Hydrate Formation and Inhibition in Offshore Natural Gas Processing

Abstract

Natural gas hydrates are crystalline water-based solids physically resembling ice, with a crystalline structure comprised of water and light hydrocarbon molecules (mainly CH₄). Such solids can be formed above the freezing temperature of water, and, for this reason, represent a major flow assurance concern, especially at high pressures. Gas hydrate structures are characterized by repetitive crystal units composed of asymmetric, spherical-like “cages” of hydrogen-bonded water molecules, each cage typically containing one (or more) guest molecule(s) held in its interior by dispersion forces. A lot of shortcomings might occur if gas hydrates accumulates severely in subsea flowlines. Their remediation is costly and risky and could mean production stoppages, causing economic losses, and posing hazards to the security and integrity of the pipelines. To thermodynamically inhibit hydrate formation in continuous gas pipeline systems, the most common prevention method is to continuously displace the hydrate forming boundary such that the operational temperature and pressure of the system lie on the outside of the hydrate boundary. This is accomplished by continuous injection of a certain flow rate of a THI compound, which must be proportional to the flow rate of water carried by the stream. This chapter covers the main aspects of hydrates and their structures, hydrate remediation and the mechanism of thermodynamic inhibition of hydrate formation.