Experimental Study of Local Scour Beneath Two Tandem Pipelines in Steady Current

This paper presents an experimental investigation of local scour beneath two identical pipelines placed in a tandem arrangement. Changes in the equilibrium scour depth and time scale of scour due to steady currents are explored for different spacing between the pipelines (defined in terms of a gap ratio G/D) under both live bed and clear water conditions. It is found that the smaller the gap ratio, the larger the interaction between the two tandem pipelines. More specifically, when G/D < 3 it is found that (i) the equilibrium scour depth beneath the downstream pipeline is larger than that below the upstream pipe, and (ii) the equilibrium scour depth for both pipelines is larger than for a single pipeline in isolation. Alternatively for G/D > 3 it is found that the equilibrium scour depth beneath the upstream pipeline can be deeper than the downstream pipeline, however both pipelines have a depth that is not significantly different to the equilibrium depth for an isolated pipeline. In terms of the time scale of scour it is observed that the downstream pipeline has a similar or larger time scale than the upstream pipeline over all gap ratios analyzed. In comparison to an isolated pipeline the time scale for both tandem pipelines is larger when G/D < 3, whilst for G/D > 3 and G/D > 6, respectively, the upstream and downstream pipelines have a similar time scale to an isolated pipeline. The trends in the experimental results are shown to agree well with recent numerical results in the literature. Empirical formulas for predicting the time development of scour beneath two tandem pipelines are proposed.