Abstract
The subsea production system is presently widely adopted in deepwater oil and gas development. The throttling valve is the key piece of equipment of the subsea production system, controlling the safety of oil and gas production. There are many valves with serious throttling effect in the subsea X-tree, so the hydrate formation risk is relatively high. In this work, a 3D cage-sleeve throttling valve model was established by the numerical simulation method. The temperature and pressure field of the subsea throttling valve was accurately characterized under different prefilling pressure, throttling valve opening degree, and fluid production. During the well startup period, the temperature of the subsea pipeline is low. If the pressure difference between the two ends of the pipeline is large, the throttling effect is obvious, and low temperature will lead to hydrate formation and affect the choice of throttling valve material. Based on the analysis of simulation results, this study recommends that the prefilling pressure of the subsea pipe is 7–8 MPa, which can effectively reduce the influence of the throttling effect so that the downstream temperature can be kept above 0°C. At the same time, in regular production, a suitable choke size is opened to match the production, preventing the serious throttling effect from a small choke size. According to the API temperature rating table, the negative impact of local low temperature caused by the throttling effect on the temperature resistance of the pipe was considered, and the appropriate subsea X-tree manifold material was selected to ensure production safety. The hydrate phase equilibrium curve is used to estimate the hydrate formation risk under thermodynamic conditions. Hydrate inhibitors are injected to ensure downstream flow safety.
Highlights
In 1947, American engineers first proposed the concept of a “subsea wellhead” (Li and An, 2020), which laid the foundation for the development of a subsea production system
The prefilling pressure of downstream sea pipe is analyzed and the reasonable pressure design value is recommended
Under a certain flow rate, if the throttling valve is selected with a small opening degree, the downhole pressure needs to be reduced more to ensure the stability of production, and the throttling effect will be intensified
Summary
In 1947, American engineers first proposed the concept of a “subsea wellhead” (Li and An, 2020), which laid the foundation for the development of a subsea production system. Under a certain flow rate, if the throttling valve is selected with a small opening degree, the downhole pressure needs to be reduced more to ensure the stability of production, and the throttling effect will be intensified. It shows that the downstream velocity is slightly higher than the upstream flow rate when the upstream and downstream pipe diameters are the same. In the stable stage of production, the temperature upstream of the subsea throttling valve can reach 70°C and the pressure is up to 30 MPa. Even if the temperature and pressure drop rapidly after the gas passes through the throttling valve, the risk of hydrate formation is low. In the early design and valve selection, it is necessary to accurately consider the throttling valve temperature change
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