Erosion and scour behaviour of marine sediments

    Research output: ThesisDoctoral Thesis

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    Offshore oil and gas developments involve the installation of numerous structures on the seabed, such as pipelines and subsea foundations, which must remain in-situ. Periodic tidal currents, internal waves and tropical storms cause localised increases in hydrodynamic flow around these structures. This can lead to local scour of seabed sediments resulting in self-burial or settlement of structures without scour protection. To predict the effect of scour on structures, the erosion behaviour of the underlying marine sediment must be quantified. Improved erosion and scour prediction formulas can help ensure safety, reduce project risk and may allow optimisation of infrastructure investment.

    This thesis first undertakes a preliminary assessment of sediment mobility on Australia’s North West Shelf (NWS). The erosion properties of marine sediments recovered from the NWS are measured and, when combined with realistic metocean data, indicate a high likelihood of sediment mobility on the NWS. This suggests a need for subsea design that accounts for sediment mobility.

    Secondly, an extensive series of experimental studies was undertaken to investigate the performance of a recirculating flume, known as an O-tube. An O-tube has the ability to reproduce large combined wave and current conditions near the seabed, typical of storm conditions under which mobility of marine sediments may occur. The O-tube has been used throughout this thesis to conduct erosion and pipeline scour experiments. The hydrodynamics of an O-tube are investigated theoretically and experimentally, and a set of governing equations are defined that couple the pressure (supplied by the pump in the O-tube) to the bulk flow rate. Together with a series of velocity measurements which give a detailed description of the velocity field within the O-tube, this work provides the hydrodynamic basis for subsequent detailed laboratory erosion studies on marine sediments.

    Following the preliminary assessment, a larger laboratory study was undertaken using the O-tube to measure the erosion properties of artificial silica and carbonate sediments, as well as marine sediments originating from the NWS that were prepared to represent a range of field conditions. For marine sediments with no fines content (i.e. minimal grains with diameter less than 75 μm), the threshold shear stress and erosion rate were found to be in good agreement with the empirical Shields curve and classical bedload transport formulas. Density and angularity did not appear to have a significant effect on the measured erosion properties of these sediments.

    For finer marine sediments, the erosion properties were found to be highly correlated with the amount of fine material (i.e. the mass fraction of grains with diameter less than 75 μm) and the bulk density. Measurements showed an increase in threshold shear stress (and a reduction in erosion rate) with increasing bulk density and fines content. A comprehensive comparison of the erosion measurements is made with existing predictive models in the literature for fine grained sediments. These existing models, however, show a lack of consensus. Consequently, a new theoretical model has been developed in this thesis to interpret the threshold shear stress and erosion rate measurements assuming the seabed to be an inelastic porous medium. The new model considers a force balance between the disturbing hydrodynamic shear stress across a particle surface, the particle weight and a suction force defined in terms of the erosion rate and soil permeability. The resulting predictive formula is calibrated and found to agree well with erosion data from this study and existing literature concerning erosion measurements of fine quartz sediments and mud-sand mixtures.

    Finally, model scale experiments of scour beneath a pipeline have been conducted to investigate the extent and rate of the developing scour process. Comparing these results with the element-scale erosion data, theoretical analysis was used to develop a new empirical formula describing the rate of pipeline scour for fine grained sediments that move predominately due to entrainment into suspension. This formula, together with an existing empirical formula for coarser sediments moving in bedload, can be used to assess the temporal development of scour beneath offshore pipelines assuming erosion threshold and rate parameters are known from erosion testing. This represents the first example of erosion rate measurements used directly to estimate the scour rate for pipelines and improve design predictions for scour.

    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - Apr 2015

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