Loading...
Study of hydrate deposition and sloughing of gas-dominated pipelines using numerical and analytical models
Liu, Zhijian
Liu, Zhijian
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2017
Date Submitted
Keywords
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
Abstract
Nowadays, one of the most important technical issues facing the petroleum industry is flow assurance. Within flow assurance, gas hydrate formation poses the most severe threat to the transportation of hydrocarbons through long-distance pipelines. The prevention and remediation of hydrate problems cost billions of dollars in gas transmission pipelines. Transportation of oil and gas for deep-water reservoirs involves handling multiphase flows over long distances in cold sub-sea environments. The temperature of the flowing fluids decreases along the pipe due to the cold surrounding sea water. If the temperature in the subsea pipeline is low enough, hydrate formation may be induced. This doctoral thesis will study hydrate deposition and sloughing of gas-dominated pipelines using numerical and analytical models. This work proposed the following hypothesized plug evolution processes for gas-dominated system: (1) hydrate formation; (2) deposition; (3) sloughing;(4) jamming; and finally (5) plugging. Hydrate formation mechanism and prediction models are discussed first. Presence of water, high pressure and low temperature are required for gas hydrate formation. Fluid composition, pipeline surface roughness and wettability can significantly affect the deposition process. The heat transfer principle is also studied. An equation is developed to predict hydrate deposit thickness growth assuming that hydrate formation only occurs at interface area between water and gas (i.e., water lm attached on the wall or water droplets entrained in the gas). The growth of hydrate deposits is mainly due to two parts: (1) water condensation on the pipeline wall; and (2) direct hydrate particle deposition from the gas phase. The model is verified using published experimental data. To investigate hydrate deposit sloughing risk, the mechanical properties of hydrate deposits are studied. Four different packing patterns are proposed: (1) simple cubic packing;(2) rhombohedral packing; (3) tetrahedral packing and (4) orthorhombic packing. The packing patterns are based on the proposed assumption that all hydrate particles possess spherical shapes and uniform diameters. Different packing patterns will result in different porosity and mechanical properties. The reduced porosity of the hydrate deposits due to hydrate particle sintering is also studied. Two models are proposed to study the relationship between bridge diameter and porosity: (1) evenly coating model and (2) corner growth model. These two models contribute to the understanding of the annealing effect on the decrease of the hydrate deposit porosity. To investigate the sloughing phenomenon in pipelines, the finite element method (FEM) is applied to study the internal stress distribution inside the hydrate deposit. The compact force induced by the momentum of upstream gas molecules will result in collapse of hydrate deposit from the wall. This work also investigated the methods to characterize hydrate deposit: (1) pressure-temperature profile method, (2) back-pressure method, (3) average pressure method, and (4) pressure transient method. The combination of these methods can provide more detailed information about the deposit. The original models proposed by this work advance the understanding of how hydrate deposition and sloughing will eventually plug pipelines. This work made a good progress in the area of hydrate deposition and sloughing mechanism for gas dominated systems.
Associated Publications
Rights
Copyright of the original work is retained by the author.