An integrated sediment mobility and scour assessment: Characterisation, calibration and mitigation studies for a pipeline in the South China Sea

We present a shallow water (200 m to 600 m) export pipeline case study from the South China Sea where different kinds of sediment bedforms and variations in seafloor roughness meant that a sediment mobility assessment was necessary to determine which sections of the pipeline would require burial. Our phased approach began with a qualitative geomorphological assessment, progressed to theoretical and empirical evaluations of sediment mobility potential, and culminated in a probabilistic estimation of the morphological baseline below which sediment mobility is not expected. The geomorphological assessment phase included a review of current and past metocean regimes, delineation of bedforms indicative of seafloor mobility, and identification, confirmed by targeted coring and sedimentological analysis, of young and presumably mobile bedforms superimposed on older megaripples. Critical shear stress values were estimated from classical Shields curves and erosion function apparatus (EFA) testing of site-specific samples. Plotting local sediment flow conditions on bedform equilibrium diagrams led us to conclude that not all of the observed bedforms are in equilibrium under present conditions. One active bedform field, where sediment transport rates are predicted to be as high as 1.8 m per year during extreme conditions, was identified from our analysis of bedform equilibrium conditions. Finally, safe burial depths across the active bedform field were probabilistically estimated using a spectral approach to separate mobile from immobile bedform components in high resolution multibeam echosounder (MBES) bathymetric profiles. The result was a study that fully integrated several kinds of data and fields of technical expertise to provide a sediment mobility assessment that is geologically credible, temporally relevant, and well calibrated. Perhaps most importantly, it avoided the potential pitfalls of over- or under-design that might have occurred had only one of the assessment methods been used.