Study On Submarine Suspended Pipelines About Stress Comprehensive Analysis

Submarine pipelines play an important role in offore oil and gas gathering transpotation and it is susceptible to suspend under complex marine environment,which have significant impact on offore oil and gas production.Under the backgrand about China develope Blue Ocean Strategy of increasing development of offore oil and gas resources production.Study on the submarine suspended pipelines about comprehensive stress distribution properties has significant reference for pipeline maintenance and management to achieve secondary application.This paper takes rhe submarine suspended pipelines of chegndao oilfield as reaserach object.A comprehensive analysis of the stress distribution characteristics of pipelines from the aspects of multi-phase flow in pipes,the role of marine dynamic environment outside the pipe,and calculation of the maximum allowable suspension length.Analyzing the effece of submarine suspended pipeline deformation on multiphase flow with the methord of theoretical explanations and numerical simulation mutual authentication base on the software of Fluent and AutoPIPE.According to the flow field explaning the formation mechanism of slug flow and then verification by theoretical explanations;As for the impact of marine dynamic environment on pipeline,had calculated the effect of sea floor soil load on pipeline stress,The stress effects of different storm conditions(wave height,cycle,and flow rate)and different seismic conditions on different suspended heights,and verifying AutoPIPE calculation reliability with Fluent.Finally,calculated the maximum allowable suspended length of pipeline.The research results indicate that the suspension deformation of the pipeline causes changes in the liquid retention rate at the bottom of the pipeline,which makes the gas-liquid interface destabilize and causes the gas phase into downstream of the pipeline with high speed.Then,the Multi-phase flow is severely disturbed at the upper bend of the pipeline to form a slug;The axial force of the pipeline is higher than vertical direction,and the stress concentration of pipelines in slope shoulder and the upper tilting buckling section is too large,which has a strong damage effect under slug flow condition.In the absence of a slug,the main reason of pipeline vibration caused by multi-phase flow is pulsating pressure excitation.Under stable wave flow conditions,the maximum equivalent stress of the pipeline decreases with the increase of the suspended height of the pipeline,but there is also an inverse change of stress during the sinking of the pipeline,mainly due to the self-weight factors and flow field changes caused by the depression of the seabed;The hoop stress and axial stress of the pipe has no change with the suspending height.Comparing Fluent with AutoPIPE calculation results,the error of C_d is 5%,which verifies that the calculation result of AutoPIPE is reliable and satisfied for practical engineering.At the same suspended height,the effect of wave height and velocity on the maximum equivalent stress of the pipeline is more significant than wave period;The maximum equivalent stress changes of buried pipeline is positively correlated with seismic intensity,and the influence of seismic that shear waves on pipeline stress is more obvious than longitudinal waves,but the influence of earthquake on pipeline buried depth is not significant.