Assesment Of Hydrate Plugging Probability In Gas-condensate Pipelines

The blockage of hydrates will bring severe safety problems and economic losses to the operation of natural gas condensate pipelines.So that to determine the reasonable conditions of safe operation requires systematic evaluation of flow safety under different working conditions.Various factors that affect the safety situation of pipeline flow are obviously uncertain.The traditional deterministic evaluation method cannot fully reflect the pipe flow safety condition systematically and comprehensively.Therefore,this paper presents an evaluation method of gas condensate pipeline flow safety based on reliability,which describes the safety of pipeline operation quantitatively by hydrate plugging probability.In this study,a quasi-steady-state mathematic model and a numerical solution method based on hydrate formation and gas condensate pipeline flow characteristics are established.According to the hydrate shell inward and outward growth model,the formation of hydrates along the pipeline and the pressure and temperature changes along the line can be predicted.The influence of pressure,temperature and flow rate on the growth and the plugging process of hydrate were investigated,and the main influencing factors of hydrate formation and plugging were clarified.Based on the hydrate slurry flow mechanism model,the limit state was described by the volume fraction of hydrate,and the critical volume fraction was determined according to literature and experimental data.So that the risk probability of hydrate plugging in gas condensate pipelines can be quantitatively estimated with probabilities solved based on reliability.When it comes to evaluating the safety of the pipeline with the module above,there are two problems which cannot be ignored: the low efficiency of ordinary sampling method and the large amount of small probability event demands.Aiming at these two problems,Latin hypercube sampling(LHS)method is introduced to deal with the poor convergence performance of the traditional Monte Carlo method,which reduces the number of sampling times in to a large extinct.Meanwhile,the Proper Orthogonal Decomposition(POD)algorithm is applied to calculate the related value of gas condensate pipeline,and the calculation efficiency is greatly improved.In this paper,the evaluation method based on reliability is applied to analyze the uncertainty parameters of hydration risk assessment concerned of the operating condition of gas condensate pipeline,as the influence of random parameters like pressure and temperature on the probability of plugging is studied quantitatively.And the method of reducing the plugging probability and its effect are studied.Different random variables and different sample size will have variable impacts on the calculated value of hydrate plugging probability.The effect of water cut on the hydrate plugging probability is obvious,so the water cut of the gas condensate pipeline should be strictly controlled.With the increase of time,the probability of plugging at different positions of pipeline shows a tendency of changing,and the plugging probability needs to be analyzed in combination with the pipeline elevation change.When the bulk phase flow rate is lower than the critical suspension flow rate,the hydrate deposition layer will appear.So that hydrate is not the volume fraction of hydrate only factor influencing pipe plugging which should be concerned.The flow model of hydrate slurry introducing deposition theory was established,and the fluid-solid coupling model was applied to the prediction of pipeline hydrate plugging probability.The PFC3 D particle flow simulation software was used to simulate the flow of hydrate slurry,and the blockage probability of hydrate slurry under specific working conditions was analyzed.The influence of pipe flow path and flow velocity on the probability of hydrate plugging was clarified.