Page 123 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 119

hydrate dissociation this insulation limits the heat which can enter the pipeline and dissociate
the hydrate. This insulation explains why it takes so long to dissociate hydrate in a subsea
flowline. Energy flows from a cold 4 °C seawater into the pipeline because hydrate gets even
colder when it dissociates and reaches the equilibrium temperature at the flowline pressure.
The dissociation temperature may be lower than the freezing temperature of water. Fresh
water is released from a melting gas hydrate. If the dissociation occurs in conditions where
ambient temperature is below the freezing temperature of water, the water released from
hydrate will freeze as ice and will remain as ice until the ambient conditions warm up above
the freezing temperature of water. This should be taken into account in arctic operations and
in permafrost regions. A consideration that a hydrate blockage cannot be dissociated in a
gas pipeline operated at below freezing conditions was first specified by Makogon in 1961; a
partial hydrate blockage may be removed either by heat or by addition of methanol to the gas
stream (Makogon, 1961).
When hydrate forms, it releases heat and thus limits the rate of hydrate growth. When hy-
drate melts, it consumes heat and limits the rate of hydrate dissociation. Hydrate dissociation
or formation is a phase transition process. It is similar to boiling of water phase transition:
temperature will remain fixed until the whole phase transition from liquid to vapor completes.
The hydrate melting temperature typically is close to the water freezing temperature.
Hydrate takes up energy from the water present in the flowline until water freezes, which
depending on salinity occurs between 0 and approx. −6 °C.
Chemicals may be used to help dissociate hydrate.
Their effectiveness by weight decreases as the molecule size increases.
For example, to prevent hydrate at 4 °C and 100 atm it takes around 30 wt% methanol,
45 wt% MEG and 60 wt% TEG (Wood Virtuoso GUTS 6.2 software).
However, toxicity and hazardousness decrease as the molecule size increases. Methanol
is flammable and poisonous, while TEG is toxic if ingested and may be combustible at high
temperature.
In each of these inhibitors the active components are the oxygen and hydroxyl groups O
and OH.
Methanol formula is CH 3 -OH.
MEG or mono-ethyleneglycol formula is HO-CH 2 CH 2 -OH and easy to remember as two
methanols CH 3 -OH.
TEG or tri-ethyleneglycol formula is HO-CH 2 CH 2 -O-CH 2 CH 2 -O-CH 2 CH 2 -OH and easy to
remember as three MEG.
Furthermore, the chemicals get diluted and become ineffective as hydrate dissociates and
water is released from melting hydrate.
In one instance, a hydrate blockage formed in a deepwater dry tree riser remained stable
for weeks despite MEG being placed on top of the hydrate plug. After few months of waiting,
coiled tubing was deployed to safely jet out the hydrate blockage with a lukewarm KCl brine.
Combinations of pressure, chemical and thermal methods as well as points of access to
deliver the solutions allow for a multitude of implementation options. Table 5.4 below sum-
marizes over 50 ways to melt a hydrate plug, many of which have been attempted in the past.
Some methods are more effective than others and some are more novel than others. Technical
feasibility, safety, duration and economics of each option should be considered when planning
a removal operation. Two-sided depressurization is considered to be safer than other options.