Once the gas hydrate is formed, immeasurable damage will be imposed on pipelines especially for long-distance, high-pressure, deep-water subsea wet gas pipeline. Therefore, the normal solution is strict control and precaution the hydrate's formation. The thermodynamic inhibitors, such as ethylene glycol (MEG), methyl alcohol (MeOH) always be injected into wet-gas pipeline. Injection suitable dosage inhibitor helps to lower down hydrate formation temperature to prevent hydrate's formation in the stage of commissioning and continuous production. However, there is hardly any reports on hydrate's precaution when in shut-in. In this paper how to economically and safely avoid the formation of hydrate in the phase of shutdown is studied. Based on a subsea wet gas pipeline ZB which lying in the Bohai bay of China, and with the help of dynamic multiphase flow simulator OLGA, key parameters along the pipeline are computed, which show that hydrate will be formed after several hours' shutdown. For a safe operation, measures should be taken to both the planned and emergence shutdown: To the planned shut-in of the ZB, the regular practice of elevating dosage of MEG doesn't work as to the topography. To the emergence shut-down, method of venting and burning is chosen against the hydrate's formation, which can be also used in planned shutdown when others method is failure. In order to cut down the venting gas amount to the greatest extent, the law between venting rates and the total amount of burned gas is depicted and analyzed. Finally, an optimal venting rate of 2000 Sm3/d is achieved for pressure relieving after shutdown. The results and the analytical approach adopted in the paper provide some technical support for the similar offshore wet gas pipeline and other multi-fluctuate onshore wet-gas pipeline.
A large amount of experimental and theoretical studies have been conducted by Researchers, and some measures for prediction, control, and elimination of gas hydrate when pipelining wet-gas in deep-water (Guo, Sun, and Gao, 2016, Li, Wang, and Rao, 2014), cold-environment (Zhao, Deng, and Liu, 2014) have been developed since the Davy first discovered the gas-hydrate in 1810, and Hammer Schmidt found that the gas hydrate would plug the gas pipeline in 1934. Hydrate is a combination product of high pressure, low temperature and free water, which will be formed in the process of drilling (Guan, Ren, and Sun, 2014), recovery, processing, and transportation (Bi and Jia, 2009, Guang and Tian, 1996). Once the three factors encountered simultaneously in nature gas pipeline or pressure vessel, the gas hydrate will be formed to plug the gas pipeline resulting in dramatic damage to the oil and gas industry (Hou, Lei, and Zhao, 2013, Yu and Zhang, 2005). Therefore, the normal solution is strict control and precaution the hydrate's formation in the stage of commissioning and operation (Li, Wang, and Zhou, 2014, Ng and Robinson, 1976). There are several ways, including dehydration, insulation, heating, inhibitor injection and depressurization, to avoid the formation (Notz, Burke, and Hawker, 1991). Among them, inhibitor injection method is extensively used in offshore oil and gas industry when consideration of limited platform space, expensive installation and procurement fee, which has a significant different with onshore (Xu, Li, and Chen, 2006, Yi, Zhou, and Zhu, 2012). Injection suitable dosage inhibitor help to lower down the gas hydrate equilibrium temperature and prevent the formation of hydrate to benefit the high pressure wet-gas pipeline's safe running in the cold environment.
