1,721,023 research outputs found
Sediment transport and trench development beneath a cylinder oscillating normal to a sandy seabed
No full textThe purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinders – such as pipelines, cables or idealised chains“– close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (KC number). For KC≲4 sediment motion results in bed load transport that is symmetric about the cylinder centre line. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4≲KC≲9 sedimen tmotion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9≲KC≲12 the sediment motion is characterised by localised transport events and asymmetric bed load transport driven by overall vortex dynamics. In terms of trench size, the maximum(equilibrium) depth is found to increase with KC and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with KC number and ψ, with an additional dependency on the cylinder β number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the KC number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue
The effect of permeability on the erosion threshold of fine-grained sediments
No full textThe erosion of marine sediments, although difficult to predict, can lead to important implications in offshore engineering, sedimentology and coastal management. Continued research is, therefore, warranted to compile high-quality erosion data from which to develop models to better predict the erosion resistance of different types of marine sediments. In this paper, dimensional analysis is performed to express the threshold shear stress as a function of a selection of soil properties that are commonly linked to the erosion process of sediments. To identify the dominant dimensionless group, an experimental investigation on the erosion threshold was carried out using fine-grained sediments that were systematically prepared to ensure variations in (i) particle size distribution (i.e. fines content), (ii) bulk density, and (iii) hydraulic permeability. The samples included silica, carbonate and marine sediments, each of which are expected to have limited or no clay-mineral content. The measurements were analysed and compared with existing literature and predictive models. It was found that marine sediment samples with limited fines content showed good agreement with the empirical Shields curve, irrespective of particle size distribution, bulk density and permeability. In contrast, for finer marine sediment it was found that variations in these soil properties modify the threshold shear stress away from the Shields curve. Across each of these parameters only permeability appeared to independently correlate with the observed range of threshold measurements. Motivated by this finding, a model is introduced to predict the threshold shear stress as a function of permeability and the reference erosion rate that is used to define when the threshold is reached. The resulting expression is shown to quantitatively explain the experimental data and is found to also agree with existing data from the literature for quartz sediments with a wide range in fines content. An apparent advantage of the new model is that it is consistent with existing studies that identify variations in threshold shear stress due to changes in bulk soil parameters – including fines content and bulk density – since each of these parameters also affect permeability.</p
Unlocking the benefits of long-term pipeline-embedment processes: Image analysis-based processing of historic survey data
No full textAn image analysis-based method for retrieving high-resolution bathymetry data from historic pipeline inspection video is detailed. The resulting time-dependant bathymetric datasets are used to inform an improved understanding of pipe-soil and pipe-soil-fluid interaction. This allows better management of existing pipelines and improved design of new pipelines. The pipeline and seabed positions are extracted automatically from the survey video, spurious points removed, the data scaled, and interpolation applied. The data can then be projected in various forms, for different applications. Drawing on field data from Australia's North West Shelf, applications of the method are described. For pipeline design, it is shown how improvements in on-bottom stability and changes in seabed friction can be quantified for mobile seabeds and through buckling sections of pipeline. This approach unlocks advances in existing design practice by providing quantification of the effects of seabed mobility. The processes and mechanisms that lead to through-life changes in pipeline embedment, soil support, and hydrodynamic shielding can be quantified, allowing design to move beyond the usual assumptions of a pipeline embedment that is invariant in time and space.</p
The influence of permeability on the erosion rate of fine-grained marine sediments
No full textLaboratory experiments have been carried out to investigate the erosion behaviour of a number of marine sediments which were reconstituted and sieved in various ways to ensure a large variation in soil properties such as (i) particle size distribution (including median grain size and fines content), (ii) bulk density, and (iii) hydraulic permeability. Based on the experimental results, it was found that for finer marine sediments the erosion rate at a given shear stress was sensitive to changes in these soil properties. No unique relationship was found between changes in the erosion rate and changes in the bulk density or fines content. More specifically, we find that the erosion rate at a given shear stress reduces with increases in density and fines content, but by an amount that is different for different sediments. In contrast, we show that there appears to be a unique relationship between permeability and erosion rate, such that permeability may be used to predict the erosion rate for the marine sediments at any density. By reinterpreting existing experimental results in the literature, we find that this same relationship between permeability and erosion rate is apparent for quartz sediments. We propose an empirical relationship, which fits well the erosion rate behaviour of finer sediments close to the threshold shear stress
Free field sediment mobility on Australia's North West Shelf
No full textUnder 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.</p
Risk-based assessment of scour around subsea infrastructure
No full textScour poses a significant risk to infrastructure placed on mobile seabeds. Seabed mobility is common on the North West Shelf of Australia, in parts of the North Sea, and also occurs in the deepwater Gulf of Mexico, due to loop currents. Scour can undermine structures and, for shallow-skirted mudmat foundations, there can be significant consequences including excessive settlements, tilt and loss of bearing and sliding capacity. However, scour mitigation via engineered protection is costly, and to be avoided if possible. This paper describes a new quantitative risk-based approach for assessing the susceptibility of subsea infrastructure to scour processes. This probabilistic scour assessment accommodates measurable uncertainties in metocean and seabed conditions, using new characterization techniques. The approach allows operators and owners to better assess the optimum strategy to address scour risk, selecting from mitigation during installation or in-service monitoring, prediction and remediation. The paper describes (i) best practice approaches for assessing scour susceptibility and propagation rates with and without engineered protection, (ii) new methods for determining the applicable seabed and metocean inputs, (iii) a probabilistic framework for encompassing uncertainties, and (iv) how this approach can be applied in project applications. Our probabilistic method of assessing and presenting scour risk produces a distribution of estimates of scour depth and time rate. By capturing and quantifying the full range of uncertainties, this method facilitates decision-making by showing the range of potential outcomes and allowing the associated costs and consequences to be evaluated. This approach is superior to deterministic 'worst case' calculations, which are often used to assess scour susceptibility. In summary, this paper provides operators and owners with an improved methodology to unlock Capex and Opex savings through more accurate and informed scour assessments.</p
Effect of wave boundary layer on hydrodynamic forces on small diameter pipelines
No full textEffect of wave boundary layer on hydrodynamic forces on small diameter pipelines is speculated in this paper. It is found that use of the recommended calculation methods in DNV-RP-F109 leads to unrealistic predictions of high specific gravity (SG) requirements for small diameter pipelines subject to wave loading. Potential reasons for such high SG requirements under wave loading are speculated based on existing knowledge and experimental evidence, and a way forward is proposed. It is suspected that the unrealistic predictions are because the calculation methods ignore wave boundary layer effects on hydrodynamic forces.</p
Stability of subsea pipelines during large storms
No full textpipeline stability, scour, offshore hydrodynamics Author for correspondence
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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