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More often than not,the subsea pipelines are oil and gas multiphase flow pipelines.To the subsea pipeline of low oil gas ratio,the gas flowrate and delivery pressure are much high.In the winter season,the ambient temperature is low,and moreover,because of the throttle influence caused by wax precipitation,the pipeline outlet temperature is lower.In addition,there is water in the oil and gas well products,hydrate may be formed in someplace of the pipeline in the process of flow.The wax slug and hydrate slug maybe simultaneously exsit in the pipeline,which is not good for normal delivery of the fluid and has bad effct on the pipeline safety. In this paper, based on theoretical analysis and numerical simulation,the influence of the liquid hydrocarbon existence and wax precipitation on hydrate formation in the subsea pipeline has been studied.In this paper,the Phase Equilibrium Simulation software has been used to simulate and cauculate the influence.According to the results,the existence of the liquid hydrocarbon can inhibit the formation of hydrate, and the inhibitory effect of mixtures of liquid hydrocarbons turns out to be additive;With increasing amount of the wax,the distribution of components changes,which results in the reduction of hydrate formation pressure at the same temperature,that is to say,the hydrate pressure-temperature curve moves to right,which means that it can promote the formation of hydrate.In this paper,the subsea pipeline from JZ21-1 WHPA to JZ20-2 BOP has been taken for example,the change of temperature caused by wax throttle has been cauculated to analyze the hydrate formation.According to the results,under subcritical flow,with the increase of diameter of orifice,the throttle pressure ratio increases,and the pressure after throttle increases,and moreover,the pressure loss reduces;with the reduction of diameter of orifice,the temperature reduction caused by throttle increases;If the diameter of orifice caused by wax precipitation is less than 12.3mm,the temperature reduces at the time of passage of gas,which can reach the temperature demand of hydrate formation and result in the simultaneous existence of wax and hydrate.In addition,the solution of wax and hydrate slug has been suggested in the end of the paper.The result is important to the operation and management of the offshore gas pipelines.
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