Research On The Preheating And Commissioning Process Of Subsea Hot Oil Pipeline Based On Numerical Simulation Method

Being the main procedure before commissioning, preheating is closely related to whether a newly-built hot oil pipeline can be put into operation safely. However, subsea pipelines have the features of varable conditions, complicated structure, unusual environment and the fact that the accurate temperature field around subsea pipeline is difficult to measure. Meanwhile, there is a few papers focus on heat transfer of subsea pipeline, and on treating preheating and commissioning as a whole. So, the changing regularity of operational parameters is not clear, which makes the preheating schedules adopted spend much more energy. Hence, analyzing the preheating and commissioning process of subsea pipeline in detail, and establishing a simulation method, is not only beneficial to predict the parameter changing regularity during preheating and commissioning process, but also to understand the operation characteristics of subsea pipeline at the initial stage of production. It will make sense in carrying out reasonable commissioning programs, and guiding practical operation.In consideration of the unsteady thermodynamic and hydraulic problems, a newly-built subsea hot oil pipeline located in Bohai Sea was selected as the research object. Based on the fundamental of fluid mechanic and heat transfer theory, the numerical simulation was adopted. By the method, it was deeply studied that parameter changing regularity, influencing factors, optimal plans, time for starting to transport oil, etc. The main research contents and results are shown as follows:(1) Based on the field procedure, the thermodynamic system of the subsea pipeline was analyzed, and the physical and mathematical models for both the preheating and commissioning process were established. Adding the equation of pig motion, the real situation was described that a pig was used to separate oil and water. Studying the period that crude oil, pig and water co-exist in the pipeline carefully, a comprehensive coupling algorithm for preheating and commissioning the subsea pipeline was addressed, which was programmed by using Matlab. Simulation result was verified by field data.(2) According to the established simulation algorithm for preheating process, it was studied that the thermodynamic parameter changing regularity and the influence of different factors of the selected pipeline during preheating process. Results showed subsea pipeline had a little impact on the temperature field distribution of sea mud during preheating process, which reavealed the fact that the preheating medium mainly heated the pipeline. Equivalent total heat transfer coefficient can reflect the transmission of hot water and temperature more realistic than that of the total heat transfer coefficient obtained by reversing calculation. For the specified pipeline, heat loss caused by risers was weak, while the thermal conductivity played a dominant role on outlet temperature among the sea mud properties. The result also showed, subjected to the equal heating power, a better preheating effect was obtained at a higher flow rater of the preheating medium and a lower inlet temperature.(3) According to the established simulation algorithm for commissioning process, it was studied that the changing behavior of operational parameters and the influence of different factors of the selected pipeline during commissioning process. The difference and reason between subsea and onshore pipeline in temperature changing regularity during commissioning process was reavealed, too. Result showed that, upon the arrival of oil head, the outlet temperature would decrease sharply and turned to stable state generally, instead of decreasing followed by increasing which happened on onshore buried pipeline. Result also showed, in the trasition stage, the oil head temperature even become higher than inlet temperature. With the distance and time became longer, the increasing disappeared. Stable oil temperature was lower than that of preheating water at the same location.(4) The outlet water temperature, minimum oil head temperature and thermal equilibrium principle were adopted to determine the time of starting transportation of crude oil for the specified subsea pipeline. Result showed that, the preheating duration calculated by minimum oil head temperature was much lower than other methods, but lacked verification by real case. The thermal equilibrium principle being also suitable for subsea pipeline, its realization procedure was too complicated. Although the result calculated by outlet water temperature took the longset preheating time, it showed the obvious advantage in convenience. A subsea pipeline could be started to transport crude oil as soon as the outlet temperature was preheated higher than the freezing point. In comprehensive consideration of safety, successful case and convenience, the outlet water temperature was recommened to determine whether the transportation of crude oil could be started in field.(5) Specific to the possible situation that commissioning cannot be implemented immediately after preheating, case of delaying transportation of crude oil was researched. By simulating the temperature drop of preheating water during shutdown period and commissioning process, the maximum shutdown time of the selected pipeline was carried out as 107h and 89h respectively for normal environment and extreme environment in winter.(6) The subsea pipeline could be preheated from both sides. With the established model, the order and reverse preheating process, and the corresponding commissioning process were simulated, temperature changing regularity of pipeline entrance, exit, and oil-head occurred during the preheating and commissioning process under different environment conditions and preheating time were explored. The result showed that, the order and reverse preheating operation can reduce water consumption, while the preheating time will be extended. For commissioning process, temperature changing regularity was relevant to reverse preheating time, and the steady temperature and time needed were in accordance with that of the order preheating and commissioning process.