Quantitative Evaluation Method For Hydrate Blockage Risk In Multiphase Transportation Pipeline

The nucleation,growth,aggregation and adhesion-deposition of hydrates in multiphase transportation pipelines can lead to blockages.Hydrate blockage has become one of the key flow assurance issues in the oil and gas industry.The primary key to efficiently and economically controlling hydrate blockage in high-pressure pipeline systems is an effective and accurate evaluation of the risk of hydrate blockage in pipelines.The quantitative evaluation of hydrate plugging risk is still in the initial stage,and there is a lack of research on hydrate plugging risk under equipment failure.In this paper,the flow and blockage characteristics of hydrate in pure water system and oil-dominated system are explored based on experimental study,and the evaluation index of hydrate plugging in pure water system and oil-dominated system are proposed respectively.Secondly,an efficient numerical algorithm has been used to calculate the hydrate plugging probability by the limit state method,enabling the quantitative evaluation of hydrate plugging risk.Finally,the reliability index of hydrate prevention and control is proposed considering the hydrate formation risk in multi-phase pipelines,and a method for calculating the probability of hydrate blockage in multi-phase pipelines under unit failure is formed.The details of the study are as follows:(1)Through the analysis of the experimental data of hydrate formation in the highpressure flowloop of the pure water flow system,combined with the changes of the macro data of pressure drop,flow rate and hydrate volume fraction and high-pressure visual window observation,it is found that the continuous accumulation of hydrate particles induces the formation of stationary deposit layer,which is the main cause hydrate blockage in the pipeline of the pure water.Three flow types of pure water system are observed: unplugged,slow plugged and fast plugged.Based on the critical velocity calculation of solid-liquid three-layer flow model,Fr is proposed as the evaluation index of hydrate blockage in pure water system.In addition,based on the numerical simulation methods of computational fluid dynamics(CFD)and discrete element method(DEM),the commercial software Fluent and EDEM are used to calculate the established threedimensional model of the solid-liquid pipeline containing hydrate particles,to simulate the process of hydrate blockage caused by accumulation and deposition.Two simulation strategies,adding a throttling element and setting up a particle plant,are implemented to analyse the variation characteristics of hydrate particle concentration distribution,particle accumulation state and particle velocity distribution state before throttling element,and to investigate the particle concentration distribution and the pressure drop variation for single and multiple collisions between upstream and downstream hydrate aggregates.(2)The experimental data of hydrate formation in the oil-dominated flow system are analyzed to explore the hydrate flow and blockage characteristics in the oil-dominated system.The experimental results show that the continuous formation and accumulation of hydrate deposits is the key factor for hydrate blockage in the oil-dominated system.Three flow types of the oil-dominated system are observed: rapid blockage type,slowsecurity type,and slow-blockage type.Based on the theory of shear-limited aggregation,the hydrate particles forces are divided into two types of forces that promote and inhibit the particle aggregation.The hydrate blockage risk evaluation index HBRI for the oildominated system is established from the perspective of particle force analysis.(3)A probabilistic method is developed to evaluate the risk of hydrate blockage.The hydrate blockage limit state equations with hydrate volume fraction and HBRI as evaluation indexes are established respectively.A large number of samples are generated using Latin Hypercube sampling(LHS)according to the distribution characteristics of variables,and the hydrate volume fraction under large samples is quickly obtained by combining with the Proper orthogonal decomposition(POD)method,which realized the reconstruction of the physical field of OLGA-CSMHy K simulation.This method can predict the hydrate blockage probability at any time and at any location of the multiphase pipeline under multiple variables.In addition,the reliability of the method is verified by using the field data of the actual oil and gas multiphase pipeline in the South China Sea.The calculation results of hydrate volume fraction and HBRI as limit state equation are compared.The uncertainties of pipeline operating parameters(inlet temperature,outlet pressure,flow rate after reduced production,heat transfer coefficient and initial droplet diameter),as well as the effects of running time and topography on the hydrate blockage probability are analyzed.(4)Considering the hydrate formation risk of multiphase pipeline,the hydrate control reliability index is proposed to characterize the ability of the system to completely avoid hydrate formation in the multiphase transportation pipeline under specified time and conditions.Relying on the failure data of OREDA’s subsea production equipment,the stochastic process of system state transfer is simulated by Monte Carlo method and the system reliability considering hydrate risk is evaluated.Combined with the limit state equation for hydrate plugging using HBRI as the evaluation indicator,the calculation method of hydrate plugging probability under equipment failure is formed;the influence of equipment failure on hydrate blockage in multiphase transportation pipelines is analysed.