Study On Strength Properties Of Submarine Slides And Their Impact On Pipelines

In recent years,with the acceleration of the ocean development and utilization,the number,length and working depth of submarine pipelines have been increasing.During the service period of submarine pipelines,their stability directly affects the safety of the exploitation of ocean resources,cross-sea telecommunications,lives and property of workers,as well as ocean environment,which has a far-reaching influence on the ocean management and planning.However,due to the limitation of the submarine terrain conditions,the laying route of submarine pipelines is always close to the potential submarine slide regions,and even directly crosses the high frequency regions of slides in some cases,which poses a major challenge for the safety of ocean pipeline projects.The submarine slide is in a catalogue of marine geo-hazards with the following characteristics:occurrence frequently,affecting a wide zone,and prediction difficulty.After it triggered,the submarine slides will evolve to a dilute sliding body by the complex soil-water interaction.The sliding body shows the material properties of both the soil and fluid with the characteristics of large volume,high velocity,long run-out distance and strong impact force,with a huge threatening.Therefore,it is of great theoretical and practical significance to explore the strength properties of submarine slides and to improve the evaluation of hazard effects of submarine slides on pipelines.Based on above reasons,the undrained shear strength of typical marine soft soils is firstly tested and analyzed in this paper using an improved new full-flow penetrometer.Furtherly,combined with the rheometer test method,the rheological strength characteristics of submarine slides in a run-out stage are systematically discussed at different shear strain rates.Then,through the flume model tests,the occurrence conditions of the "Hydroplaning phenomenon" at the front of submarine slides and the deformation mechanism of the sliding front under its influence are studied.Besides,the interface mass transfer between the submarine sliding body and its surrounding water at different movement velocities is investigated.Finally,the computational fluid dynamics(CFD)and fluid structure interaction(FSI)numerical methods are employed to study the vertical impact of submarine slides on pipelines and the vertical displacement responses of pipelines encountered submarine slides,respectively.In addition,the submarine pipelines with a streamlined protection design and their hazard reduction effects are discussed.Due to the above works,the following results have been achieved:(1)Based on the principle of full flow strength test for soils,a new full-flow penetrometer is developed to suit for testing the strength characteristics of the marine soft soils with a very low strength,high water content and sensitivity.Strength distributions,soil structures,resistance coefficients of full-flow penetrometer test for marine soft soils in the northern slope of the South China Sea are revealed.Based on the power-law relationship between the undrained shear strength of remolded soils and the comprehensive index of water contents,a normalized undrained shear strength prediction model is proposed to suitable for the marine soft soils in the northern slope of the South China Sea,which is verified by previous studies.(2)Combined the benefits of the rheometer in shear strain rate tests and the full-flow penetrometer in strength tests,a combination test method is proposed and verified.For the three representative marine soft soils,the relationship between the rheological property parameters and the comprehensive index of water contents,and the effect of the shear strain rate on the soil sensitivity test results are obtained.Based on the theory of shear thinning behavior of non-Newtonian fluids,a piecewise rheological strength model is proposed,which can be used to describe the rheological strength characteristics of submarine slides in the full range of shear strain rates.(3)Based on the self-designed flume test system,the "Hydroplaning phenomenon" at the front of sliding body is successfully captured and monitored,the critical occur conditions of"Hydroplaning phenomenon" is verified,and the deformation mechanism of the sliding front under the "Hydroplaning phenomenon" influence is improved.Based on the critical movement conditions on the shear stress of soil particles,a flume test system is designed for testing the mass transfer of soil particles into water,and the variation law of the mass transfer flux at the soil-water interface of submarine slides under different movement velocity conditions is revealed using this test system.(4)Based on the CFD numerical method,the curve characteristics and load magnitude of the vertical forces of slide-pipeline interaction and the effect of the pipeline-seabed gaps on them are obtained.Based on the hybrid geotechnical and fluid mechanics framework,the influence of pipeline-seabed gaps on the geotechnical strength component and fluid inertial components of the vertical forces is explained step by step,and the concept and calculation method of a critical pipeline-seabed gap are recommended.Furthermore,the evaluation equation of vertical submarine slide-pipeline forces considering the influence of pipeline-seabed gaps is proposed and verified.(5)Based on the FSI numerical analysis with the arbitrary Lagrangian-Eulerian method,the numerical simulation of the coupling interaction between the submarine slides and pipelines is realized,and the simulation process is verified by a classical fluid mechanics question about the free settling of a small ball in water.The vertical displacement responses of pipelines encountered submarine slides is reveals.Through comparing their difference with a fixed pipeline,the influence of vertical displacement of pipeline under the slides impact on the design parameters of submarine pipeline projects is clarified.(6)Based on the resistance reduction theory of streamline,the streamlined design concept is applied into the outer layer design of submarine pipelines,and four specific streamlined contours and corresponding dimensional designs are given.Based on the CFD numerical method,the effect and mechanism of resistance reduction of the streamlined pipelines under the impact of submarine slides are revealed.and the prediction models on the impact force of slides for every streamlined pipeline are established.According to the general process from the design stage to the operation in the submarine pipeline projects,the influence of streamlined design on the performance and design parameters of every stage of pipeline projects is clarified.