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

1.4.3 Gas Hydrates Formation: Subsea and/or Downhole Separation 
Downhole and subsea separations are suggested to reduce the operation and 
capital costs in offshore operations. The objective is to separate water from 
hydrocarbon and re-inject it back to the reservoir. 
This will result in lower water cut in the hydrocarbon phase, reducing the size 
of the pipeline and pumping facilities. Also, it has been suggested that water-
oil separation under high-pressure conditions is more efficient than at surface 
conditions. 
Subsea separation has been successfully tested and commissioned in the 
Troll field in Norway by Norsk Hydro, see Figure 13. 
Figure 13: Flow Diagram of Typical Troll Subsea Operation Unit 

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TOPIC 1: Gas Hydrates 
 
 
 
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Downhole and/or subsea separation is of significant benefit to flow assurance 
policy (minimising gas hydrate problems) for the following reasons: 
•
As the amount of water has been reduced, less inhibitor is required. 
This will result in significant savings in chemical, pumping costs, space 
requirements and logistics. 
•
If gas hydrates are formed, their quantity is limited due to low water cut 
and this in turn reduces the risk of a major gas hydrate blockage. 
•
As hydrates exclude salts from their structure, low water cut means 
that any gas hydrate formation will result in a greater increase in the 
salinity of the remaining water – and this fact helps to further limit gas 
hydrate formation. 
•
The last two factors indicate that any gas hydrate formation is very 
limited in the transfer lines – this decrease in quantity in turn increases 
the transportability of any gas hydrates that do form. 
However, due to inaccessibility of subsea separators (the water depth), 
measures should be taken to minimise down times and the need for expensive 
operations. 
Minimising the risks associated with wax, asphaltene and gas hydrate 
formation are important flow assurance considerations. Measures should be 
taken to avoid gas hydrate problems during start-up and shutdowns (and 
normal operation, if necessary). Circulation of hot fluids and injection of 
inhibitor are two available options.