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The interaction between a spudcan and an existing footprint is one of the major concerns during the reinstallation of jack-up rigs. This paper reports the results from a series of centrifuge model tests undertaken to assess the efficiency of a recently developed novel spudcan shape with a flat base and four holes through the base to ease spudcan–footprint interactions. The experimental program was carried out at 200 g in a beam centrifuge. The footprints were created either manually using a cutting tool or through spudcan penetration and extraction allowing a typical operation period as well as an elapsed time prior to reinstallation. While the former allowed a consistent evaluation of the benefits of spudcan shape, removing the additional variability due to soil strength heterogeneity, the latter allowed for examining the effect of real field footprints. For manually created shallow footprints in soft clay, comparing the responses with those on a generic spudcan, it was possible to show that the novel spudcan reduced the maximum horizontal force and moment by up to 47%–62% and 28%–36%, respectively. The critical offset distance was found at 0.75D. For manually created deep footprints in stiff clay, the reductions were 20%–59% and 9%–45%, respectively. The critical offset distance was found at 0.5D. For footprints created by penetrating and extracting a spudcan, the critical offset distance was identified at 0.75D. The efficiency of the novel spudcan was calculated with the reduction of maximum horizontal force and moment by 42%–59% and 21%–62%, respectively. [ABSTRACT FROM AUTHOR]

Assessing the potential for a punch-through failure during spudcan installation in sand-over-clay is crucial for reducing risk in the operations of mobile jack-up platforms. Typically, in the offshore industry, the peak penetration resistance and the depth at which it occurs are determined deterministically without rigorously considering the uncertainties in the soil. This paper proposes a probabilistic approach to estimate the peak resistance and the corresponding depth, as well as a Bayesian method of incorporating installation data to update the predictions. Instead of a single value in the deterministic analysis, a range of the potential peak resistances and depths can be estimated by accounting for the uncertainties in the soil, the spudcan's geometry and in the calculation method itself, with a database of 66 geotechnical centrifuge tests characterising the model. This prior probability is then updated using the monitored data, allowing a real-time update of the probabilities associated with candidate values of peak resistance and depth during the installation. The advantage of such a probabilistic updating model is shown in a retrospective simulation of a mobile jack-up platform in sand-over-clay conditions in the Gulf of Mexico. The results show that the prior estimation can be effectively refined by incorporating the monitored data. The proposed method provides a powerful tool for assisting decision-making during the installation of jack-ups offshore. [ABSTRACT FROM AUTHOR]

Suction caissons are being increasingly considered as an alternative foundation type to monopiles for offshore wind turbines. Single caisson foundations (or monopods) for offshore wind turbines are subjected to lateral cyclic loading from wind and waves acting on the structure. Recent studies have considered the response of suction caissons to such loading in sand, but have generally been limited to a few thousand cycles, whereas offshore wind turbines will generally experience millions of loading cycles over their lifetime. This paper presents the results from a programme of caisson tests in sand, clay and sand over clay seabed profiles, where each test involved about one million cycles of lateral load. The capacity and rotation response is shown to approach that measured in the sand seabed when the sand–clay interface is located at or beneath the caisson skirt tip. In contrast to previously published studies in sand, one-way cyclic loading is the most onerous loading symmetry for a layered seabed with a sand thickness equal to half the skirt length. However, the rotation for this seabed profile is essentially identical if the load is sustained or cyclic, provided that the cyclic loading remains one way. Lateral cyclic loading was seen to increase caisson capacity by up to 30% – with a bias towards clay-dominated seabed profiles – and stiffness by up to 50%. Such stiffness increases need to be considered when assessing the system dynamics for the offshore wind turbine, as demonstrated in the paper. [ABSTRACT FROM AUTHOR]

Slope stability analysis has traditionally been performed using a deterministic approach. However, it has strongly been debated that the use of only the factor of safety in slope stability analysis does not explicitly account for all the uncertainties in soil parameters. Therefore, to investigate the effect of uncertainties in the stability of a fill slope, this paper uses the finite-element limit analysis methods to conduct a simple probabilistic-based analysis of two-layered undrained clay slopes. The results obtained show that although the recently developed stability charts for two-layered undrained clay may be convenient tools, the factors of safety calculated from the charts may not be sufficient. Particularly, it is shown that the probability of failure of this type of slope can be highly influenced by uncertainties in the soil properties. Various probabilistic charts based on different coefficients of variation in soil properties have been proposed in this paper. Based on this study, the recently developed charts for two-layered undrained clay slopes can be better applied. Additionally, this study also demonstrates the applicability of the finite-element upper and lower bound limit analysis methods to conduct probabilistic slope stability analyses. [ABSTRACT FROM AUTHOR]

The use of plate anchors in permanent moorings for deep-water floating structures requires a thorough understanding of their performance under sustained and cyclic loading. This paper addresses the former aspect through a series of centrifuge tests on vertically loaded plate anchors in normally consolidated kaolin clay, supported by large-deformation finite-element analyses. The centrifuge experiments, which included particle image velocimetry measurements to identify the conditions leading to anchor failure, demonstrate that sustained loading can be maintained indefinitely if the anchor displacement at application of the sustained loading does not exceed 40% of the displacement reached at monotonic failure. This corresponds to a sustained load that is about 88% of the monotonic capacity, as identified from the numerical analyses. An important observation from both the centrifuge tests and the numerical analyses was a gap at the base of the anchor that developed under moderate load levels when the vertical effective stress at the base of the anchor reached zero. Although this so-called breakaway condition is generally associated with a lower capacity factor and a heightened potential for anchor failure, the conditions leading to breakaway also allow for consolidation-induced strength increase at the top face of the anchor and the mobilisation of partially drained or drained soil strength, both of which maintain anchor stability. [ABSTRACT FROM AUTHOR]

This paper investigates slope stability and produces a set of stability charts for three-dimensional (3D) slopes for a specific case in which frictional fill materials are placed on purely cohesive clay. As slopes are not usually plane strain in nature and are influenced by physical boundaries, this study uses a 3D analysis using the finite-element LB limit analysis method. Stability charts are convenient tools for geotechnical engineers during design in practice. For comparison purposes, the results from two-dimensional (2D) analyses are also discussed. The results from this study quantify the increase in the factors of safety obtained when 3D conditions are analyzed as opposed to the more traditional 2D. [ABSTRACT FROM AUTHOR]

Identification of soil stratification is vital to geotechnical structural design and construction where the soil layer, soil type and properties are necessary inputs. Although methods are available for classifying the soil profiling using measured cone penetration test (CPT) data, the identification of soil stratification at unsampled locations is still difficult due to significant variability of natural soil. The identification is further complicated by the considerable uncertainties in the CPT measurements and soil classification methods. This study aims to develop a probabilistic method to predict soil stratification at unsampled locations by explicitly filtering the uncertainties in soil classification systems. An established Kriging interpolation technique is used to estimate the CPT parameters which are further interpreted to identify the soil stratification. Equations are derived to quantify the degree of uncertainties reduced by this method. The approaches are illustrated using a database of 26 CPT tests recently sourced from a dike near Ballina, Australia. Results show that the majority of the uncertainties in the soil parameters are screened by a soil classification index. The remaining uncertainties are further filtered by the soil classification systems. A clear stratification with a high degree of confidence is obtained in both horizontal plane and vertical unsampled locations, which shows excellent agreement with the existing CPT tests. This study provides a methodology to clearly identify the soil strata and reduce the uncertainties in prediction of design properties, paving the way for a more cost-effective geotechnical design. [ABSTRACT FROM AUTHOR]

Centrifuge tests investigating the effect of compressive preloading and consolidation on the subsequent uplift capacity of shallow skirted foundations that rest on lightly over-consolidated kaolin clay are reported. The uplift resistance, total and pore pressures at the foundation invert were monitored during the installation, consolidation and uplift. The results show that uplift performed immediately after the preloading generates a lower capacity due to the remoulding of the clay and reduction in soil strengths that result from the application of preloading. The soil strength can be recovered and eventually enhanced by the subsequent consolidation process. A theoretical framework, where the variation of the soil operative strength with preloading and consolidation is interpreted using the critical state concept, is proposed to explain and justify the experimental observations. The framework is validated through retrospective prediction of the uplift resistance of the centrifuge model tests with good agreement for a wide range of levels of preloading and degrees of consolidation demonstrated. [ABSTRACT FROM AUTHOR]

A complete analytical method to describe the full load-penetration resistance profile of a mobile jack-up spudcan footing penetrating a sand over clay stratigraphy is described. It is based on both large deformation finite-element analyses and geotechnical centrifuge experiments. The coupled Eulerian-Lagrangian (CEL) approach is used to accommodate the large deformations of a spudcan footing penetrating sand overlying clay. Modified Mohr-Coulomb and Tresca models describe the sand and clay behaviour, with modifications accounting for the effects of strain softening on the response of the soil. The CEL results are shown to match centrifuge tests well, allowing the numerical study to be extended parametrically, and with confidence, to cover the range of layer geometries, sand relative densities and footing shapes that are of practical interest to offshore jack-ups. The results are used to ( a) assess the performance of an existing model to predict the peak resistance in the sand layer (extending its range of application to medium dense to dense sands and to conical footings of angle 0° to 21°), and ( b) develop an expression for the bearing capacity factor when the footing penetrates into the underlying clay. Using the analytical formulas proposed, retrospective simulations of centrifuge tests show that the method provides a reasonable estimate of the peak punch-through load, the behaviour in the underlying clay, as well as the punch-through distance; the latter being a basic reflection of the severity of a potential punch-through failure. [ABSTRACT FROM AUTHOR]

This paper proposes an analytical approach to evaluate the ultimate embedment depth and holding capacity that plate anchors can potentially achieve. Based on a plasticity model for anchor-soil interaction and compatible chain solution, detailed derivations are presented that allow the main dimensionless groups of input parameters to be identified. For typical cases where the weight of the anchor is negligible relative to its holding capacity, explicit expressions are provided in non-dimensional form for ultimate embedment and anchor capacity. A thorough parametric sensitivity study highlights the major factors affecting these quantities. Two practical examples are considered that demonstrate the proposed analytical approach for different types of anchors, in one case revealing significant scope for improved design of plate anchors in order to optimise performance. [ABSTRACT FROM AUTHOR]

Accurately predicting peak penetration resistance qpeak during spudcan installation into sand overlying clay is crucial to an offshore mobile jack-up industry still suffering regular punch-through failures. This paper describes a series of spudcan penetration tests performed on medium-loose sand overlying clay and compares the response to existing centrifuge data from tests performed on dense sand overlying clay. Together these data demonstrate that punch-through is a potential problem for both dense and loose sand overlying clay soil stratigraphies. Using this experimental database, a failure-stress-dependent model has been modified to account for the embedment depth, and the depth of occurrence of qpeak is shown to be a function of the sand thickness Hs. The model then was recalibrated, taking these findings into account, for a larger range of material properties and ratios of sand thickness to spudcan diameter (Hs=D). Finally, the performance of the modified and recalibrated model is verified by comparing its predictions with those calculated using current guidelines. The comparisons show that the modified model yields more accurate predictions of qpeak over the range of Hs=D ratios of practical interest, which when used in practice will potentially mitigate the risk of unexpected punch-through on sand overlying clay stratigraphies. [ABSTRACT FROM AUTHOR]

Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. During installation and operation, these structures are subjected to repetitive motions which can cause the surrounding seabed soil to be remolded and soften. This disturbance leads to significant changes in the operative shear strength, which must be assessed in design. This paper presents an analytical framework that aims to quantify the degradation in undrained shear strength as a result of gross disturbance—in this case through repeated vertical movement of a cylindrical object embedded in undrained soil. The parameters of the framework were calibrated using data obtained in a geotechnical centrifuge test. In this test a T-bar penetrometer, which is a cylindrical tool used to characterize the strength of soft soil, was cycled vertically in soil with strength characteristics typical of a deep water seabed. Using simple assumptions regarding the spatial distribution of “damage” resulting from movement of the cylinder, and by linking this damage to the changing undrained shear strength via a simple degradation model, the framework is shown to simulate well the behavior observed in a cyclic T-bar test. This framework can potentially be extended to the similar near-surface behavior associated with seabed pipelines and risers. [ABSTRACT FROM AUTHOR]

Offshore jack-up drilling rigs are subjected to loading from wind, waves, and current in addition to their self-weight. This applies combined loading in all six degrees-of-freedom in space on the footings. Although the foundation–soil interaction is crucial to the overall response of a jack-up structure, current state-of-the-art models to predict jack-up footing behaviour, developed using data from single footing experiments, have not been validated for such multi-footing systems under general combined loading. This paper introduces the experimental development of a three-legged model jack-up and loading apparatus designed to investigate the rig’s response — in particular the footing load paths — under combined loading in three dimensions. Push-over experiments were performed in a geotechnical beam centrifuge on silica sand. Experimental results of two tests on dense sand are discussed, highlighting differences in response and mode of failure depending on the loading direction of the jack-up. The importance of three-dimensional modelling is also stressed by experimentally demonstrating that the symmetric load case is not necessarily conservative. Les plates-formes de forages en mer subissent des charges provenant du vent, des vagues et des courants en plus de leur masse. Ceci implique des charges combinées dans les six degrés de liberté dans l’espace sur les semelles. Malgré que les interactions fondations–sol soient cruciales dans l’établissement de la réponse de la plate-forme, les modèles intégrant les connaissances actuelles pour la prédiction du comportement des semelles des plates-formes, qui ont été développés à partir de données provenant d’essais à une semelle, n’ont pas été validés pour des systèmes avec semelles multiples soumis à des charges combinées. Ce papier présente le développement expérimental d’un modèle de plate-forme à trois pattes et d’un appareil de chargement conçu pour investiguer la réponse de la plate-forme - plus particulièrement le cheminement de charge de la semelle – soumis à un chargement combiné en trois dimensions. Des expériences en déplacement latéraux ont été effectuées dans une centrifuge géotechnique sur du sable de silice. Les résultats expérimentaux des deux essais sur du sable dense sont discutés, démontrant les différences de réponses et de mode de rupture dépendamment de la direction de la charge sur la plate-forme. L’importance de la modélisation en trois dimensions est valorisée en démontrant expérimentalement que le scénario de charge symétrique n’est pas nécessairement conservateur. [ABSTRACT FROM AUTHOR]

Jack-up drilling rigs are usually founded on three shallow footings. Under wind, wave, and current loading offshore, the footings of these tall multi-footing systems transfer large moment loads in addition to self-weight, horizontal load, and even torsion to the underlying soil. To be able to deploy a jack-up safely at a particular offshore site, the unit’s capacity to withstand a 50 year return period storm is required to be checked in accordance with current guidelines (Site specific assessment of mobile jack-up units, The Society of Naval Architects & Marine Engineers). As the overall system behaviour is influenced significantly by the footing restraint, models that account for the complex nonlinear foundation–soil interaction behaviour are required to be integrated with the structural and loading models. Displacement-hardening plasticity theory has been suggested as an appropriate framework to formulate force-resultant models to predict shallow foundation behaviour. Recent research has extended such a model to account for six degree-of-freedom loading of circular footings on sand, allowing integrated structure–soil analysis in three dimensions. This paper discusses “class A” numerical predictions of experiments on a model jack-up in a geotechnical centrifuge, using the integrated modelling approach, and critically evaluates the predictive performance. The numerical simulations are shown to represent a significant improvement compared with the method outlined in the current guidelines. Les plates-formes de forage sont généralement installées sur trois semelles peu profondes. Sous l’effet des charges provenant du vent, des vagues et des courants marins, les semelles de ces hautes structures transfèrent des grandes charges de moment en plus de son propre poids, des charges horizontales et même de torsions du sol sous-jacent. Afin de pouvoir déployer de façon sécuritaire une plate-forme de forage sur un site spécifique en mer, sa capacité à supporter une tempête ayant une période de retour de 50 ans doit être vérifiée conformément aux directives actuelles (Site specific assessment of mobile jack-up units, The Society of Naval Architects & Marine Engineers). Comme le comportement du système est influencé significativement par les semelles, les modèles qui tiennent compte des interactions non-linéaires complexes sol – fondations doivent être intégrés aux modèles de structure et de chargement. La théorie de déplacement – durcissement plastique a été suggérée comme base pour formuler des modèles de type force-résultante pour prédire le comportement de fondations peu profondes. La recherche récente a étendu ce type de modèle pour tenir compte du chargement avec six degrés de liberté sur des semelles circulaires sur du sable, ce qui permet d’effectuer une analyse intégrée structure – sol en trois dimensions. Cet article discute de résultats de prédictions numériques de « classe A »d’expériences sur un modèle de plate-forme dans une centrifuge géotechnique en utilisant l’approche de modélisation intégrée, et évalue de façon critique les performances prédites. Il a été démontré que les simulations numériques sont une amélioration significative comparativement à la méthode suggérée dans les directives actuelles. [ABSTRACT FROM AUTHOR]

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The article presents a study which describes the combined loading behavior of circular footings on loose silica sand. The study which was conducted at an acceleration of 100 times that of Earth gravity in the drum centrifuge at the University of Western Australia aims to examine the components of the plasticity models determined from the results of the G. Gottardi and colleagues study on sand in 1999, and B. Byrne and co-worker in 2001. Information on the result was offered after the study.

Copyright of Géotechnique is the property of Thomas Telford Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Géotechnique is the property of Thomas Telford Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Géotechnique is the property of Thomas Telford Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A case is reported of a renal arteriovenous fistula following percutaneous needle biopsy resulting in heavy haematuria that was successfully occluded by transcatheter embolization with a Gianturco Mini Coil. [ABSTRACT FROM PUBLISHER]

Endothelin is a 21 -residue peptide vasoconstrictor produced by endothelium. Using a radioimmunoassay, endothelin values were measured in four groups of individuals: normal controls (0.54±0.12 pmol/1, n = 20); undialysed patients with chronic renal failure (CRF)(0.82±0.13 pmol/1, n = 38); chronic renal failure patients on continuous ambulatory peritoneal dialysis (CAPD) (2.81±0.63 pmol/1, n = 20); and patients with CRF on haemodialysis (HD) (4.52±1.21 pmol/1, n=14). The endothelin values were significantly greater in undialysed patients with CRF when compared with controls (P<0.001) and significantly greater in both dialysis groups when compared with controls and the undialysed CRF group (P≪0.001). The difference between the two dialysis groups was not significant (P=0.07). There was no correlation between endothelin and serum creatinine, mean arterial pressure, presence of chronic hypertension and/or diabetes, use of calcium-channel blockers and/or ACE inhibitors, or primary renal diagnostic category. A single haemodialysis session had no significant effect on endothelin values in the ten patients in whom this was assessed. Fast protein liquid chromatography (FPLC) appeared to confirm that the molecular species found in chronic renal failure were the same as those found in diabetic patients. [ABSTRACT FROM PUBLISHER]

Discusses the problems regarding renal impairment which are associated with elderly patients who suffer from hypertension. Consideration of the patients that need chronic dialysis; Status of the renal registry report for the United Kingdom in 2002; Causes of impaired mobility; Priorities associated with the treatment of elderly patients; Importance of quality of life for older patients.