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

1.4.1 Gas Hydrates Formation: Pipeline Start-up and Shutdown 
Generally, in offshore pipelines, there is a high risk of gas hydrate formation 
during start-up and shutdown. This is mainly due to a decrease in pipeline 
temperature to ambient temperature (as discussed, for subsea pipelines this is 
around 4
o
C). In this event hydrate formation may occur either during pipeline 
pressurisation (start-ups) or, as discussed, during pipeline depressurisation 
(shutdown). 

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TOPIC 1: Gas Hydrates 
 
 
 
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Figure 10 shows pipeline conditions during normal operation, which is well 
outside the gas hydrate stability zone. However, during shutdown when the 
flow rate is reduced, the pipeline temperature will drop to that of seawater 
temperature, which could result in gas hydrate formation in the whole or some 
sections of the pipeline. Hence, great care should be taken to avoid gas 
hydrate problems during such operations. 
Figure 10: Possible Pipeline Conditions – Flowing and Shutdown 
Conditions 
To avoid gas hydrate problems during start-up and shutdown: 
1. Analyse the hydrate phase boundary and the phase behaviour of the 
system at normal operating and at shutdown conditions; predictive 
techniques could be used for this purpose, although some companies 
may prefer experimental measurements. 
2. After considering the minimum seawater temperature and the 
maximum pressure (during shutdown and at initial start-up), inhibitor 
could be injected whenever a shutdown is planned. The amount of 
inhibitor injected will depend on the amount of water in the system. 
The injection of a thermodynamic inhibitor will shift the gas hydrate 
phase boundary to the left. 
3. In the case of start-up, after an unplanned shutdown, it could be 
necessary to inject inhibitor prior to start-up. The choice of inhibitor 
depends on the condition of the pipeline, though generally methanol is 
used due to its high vapour pressure. 
4. Alternatively, pipeline heating could be used if the necessary 
infrastructure is in place. 

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TOPIC 1: Gas Hydrates 
 
 
 
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