EnerSea’s proprietary gas handling system used for loading and unloading gas maximises the efficiency of gas cargo storage. The loading system, which compresses and cools the gas to storage conditions, increases the gas to steel weight ratio as explained in Gas Containment system. To unload the gas from the gas containment system, a fluid (ethylene glycol/water mixture – similar to antifreeze) is pumped into the containment system, displacing the gas at constant pressure and temperature.
This operation is a closed loop process and is performed sequentially from tank group to tank group, thus minimizing the amount of ethylene glycol-water inventory required to a level of approximately 8-10% of the total containment system capacity.
Control of the temperature behavior within the containment system allows gas to be offloaded at very high rates while still utilizing carbon steel materials in its construction. Gas is heated prior to its transfer to the customer to ensure that the specified delivery conditions are achieved. This provides a highly efficient offloading method to minimize the residual gas left in containment, while preventing potentially detrimental temperature dynamics from occurring within the cargo containment system.
In addition, the positive displacement aspect of the VOTRANS™ cargo delivery system efficiently overcomes the problems of downstream back pressure, which limits the ability of pressure depletion methods (i.e. “blowdown”) commonly used in alternative CNG designs. Expensive scavenging compression is not required, and yet the VOTRANS system can still deliver up to 98% of the stored cargo to the market, compared to only 85-90% for the less efficient blowdown systems.
In summary, the advantages of this process are:
- Lower residual gas volumes (2-5%) for VOTRANS compared to about 10-15% for conventional high pressure blowdown systems
- Pressure and temperature dynamics are managed during offloading
- Minimal impact by downstream receiving system back-pressure
- Evacuates all NGLs, therefore can accommodate a wide range of gas composition, including rich and associated gas
- Low cost liquid pumps required for offloading compared to scavenging gas compression required for blowdown method
The complete gas handling system, including automation and control systems, have been designed, built and tested during EnerSea’s Prototype Test Program, the results of which were approved by ABS.