Effect and mechanism of disaster reduction of pipelines with double-elliptic streamline contour against impact of submarine landslides

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With the rapid development of marine energy extraction, the demand for submarine pipelines has been increasing in recent years. The stability of pipelines during their service period directly affects the safety of the oil and gas exploitation, the workers' life and the marine ecological environment. In view of the fact that submarine pipelines are vulnerable to the damage from landslides, a novel type of submarine pipeline with a double-elliptic contour is developed. Then, the effect and mechanism of disaster reduction of the pipeline under the impact of landslides are analyzed based on the computational fluid dynamics (CFD) method. The results show that the developed pipeline, no matter in a suspended or laid-on-seafloor status, can delay the separation of velocity boundary layer near the pipeline surface to reduce the influence of Karman Vortex Street. Thus, the drag force and lift force of pipelines imposed by submarine landslides are effectively reduced,with a maximum lessening percentage of 71.01% for drag force coefficients and 32.14% for lift force coefficients. Moreover, the equations for estimating the drag force and lift force coefficients of double-elliptic contour pipeline are recommended, which provides a new reference for the disaster fortification and mitigation engineering on submarine pipelines.

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High temperature and high pressure can lead to a lateral global buckling of unburied or shallowly buried submarine pipelines. Because soil resistance determines the deformation and stress distribution of post-buckling pipelines, it is important to characterize the soil resistance in the pipeline global buckling analysis. A series of model tests based on the sand sampled from Bohai Gulf is conducted. The soil resistance to pipelines with different buried depths are measured. A dynamical soil resistance model varying with the buried depth of pipeline is developed, and the influence of buried depth on peak soil resistance and final soil resistance is analyzed. As the built-in penalty interaction model in ABAQUS software cannot simulate the dynamical friction in pipe-soil interaction, a user-defined subroutine VFRIC is developed to implement the established soil resistance model and used to simulate the variation of soil resistance with pipeline displacement for accurate pipeline global buckling analysis. The study shows that the soil resistance model can significantly affect the results of post-buckling pipeline. Because the dynamical soil resistance model has a peak value and an attenuation process, the critical buckling force of post-buckling pipelines with dynamical soil resistance model is larger than pipeline with constant soil resistance, the deformation of post-buckling pipelines with dynamical soil resistance model is more concentrated, and the maximum bending moment and strain are larger.
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Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe-soil velocities of up to 4.2 m/s. The changing density and shear strength of the samples were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature.
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Deepwater offshore oil and gas developments require an assessment to be made of the risk of infrastructure damage from submarine slides. The likelihood and magnitude of submarine slides, and the consequent impact loading on seabed infrastructure in the path of the debris from the slide, must be estimated. Export pipelines are especially vulnerable to impact from submarine slides, because of their length and the need to cross canyons and other seabed features that are potential paths for the flowing debris. Characterising the debris material represents a particular challenge, as the original soil, which is typically characterised using conventional geotechnical methods, evolves through remoulding and water entrainment into a viscous fluid. Because of this transition from soil to fluid, characterisation of the strength of flowing fine-grained sediment has been addressed separately within a soil mechanics framework and a fluid mechanics framework, resulting in two different approaches for expressing the strain-rate-dependent strength of debris flows, and the consequential impact loads on pipelines. In this paper we compare the two approaches, and show that the geotechnical characterisation of fine-grained sediments can be extended into the liquid range in a continuous fashion. This is supported by a series of undrained shear strength measurements on two different remoulded soils, from fall cone tests, vane shear (including viscometer) tests, T-bar and ball penetrometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content, for a given soil. Furthermore, the effects of rate of shearing are well captured by a dimensionless function of the normalised strain rate. The geotechnical approach also accounts for the observed strength reduction due to intense shearing.
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Results are presented from experiments and computational fluid dynamics (CFD) simulations involving the streamlining of suspended underwater pipelines or structures subject to impacts from sediment/water mixtures. The experiments showed that a considerable reduction in force may be achieved by streamlining, and this reduction in force is most significant at lower Reynolds numbers. Several shapes were tested: round, airfoil, and wedge. Of these, the wedge exhibited the lowest drag coefficient-about one-fifth of the cylindrical pipe at low values of the Reynolds number, and about half at higher Reynolds numbers. DOI: 10.1061/(ASCE)WW.1943-5460.0000113. (C) 2012 American Society of Civil Engineers.
Impact forces induced by submarine landslides on pipelines are estimated using a computational fluid dynamics (CFD) approach. It is found that the predicted forces using the CFD approach are consistent with those predicted using the conventional geotechnical approach. It is also found that the impact angle of the debris flow induced by landslides affects the normal drag factor but hardly has any effect on the longitudinal drag factor. Empirical formulae for estimating normal and axial impact forces on a free spanned pipeline induced by an oblique debris flow are developed based on numerical test results. A new failure envelope that can be used in conventional geotechnical approaches is also obtained based on the numerical test results.
Computational Fluid Dynamics (CFD) numerical analysis was employed to analyze the situations tested experimentally, as described in Part I. The methodology and results of the CFD analyses are discussed and compared with the observations made from the experiments. The numerical model performed satisfactorily with regard to obtaining the impact forces exerted on the model pipe as well as simulating the hydroplaning phenomenon and estimating slurry flow heights. The experimental results were combined with the results of the CFD analyses to develop a practical method to compute the drag force caused by a submarine debris flow impact on a pipeline. The CFD analyses provided some insight to the separated region characterization, but the attempt to analyze the vortex shedding phenomenon as observed in the experiments was unsuccessful. Additional studies are required for better understanding of both the separated region characteristics and vortex shedding.
Estimating the impact forces exerted by a submarine debris flow on a pipeline is a challenge, and there is room for considerably more work to advance the state of the art. To this end, an experimental program was performed to investigate the impact on two pipeline installation scenarios: 1) suspended pipeline and 2) laid-on-seafloor pipeline. The results and observations from the experimental investigation are discussed. The definition of Reynolds number was modified for non-Newtonian fluids and an ad hoc method was developed to estimate the drag force exerted by an impact perpendicular to the pipe axis. The method may be used in prototype situations to estimate the drag force from submarine debris flow impact on pipelines. The experimental program was complemented by Computational Fluid Dynamics (CFD) analyses, the details of which are discussed in the accompanying paper.
Landslides are common on inclined areas of the seafloor, particularly in environments where weak geologic materials such as rapidly deposited, fine-grained sediment or fractured rock are subjected to strong environmental stresses such as earthquakes, large storm waves, and high internal pore pressures. Submarine landslides can involve huge amounts of material and can move great distances: slide volumes as large as 20,000 km³ and runout distances in excess of 140 km have been reported. They occur at locations where the downslope component of stress exceeds the resisting stress, causing movement along one or several concave to planar rupture surfaces. Some recent slides that originated nearshore and retrogressed back across the shoreline were conspicuous by their direct impact on human life and activities. Most known slides, however, occurred far from land in prehistoric time and were discovered by noting distinct to subtle characteristics, such as headwall scarps and displaced sediment or rock masses, on acoustic-reflection profiles and side-scan sonar images. Submarine landslides can be analyzed using the same mechanics principles as are used for occurrences on land. However, some loading mechanisms are unique, for example, storm waves, and some, such as earthquakes, can have greater impact. The potential for limited-deformation landslides to transform into sediment flows that can travel exceedingly long distances is related to the density of the slope-forming material and the amount of shear strength that is lost when the slope fails.
To predict debris flow characteristics it is necessary to determine the rheological properties of these mixtures. It is suggested that such a mixture can be obtained by adding successively to clear water, coarser particles derived from a sample of the debris flow material. At each addition different suspensions are obtained that can be considered as progressively coarser interstitial fluids of the complete mixture. These suspensions are easier to test and their rheological properties can thus be determined. The behavior of the complete material can then be inferred from these results. This procedure has been adopted for a debris flow which occurred on Moscardo Torrent (Friuli Region, Northeastern Italy) in 1995. A wide range of rheometrical techniques have been used to determine the rheological characteristics, such as laboratory rheometer, inclined plane test, large-scale rheometer, and field tests. The rheological parameters and behavior type of the complete material have been inferred from the yield stress-solid fraction curve and from the flow curve evolution. The behavior has been found to be mainly viscoplastic and representable by a Herschel-Bulkley model.
The prehistoric Storegga Slide, one of the world largest known submarine slides, took place about 8200 years ago. Most likely, the slide was triggered by an earthquake in a steeper slope area in the distal part of the slide. The present-day's morphology of the slide area indicates that major parts of the slide took place as a sequential failure process spreading successively from the far end to the present shelf edge. Numerical simulations illustrating this retrogressive, back-stepping behaviour are presented for the last phase of the Storegga Slide as it reached today's upper headwall. The applied rheological model is based on a Bingham fluid with a history dependent yield strength and consistency.
Due to the recent development of well-integrated surveying techniques of the sea floor, significant improvements were achieved in mapping and describing the morphology and architecture of submarine mass movements. Except for the occurrence of turbidity currents, the aquatic environment (marine and fresh water) experiences the same type of mass failure as that found on land. Submarine mass movements, however, can have run-out distances in excess of 100 km, so their impact on any offshore activity needs to be integrated over a wide area. This great mobility of submarine mass movements is still not very well understood, particularly for cases like the far-reaching debris flows mapped on the Mississippi Fan and the large submarine rock avalanches found around many volcanic islands. A major challenge ahead is the integration of mass movement mechanics in an appropriate evaluation of the hazard so that proper risk assessment methodologies can be developed and implemented for various human activities offshore, including the development of natural resources and the establishment of reliable communication corridors. [Journal Article; 126 Refs; In English; Summary in English and French]
Electronic document. Mode of access: World Wide Web. Title from title screen; viewed on 02/13/07. "Project report prepared for the Minerals Management Service, under the MMS/OTRC cooperative research agreement, 1465-01-99 CA-31003, task order 18217, MMS project 421." "August 2003." "OTRC Library Number: 8/03B121." Thesis (M.S. in Engineering)--University of Texas at Austin, 2003. Includes bibliographical references.
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