Gas storage and transmission systems

Liquefiable gases such as associated natural gas may be liquified, stored in the liquid form and re-vaporized by cooling a pressurized first liquified gas stream by indirect heat exchange with a first stream of a refrigerant, such as liquid nitrogen, to form a second liquified gas stream whose temperature is less than its initial boiling point at ambient pressure and a first warmed refrigerant stream. The second liquified gas stream is passed to a vessel wherein the liquified gas is stored. When the gas is required the second liquified gas is removed from the storage vessel and warmed by indirect heat with a second warmed refrigerant stream, to form a third liquified gas stream and said first refrigerant stream. The warmed refrigerant stream is also in indirect heat exchange with said first warmed refrigerant stream and after heating comprises second warmed refrigerant stream. Thereafter, the third liquified gas stream is vaporized.

This invention relates to a method and apparatus for the storage and 
transmission of chemical compositions which are normally gaseous, but 
which may be liquified. More particularly, the invention relates to the 
transmission and storage of gaseous fuels such as natural gas associated 
with oil. 
Associated gas is normally liquified on the production platform or on board 
specially constructed barges or ships, transferred to a land-based 
terminal as liquified natural gas (LNG) stored at the terminal as LNG and, 
when it is required, vaporised into the gas transmission system. 
Hitherto, LNG has been transported at atmospheric pressure and considerable 
amounts of energy are required to cool the liquid to a sufficiently low 
temperature to maintain it in the liquid phase at ambient pressure. It is 
more economical to transport the LNG at higher temperatures and at higher 
pressures. However, there are problems in the storage of LNG at super 
atmospheric pressure. 
The present invention proposes apparatus whereby a normally gaseous 
chemical composition such as LNG can be received in liquified form at a 
pressure above atmospheric pressure, stored in liquified form at 
atmospheric pressure and transmitted as a gas at a predetermined line 
pressure. 
In accordance with the present Invention, there is provided method for 
storing and transmitting liquifiable gases, which method comprises cooling 
a pressurised flat liquified gas stream by indirect heat exchange with a 
first refrigerant stream, to form a second liquified gas stream whose 
temperature is less than its initial boiling point at ambient pressure and 
a first warmed refrigerant stream, passing the second liquified gas stream 
to a storage vessel, removing second liquified gas from said storage 
vessel and warming it by indirect heat exchange with a second warmed 
refrigerant stream, to form a third liquified gas stream and said first 
refrigerant stream, said warmed refrigerant stream also also being in 
indirect heat exchange with said first warmed refrigerant stream to form 
said second warmed refrigerant stream; and vaporising said third liquified 
gas stream. 
The present invention also provides apparatus for storing and transmitting 
liquifiable gas comprising a first indirect heat exchange means for 
cooling a pressurised liquified gas with a refrigerant, means for storing 
said cooled gas, second indirect heat exchange means for warming said 
stored cooled gas, said second indirect heat exchange means also including 
separate means for warming said refrigerant received from said first heat 
exchange means.

Pressurised LNG at a temperature above its ambient pressure bubble 
temperature is passed from the carrier 1, through a heat exchanger 2, 
where it subcools to its ambient pressure bubble temperature, and is 
reduced in pressure through valve 3 to the LNG storage tank 4. 
Meanwhile, LNG from the tank 4 is pumped up to pipeline pressure in pump 5 
warmed in exchanger 6 and vaporised in the base load vaporiser 7 into the 
pipeline. 
A refrigerant circuit of nitrogen acts as a heat pump between exchangers 2 
and 6. Nitrogen is compressed to a high pressure in compressor 8 and 
cooled and partially liquified against the base load LNG output and 
returning low pressure nitrogen in Exchanger 6. The nitrogen is expanded 
through valve 6 where it further cools. The cold nitrogen warms in 
exchanger 2 as it subcools the LNG unloaded from the carrier.