Modelling the dynamic embedment of seabed pipelines / C. Y. Cheuk in Géotechnique, Vol. LXI N° 1 (Janvier 2011)  

Public ISBD [article]  inGéotechnique > Vol. LXI N° 1 (Janvier 2011) . - pp. 39-57 Titre : Modelling the dynamic embedment of seabed pipelines Type de document : texte imprimé Auteurs : C. Y. Cheuk, Auteur ; D. J. White, Auteur Année de publication : 2011 Article en page(s) : pp. 39-57 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Plasticity Clays Failure Theoretical analysis Centrifuge modelling Offshore engineering Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : The as-laid embedment of a seabed pipeline is an important design parameter. As a pipe is laid on the seabed it oscillates, owing to vessel motion and hydrodynamic loading of the hanging pipe. This movement significantly increases the pipe embedment beyond the theoretical value related to the static pipe weight, even when corrected for any stress concentration caused by the hanging catenary. Dynamic lay effects are either ignored in practice, or are accounted for by scaling up the static embedment by an empirical factor, leading to significant uncertainty in this important design parameter. A series of centrifuge model tests has been conducted using two clays – kaolin and a high-plasticity natural clay – to simulate the dynamic embedment process. The results indicate that only a few cycles of small-amplitude oscillation (±0·05D) are required to double or triple the pipe embedment, owing to the combined effect of lateral ploughing and soil softening. In these experiments the pipe embedment increased to up to eight times the static embedment after 100 cycles of motion, which represents a typical lay process. A model is proposed for the cycle-by-cycle embedment of a pipeline under a given sequence of small-amplitude oscillations at a given applied vertical force. The trajectory of the pipe movement is assessed using a flow rule derived from plasticity-based yield envelopes. The effect of soil remoulding is explicitly captured by linking the accumulated disturbance to the decay in soil strength. Using input parameters derived from theoretical considerations and T-bar penetrometer tests, the model captures the essential features of the dynamic embedment process. With modest optimisation of the model parameters, the mean discrepancy between the calculated and measured embedment is only 12% for both clays. The ultimate states predicted by this cycle-by-cycle model also provide a rough estimate of the maximum pipe embedment for fully remoulded conditions, which include some degree of water entrainment caused by the lay process, evident in the optimised parameters. This ultimate embedment is governed by the remoulded soil strength and the pipe weight (augmented by any stress concentration). The amplitude of the cyclic motion affects the rate of softening, and hence the rate of settlement. This model provides a framework for assessing the as-laid embedment of seabed pipelines on a more rigorous basis than current practice. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.148 [article] Modelling the dynamic embedment of seabed pipelines [texte imprimé] / C. Y. Cheuk, Auteur ; D. J. White, Auteur . - 2011 . - pp. 39-57. Génie Civil Langues : Anglais (eng) in Géotechnique > Vol. LXI N° 1 (Janvier 2011) . - pp. 39-57 Mots-clés : Plasticity Clays Failure Theoretical analysis Centrifuge modelling Offshore engineering Index. décimale : 624 Constructions du génie civil et du bâtiment. Infrastructures. Ouvrages en terres. Fondations. Tunnels. Ponts et charpentes Résumé : The as-laid embedment of a seabed pipeline is an important design parameter. As a pipe is laid on the seabed it oscillates, owing to vessel motion and hydrodynamic loading of the hanging pipe. This movement significantly increases the pipe embedment beyond the theoretical value related to the static pipe weight, even when corrected for any stress concentration caused by the hanging catenary. Dynamic lay effects are either ignored in practice, or are accounted for by scaling up the static embedment by an empirical factor, leading to significant uncertainty in this important design parameter. A series of centrifuge model tests has been conducted using two clays – kaolin and a high-plasticity natural clay – to simulate the dynamic embedment process. The results indicate that only a few cycles of small-amplitude oscillation (±0·05D) are required to double or triple the pipe embedment, owing to the combined effect of lateral ploughing and soil softening. In these experiments the pipe embedment increased to up to eight times the static embedment after 100 cycles of motion, which represents a typical lay process. A model is proposed for the cycle-by-cycle embedment of a pipeline under a given sequence of small-amplitude oscillations at a given applied vertical force. The trajectory of the pipe movement is assessed using a flow rule derived from plasticity-based yield envelopes. The effect of soil remoulding is explicitly captured by linking the accumulated disturbance to the decay in soil strength. Using input parameters derived from theoretical considerations and T-bar penetrometer tests, the model captures the essential features of the dynamic embedment process. With modest optimisation of the model parameters, the mean discrepancy between the calculated and measured embedment is only 12% for both clays. The ultimate states predicted by this cycle-by-cycle model also provide a rough estimate of the maximum pipe embedment for fully remoulded conditions, which include some degree of water entrainment caused by the lay process, evident in the optimised parameters. This ultimate embedment is governed by the remoulded soil strength and the pipe weight (augmented by any stress concentration). The amplitude of the cyclic motion affects the rate of softening, and hence the rate of settlement. This model provides a framework for assessing the as-laid embedment of seabed pipelines on a more rigorous basis than current practice. DEWEY : 624.15 ISSN : 0016-8505 En ligne : http://www.icevirtuallibrary.com/content/article/10.1680/geot.8.p.148

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Uplift mechanisms of pipes buried in sand / C. Y. Cheuk in Journal of geotechnical and geoenvironmental engineering, Vol. 134 N°2 (Fevrier 2008)  

Public ISBD [article]  inJournal of geotechnical and geoenvironmental engineering > Vol. 134 N°2 (Fevrier 2008) . - pp. 154–163 Titre : Uplift mechanisms of pipes buried in sand Type de document : texte imprimé Auteurs : C. Y. Cheuk, Auteur ; D. J. White, Auteur ; M. D. Bolton, Auteur Année de publication : 2008 Article en page(s) : pp. 154–163 Note générale : Geotechnical and geoenvironmental engineeringa Langues : Anglais (eng) Mots-clés : Buried pipes Soil deformation Sand Uplift resistance Particle size Imaging techniques Résumé : Reliable design against upheaval buckling of offshore pipelines requires the uplift response to be predicted. This paper describes a model-scale investigation into the mechanisms by which uplift resistance is mobilized in silica sand, and illustrates how the observed mechanisms are captured in prediction models. A novel image-based deformation measurement technique has been used. The results show that peak uplift resistance is mobilized through the formation of an inverted trapezoidal block, bounded by a pair of distributed shear zones. The inclination of the shear zone is dependent on the soil density, and therefore dilatancy. After peak resistance, shear bands form and softening behavior is observed. At large pipe displacements, either a combination of a vertical sliding block mechanism and a flow-around mechanism near the pipe or a localized flow-around mechanism without surface heave is observed, depending on the soil density and particle size. En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%291090-0241%282008%29134%3A2%2815 [...] [article] Uplift mechanisms of pipes buried in sand [texte imprimé] / C. Y. Cheuk, Auteur ; D. J. White, Auteur ; M. D. Bolton, Auteur . - 2008 . - pp. 154–163. Geotechnical and geoenvironmental engineeringa Langues : Anglais (eng) in Journal of geotechnical and geoenvironmental engineering > Vol. 134 N°2 (Fevrier 2008) . - pp. 154–163 Mots-clés : Buried pipes Soil deformation Sand Uplift resistance Particle size Imaging techniques Résumé : Reliable design against upheaval buckling of offshore pipelines requires the uplift response to be predicted. This paper describes a model-scale investigation into the mechanisms by which uplift resistance is mobilized in silica sand, and illustrates how the observed mechanisms are captured in prediction models. A novel image-based deformation measurement technique has been used. The results show that peak uplift resistance is mobilized through the formation of an inverted trapezoidal block, bounded by a pair of distributed shear zones. The inclination of the shear zone is dependent on the soil density, and therefore dilatancy. After peak resistance, shear bands form and softening behavior is observed. At large pipe displacements, either a combination of a vertical sliding block mechanism and a flow-around mechanism near the pipe or a localized flow-around mechanism without surface heave is observed, depending on the soil density and particle size. En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%291090-0241%282008%29134%3A2%2815 [...]

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