ESTIMATING THE RATE OF SCOUR PROPAGATION ALONG A SUBMARINE PIPELINE IN TIME-VARYING CURRENTS AND IN FINE GRAINED SEDIMENT

摘要

Estimating the horizontal rate of scour propagation along a submarine pipeline is a key step in estimating changes in the pipelines burial state and on-bottom stability due to sediment transport. However, whilst recent work has been undertaken to estimate the horizontal rate for non-cohesive uniform sands in steady current, in field conditions currents can vary in time and the sediment can be fine grained and exhibit very different erosion properties to sand. As a first attempt to account for these complications, in this paper we present results from a series of experiments designed to measure the rate of scour along a model pipeline in time varying currents and in a fine grained sediment. The scour experiments are also supplemented by erosion testing, which indicate that the erosion resistance of the fine grained sediment is larger than that predicted by the well-known Shields curve. Based on the scour experiments, it is found that in time-varying currents the scour rate can be predicted using an amalgamation of the results obtained for steady current conditions; this is a convenient result because theoretical predictions already exist for the scour rate in steady current conditions. In the fine grained sediment experiments, it is found that the horizontal rate of scour is much lower than that predicted by existing theoretical models that assume a non-cohesive sandy seabed. To provide an improved estimate of the horizontal rate of scour, a new theoretical model is introduced that relates the horizontal rate of scour to the measured erosion properties of the sediment. This new model is found to agree well with the experimental measurements. Although further experimental testing is recommended, in combination, it appears that these results may be used to better estimate the horizontal rate of scour in both time-varying and fine grained sediment.