B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

Subsea Separators: 
Separation of water at sea floor has many 
benefits. However, these facilities should be protected against gas 
hydrate formation – this is because the combination of low seawater 
temperature and high separator pressure could put the system inside 
the hydrate stability zone.
 
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Long Tiebacks and Deepwater Production: 
The economy of many 
marginal oil fields could be improved by using long tiebacks. However, 
this will result in longer transportation times in a cold seawater 
environment this, together with high fluid pressure, may again favour 
gas hydrate formation.
 
There are three common misconceptions regarding gas hydrate formation in 
pipelines:
1. A free water phase is absolutely necessary for the formation of gas 
hydrates. A droplet of water (suspended and carried by the 
hydrocarbon phase) can provide enough water molecules for 
hydrogen bonding and gas hydrate formation. 
Therefore, there is no need for the presence of a free water phase. 
However, from a practical view point the water concentration in 
hydrocarbon phase is very low (<100 ppm) and very long time may be 
required for water molecules to accumulate into a significant hydrate 
mass. Nonetheless, it is possible to have H-V (Hydrate-Vapour) or H-
LHC (Hydrate-Liquid Hydrocarbon) phase equilibria. 
2. Ice is the solid phase at low temperature: This is not completely true, 
because in the presence of gas molecules hydrates could be the 
stable phase – or even a combination of hydrates and ice. 
However, if ice is formed initially, it might take a long time for ice to be 
converted into gas hydrates, as mass transfer in the solid phase is 
generally very slow. 
3. Gas hydrates do not form in liquid hydrocarbon lines: There is no 
reason why gas hydrates cannot form in liquid hydrocarbon lines, if the 
necessary conditions exist. 
Liquid hydrocarbons generally have some light components (capable 
of gas hydrate formation) dissolved in them. In the presence of water 
and favourable temperature and pressure conditions gas hydrates 
could form. 
One might expect less hydrate blockage problems in liquid 
hydrocarbon lines. This is mainly due to the fact that that the presence 
of a liquid hydrocarbon lubricates the pipe internal wall. Also gas 
hydrates could be suspended in a liquid hydrocarbon phase 
continuous phase. 

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TOPIC 1: Gas Hydrates 
 
 
 
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Finally, the amount of gas hydrates is limited due to limitation in the 
available gas or water in a liquid hydrocarbon system. In reality gas 
hydrates could form in liquid hydrocarbon phase, as was the case in 
Eldfisk field in Norway which resulted in 188 days of pipeline blockage. 
Once a condensate pipeline is blocked due to gas hydrate formation 
the opportunities for dissociation are much more limited than that of a 
gas pipeline, as it is difficult to de-pressurise (due to vaporisation) the 
pipeline and the difficulty in getting a heat source and/or chemical 
inhibitor to the point of blockage. 
There are several processes that could result in gas hydrate 
formation/disassociation in a pipeline. Pipeline depressurisation can either 
result in gas hydrate formation, (due, for most fluids, to the Joule-Thomson 
effect), or gas hydrate dissociation (caused by moving the system to outside 
gas hydrate stability zone – given that hydrates have already been formed). 
As shown in the Figure 8, the system at point A is inside hydrate stability zone. 
An isothermal depressurisation will move the system to the conditions 
described by point B, outside hydrate stability zone. 
Figure 8: Effect of Depressurisation on Hydrates – Initial Conditions is 
Inside Hydrate Stability Zone 
However, if depressurisation occurs over a short distance (e.g., a control valve 
or a choke) heat transfer is very limited and the system could be regarded as 
adiabatic. This will result in the system moving further into the gas hydrate 
stability zone as represented by point D. For an adiabatic-reversible process 
(i.e., isentropic) the pipeline conditions is represented by point F. 

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TOPIC 1: Gas Hydrates 
 
 
 
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If the system is initially outside gas hydrate stability zone, the above processes 
could result in gas hydrate formation, as presented in Figure 9. However, one 
main difference is that for a closed system the formation of gas hydrates will 
result in pressure reduction as presented by ACGF. 
Figure 9: Effect of Depressurisation on Hydrates – Initial Condition is 
Outside Hydrate Stability Zone 
From a practical viewpoint, isothermal expansion is generally very slow and 
perhaps impossible in real systems. Hence, to avoid gas hydrate formation 
due to depressurisation and fluid expansion, the following options are 
available: 
•
The system should be at higher temperature – by heating the fluid or 
using the waste heat. 
•
Breakdown the pressure reduction into several stages – thus providing 
adequate pipe length for heat transfer. 
•
Finally it is possible to use inhibitor (generally injected upstream) – this 
shifts the gas hydrate stability zone and avoids gas hydrate formation.