We present an export pipeline case study in the South China Sea in water depths between 200 m and 600 m. Several types of sediment bedforms and variations in seabed roughness were identified at seafloor; hence the requirement for a sediment mobility assessment. Data types acquired include seafloor and sub-bottom geophysical data, geotechnical samples, in situ tests, geological analyses, hydrological testing and metocean measurements. The outcome of the testing determined sections of pipelines requiring burial.

A phased approach was taken from a qualitative, geomorphological assessment, to a quantitative determination of a morphological baseline. This approach is applicable to scour assessment for cable, pipeline routes and other seafloor structures and ensures that unnecessary effort is not wasted at early stages of assessment.

A review was initially performed of regional present day and past metocean regimes and a geomorphological assessment of features characteristic of sediment mobility. A geometrical characterisation identified the superposition of small, younger bedforms on megaripples. A targeted sampling programme acquired cores for detailed sedimentological and geochronological analysis to validate these interpretations. Critical shear stress values for mobility were determined from theoretical relationships (classical Shields curve) and from erosion function apparatus (EFA) testing performed in the laboratory on site-specific samples. The local sediment flow conditions were plotted on bedform regime diagrams for oceanic flows identifying that not all of the observed bedforms could be considered in equilibrium under present day near-bed current conditions. A predictive relationship for bedload sediment transport was used to assess sediment transport rates over the bedforms. An active bedform field was identified, with predicted migration rates of up to 1.8 m during one year extreme background conditions. This was mitigated by using a spectral decomposition approach to identify the amplitudes of mobile versus immobile bedforms and obtain probabilistic estimates of a morphological baseline.

Assessments of sediment mobility and scour need to be geologically credible, temporally relevant, and well calibrated. An assessment of sediment mobility from just one of the methods may provide either over- or under-conservative design solutions. This study demonstrates the importance of full integration and the added benefit of inputs from multiple different disciplines.

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