Estimating the Rate of Scour Propagation Along a Submarine Pipeline in Time-Varying Currents and in Fine Grained Sediment

Abstract

Estimating the horizontal rate of scour propagation along a submarine pipeline is a key step in estimating changes in the pipelines burial state and on-bottom stability due to sediment transport. However, whilst recent work has been undertaken to estimate the horizontal rate for non-cohesive uniform sands in steady current, in field conditions currents can vary in time and the sediment can be fine grained and exhibit very different erosion properties to sand. As a first attempt to account for these complications, in this paper we present results from a series of experiments designed to measure the rate of scour along a model pipeline in time varying currents and in a fine grained sediment. The scour experiments are also supplemented by erosion testing, which indicate that the erosion resistance of the fine grained sediment is larger than that predicted by the well-known Shields curve. Based on the scour experiments, it is found that in time-varying currents the scour rate can be predicted using an amalgamation of the results obtained for steady current conditions; this is a convenient result because theoretical predictions already exist for the scour rate in steady current conditions. In the fine grained sediment experiments, it is found that the horizontal rate of scour is much lower than that predicted by existing theoretical models that assume a non-cohesive sandy seabed. To provide an improved estimate of the horizontal rate of scour, a new theoretical model is introduced that relates the horizontal rate of scour to the measured erosion properties of the sediment. This new model is found to agree well with the experimental measurements. Although further experimental testing is recommended, in combination, it appears that these results may be used to better estimate the horizontal rate of scour in both time-varying and fine grained sediment.

Numerical Study of Local Scour around a Submarine Pipeline with a Spoiler Using a Symmetry Boundary Condition

“…It can be seen from Figure 10 that the profiles of the amplification factor α are very similar for different spoiler lengths. For α beneath the submarine pipeline, it is clearly shown that a In the present study, the maximum amplification factor of the bed shear stress is presented, because the vertical scour rate is highly dependent on this variable [27]. It can be seen from Figure 10 that the profiles of the amplification factor α are very similar for different spoiler lengths.…”

“…The amplification factor α associated with L/D = 0.33 is 23.8% larger than that corresponding to L/D = 0.0. As reported by [27], the vertical scour rate (represented by ξ) of a submarine pipeline can be estimated by the relationship ξ ∞ Further examination shows that the correlation coefficient is R 2 = 0.9974, demonstrating that the scour depth is highly correlating to the spoiler length.…”

“…The horizontal velocity corresponding to the first layer of mesh nodes located at a distance Δ1 away from the seabed can be obtained, then the friction velocity uτ and the bed shear stress τ = ρuτ 2 can also be determined. In the present study, the maximum amplification factor of the bed shear stress is presented, because the vertical scour rate is highly dependent on this variable [27]. It can be seen from Figure 10 that the profiles of the amplification factor α are very similar for different spoiler lengths.…”

“…The amplification factor α associated with L/D = 0.33 is 23.8% larger than that corresponding to L/D = 0.0. As reported by [27], the vertical scour rate (represented by ξ) of a submarine pipeline can be estimated by the relationship ξ ∞ A(ατ 0 -τ cr ) B , where A and B are two positive constant parameters and τ cr is the critical shear stress. The above-mentioned parameters/variables can be obtained through apparent erosion rate measurements [28].…”

The offshore renewables industry may be better served by new bespoke design guidelines than by automatic adoption of recommended practices developed for oil and gas infrastructure: A recommendation illustrated by subsea cable design

“…While this is acknowledged in F109 Section 8.5, F109 does not provide any useful design method guidance but instead refers to Griffiths et al (2018d). This reference describes the extensive research program which has been completed through the STABLEpipe JIP, using the UWA recirculating O-tube (Cheng et al, 2014) as a transformational research tool in the development of a new design guideline which has now been co-developed with DNV using the design methods described by Draper et al (2018a); Draper et al (2018b). The STABLEpipe guideline has been used on a number of projects and remains the most thorough DNVendorsed description on how to design pipelines on erodible seabeds…”