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

1.5.2 Blockage Removal: Depressurisation 
Depressurisation will move the system conditions outside the hydrate stability 
zone (at least temporarily), as shown in Figure 15. This means that the gas 
hydrate in the pipeline should dissociate. 
However, gas hydrate dissociation is an endothermic process and cannot 
proceed unless heat is supplied. In reality when the system pressure is 
reduced some gas hydrates dissociate by removing heat from the local 
surroundings which can result in a fall in system temperature. 
Hydrate dissociation could cease due to an increase in pressure, due to gas 
release, and/or temperature reduction, due to heat removal – either process 
will result in the system returning to the hydrate stability zone. 
It is possible to release the extra gas maintaining the system pressure. In this 
case the system temperature will reduce to the corresponding temperature on 
gas hydrate phase boundary, ceasing gas hydrate dissociation. Further 
pressure reduction will dissociate some more gas hydrate which will again stop 
when the temperature drops further, see Figure 15. 
Figure 15: Hydrate Dissociation: Depressurisation 

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TOPIC 1: Gas Hydrates 
 
 
 
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The temperature drop following any pressure drop will return the system 
conditions to a point of gas hydrate phase equilibria (i.e. gas hydrate 
dissociation will stop). The important fact is that pressure reduction alone is 
not adequate for gas hydrate dissociation and heat should also be supplied to 
the system. 
Another problem with depressurisation is that the consequent temperature 
reduction (from dissociation) will result in a greater temperature difference 
between the hydrates in the pipeline and the surroundings. This will increase 
the rate of heat transfer from outside to the pipeline, supplying heat for extra 
gas hydrate dissociation. 
Looking at this situation in more detail, we see that initial depressurisation will 
occur at the plug ends – thus, removing heat from the system. However, 
further plug dissociation will occur radially, as heat is transferred radially from 
outside to the pipeline – hence, over time the plug dissociates along the inside 
surface of the pipeline. If there is a pressure difference across the plug, then 
this could result in plug dislodge and the risk of projectiles. 
Another important factor is the potential formation of ice. If the pressure is 
reduced to the extent that the equilibrium temperature is outside the gas 
hydrate stability zone but below ice point, the water released from gas hydrate 
dissociation could covert to ice. 
Ice formation could hamper the plug removal process; this is because ice is a 
good insulator and will reduce the rate of heat transfer to gas hydrates. In fact 
ice will form a protective layer on gas hydrates (reducing the rate of gas 
hydrate dissociation). Furthermore, ice, unlike gas hydrates, only responds to 
temperature increase. Therefore, it might take considerable time to supply 
heat to the system and melt the ice. 
Further investigation is underway to optimise the condition for gas hydrate 
dissociation through depressurisation, but we can summarise: 
•
Ice formation should be avoided, while reducing the pressure as low as 
possible. 
•
Furthermore, pressure reduction from both sides is preferred to avoid 
any potential projectile. 
•
It seems that the best practise in depressurisation is to reduce the 
pressure in steps, giving adequate time between the steps – the 
system should be monitored carefully. 
•
In all cases the most important factor, to avoid any danger to personnel 
and installations, is patience. 

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TOPIC 1: Gas Hydrates 
 
 
 
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NIVERSITY B41OA December 2018 v3