Free Field Sediment Mobility on Australia’s North West Shelf

Abstract

Under cyclonic conditions, sediment on the North West Shelf (NWS) of Australia may become mobile in shallow water due to classical sediment transport or local liquefaction, and this can affect, for example, the on-bottom stability of subsea pipelines. In this paper, three calcareous sediments sampled from the NWS are analysed, together with realistic metocean data, to illustrate this potential for sediment mobility on the NWS. Specifically, experiments are performed in a recirculating flume (known as an O-Tube) to measure the erosional behaviour and an additional series of experiments are performed using a shaking table, on which each of the sediments have been liquefied and excess pore pressure measurements recorded to back calculate the consolidation coefficient. Soil characterisation data, threshold velocity measurements and shaking table results have then combined to illustrate the potential for sediment mobility for each of the NWS sediments. Best practice models are used to calculate wave and current combined shear stress at the seabed and excess pore pressure accumulation. We find that for these sediments, freshly deposited in laboratory samples, mobility due to sediment transport or liquefaction is very likely in cyclonic conditions on the NWS. Liquefaction is most likely for loosely packed silt, whilst sediment transport is most likely for sand. However, we also show that in more extreme cyclonic conditions there are a subset of sediments that can become mobile due to both sediment transport and liquefaction.

Lifelong embedment and spanning of a pipeline on a mobile seabed

“…This is consistent with some of the carbonate sediment results of Mohr et al (2013), and other work which highlights the influence of cohesion, soil mixtures and other factors in determining erosion resistance (see for example Mitchener and Torfs (1996) for sand/mud mixtures and Grabowski et al (2011) for a more general review). The mean threshold shear stress of the samples is approximately 0.85 N/m 2 .…”

“…The threshold shear stress measurements for these samples are shown in Fig. 8, together with the data for a variety of silica sands from Shields (1936) and Soulsby (1997), and both silica and calcareous sediments from Mohr et al (2013). For context the modified Shields curve presented in Soulsby (1997) is also shown.…”

“…These samples were subjected to erosion testing at the University of Western Australia's Mini O-Tube (MOT) facility using a methodology identical to that outlined in Mohr et al (2013), except that the half-cylindrical samples discussed here were obtained by splitting the intact core with a wire cutter rather than by reconstituting the sample. The threshold shear stress measurements for these samples are shown in Fig.…”

“…Threshold shear stress measurements. Comparison is made with data collated byShields (1936),Soulsby (1997) andMohr et al (2013).…”

The influence of permeability on the erosion rate of fine-grained marine sediments

“…the threshold shear stress) and (ii) the erosion rate as a function of the excess shear stress (McCave 1984). Differences in the threshold shear stress of NWS sediments, compared with uniform silica sand, have been reported by Mohr et al (2013) and Mohr (2015). These results showed that particle size and grain density measurements are insufficient to isolate the threshold shear stress for NWS sediments.…”