| In recent years,the development of China's marine resources has entered the deep-sea areas.As the lifeline of oil and gas resources exploitation,the safe operation of submarine pipelines has attracted increasing attention.Deep-sea pipelines are often laid on the seabed surface,lacking effective protection measures,and are easily damaged,causing immeasurable economic losses and environmental pollution.In general,the site selection of submarine pipelines should avoid marine geological disasters as much as possible,especially in areas prone to submarine landslides;and thus,it is necessary to carry out submarine landslide susceptibility assessment in the early stage of submarine pipeline deployment.However,during the long-distance transportation of deep-sea pipelines,the pipeline inevitably crosses the submarine landslide-prone area or landslide migration area.In such cases,it is essential to evaluate impact forces on the submarine pipelines imposed by the destructive accidental load(e.g.,submarine landslides),to optimize the submarine pipeline design,which is an important way to resist the impact of submarine landslides.Furthermore,when the impact forces are extremely large and traditional pipeline structure design cannot meet the safety capacity,necessary protective measures should be provided to reduce the submarine landslide impact.These studies are of great scientific significance and engineering value to ensure the operational safety of submarine pipelines.In this thesis,first,the undrained strength evaluation method of marine surficial soft soil under the mechanism of non-full flow penetration and the susceptibility assessment method of regional submarine landslides are proposed.The methodology is applied to the seismic susceptibility assessment of surficial submarine landslides in the northern continental slope of the South China Sea,and then the seismic susceptibility mapping is compiled,which provides an important reference for route selection of submarine pipelines.Second,the low temperature-water content coupled rheological model of submarine landslides and the determination method of the flow state of fluidized landslides are established.Considering the application range of rheological models,the model design theory of landslide-pipeline interaction is proposed based on Reynolds criterion.The CFD modelling of submarine landslide-water-pipeline interaction is improved and verified,and the evolutionary process and formation mechanism of landslide impact forces are revealed.The prediction formulas of impact forces on pipelines under complex conditions are established,which provides key theories for pipeline design.Finally,the self-protection design concept of honeycomb-hole submarine pipeline to reduce the landslide impact is proposed,the drag reduction effect and mechanism of the honeycomb-hole pipeline are analyzed,and the optimization design method is given,which provide an effective way for pipeline protection.The specific research work and results obtained are as following.(1)A multi-probe penetrometer is developed,the centrifugal tests of three probes(i.e.,ball,cone and streamline)penetrating the South China Sea soil and kaolin clay under water environment are carried out,and the variation of ball penetration resistance in the surficial penetration process is given considering the ambient water.CFD modelling based on the Euler-Euler two-phase flow model is developed.The accuracy of the proposed CFD modelling is verified by the results of centrifugal tests using the results of centrifugal tests.Based on CFD results,the evolution law of ball penetration resistance during the surficial penetration process is analyzed,and the effect mechanism of ambient water is revealed.Taking the critical depth as the boundary,the formulas of the surficial and stable penetration resistance coefficients are given,and the methods of testing and evaluating the undrained shear strength of marine surficial soil by the ball penetration are established.(2)Considering the bidirectional quasi-static seismic loads and marine soil strength weakening model,a stability analysis model of submarine slope with multi-layer soils is established.Then,the stability analysis of surficial submarine slopes at discrete stations is carried out.Taking the quantitative safety factor as the evaluation standard,a regional submarine landslide susceptibility assessment method is proposed by extending these safety factors of discrete stations to the entire study area based on the spatial interpolation theory.The database of geological background,seismic distribution and physical and mechanical parameters of soils in the northern South China Sea are established.Based on the existing data,the seismic susceptibility assessment of surficial submarine landslides in the northern continental slope is implemented,the prone areas of landslides are identified,and the seismic susceptibility mapping of submarine landslides is compiled.(3)Considering the seabed temperature environment,low-temperature rheological tests of four water content submarine mudflow samples in the submarine landslide-prone area of the South China Sea are carried out.The effect mechanism of temperature and water content on the rheological properties of the samples is revealed by the interparticle interaction and Brownian motion.Then,based on the Herschel-Bulkley model framework,the low temperature-water content coupled rheological model of samples is established.Depending on the rheological test principle,the formula of Reynolds number of the fluidized landslide in the rheological test is derived.Considering the critical Reynolds number as the evaluation standard,a method to determine the flow state of the submarine landslide is established,and the internal mechanism of the flow state transition of samples is analyzed.This method is applied to analyze the flow state of submarine mudflow samples of the South China Sea.(4)Considering the applicable range of the submarine mud/debris flow rheological model,the formulas for determining the pipeline diameter and impact velocity of submarine landslide in the submarine landslide-pipeline interaction test and numerical models are derived based on the Reynolds criterion,and the corresponding design theory is established.Depending on the model design theory,the CFD model of submarine landslide-pipeline interaction is improved based on the CFD incompressible Eulerian-Eulerian two-phase flow model.The accuracy of the proposed CFD model is verified by a combination of qualitative and quantitative means by using the results of the classical indoor flume test and previous CFD numerical results.Finally,a generalized CFD model is proposed for analyzing submarine landslide-pipeline interaction under complex conditions.(5)The relative size and spatial relation between the submarine landslide and pipeline are analyzed,and CFD numerical simulations of submarine landslide-ambient water-pipeline interaction under complex conditions are carried out.The results show that the low-temperature environment and pipeline span height have a significant effect on impact forces.Three modes of pipeline force are summarized,and two necessary conditions of the lift oscillation are given.The concept of peak and stable impact forces(i.e.,drag force and lift force)on pipelines impacted by landslides is proposed,and the formation mechanism of peak and stable impact forces is explained.The prediction formulas of the peak drag force coefficient,stable drag force coefficient,peak lift force coefficient and stable lift force coefficient are established.Furthermore,considering the submarine landslide thickness above the pipeline,a standard chart method is proposed to improve the prediction formulas of the peak and stable drag forces.(6)To reduce the impact of submarine landslide,the self-protection design concept of a honeycomb-hole submarine pipeline is proposed,and the design standard of honeycomb-hole pipelines is given.Compared with circular pipelines,the honeycomb-hole pipelines effectively reduce the peak drag forces,and narrow the gap between the peak and stable drag forces,resulting in a smooth transition.The maximum drag reduction rate is close to 23%.Honeycomb-hole pipelines also effectively suppress the lift force fluctuations,and greatly decrease the amplitude of lift force vibration,which can reduce the risk of pipeline damage caused by vibration.Based on the evolutionary theories of the boundary layer and flow field,the drag reduction mechanism is revealed.Finally,the optimization design method of the honeycomb-hole pipeline is proposed,and the optimal design solution is given. |
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