Safety Evaluation Of Suspended Submarine Pipeline

The environment of submarine pipeline is complex.During long-term service,it will be affected by various loads and other factors,especially the combined action of wave load and ocean current will expose the pipeline,resulting in the suspended section.The vortex induced resonance phenomenon of the suspended section of the pipeline and the random action of wave load will lead to fatigue failure and damage the integrity of the pipeline.For the suspended section of submarine pipeline,this thesis comprehensively considers the influence of wave,current and nonlinear pipe soil interaction,and the fatigue life evaluation method of suspended pipeline,which provides a basis for the technical standards of subsequent submarine pipeline research.Firstly,considering the effects of internal pressure,wave load and nonlinear soil spring,the finite element mechanical model of submarine suspended span pipeline is established,and the influence of wave load on the stress and displacement of pipeline is calculated.It is concluded that the axial characteristics of pipeline are almost the same before and after wave load is applied.Discuss the influence of different factors on the suspended section: the variation law of the maximum Mises stress,displacement and axial strain in the static analysis of the suspended section of the pipeline with the influence of the suspended length,operating internal pressure and seawater velocity,among which the suspended length is the most important factor,which lays a foundation for the study of the dynamic vibration characteristics of the submarine suspended pipeline.Secondly,the transient dynamic model of submarine suspended pipeline is established,and the nonlinear modal analysis and vortex induced response calculation are carried out.At the same time,the influence of velocity and frequency on the dynamic response of suspended pipeline is analyzed.It is calculated that the maximum suspended length of suspended pipeline in the sea area where the pipeline is located is30 m.Considering the dynamic response of suspended pipeline under different wave loads: other conditions are the same,with the increase of wave height and wave period,the maximum bending stress and maximum response amplitude of the pipeline increase;The stress and deformation of the pipeline are different and fluctuate greatly with different incident directions.Finally,using rain flow counting method,random fatigue theory and miner’s linear cumulative damage law,the fatigue damage calculation method of suspended pipeline under the action of wave and current is established.Combined with det Norske Veritas standard DNV-RP-C205,the fatigue life of the dynamic response results of the pipeline is evaluated,and the fatigue life of the suspended section of an actual operating pipeline in the Bohai Sea area is calculated: the fatigue life of 24 m suspended pipeline is 9.9A,The fatigue life of 17 m suspended pipeline is 33 A.The influence of different wave loading conditions on the fatigue life of suspended pipeline and the influence of different suspended length on the fatigue life of pipeline are discussed: the period,height and incident direction of wave have relatively little influence on the fatigue life of pipeline with the same suspended length;The fatigue life of pipelines with different suspended length is quite different.