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

1.5 Gas Hydrate Blockage Removal 
Gas hydrate plugs in pipeline have some differences with in-situ gas hydrates. 
Some of the differences (or properties) are: 
•
They are porous and permeable (unlike our understanding of in-situ 
gas hydrates) – this is because pipeline gas hydrates are formed very 
rapidly. Recent results show that they get harder with time. 

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TOPIC 1: Gas Hydrates 
 
 
 
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•
They transfer pressure but are limited in the transfer of flow. This 
means that over long periods of time the pressure across a plug can 
equalise. Clearly, the time required for the pressure to equalise 
depends on pressure difference, plug length and its permeability. 
•
Generally, some free water is trapped between gas hydrate crystals, 
due to their rapid formation. 
Various methods could be used for gas hydrate plug removal. The choice of 
one, or a combination of methods, depends on the pipeline and its specific 
characteristics. Some potential options are heating, depressurisation and 
inhibitor injection. 
It is important to remove the water inside the pipeline by running a pig, as the 
water resulting from gas hydrate dissociation has history of gas hydrate 
formation – this means that it is likely to form gas hydrate more easily and with 
a smaller degree of subcooling. 
 
1.5.1 Blockage Removal: Heating 
The objective is to supply heat and move the system to outside the gas 
hydrate stability zone, as shown in Figure 14. As the pipeline is blocked it is 
not possible to flow a hot fluid through the pipeline and dissociate gas 
hydrates. 
Therefore it is necessary to locate the plug and supply the heat locally. 
Furthermore, it is important to supply the heat to the end of gas hydrate plug. 
Application of heat to other parts of the plug could result in excessive pressure 
due to gas hydrate dissociation and gas release. This could result in 
equipment failure, blowouts, or hydrate projectiles in the pipeline. 
Figure 14: Hydrate Dissociation: Heating 

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TOPIC 1: Gas Hydrates 
 
 
 
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Parallel to the application of heat (to the end of gas hydrate plug), the gas 
resulting from hydrate dissociation should be released to avoid pressure build 
up in the pipeline – this could lead to gas hydrate formation in another location.
Various physical and analytical methods can be used to find the location of 
gas hydrate plugs in onshore pipelines. However, locating gas hydrate plugs in 
offshore pipelines can prove difficult. One attractive option, where possible, is 
to use coiled tubing to locate and supply heat to the system.