various pressure and flow-control systems within relatively small- diameter transport lines through which the gas may easily be subjected to the Joule-Thompson effect, similar to those encountered in offshore oil and gas production systems. The formation of hydrate in flowlines in CNG-transporting ships during loading and unloading operations can also potentially pose severe risk factors to such systems.

While many other factors influence hydrate formation, the temperature and pressure at locations across the system are the major factors to be considered when predicting the likelihood of hydrate formation.13

The exact temperature and pressure at which hydrates

Hydrates can pose a major risk in all high-pressure oil and natural gas transport lines.

form depend on the composition of the gas and the water. For any particular composition of gas at a given pressure, there is a temperature below which hydrates will form. As the pressure increases, the hydrate-formation temperature also increases. As a general rule, as pressure increases or as the system becomes colder, the tendency to form hydrates increases.14

Therefore, oil and gas

production and transport systems are at significant risk of forming hydrate plugs during shut-in and subsequent start-up.

Hydrate propagation tends to gradually form a plug that separates the pipe into two pressure sections: a high-pressure section between the well or high-pressure gas source and the plug and a second section at low pressure between the plug and the gas-recovery division. In the upstream section, a pipe blast can occur due to a pressure rise. The plug can also behave as a projectile that destroys the pipe when the pressure difference between the upstream and downstream sections increases. Both events can endanger personnel and damage production equipment.8

Economic Risks

Hammerschmidt15

In addition, offshore operations spend substantially on insulation of subsea pipelines to prevent hydrates.16 Lysne17

indicated that the formation of gas hydrate was responsible for blocking pipelines. Considering the significant economic risks in the gas and oil industry, a great deal of research has been conducted by the petroleum industry to inhibit this undesirable phenomenon. Annually, an enormous operating expense is allocated to hydrate prevention, a significant portion of which is devoted to inhibition.16

Furthermore,

listed three incidents in which hydrate projectiles erupted from pipelines at elbows and caused the loss of three lives and >US$7 million in capital costs. Accordingly, concerns have been expressed about the effect of hydrates on foundations of platforms and pipelines and offshore drilling.

Oil and gas wells always produce undesired water along with hydrocarbons that are in the hydrate guest size range. As the flowing

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