Experimental and Computational Study of Thermodynamic Hydrate Inhibitor Mixing and Displacement in Subsea Jumper

Abstract

Abstract The risk of gas hydrate formation is one of the major concerns for offshore flow assurance. Solutions of methanol (MeOH) and mono-ethylene-glycol (MEG) are most often employed as thermodynamic inhibitors (THI) to prevent the formation of hydrate plugs. This study involves experimental and modeling work to investigate and compare the suitability and effectiveness of methanol and MEG on inhibiting and displacing the water in jumper type configurations during flushing procedures. Numerical analysis aims to investigate the mixing and displacing mechanisms that occur during jumper flushing to optimize key factors such as the position of the injection port and the flowrate of a required chemical inhibitor. This paper presents the experimental results with respect to effect of inhibitor type, injection rate, brine salinity and liquid loading. All experiments were carried out in a 3" jumper system at The University of Tulsa. Simulations using 1D transient multiphase flow simulator were conducted to evaluate its capacity to predict the thermodynamic inhibitor dispersion by using the inhibitor tracking module. The 3D computational fluid dynamic (CFD) simulations were performed with the commercial software to help predict the amount and flowrates of chemicals required, and to serve optimizing the location of injection ports. Comparisons were made between the simulation results and experimental data from full fresh water loading jumper displacement tests with MEG and methanol. From the experiments, several conclusions are drawn. Different dispersion and partitioning mechanisms were observed for methanol and MEG through experiments, especially in the vertical sections and low spots of the jumper. Methanol overriding the water phase at horizontal low spots was captured for the low velocity experimental cases, leaving a large amount of water behind that could result in under-inhibited situations. For the 12% brine tests, less water was displaced for each MEG injection rate, whereas a better mixing of methanol with brine was measured. The 1D and 3D simulations provided reasonable prediction for THI distribution along the jumper after displacement tests, except that neither was able to reproduce methanol overriding the water phase at both low spots. 3D CFD simulations results obtained generally gave better agreement with the results from the experiment. This study provides a better understanding of the displacement and mixing mechanisms of thermodynamic hydrate inhibitors. It helps operators minimize operational risks, reducing the size of the umbilical and decreasing capital costs related to equipment and chemicals.