Oil pipelines buried in sea mud are easily affected by adverse environments such as ocean currents and storm surges,and are washed away until exposed on the seabed,thereby losing the heat storage and insulation function of sea mud.To study the shutdown temperature drop and condensation process of semi-buried subsea oil pipelines,a three-dimensional unsteady heat transfer model containing multiple elements such as sea mud,seawater,crude oil,pipelines,insulation layer,etc.was established.The temperature field near the pipeline before shutdown was used as the initial condition to calculate the heat dissipation and condensation process of crude oil after shutdown.The variation patterns of crude oil temperature and condensation rate with position and shutdown time were analyzed.The results show that after the shutdown of the semi-buried pipeline,the crude oil temperature drops first quickly and then slowly.When the shutdown lasts for 80 hours,the crude oil temperature drops by more than 25 ℃.During this process,the central oil temperature of the pipeline is always higher than the temperature at the wall,and the temperature difference between the two first increases and then decreases.The maximum temperature difference is up to 26.8 ℃ when the pipeline is shut down for 20 hours.After the semi-buried pipeline is shut down for 20 hours,the crude oil begins to solidify from the wall and gradually expanded towards the center of the pipeline,with the solidification amount continuously increasing.When the shutdown lasts for 80 hours,the crude oil solidification reaches 30％.The temperature drop of crude oil in semi-buried pipelines is faster than that in fully buried pipelines.At the same time,the average temperature is 1.8 ℃ lower and the average solidification rate is 2 percentage points higher.There are significant differences in the condensation time and amount of crude oil in different regions,which should be distinguished.The findings can provide reference for the shutdown and restart operations of semi-buried subsea oil pipelines.
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