The compressed natural gas (CNG) emerging as new gas technologies - CNG carriers
Transporting The Compressed Natural Gas (CNG):
Compressed Natural Gas (CNG) has been developed as a shipping concept, relying on the
high pressure containment technology used in conventional pressure vessel CNG storage and in
pipeline systems to provide a form of concentrated gas storage which can be fitted onboard ships.
The CNG concept that has been around for a long time, seeks to offer savings by removing the
need for the expensive onshore refrigeration trains required for LNG. However, CNG still needs
separate gas conditioning and process systems to get the production gas segregated and suitable
for transportation. A separate LPG processing and transportation system is also needed.
Although gas demand is growing, a simple development approach, with appropriately lower
CAPEX and OPEX, is required to serve the smaller or “stranded” reserves. Hence, there has
been a re-emergence of CNG proposals.
Processing is still required to condition the natural gas
for shipment as CNG. Condensates, NGL’s and water are removed from the product before it is
able to be compressed for storage in marine transportation containers. Like LNG, the LPG’s are
processed and shipped separately.
Fig:LNG carrier underway
Proponents of CNG have tested various combinations of pressure and temperature in an attempt
to achieve optimal volumetric efficiency compared to containment system mass and cost. It is
reported that CNG technology offers refrigerated compressed storage systems typically operating
at 1800 psig at -20°F (-29°C), or pressure only systems operating at 3000 to 3600 psig at
atmospheric temperatures in order to achieve compressed volumetric storage ratios in the order
of 250 to 300 times the natural gas density at STP.
Yet, CNG systems have not yet entered full
commercial development. Equipment development and economic costs are some of the major
hurdles being addressed by various groups attempting to develop this technology, over the past
10 years.
The practicality of the proposed patented process solutions and offloading the CNG product from
storage, at the required volumetric ratio levels previously mentioned, are proving to be energy
intensive and time consuming to realize. The energy value of the production gas is not realized at
market. Commercial implementation of this technology has been slow to emerge as there are no
compelling economics.
In general, the largest vessel designs proposed will deliver 0.7 to 1.0 BCF
of CNG product. Published figures concerning their economic realm of operation appear to place
CNG ships in coastal or regional, as opposed to mid or trans-ocean service.
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