Study On The Shutdown And Restart-up Characteristics For Heavy Oil Mixed With Water In Offshore Pipeline

In the event of a sudden power outage or system failure,the hot oil pipeline will shutdown.During the pipeline shutdown process,the crude oil fluidity will continue to deteriorate as the oil temperature decreases,which may cause a condensation accident.Therefore,the problem of the shutdown and the restart of hot oil pipelines has become one of the hot topics of safety flow protection.However,the existing research is mostly limited to the heat transfer pipeline of waxy crude oil on the land,but the research on the shutdown and restart of heavy oil pipelines is less.However,with the continuous development of marine oil and gas resources,especially offshore heavy oil resources,oil-water mixed transportation has become one of the common transportation methods for offshore heavy oils.Because the viscosity and shear stress of heavy oil are higher at higher temperatures,Moreover,the offshore pipeline has a low ambient temperature.In the early stage of heavy oil well exploitation,it is often necessary to mix water to achieve heavy oil drag reduction.The offshore pipeline shutdnow and restart has a large safety risk.To this end,LD thermal recovery heavy oil is taken as the research object,and the loop experiment,numerical simulation and theoretical analysis is used to study the shutdown and restart behavior of offshore heavy oil mixed water pipelines,in order to lay the theoretical foundation for ensuring the safety of offshore heavy oil mixed water pipelines.The main research work and understanding are as follows:(1)Using the relevant standard specifications and supporting equipment,the basic compositional parameters of SARA four components,freezing point and density of LD heavy oil were analyzed and tested;Anton Paar Rheolab QC viscometer was used to test and analyze LD heavy oil and its blending Rheological properties and viscosity-temperature characteristics of 10%to 40%W/O emulsions at different temperatures.The results showed that the LD heavy oil had higher asphaltene and colloid content and lower freezing point.The apparent viscosity at 50? and 10 s-1 was 164.4 mPa s,and the density at 20? was 945 kg/m3,it belongs to ordinary heavy oil.LD heavy oil and its W/O emulsions are mostly non-Newtonian fluid properties in the study temperature range,and can be well fitted with power law mode.(2)Using Anton Paar Rheolab QC viscometer to test the starting stress characteristics of LD heavy oil and its W/O emulsions under different conditions,the effects of water content,constant temperature standing time,temperature and shear rate on the maximum stress of heavy oil were studied.The results showed that before the reverse phase,the maximum stress of heavy oil will increase with the increase of water content.After the reverse phase,the maximum stress of heavy oil starts to decrease significantly with the increase of water content.The maximum stress of heavy oil increases with the increase of shear rate.The maximum stress of the heavy oil starts to decrease with the increase of the starting temperature.The maximum stress of the heavy oil is less affected by the constant temperature standing time.(3)Experimental pipeline was used to simulate the shutdown and restart process,restart pressure and flow changes of heavy oil pipelines under different conditions.Analyzed the effect of the following aspects on the restart pressure:water content,constant temperature shutdown time,oil temperature inside the pipeline,and starting flow;the loop calculation pressure drop and measured loop restart pressure,the wall stress calculated by the loop data and the measured maximum stress were compared and analyzed;the measured restart pressure was used to predict the required restart pressure of the heavy oil in the on-site pipeline under the same operating conditions.It was found that the restart pressure of the heavy oil pipeline mainly depends on the viscosity and structural parameters.The maximum stress measured in the measured test is about three times the calculated value;the predicted on-site pipeline restart pressure is larger.(4)The multi-phase flow software OLGA was used to simulate and analyze the shutdown and restart process of the offshore heavy oil water-mixed pipeline,the law of shutdown temperature drop and restart pressure.The results showed that the optimal water mixing rate and safe shutdown time of the simulated pipeline are 50%and 32 h.