Some aspects of local scour mechanics around subsea pipelines

Qin Zhang

    Research output: ThesisDoctoral Thesis

    371 Downloads (Pure)

    Abstract

    [Truncated] The research carried out in this thesis is concentrated on physical experiments of local scour around subsea pipelines. The main findings are listed as following:
    In chapter three a series of experiments performed in the recirculating Mini O-Tube (MOT) flume at UWA concerning the onset of scour below pipelines in steady current is reported. The experiments were performed on model seabed with two different lengths to investigate the effects of variations in sediment supply on onset of scour. In each of the experiments the flow velocity and pipeline embedment were recorded simultaneously and continuously. The test results are compared with an existing empirical formula for a partially buried pipeline on a flat seabed. It is found that when the upstream sediment supply is limited, onset of scour occurs at a velocity well below that predicted by the existing empirical formula, following significant local scour around the pipe. This result suggests the possibility of onset of scour for embedded pipelines in the field where sediment supply upstream of the pipeline is interrupted by rock outcrops or seabed slopes. To interpret the experiments, a series of numerical simulations have also been conducted to investigate the dynamic pressure difference between upstream and downstream of the pipeline prior to onset of scour. The numerical results show that despite local scour significantly altering the surrounding seabed for an embedded pipeline, the pressure gradient across the pipeline is similar in magnitude to that measured for a flat seabed and is sufficient to initiate piping.

    In chapter four scour around a fixed pipeline under currents, waves, and combined waves and current conditions has been studied experimentally using both the MOT and Large O-Tube (LOT) facilities at UWA. The model pipe diameter ranges from 50 mm to 196 mm. Comparisons of equilibrium scour depth and scour width have been made between present experimental data and existing empirical formulae and data in all three flow conditions. The empirical formula for equilibrium scour depth in steady current has been updated based on the existing data. A new method of predicting the equilibrium scour depth in combined waves and current is also proposed based on the current component of equilibrium scour depth and the wave component equilibrium scour depth. Bed features (e.g. sand ripples) which occur in the model tests are discussed in detail using three dimensional scour profile measurements. These results show that different regimes of bed forms occur, but the average scour hole shape is generally not affected by bed forms. Secondly, the wall boundary effect between the pipeline model and o-tube side walls is discussed and quantified using experimental results and three dimensional numerical simulations. Collectively, the investigations of sand ripples and the wall effect provide a better understanding of the physical experiments and laboratory data.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - Jun 2015

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