Vibration Characteristics of Riser Induced by Gas-Liquid-Solid Three-Phase Flow in Deep-Sea Hydrate Extraction Under VIV Effect

Natural gas hydrate, as a low-carbon unconventional energy source, is increasingly receiving attention from the scientific community. It is considered the most promising alternative energy source in the 21st century. At present, the extraction of deep-sea hydrates is mainly carried out through riser transportation. Due to the state instability of the hydrate, it is easy to decompose during migration, forming a typical vibration phenomenon induced by gas-liquid-solid three-phase flow, which can easily lead to nonlinear vibration of the riser. This nonlinear vibration mechanism is completely different from the vibration mechanisms induced by single-phase and two-phase flows. It is particularly important to reveal the vibration mechanism of the riser in gas-liquid-solid three-phase flow.

Due to external ocean loads during the process of mining riser operations, it is easy to cause vortex-induced vibrations (VIV). The riser was subjected to the action of internal gas-liquid-solid three-phase flow of hydrate. Moreover, the upper end of the riser was subjected to the heave motion of the ocean platform. These factors require the use of multiple methods in combination for the nonlinear vibration model of the riser. Therefore, the gas-liquid-solid three-phase FIV model for deep-sea hydrate extraction riser was established using the finite element method, Hamiltonian principle, and energy method. The fluid structure coupling effect between ocean flow field and mining riser was achieved through the wake oscillator model. The coupling effect between internal multiphase flow and mining riser was achieved through additional mass, collision energy loss, and flow velocity changes. Simultaneously, a dynamic decomposition model for hydrates was established to identify changes in the content of gas, solid, and liquid phases. The platform was prone to six degrees of freedom motion under the action of wind, waves, and currents. However, during the process of hydrate extraction, the platform was most prone to heave movement. With the help of previous research, a platform heave motion model was established, which can effectively obtain the displacement boundary of the upper end of the mining riser. Then, in order to achieve a numerical solution of the model, the vibration model was solved using the combined iterative method of the incrementally applied Newmark-β method and Newton-Raphson method, obtaining the vibration response of the mining riser. Due to the excessive consideration of nonlinear factors in the model, the correctness and effectiveness of its numerical solution need to be rigorously verified. However, the actual vibration data on site could not be obtained. The simulation experiment of the water tank was particularly important. Therefore, by using the principle of similarity, a nonlinear vibration simulation experiment system for a hydrate mining riser under internal and external flow excitation was developed. The root mean square of the vibration displacement and amplitude frequency curves of the mining riser were experimentally measured. At the same time, the calculation parameters of the theoretical model were set exactly the same as the experimental parameters to compare the theoretical calculation results and the experimental test results. On this basis, the root mean square of the vibration displacement and the amplitude frequency curve of the mining riser were calculated. Comparing the experimental results with the model calculation results, the correctness of the established model was verified, and the comparison accuracy could be higher than 90%. By using frequency domain and time domain analysis methods, the influence of external environmental parameters and multiphase flow parameters on the nonlinear FIV response of the riser was explored.

The results show that the vibration amplitude of the riser in the cross-flow (CF) direction is higher than that in the in-line flow (IL) direction, and the variation of internal flow parameters has a more significant impact on the CF vibration of the riser. This phenomenon indicates a close relationship between the internal gas-liquid-solid three-phase flow and the vortex-induced effect. The axial vibration of the riser mainly consists of two parts: One part is dominated by gravity and platform heave motion, exhibiting low-frequency and high-amplitude vibration characteristics, which may cause the strength failure of the riser. The other part is induced by internal and external flow field loads, exhibiting high-frequency and low-amplitude vibration characteristics, which may accelerate the fatigue failure of the riser. With the shear flow velocity increases, the displacement of the riser in the IL direction gradually increases, and the axial second-order main frequency amplitude increases accordingly. However, in the CF direction, the amplitude and root mean square of displacement show a decreasing trend, indicating that the increase in displacement in the IL direction has a certain inhibitory effect on the CF vibration. With the increase of internal output volume, the flow velocity of each phase increases, resulting in an increase in displacement and amplitude of the riser in the IL direction, as well as an increase in amplitude in the CF direction. This widens the frequency band, expands the displacement envelope range, and reduces the number of modes in the CF direction. As the particle size of hydrates increases, the solid-phase flow velocity decreases, and the collision energy loss increases, resulting in a decrease in the vibration amplitude of the riser in the IL direction, a narrowing of the vibration frequency band, and an increase in amplitude in the CF direction. The characteristic of more concentrated energy makes it possible to control it more effectively in the future. When the shear flow velocity reaches 1.4 m/s, the hydrate abundance reaches 80%, or the hydrate particle size reaches 7 mm, the vibration amplitude of the riser in two or three directions will significantly increase, and the vibration response of the riser will not change with the variation of other parameter values. At these specific values, the frequency of the interaction between the internal and external fluids on the riser tends to approach the natural frequency of the riser system, resulting in resonance phenomena. In the actual operation process, these parameter values should be avoided to ensure the safety and stability of the riser.

Once commercial exploitation of deep-sea hydrates is achieved, the nonlinear vibration model of deep-sea hydrate mining riser can effectively guide the parameter configuration in the later hydrate mining process. It can evaluate the safety and service life of deep-sea hydrate mining risers, ensuring the safe operation of commercial mining cycles of hydrate.