Study on mechanical properties of natural gas hydrate production riser considering hydrate phase transition and marine environmental loads

Abstract A dynamic analysis model for natural gas hydrate (NGH) production riser was established considering the internal phase transition of hydrate multiphase flow and the external marine environmental loads. The model was numerically discretized by the finite element method (FEM) and solved by Newmark β method. The correctness of the analysis model was verified by an experiment and field testing data. The results indicate that the NGH begins to rapidly decompose in the middle of the riser, causing the sudden pressure drop in the riser. The internal NGH decomposition will significantly increase the axial tension and Von Mises stress of the riser. The Von Mises stress reaches maximum at the top end of the riser subjected to the distribution of effective axial tension. The lateral displacement and bending moment of the riser increase with the increase of flow rate and mixed fluid density in the middle and lower sections, and the stress decreases with the increase of flow rate and mixed fluid density. The increase of hydrate content and seawater temperature will decrease the lateral displacement and bending moment and the upper part stress of the riser, but it will significantly increase the stress in the lower part.