Study On Rebirth Of Submarine Oil Pipeline

During the commissioning or operation of subsea pipelines, the shutdown process is inevitable due to accident or planned maintenance. In order to ensure the safety of offshore oil and gas production and pipeline gathering systems, it’s significant in oil production to study the rule of shutdown and restart process of subsea pipelines.At present researches on shutdown and restart of hot oil pipelines are mainly targeted at terrestrial high waxy crude oil pipelines. Subsea pipelines have much in common with terrestrial long-distance pipelines, but they still have many distinct differences from terrestrial pipes. In this paper, by drawing on the methods of terrestrial pipelines shutdown and restart, models of steady-state and transient heat transfer process have been built up, based on the analysis of submarine pipeline thermodynamic system. For the restart process, the start filling which is caused by the compression and contraction of crude oil has been taken into account. The models are solved by the finite element method combined with thermal shock theory and characteristic method with the help of the Matlab program. The program has been used in this paper to simulate the shutdown and restart of a submarine buried pipelines and a floating pipeline. The results are as follows:(1) For the subsea buried pipeline, in the case of low ambient temperature in winter, the cooling process after shutdown can be divided into three stages. Because the soil temperature below the pipe is higher than that above the pipe in winter, the highest oil temperatures is moving to the place below the axis, there will be a atage that center oil temperature is almost constant during the cooling process. In summer the ambient temperature is high, the highest temperature zone of the oil has been located on the upper side of the axis of the pipe, the temperature drop process during shutdown has only two stages.(2) For the shutdown of floating submarine pipeline, because of relatively uniform ambient seawater temperature, the similar phenomenon does not appear.(3) The oil temperature and flow rate increase slowly at first in the restart process. After the hot oil reaches both temperature and flow rate increases rapidly within a short time and finally stabilized. For thixotropic fluid, its wall shear stress decay quickly in the start the filling process.