Centrifuge Modelling Tests And CFD Simulation Of The Impact On Pipeline Exerted By Submarine Landslides

Offshore oil and gas industry in our country has been developing rapidly since the 21st Century.As one of the most important structures for the exploitation and transportation of marine resources,the application of submarine pipelines has become more and more widespread.During service,the impact damage caused by submarine landslides to pipelines is one of the main risks faced by the safe operation of submarine pipelines.Accurate evaluation the impact of submarine landslides at different stages of migration with different impact angle on submarine pipelines under different suspended height is an important part of submarine pipeline's design.Relevant research results show that the current research results of submarine landslide impact on pipelines are mostly focused on theoretical analysis and numerical simulation.Physical model tests were rarely used to simulate the impact of submarine landslides on pipelines,especially the test results considering the impact angle.There are also limited discussions of the vertical forces created by submarine landslide on pipelines.In this thesis,the impacts of the submarine landslide on the pipeline during the initial stage and slip stage of debris flow are discussed through the centrifugal model test and the method of Computational Fluid Dynamics(CFD),and the key influence factors are analyzed.The content of this thesis mainly includes:(1)When submarine landslides encounter a submarine pipeline at a certain incident angle,the pipeline will be subjected to both normal and axial forces along the impact direction,and-existing instruments cannot achieve simultaneous measurements of those two forces.In order to meet the testing requirements of centrifugal model tests,a novel force-measurement device.named by Lateral Resistance Measured Penetrometer was independently developed,that is abbreviated to LRMP in the later chapters.The traditional 1g tests results show that the error of the LRMP in measuring the magnitude,direction and action position of the loading exceeded no more than 3%,2%and ± 1.5 mm,respectively.The reliability and stability of the LRMP in the centrifugal environment were verified by a series of centrifuge tests at 40g level.In addition,the application of the LRMP was extended for measuring the strength of shallow soft clay.Compared with the traditional T-bar penetrometer,the LRMP has the advantage of simple application and its feasibility of obtaining the continuous profile of the horizontal distribution of the undrained shear strength of the soil.(2)Using the UK kaolin clay,the self-weight consolidated soil sample was repeatably prepared under the condition of 40g.The soil samples with good repeatability could be obtained by controlling the initial water content of soil sample,the rotation speed of the centrifuge,and the criteria for judging the degree of consolidation of the soil sample to meet the testing requirements.And expressions describing the regularity of the variation of the degree of consolidation and void ratio of kaolin clay with the consolidation time under the specified test conditions were obtained.On this basis,the active pipe-dragging method was used to simulate the impact of submarine landslides with an impact velocity less than 0.1 m/s and a shear strength of 0.4-1.2 kPa on the pipeline.The tests data of different impact velocities and different impact angles were analyzed within a geotechnical mechanics framework.It was found that under the conditions of soil strength 0.96 kPa and impact velocity less than 0.1 m/s,the contribution of inertial force to the normal force was very small and could be ignored.Thus,the influence of impact velocity on the impact force can be expressed completely by adjusting the shear strength of the soil.Through comparison and analyzing the existing CFD calculation results,it shows that the test envelope and CFD envelope have their respective application ranges,and the Reynolds number of the submarine landslide can be used as the demarcation standard for judging the application of these two envelopes.In addition,the influences of both the thickness of the free soil surface of slides above the top of the pipeline and the impact angle of the submarine landslide to the impact force on pipelines were simulated by 40g centrifugal model tests.The analytical model proposed by Oliveira et al.(2010)was applied and extended to reanalyzing the centrifugal testing results,and an envelope was obtained that can consider the influences of the overburden thickness and impact angle of the submarine landslide to the impact forces on the pipeline.(3)The Computational Fluid Dynamic calculation(CFD)was first tried for simulation the impact of debris flow at the initial stage on the pipeline.Compared with the results of centrifugal model tests under the condition of normal impact,the bearing factor of pipeline calculated by CFD method was 7.9,which is slightly greater than the experimentally measured value 7.5.This comparison could help us to understand that the simulation of impact of debris flow at the initial stage on the pipeline by the CFD method is reliable.At other impact angles,the bearing factors of the pipeline calculated by CFD method is close to the experimentally measured values.Both the numerical simulation and the experimental conclusions regarding the influence of the thickness of the slide and the impact angle to the impact forces on the pipeline are consistent.(4)The CFD method based on the Herschel-Bulkley model was also used to simulate the impact of debris flow at slip stage on the pipeline laid on the seabed surface as well as the pipeline with different suspended heights at different impact angles.The calculation results were analyzed through both fluid mechanics and Hybrid approaches.The variations of normal and axial impact forces of the debris flow on the pipeline with the pipeline suspension height were discussed.Besides,the influence of the impact angle,the impact velocity of debris flow and the suspended height of the pipeline to the lift force on the pipeline were all discussed.The hydrodynamic expressions of the normal and axial impact forces of the submarine landslide that can take into account the impact angle of debris flow and suspended height of the pipeline at the same time were provided,as well as the relationship between the lift force coefficient and the pipeline suspended height.The analysis results show that the axial force on the pipeline laid on the seafloor surface is about 95%of the axial force on the pipeline at the critical suspended height.When the Reynolds number is small,the normal force on the pipeline laid on the seafloor surface may be 81%or lower of the normal force on the pipeline at the critical suspended height.However,the change in the suspended height of the pipeline had negligible effects on the normal force of the landslide on the pipeline when the Reynolds number is large.When the suspended height of the pipeline exceeds 2.5 times the pipeline diameter,the impact forces on the pipeline will no longer change with the further increase of the suspended height.In addition,the strength characteristics of the sliding sample itself is the most important factor affecting the impact force of the pipeline.Normally,the normal force of the pipeline under normal impact condition is the largest.However,when the pipeline that is laid on seafloor surface or at a low suspended height is impacted by debris flow with a higher Re number,the magnitude of the lifting force and normal force on the pipeline maybe is in the same order.At this time,the stability of the pipeline undertaking lifting force should be fully considered.