1,722,584 research outputs found
In situ decommissioning of subsea infrastructure
No full textFrom an engineering and ecological perspective is it more rational to leave offshore oil and gas infrastructure in situ after decommissioning? This paper addresses that question. Options for decommissioning offshore infrastructure are set out. The emphasis of this paper is decommissioning of subsea infrastructure, which comprise a large proportion of current oil and gas offshore infrastructure, and an increasing proportion of total offshore assets in the future. This is due to the global trend towards multi-field developments tied back to a host platform or directly to shore, or to a floating LNG production platform. Potential challenges associated with retrieval of subsea structures and pipelines, and their long term stability, are considered from a geotechnical perspective. Viewed through the lens of in situ decommissioning, the evidence of a general long term rise in bearing capacity and stability indicates potential for infrastructure to have a safe and valuable post-operational afterlife as part of the marine ecosystem
Partially mobile shallow subsea foundations: a practical analysis framework
No full textThe geotechnical design of partially mobile subsea foundations (mudmats) for pipeline/flowline end terminals (PLETs) is presented in this paper. A partially mobile mudmat represents a fit-for-purpose engineering solution that has significant commercial competitiveness. The partially mobile design lies between that of a fully anchored mudmat (which is designed for negligible movements but may be too large, causing installation issues or requiring corner piles to anchor) and a fully mobile mudmat (which moves to fully accommodate the expansion of the connected pipeline, but may suffer excessive settlements that compromise the structural integrity). The partially mobile mudmat is suited to deepwater soft soil conditions. The aim of this work is to help mature this new concept and technology for practical design and to inspire future research to improve the accuracy of predictions. The objective of the paper is to present simple new analytical solutions to predict the long-term accumulated displacements and rotations of a partially mobile mudmat on soft clayey deposits subjected to cyclic loading. The proposed displacement prediction framework combines established elements of consolidation theory, plasticity theory, and critical-state soil mechanics (CSSM). Typical ranges of soil properties pertinent to a partially mobile mudmat are provided for deepwater Gulf of Mexico (GoM) soft clays, and a design analysis example is provided. For these conditions, it is concluded that the dominant displacements of a partially mobile mudmat are caused by primary consolidation and plastic failure. Recommendations for further improvement are provided to inspire further research. </p
Geotechnical centrifuge modelling – current practice
No full textGeotechnical centrifuge studies are used during the design of a growing range of structures, especially to study soil-structure interaction. They are particularly useful to both clients and design engineers when a new design concept or ground improvement process is proposed for which full-scale experience and performance data are lacking, when an unusual soil type or stratigraphy is encountered, and when soil properties evolve over time due to complex loading or changing environmental boundary conditions. The accelerated time scale that the centrifuge creates allows whole-life studies to be made that would be impossible by any other means.</p
An investigation into the behaviour of pressed-in piles
Full text linkAn investigation into the behaviour of pressed-in piles has been conducted. The press-in method of pile installation allows large pre-formed foundation piles to be constructed without the noise and vibration associated with conventional dynamic techniques, and with minimal requirement for temporary works. This investigation is divided into two parts; a fundamental study of the mechanics of press-in pile installation in sand and a sequence of field tests to examine the behaviour of pressed-in piles at full scale.The mechanics of pile installation have been studied using a plane strain calibration chamber. A new system for deformation measurement in plane strain modelling and other forms of geotechnical testing has been developed. This system combines techniques of digital photography, Particle Image Velocimetry (PIV) and close range photogrammetry. A series of validation experiments demonstrated that the system offers greater accuracy and precision than existing measurement techniques. This improved performance is achieved concurrent with an order-of-magnitude increase in the number of measurement points that can be established within the observed soil.A series of 8 calibration chamber tests is reported. The pattern of soil displacement during pile installation was measured. These measurements were of sufficient quality to allow soil strain paths during installation to be calculated. The influence of soil type and initial density was examined, and the post-installation strain distribution was found. The concentration of shear and volumetric strain close to the pile tip was quantified, and a reversal of strain direction as the soil passes around the pile shoulder was observed.A zone of highly compacted soil was observed immediately below the pile tip and along the pile shaft. Contraction of this sleeve of broken soil grains was observed with continued penetration of the pile. A mechanism is proposed to link this kinematic observation to the distribution of shaft friction close to the tip of displacement piles. A further mechanism is proposed to predict the distribution of external shaft friction along the upper part of a pile shaft. This mechanism is based on vertical arching theory, and is an extension of a previous approach for the prediction of internal shaft friction.Four series of field tests using pressed-in piles were conducted. The first series demonstrated that internal shaft friction is well predicted by vertical arching theory. Since vertical arching evolves according to an exponential function, pile performance can be dramatically influenced by only small changes to the governing parameters. The improvement of driveability using an internal driving shoe was investigated.The final series of load tests demonstrated a novel foundation solution in which the high shaft friction created by vertical arching can be ‘switched on’ after installation. This is achieved using a construction sequence involving H-section piles. During press-in installation of each pile, the geometry does not create vertical arching. During loading of the entire structure, arching occurs. This leads to a high positive group effect, and an efficient foundation structure.<br/
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