1,720,982 research outputs found
Drained bearing capacity of shallowly embedded pipelines
No full textThis study establishes the drained bearing capacity of pipelines embedded up to one diameter into the seabed subject to combined vertical-horizontal loading. Non-associated flow finite element analyses are used to calculate the peak breakout resistance in a non-associated flow, frictional Mohr-Coulomb seabed. Critical state friction angles and dilation angles ranging from 25° to 45° and 0° to 25°, respectively, are considered. Analytical expressions have been fitted to the results as a function of embedment depth and soil properties, and compare well with experimental measurements from previous studies. The horizontal bearing capacity at small vertical loads is also predicted well via upper bound limit analysis using the Davis reduced friction angle that accounts for the peak friction and dilation angles. The analytical relationships presented in this study provide simple predictive tools for estimating the bearing capacity of pipelines on free-drained sandy seabeds. These fill a void in knowledge for pipeline stability and buckling design by providing general relationships between drained strength properties and pipeline bearing capacity. The insight gained through the good comparison with limit analysis techniques also gives confidence in the use of simple numerical techniques to predict the bearing capacity of pipelines for more wide-ranging (i.e. non-flat) seabed topography
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
Reliability of dynamically embedded anchors in soft clay
No full textFloating offshore infrastructure requires geotechnical anchorage for station-keeping. Dynamically embedded anchors are anchors that free-fall through the water column and embed in the seafloor through kinetic energy at impact, enabling efficient installation into soft seabeds. Dynamic installation of plate anchors is an alternative to static embedment methods, such as suction-embedded plate anchors. This paper uses a numerical installation and capacity methodology to explore the factors that control dynamically-embedded plate anchor (DEPLA) capacity and to compare DEPLA reliability with that of statically-embedded plate anchors for two sets of seafloor properties that represent different types of geotechnical uncertainty. Probabilistic Monte Carlo simulations are presented for the two representative conditions using the distribution of the tension loads for an example catenary-moored system. The results quantify the smaller influence of soil strength uncertainty on DEPLAs compared to statically-embedded plates. The smaller influence arises due to dynamically-embedded anchors reaching a deeper embedment in softer soil, counteracting the lower strength. This leads to increased reliability compared to statically-embedded equivalent anchors installed to a specified depth. We quantify the reduction in safety factor that could be used for dynamically-embedded anchor design to achieve similar levels of reliability compared to statically-installed anchors of the same size.</p
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
Effect of drainage on upheaval buckling susceptibility of buried pipelines
No full textThis paper investigates the effect of soil drainage on the uplift resistance of buried pipelines, and their susceptibility to upheaval buckling. The uplift resistance of buried pipelines is considered through analytical and numerical predictions for both drained and undrained conditions. Combinations of soil strength parameters for typical soils are estimated based on common correlations. For certain ranges of typical normally consolidated soil conditions, particularly those with high critical state friction angles, the drained uplift resistance may be lower than the undrained resistance. This observation is important because in typical practice only drained or undrained behaviour is considered depending on the general type of soil backfill used. In this case, the critical or minimum uplift resistance may be overlooked. Further, the changing undrained uplift mechanism between shallow and deep conditions is investigated. It is found that the common approach of considering the minimum of either a local (flow around) or global (vertical slip plane) failure can overestimate the uplift resistance in normally consolidated soils.</p
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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
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