeline, probability distributions for the pit dimensions and from these, pit dimensions associated with specified annual probabilities of exceedance. These can be used with ice-soil-pipe interaction models to ensure pipeline designs and burial depths will meet required reliability levels. A key aspect of the work was the statistical treatment of uncertainty in the data and processes. Pitting occurs under different ice conditions, water depths, soil types and distances to existing structures, which influence pitting rates. The influence of these factors was investigated. For example, while an overall distribution of soil types had been established, soil types for specific pits were generally not available. A probabilistic methodology developed to characterize influence of soil type was used to assess the influence of selected backfill materials. The main analysis was based on data from unbiased surveys along preselected routes without prior knowledge of potential for pits. Additional data obtained from targeted surveys, such as at locations where stamukhi had been observed, was incorporated after assessing the equivalent unbiased survey effort. Methods were also developed to account for uncertainty due to the limited number of years of data available and the selection of appropriate probability distributions for inputs.