AGL undertaked a feasibilitystudy to bring Australia’s first Floating Storageand Regasification Unit (FSRU) to Crib Point.But what is an FSRU and how does it work?
What is an FSRU?
In short, an FSRU is a Liquefied Natural Gas (LNG) storageship that has an onboard regasification plant capableof returning LNG back into a gaseous state and thensupplying it directly into the gas network.The ships themselves are big, up to 290 meters long and49 meters wide with a draft of 11-12m. A typical FSRU cantravel at 19.5 knots and has a cargo capacity of between125,000 m3 and 170,000 m3.FSRUs and LNG ships have four to six separate cargotanks inside their hull. As LNG is stored at very coldtemperatures, the cargo tanks are segregated from thesteel hull structure by thick insulation.
There are two cargo tank designs commonly in use.Membrane tanks are box shaped and LNG ships withmembrane tanks look like any other liquid bulk tanker.The other design is known as ‘Moss tanks’ that arespherical. LNG ships equipped with Moss tanks havethe recognizable dome shaped tanks visible abFSRUsand LNG tankers are powered by the LNG they carry,making them one of the most environmentally friendlyships in the world, emitting less CO2, NOx, SOx andparticulate matters than most other ship types that burnheavy petroleum fuels. As such, an LNG ship will only becarrying relatively small quantities of non-LNG fuels topower non-propulsion systems.Unlike oil tankers, FSRUs are not involved in oil cargotransfer operations where there is the possibility of aspill or pollution. LNG cargo is a clear, colourless andodourless liquid; if a small quantity is accidentally spilledit will quickly and completely evaporate leaving nofootprint behind.
How do they work?
FSRUs are usually permanently moored at a jetty,in this case it would be Crib Point, storing LNG at atemperature of -161oC in cryogenic storage tanks. Thecold temperature keeps the LNG cargo in its liquidstate until it is required for the gas network. LNGis generally stored and transported in bulk storagetanks at slightly above atmospheric pressure; usuallyless than 150 kPa (mbar) above.When it is time to convert the liquid back into a gas,seawater is used to warm the LNG causing it to returnto a gaseous state. The heater is usually a ‘tubeand shell’ heat exchanger where water is pumpedaround the shell of the heat exchanger and LNGpasses through tubes. The difference in temperaturebetween the inlet seawater and outlet seawater isinitially about -7ºC,this then blends back to ambienttemperature.An FSRU typically discharges gas into the network at apressure of around 60-80 Bar and at 5oC. Working atfull capacity, a 170,000 m3 cargo could be regasifiedin about six days.
What happens when the ship isempty?
Once the FSRU is empty, another ship may arrive torefill the FSRU. AGL would source LNG from Australia or international markets and use an LNG tanker tobring it to Crib Point where the LNG tanker moorsalongside the FSRU and refills the FSRU’s storagetanks – a process that should take less than 24hours.
What are the advantages of using anFSRU?
The FSRU can sail in with the cryogenic tanks andregasification units onboard ready to go, avoidingthe impact of building large infrastructure onshore.As the FSRU is an operating ship, when the projectconcludes the FSRU simply sails away and thecommunity is not left with redundant infrastructure.The FSRU also allows AGL to manage gas demandthrough high and low demand periods such aswinter peak demand requirements. Gas canimmediately be discharged into the network tocover network needs, be stored on board until it isrequired or sent to existing onshore storage facilitiesand more LNG imported to the FSRU. The timingand volume of LNG imported can be adjusted tomeet market needs.Importantly, importing LNG by ship allows AGL toaccess gas supply from other Australian States, andinternationally, providing certainty of supply to thesouth-eastern states.
What are the risks associated with anFSRU?
Like all big industrial and resources projects an FSRUcarries some risks. The key is to identify, minimiseand manage the risks to the greatest extent possible.Ensuring that everyone goes home safely each day isa fundamental operating principle at AGL.LNG contains large amounts of energy, however, inits liquid state it cannot explode or burn. Only in itsgaseous state, and mixed with the correct amount ofoxygen, can it ignite. (Methane needs to be diluted to5-15% concentration in the atmosphere for ignition).An explosion could only occur if the methane gas hadleaked into a confined space prior to igniting – justlike a conventional gas explosion you might see in ahouse. LNG ships are some of the most sophisticatedships in the world and are equipped with automatedleak detection and emergency shut down systems.While unlikely to occur, the most likely fire scenario isa vapour cloud fire or pool fire (where some escapedgas settled on water). These fires would be lethalto those caught in the burn zone. However, whileintense, methane fires burn inward at a rapid paceand there would be limited impact on those outsidethe burn zone.In the past, on a few rare occasions, equipment faultsand failures of safety practices overseas have resultedin incidents that have led to loss of life and propertydamage.Fortunately, very few incidents have ever occurredat LNG import terminals and there are currently hundreds of LNG ships and LNG terminals safelyoperating, so these risks are well understood and canbe managed. The FSRU and the ships that transportLNG have advanced safety features as will the jettyitself.Double hulls increase the structural strength of thehull and provide additional protection for the cargotanks in the case of an accidental collision, groundingof the vessel or a deliberate attack.All containment systems include cargo monitoring,measuring, control and safety systems designed tooperate at cryogenic temperatures. Nitrogen gasis used to purge the spaces between the tanks, theinsulation and the hull. Nitrogen is an inert nonflammable gas used to displaces the oxygen in thesespaces and prevent fire.All FSRUs and LNG ships are equipped withEmergency Shut Down (ESD) Systems. The ESDsystem is programmed to automatically stop thetransfer of LNG or methane gas and close isolatingvalves should an issue arise. If the ship and/or theFSRU moved out of position during cargo transferfor example or if the FSRU tanks are accidentallyoverfilled by the LNG ship then emergency shutdownoccurs before any damage can be done.FSRUs terminals are sited with strict separationdistances from potential ignition points. They arealso sufficiently isolated from other facilities to avoida fire damaging their neighbours. The distances arecalculated as part of a safety case which is conductedduring the project regulatory approval process.
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