Study On Hydrate Blockage Mechanism And Characteristic In Natural Gas Transportation Pipeline

Natural gas hydrate is a kind of ice-like crystal compound.which is usually formed in the mixture of gas and water with high pressure and low temperature environment condition.The environmental natural gas hydrate was early discovered in natural gas transportation pipeline.Because the formed hydrate resulted in the hydrate deposition and blockage in the pipeline,it will bring serious safety problems and economic losses to industrial production of pipeline.In order to prevent hydrate blockage,it is necessary to study the mechanism and characteristics of hydrate blockage.Therefore,the study of gas hydrate particle movement mechanism was used to analyze the cause of hydrate blockage.Clarifying the changes of hydrate slurry viscosity and using the pressure drop signal to monitor the hydrate formation and blockage process based on the flow loop system.Furtherly.the influence of different pipeline structures,operating conditions and hydrate blockage removal operations to hydrate blockage were studied.The research provided the theoretical basis for the safety of the pipeline transportation and engineering guidance advice.For studying the hydrate particles movement mechanism,this paper optimized the model of particle critical bedding velocity by force analysis and parameter optimization,and calculated the critical flow velocity which could ensure the hydrate particles stable flow without bedding in the pipeline.For studying the hydrate particles removal process,the changes of capillary force caused by liquid bridge deformation were introduced into the model.The hydrate particle removal critical flow velocity model was established,and calculated the critical flow velocity which could ensure the hydrate particles without attachment to the surface in the pipeline.The hydrate particle removal critical flow velocity was further verified by the experiments,and the critical flow velocity of particle rolling and sliding were measured respectively.Within the range of error less than 10%,the prediction accuracy of the model with rolling removal was 75%,and that with sliding removal was 90%.The model can be used to calculated the minimum flow velocity to ensure the hydrate particles without deposition and accumulation in the pipeline.In order to study the process of hydrate from formation to blockage,the change of hydrate vliscosity value in hydrate formation-accumulation-deposition process in different initial experimental conditions was measured.The regularity that the viscosity value increased during hydrate formation and agglomeration while decreased during deposition were verified.For further studying the process in the pipeline,this paper designed and built a high pressure fully visual flow loop,which can transport gas-liquid-solid mixture three phase flow under high pressure and low temperature.Hydrate formation,agglomeration,deposition and blockage in the pipeline were in-situ monitored using the change of pressure drop and combined with images in the flow loop.The water conversion rate was calculated,and the early warning judgment points of hydrate from formation to blockage stages based on the pressure drop were put forward,revealing the difference of hydrate flow characteristic from formation to blockage process under different liquid loading and pipeline flow velocity.Based on the above hydrate formation and blockage research,this paper further observed the changes of hydrate slurry flow characteristic and hydrate blockage regularities in many areas such as sloped pipes,deadleg and U-shaped pipes,and found hydrate blockage position.The influence of shut-in and re-start operation on hydrate blockage during pipeline transportation process was expounded.The results showed that too long shut-in time will increase the subcooling degree of hydrate formation in pipeline and further accelerate hydrate blockage process.In hydrate blockage removal process,it was proposed that high liquid loading can use step-wised depressurization and low liquid loading can use direct depressurization method to ensure the optimal blockage removal efficiency.