The Experts below are selected from a list of 285 Experts worldwide ranked by ideXlab platform
J B Jonas - One of the best experts on this subject based on the ideXlab platform.
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central retinal artery and vein Collapse Pressure in giant cell arteritis versus nonarteritic anterior ischaemic optic neuropathy
Eye, 2008Co-Authors: J B Jonas, B HarderAbstract:Central retinal artery and vein Collapse Pressure in giant cell arteritis versus nonarteritic anterior ischaemic optic neuropathy
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central retinal artery and vein Collapse Pressure in eyes with chronic open angle glaucoma
British Journal of Ophthalmology, 2003Co-Authors: J B JonasAbstract:Aims: To determine central retinal vessel Collapse Pressure in chronic open angle glaucoma. Methods: For 19 eyes with chronic open angle glaucoma and 27 eyes of a control group, central retinal vessel Collapse Pressure was measured by a Goldmann contact lens fitted with a Pressure sensor in its holding grip. Results: Central retinal vein Collapse Pressure was significantly (p=0.001) higher in the glaucoma group than in the control group (26.1 (SD 26.4) relative units versus 6.1 (8.4) relative units). Conclusions: Measured by a new ophthalmodynamometer, central retinal vein Collapse Pressure measurements may be abnormally high in eyes with chronic open angle glaucoma.
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reproducibility of ophthalmodynamometric measurements of central retinal artery and vein Collapse Pressure
British Journal of Ophthalmology, 2003Co-Authors: J B JonasAbstract:Background: To assess the reproducibility of ophthalmodynamometric measurements using a new, Goldmann contact lens associated, device allowing biomicroscopic visualisation of the optic disc. Methods: The prospective clinical study included 87 eyes of 58 subjects presenting with a normal fundus (n=40), or ocular diseases (n=47). With topical anaesthesia, a Goldmann contact lens, fitted with a Pressure sensor mounted into the holding ring of the contact lens, was placed onto the cornea. Pressure was applied onto the globe through the contact lens, and the Pressure values obtained when the central retinal vessels started pulsating were noted. The measurements were performed 10 times. Results: The mean coefficients of variation for redeterminations of the Collapse Pressure of the central retinal vein and artery were 16.3% (SD 11.4%), and 8.5% (4.1%), respectively. Conclusions: A simple and new, Goldmann contact lens associated, ophthalmodynamometer allows central retinal artery and vein Collapse Pressure measurements which are reproducible in a clinical setting.
Stelios Kyriakides - One of the best experts on this subject based on the ideXlab platform.
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Collapse of partially corroded or worn pipe under external Pressure
International Journal of Mechanical Sciences, 2008Co-Authors: Naoto Sakakibara, Stelios Kyriakides, Edmundo CoronaAbstract:Abstract This paper is concerned with the effect of internal corrosion or erosion defects on the Collapse of pipelines under external Pressure. Experiments are conducted on SS-304 tubes with D/t's of 21.0 and 18.7 with axially uniform grooves of several thicknesses and angular spans of 6–60°. All specimens exhibited the classical ovalization mode of Collapse with the grooved side being more deformed. For groove angles between 6° and 20° the Collapse Pressure decreases as the angle increases but remains essentially constant for angles between 20° and 60°. The Collapse Pressure was found to decrease with groove depth, reducing by nearly 50% when the groove depth reached 50% of the pipe wall thickness. The experiments were simulated numerically using both the custom computer code BEPTICO as well as an axially uniform finite element model. The predictions were found to follow closely the trends of the experimental results. The three dimensionality of grooves was examined using appropriate finite element models. It was found that grooves shorter than about 8D constitute local imperfections and accurate prediction of their effect on Collapse Pressure requires fully 3-D modeling. By contrast, grooves 10D long or longer behave essentially as “long” grooves and can be analyzed using 2-D models.
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effects of the uoe uoc pipe manufacturing processes on pipe Collapse Pressure
International Journal of Mechanical Sciences, 2007Co-Authors: M D Herynk, Stelios Kyriakides, A OnoufriouAbstract:Large-diameter pipes used in offshore applications are commonly manufactured by cold-forming plates through the UOE process. The plate is crimped along its edges, formed into a U-shape and then pressed into an O-shape between two semicircular dies. The pipe is welded closed and then circumferentially expanded to obtain a highly circular shape. Collapse experiments have demonstrated that these steps, especially the final expansion, degrade the mechanical properties of the pipe and result in a reduction in its Collapse Pressure upwards of 30%. In this study the UOE forming process has been modeled numerically using a 2-D finite element model. The model can assess the effects of press parameters of each forming step on the final geometry and mechanical properties of the pipe. The final step involves simulation of pipe Collapse under external Pressure in order to quantify the effect of the forming variables on its performance. Examples of these variables are the radii of the forming dies, the chosen displacements of the dies, the compression strain in the O-step, the expansion strain, etc. An extensive parametric study of the problem has been conducted, through which ways of optimizing the process for improved Collapse performance have been established. For example, it was found that optimum Collapse Pressure requires a tradeoff between pipe shape (ovality) and material degradation. Generally, increase in the O-strain and decrease in the expansion strain improve the Collapse Pressure. Substituting the expansion with compression can not only alleviate the UOE Collapse Pressure degradation but can result in significant increases in Collapse performance.
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optimization of uoe pipe manufacturing process for improved Collapse performance under external Pressure
Volume 1: Project Management; Design and Construction; Environmental Issues; GIS Database Development; Innovative Projects and Emerging Issues; Operat, 2006Co-Authors: Stelios Kyriakides, M D HerynkAbstract:Large-diameter pipes used in offshore applications are commonly manufactured by cold-forming plates through the UOE process. Collapse experiments have demonstrated that these steps, especially the final expansion, degrade the mechanical properties of the pipe and result in a reduction in its Collapse Pressure, upwards of 30%. In this study, the UOE forming process has been modeled numerically so that the effects of press parameters of each forming step on the final geometry and mechanical properties of the pipe can be established. The final step involves simulation of pipe Collapse under external Pressure. An extensive parametric study of the problem has been conducted, through which ways of optimizing the process for improved Collapse performance have been established. For example, it was found that optimum Collapse Pressure requires a tradeoff between pipe shape (ovality) and material degradation. Generally, increase in the O-strain and decrease in the expansion strain improve the Collapse Pressure. Substituting the expansion by compression can not only alleviate the UOE Collapse Pressure degradation but can result in a significant increase in Collapse performance.Copyright © 2006 by ASME
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on the arresting efficiency of slip on buckle arrestors for offshore pipelines
International Journal of Mechanical Sciences, 2004Co-Authors: L H Lee, Stelios KyriakidesAbstract:Slip-on buckle arrestors are tight-fitting rings placed over the pipe at intervals of several hundred meters. They locally increase the pipe resistance to Collapse by providing additional circumferential rigidity, and thus impede downstream propagation of Collapse. This type of arrestor offers important advantages over other designs as it does not require welding. Alternatively, when split in two and by the addition of flanges, it can be clamped onto a continuous pipe. This is an essential characteristic for lines installed by the reeling method. It has long been known that such devices often cannot reach higher levels of arresting efficiency. The somewhat deficient performance is due to the fact that a propagating buckle can penetrate such devices via a folded-up U-mode at Pressures that are lower than the Collapse Pressure of the pipe. Previous work on the subject from the 1970s dealt with relatively thin-walled pipes used in shallow waters. The subject has recently been revisited and bounds for this deficiency in performance have been established. The deficiency depends on the pipe D/t and on the steel grade. This paper presents the results of a more detailed study of such arrestors for pipes with lower D/t values (18–35) suitable for moderately deep and deep waters. The arresting efficiency has been studied parametrically through experiments and full scale numerical simulations. The results have also shown the previously developed efficiency bounds to be viable. A new empirical design formula has been developed. Used in conjunction with the efficiency bounds, it provides a reliable method for an expanded use of slip-on type buckle arrestors in many deepwater applications.
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efficiency limits for slip on type buckle arrestors for offshore pipelines
Journal of Engineering Mechanics-asce, 2002Co-Authors: Stelios KyriakidesAbstract:Slip-on buckle arrestors for offshore pipelines offer important advantages over other arrestor designs as they do not require welding. An additional advantage offered by the related concept of the two-part clamp-on arrestor is that it can be installed onto a continuous pipe. It has long been known that such devices often cannot reach higher arresting efficiencies. As a result, the use of slip-on arrestors in deepwater applications has been limited. The somewhat deficient performance is due to the fact that a propagating buckle can penetrate such devices via a folded-up U mode at Pressures that are lower than the Collapse Pressure of the pipe. This can take place even for very rigid rings. A methodology for generating bounds on the maximum arresting efficiency that can be expected from slip-on arrestors is developed. An upper bound of their efficiency is related to the Pressure at which a propagating buckle starts to penetrate a long and stiff circumferential confinement. A lower bound is established by relating the crossover Pressure to the lowest Pressure at which the buckle will propagate through the confinement. The parametric dependence of both of these Pressures was established experimentally. The method developed yields the range of pipe D/t's where degradation in arresting efficiency can be expected and predicts its extent. The arresting efficiency degradation was shown to depend on the yield stress of the pipe. Pipe of higher yield stress exhibits smaller degradation in arresting efficiency.
Clovis De Arruda Martins - One of the best experts on this subject based on the ideXlab platform.
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flexible pipes influence of the Pressure armor in the wet Collapse resistance
Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme, 2014Co-Authors: Clovis De Arruda MartinsAbstract:When submitted to high external Pressure, flexible pipes may Collapse. If the external sheath is damaged, all the external Pressure is directly applied on the internal polymeric layer that transmits the loading to the carcass layer, which can fail due to this effect, leading to wet Collapse. This failure mode must be taken into account in a flexible pipe design. A model can be set up neglecting the influence of the Pressure armor, but this assumption may underestimate the wet Collapse Pressure value. This work aims to include the Pressure armor effect in the numerical prediction of wet Collapse. The main contribution of the Pressure armor to the flexible pipe resistance to Collapse is to be a constraint to the radial displacement of the carcass and the internal polymeric layers. Two models were developed to find the wet Collapse Pressure in flexible pipes. A first study was done using a ring approximation three-dimensional (3D) finite element method (FEM) model. Comparisons are made with more simplified models using a 3D FEM equivalent ring approximation. The aim is to clarify the mechanical behavior of the Pressure armor in the wet Collapse scenario. Parametric studies of initial ovalization of carcass and initial gaps and interference between polymeric layer and Pressure armor are made and discussed.
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flexible pipes influence of the Pressure armor in the wet Collapse resistance
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Clovis De Arruda MartinsAbstract:When subjected to large valued external Pressures, flexible pipes may Collapse. If the external sheath is damaged, all the external Pressure is directly applied to the internal polymeric layer that transmits the loading to the carcass layer. When the carcass layer fails due to this effect, the wet Collapse occurs. This failure mode must be taken into account in the flexible pipe design. The study for this problem can be done neglecting the influence of the Pressure armor, but this assumption may underestimate the wet Collapse Pressure value. This work aims to study the Pressure armor effect in the numerical prediction of wet Collapse. The main contribution of the Pressure armor to the flexible pipe resistance to Collapse is to be a constraint to the radial displacement of the carcass and the internal polymeric layers. Two models were developed and compared with the purpose of calculating the critical value of the external Pressure that causes carcass layer to Collapse. The first and most complete study is done using a ring 3D FEM model that takes into account both the real Pressure armor and carcass real profiles. In the second model, the Pressure armor is considered adopting an equivalent ring simplification. The comparison of the results of both the models clarifies how the behavior of the Pressure armor in the wet Collapse situation is. Parametric studies of initial ovalization of the carcass and initial gaps in manufacturing of flexible pipes are made and discussed.Copyright © 2011 by ASME
A. B. Mandal - One of the best experts on this subject based on the ideXlab platform.
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two dimensional surface properties of an antimicrobial hydantoin at the air water interface an experimental and theoretical study
Colloids and Surfaces B: Biointerfaces, 2010Co-Authors: Ramya Santhana Gopala Krishnan, Satya Panigrahi, Sathiah Thennarasu, A. B. MandalAbstract:Abstract 5,5-Acetamidomethyl-5-methylimidazolidine-2,4-dione shows antimicrobial activity against bacteria at millimolar concentrations well above its critical micellar concentration (cmc). Two-dimensional surface properties were investigated using Langmuir Film Balance to understand the membrane-active nature and nanomaterial behavior of this hydantoin derivative. Hydantoin forms an expanded nanofilm at air–water interface. The maximum limiting surface area ( A 0 ) and Collapse Pressure ( π c ) are dependent on hydantoin concentration. Hydantoin undergoes a change in orientation at the interface, in the Pressure region 2.5 and 7.5 mN m −1 , corresponding to surface areas 51–15 and 41–12 A 2 molecule −1 , respectively. A large Collapse Pressure ( π c ) in LB film indicates a role for hydrophobic interactions in the self-assembly of hydantoin. Surface areas computed using Connolly method, are in good agreement with the experimental results. Monolayer studies suggest a dispersed state for hydantoin when its concentration is below cmc, suggesting a mechanism for the observed bacteriostatic activity of hydantoin. In the present study, it has been found for the first time that the minimum inhibitory concentration (MIC) of the hydantoin is very close to its cmc.
Anthony E. Walsby - One of the best experts on this subject based on the ideXlab platform.
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digital recordings of gas vesicle Collapse used to measure turgor Pressure and cell water relations of cyanobacterial cells
Journal of Microbiological Methods, 2009Co-Authors: Daryl Philip Holland, Anthony E. WalsbyAbstract:Abstract The gas vesicles of the cyanobacterium Microcystis sp. Collapse under Pressures ranging from 0.65–1.10 MPa, determined from turbidity changes in a Pressure nephelometer. In turgid cells, Collapse occurs at a lower range of Pressures; the difference is equal to the cell turgor Pressure. The turgor Pressure decreases, however, as gas vesicles Collapse; this decrease is minimised by calculating the turgor Pressure in samples with few of their gas vesicles Collapsed. Previously, Pressure and turbidity were measured in discrete steps, using analogue meters, or continuously, using chart recorders: turgor Pressure was calculated from the mean or median Collapse Pressures. We describe modifications allowing continuous digital recording; the output was modelled with polynomial or sigmoid functions, the latter providing the best fit over the full Collapse-Pressure curve; turgor Pressure could then be calculated for any point on the Collapse-Pressure curve. The shape of the Collapse-Pressure curve was affected by the rate of Pressure rise; curves were similar to those from step-wise methods when the Pressure was raised at approximately 4 kPa s − 1 . Under a rapid, almost instantaneous, rise in Pressure there was a larger initial decrease in turgor and from the subsequent recovery the hydraulic conductivity of the cell surface could be calculated; the new method gave improved measurements of the cell volumetric elastic modulus. Following Collapse of half the gas vesicles, cells recovered their full turgor Pressure after 3 h. This suggests turgor homeostasis. These methods are applicable to other bacteria with gas vesicles, including Escherichia coli , if it could be genetically modified to express transgenic gas vesicles.