Page 224 - Handbook Of Multiphase Flow Assurance

Hydrate stability and crystal growth 223

Currently, gas hydrates are considered to be an industrial nuisance. Hydrate plugs form-
ing in the pipelines prevent the normal operation of gas and oil production facilities. To date
the worldwide expenditures on hydrate prevention through methanol injection into the pipe-
line are over $150,000,000 (Long et al., 1994).

Gas hydrates as an industrial hazard
The normal operating conditions during oil and gas production establish the thermody-
namic conditions favorable for hydrate formation. Gas is transported from the well at high
pressure. Water coming from the production well is also present in the pipeline. When the
temperature of the pressurized gas-water mixture falls below the equilibrium value, thermo-
dynamic conditions are established for gas hydrate to form.
Formation of hydrate consumes gas and water present in the pipeline. Bulk of the forming hy-
drate eventually adheres to the pipeline walls and narrows the flow channel. This results in the
further pressure increase in the pipeline in front of the hydrate and accelerates growth of hydrate
on the wall of pipeline. With time hydrate plugs the pipeline and stops its normal operation.
Usually, after the hydrate formation pressure in the pipeline is lowered below equilibrium
pressure and hydrate is allowed to dissociate into gas and water. Sometimes, however, the
hydrate plugging of the pipeline may result in serious damage to the producing facilities and
cause a disaster.
Such was the case at the Piper Alpha oil rig in the North Sea (Cullen, 1990). The explosion at
that platform had happened on 6 July 1988. It caused the death of 167 persons, injury and trauma
to many of the survivors, and destruction of the platform. The economic estimate (Lovegrove,
1990) was that the British Government will have lost $2.8 billion in revenues. Cost of the platform
for Occidental Petroleum (Caledonia) LTD which operated the platform was $0.5 billion.
The investigation on the accident had shown that one possible cause of the explosion was
plugging of the condensate pump with gas hydrates and the following gas leak.
Finally, gas hydrates within the foundation sediments of deep water offshore structures may
present a hazard to the foundation of pipelines and other production facilities (Makogon, 1988).

Hydrates as an environmental buffer for holding CH 4 , CO 2
Interest on environmental aspects of gas hydrates is mostly related to the greenhouse effect
of methane and carbon dioxide. These gases may form hydrates at conditions existing on earth.
Greenhouse gases are transparent to the infrared solar radiation on its way to earth. They
absorb the energy of this radiation after it reflected from the earth surface. This causes the
temperature increase in the atmosphere.
Atmospheric temperature affects the stability of methane and natural gas hydrates lo-
cated widely on earth. More methane is released into the atmosphere as hydrate decomposes.
Hypothetically, this may result in a runaway global warming (Fig. 10.1).
Methane has a greenhouse effect which is 21 time stronger than that of carbon dioxide
(Englezos, 1993a). Several scenarios of global warming were proposed. The annual atmo-
spheric temperature increase ranged from 0.006 °C/year to 0.08 °C/year for the catastrophic
scenario (Englezos, 1993b). Industrial development has been accompanied by a release of
greenhouse gases into the atmosphere. Atmospheric temperature increase during the past
century is attributed to artificial activity. Electric power plants are among the most active sites
releasing CO 2 , another important greenhouse gas.