CNG is natural gas (NG) compressed for the purpose of simplified transport and storage.
Bulk CNG transport technology is not new, it is well-proven - CNG has been successfully transported on land by road-trailer (trucking) for over thirty years. The upscaled application of proven CNG technology to a marine (shipping) transport system is new.
For storage and transporting, natural gas is compressed into special tanks, gas containment tanks (GCTs) normally to a pressure of 200 to 250 bar or 2900 to 3600 psi. The NG capacity of a GCT, termed in standard cubic feet (scf) or metres (scm), depends on the volume of the GCT, working pressure, temperature and composition of the NG. GCTs are mainly cylindrical and vary in diameter and length and can be made of steel or lighter-weight composite materials one technology uses coiled small diameter pipe for marine transportation.
Feedstock gas for compression can be sourced from a natural gas field (onshore or offshore), an existing natural gas transmission pipeline (PLG), a coal mine as coal-seam-gas (CSG) or coal-seam-methane (CSM), biogas and also can be from flared gas.
Pressurised Natural Gas (PNG) is another term for CNG.
CNG MARINE SEA TRANSPORTATION
CNG marine transportation is a new concept in gas shipping, it is at a stage just like LNG was some 50 years ago. CNG is an emerging effective method and economic solution of transporting NG to shorter-distance niche markets.
CNG shipping technology has a wide scope of commercial applications in major worldwide NG markets. Several marine CNG projects are now pending after CNG shipping technology providers have received classification society approvals for ship design and gas (CNG) containment system technology. Two CNG technology providers have now set up manufacturing facilities for their gas containment tanks, one already in production.
A CNG project (operation) is more quickly planned and implemented than LNG. CNG shipping can be the best method to monetise stranded gas reserves quickly and can also be a temporary solution to create a market history in order to justify a pipeline, or LNG, transportation later.
CNG marine technology is particularly suited to both onshore and offshore reserves that cannot be produced because a pipeline solution is uneconomic or because an LNG option is too costly. CNG has a market niche between pipelines and LNG.
CNG technology is more suited on short distance projects. The considered economic range for a marine CNG project is up to 2000 nautical miles (nm) one-way distance to market, although up to 2500 nm can be an extreme range for CCNG (chilled CNG) shipping.
The marine CNG supply (value) chain consists of three components:
(1)The export upstream loading compression station with loading terminal (onshore or offshore)
(2)The midstream shuttle shipping
(3)The import downstream unloading terminal (onshore or offshore) and the unloading decompression station
The biggest investment cost in the CNG supply (value) chain is in the midstream shipping component (redeployable or movable assets), contributing up to 90% of the capital investment needed this makes CNG projects less risky as most of the investment is in ship assets supplied by the shipping contractor that means less investment for a project is needed for the fixed end (upstream and downstream) assets than would be required for LNG (liquefaction and regasification facilities).
The CNG supply system is optimised, modelled and designed for each specific project case as determined by case study as each project requirement is different mainly in terms of volumes and distances (many other factors are involved). There may be several CNG carriers (CNGCs) needed in the sea transport component of a project.
CNG carriers have a (cargo) gas containment system (GCS) comprising of gas containment tanks (GCTs) installed in a host ship hull (LNGC hulls are suitable). The system comprises many GCTs (can be many thousand) of up to 80 feet or 30 metres in length arranged either horizontally or vertically, depending on the GCS technology design. One technology providers GCS consists of coiled pipe (wound pipeline).
CNG is normally carried in the gas containment system at ambient temperatures.Some developing technologies offer marine CNG at chilled temperatures of -20F to -40F (CCNG) - the low temperatures when combined with high pressure results in a higher gas density (dense vapour phase) being achieved which gives a greater volume of gas in the GCS and thus a greater gas cargo (payload) can be loaded - which may allow greater economic sea distances. Chilled CNG is basically a state of gas stored between ambient temperature CNG and LNG. A compression ratio of up to 400:1 (LNG 600:1) can be achieved with chilled CNG by low temperature and high pressure.
There are several CNG containment systems (GSCs) for both ambient and chilled methods on, or in the process of being developed for, the CNG shipping market.
For promoting and providing CNG shipping services, most GSC technology providers partner with a credentialed LNG shipping company (and a shipbuilder) one CNG GCS provider also provides its own LNG shipping services. This allows the shipping company the flexible ability to offer a CNG or LNG shipping service, or both, to suit a NG delivery project.
CNGC cargo capacity (GCS) is normally designed and optimised for a specific project. A representative median cargo size CNGC is considered to be around 14 MMscm (million standard cubic metre) or 500 MMscf (million standard cubic feet).There are CNGC designs proposed for cargo capacity of 28 MMscm or 1Bscf (billion standard cubic feet). CNGCs can be designed for small, medium or large scale projects. CNGCs can be ships or barges (towed or pushed).
CNG shipping services are arranged by either the exporter / seller or the importer / buyer depending on the gas sales agreement (GSA) arrangements, that is on terms of CIF/ex-ship or FOB. CNG shipping services can be arranged and acquired on either a long-term time charter (charter party) basis, or on a toll ($/MMbtu) basis under a contract of affreightment (COA). Given that a CNGC or CNGCs are purpose-built for a project shipping service, CNGC employment, is on a long-term contract basis of 15 to 20 years usually same as the delivery contact.
A small-scale GCS, for small ships or barges, can be with CNG skids, which are cartridged GCTs, installed within an ISO container frame which can also be used for on-land storage. This can be an advantage for an intermodal transport (sea, road, rail) project requirement.
The CNG Skid can also be installed inside an ISO shipping container (CNG Container) which can be chilled down to 40F using a special (non-mechanical) liquid CO2 (LCO2) fuelled refrigeration (and low-maintenance) system which has fuel-endurance of up to thirty days. This unique Chilled CNG Container, developed by CNGI, has the ability to increase a CNG Container payload by up to 30% from ambient temperature. Using a light-weight CNG cylinder technology inside the Chilled CNG Container, a payload of up to 460,000 scf (13,000 scm) is achievable at a pressure of 3,600 psi and a compression ratio of 400:1. The Chilled CNG Container at a total gross weight of 65,000 lbs or 29,500 kg, is specially designed for intermodal transportation. (For more details on this proprietary Chilled CNG Container contact CNGI). Note: LCO2 fuel needs to be supplied for a Chilled CNG Container system projectand is usually commercially available.
A disadvantage of a CNG solution may be land storage, if required, of the delivered CNG a storage solution for CNG is in depleted gas fields, if available. CNG is best delivered (loaded/unloaded) on a continuous, uninterrupted, basis. If storage is an issue in a NG delivery project, then small-scale LNG may be the alternative solution to consider.
NEXT GENERATION CNG CARRIER THE HIGH SPEED CNG CARRIER
A new technology next generation CNGC is about to be developed. This is a High Speed CNG Carrier (HS-CNGC), capable of 60+ knots service speed, three times that of a conventional type CNGC of 19 knots.
CNGI is associated with the proprietary ship design owner on this project for developing HS-CNGC (and HS-LNGC) project applications.
Given the combination of high speed and a comparable fuel consumption of the HS-CNGC to that of a conventional CNGC, effectively the HS-CNGC would advantageously increase the economic range of CNG delivery by around three times, up to 4,500-7,500nm, LNG distances.
The HS-CNGC could also reduce the number of CNGCs in a CNG delivery system giving significant savings on the capital investment and overall costs in the shipping component of the supply chain in a CNG delivery contract.
Example case in point:
HS-CNGC of 60 knots voyage speed and conventional CNGC of 18 knots voyage speed both with same 14 MMscm (500 MMscf) deliverable cargo capacity. Route distance one-way of 1400 nautical miles.
§HS-CNGC will take 24 hours transit time, 4 days round voyage to deliver cargo load = 91 voyages per year = 1,274 MMscm per year (1.274 Bscm) or 44,972 MMscf (45 Bscf).
§CNGC will take 78 hours transit time, 9 days round voyage to deliver cargo load = 40.6 voyages per year = 568 MMscm per year = 2.3 CNGCs to deliver same annual cargo as one HS-CNGC.
A HS-CNGC with a gas containment system of 98 Chilled CNG Containers has a gross payload capacity of 35 MMscf or 1 MMscm. Over a 500nm market distance, three HS-CNGCs could delivery annually around 13 Bscf.
CNG LAND ROAD AND RAIL TRANSPORTATION
Land CNG transportation includes both road and rail projects.
A bulk CNG road delivery system is considered best suited for short range projects, normally under 200km one-way, with continuous end unloading supply and low storage requirements. A CNG project system is normally designed around the main criteria of annual and daily volume of NG required and route distance.
A total CNG supply chain system comprises: loading compression station, trucking for haulage of the CNG trailers (CNGTs), and the market unloading decompression (pressure letdown and flow control) station. All projects are different in system requirements, for a specific project the system is modelled and designed from case study.
CNG can be transported in ISO CNG-Skids (CNGSs) by either road or rail either in a separate or combined (intermodal) project. CNGSs have a cartridge cluster of cylinder gas containment tanks or GCTs (tubes) that can be of varying size and volume and NG capacity. A standard type of CNGS is 40 (12.2m) with a NG capacity of between 4,000 scm and 10,000 scm.
For road transport, the CNG-Skids are ISO containers that can be fitted onto an ISO trailer-chassis, combining to make an ISO CNG-Trailer (CNGT). A CNGT can have a NG capacity of up to 350,000 scf or 10,000 scm (365 GJ energy) at ambient temperature. A CNGT is sometimes referred to as a tube-trailer. The CNGT can double as both transport and storage. A road train of a three CNGT combination can carry a gross payload around 1,050,000 scf (30,000 scm) or 1,100 GJ of energy.
As previously mentioned, the CNG-Skid can also be installed inside an ISO shipping container (CNG Container) which can be chilled down to 40F using a special (non-mechanical) liquid CO2 (LCO2) fuelled refrigeration (proprietary) system. This unique Chilled CNG Container (developed by CNGI) has the ability to increase a CNG Container payload by up to 30% from ambient temperature. Using light-weight CNG cylinder technology inside the 45ft Chilled CNG Container, a gross payload of 460,000 scf (13,000 scm) is achievable = maximising payload. The Chilled CNG Container at a total gross weight of 65,000 lbs or 29,500 kg is specially designed for intermodal transportation, particularly suited for road application.A road-train of three Chilled CNG Containers (if regulations allow like in Australia on certain road routes) has a combined gross payload capacity capability of up to 1,380,000 scf (39,000 scm) or 1,450 GJ.
For rail transport, the ambient CNG-Skids or Chilled CNG Containers are loaded direct onto the flatbed railcar and can be double-stacked thus, in this case, one flatbed can carry up to 700,000 scf / 20,000 scm (ambient) or 920,000 scf / 26,000 scm (chilled). CNG-Skids can also be used as fuel tanks for providing NG for dual-fuel (gas-diesel) converted locomotives. A project-dedicated CNG train can economically transport (from gas source loading compression station) a high volume of NG over a long distance (virtual pipeline) in a short time. As an example, 100 MMscf or 3.5 MMscm per day (equivalent to around 2,000 tonnes LNG). The railed CNG (where a pipeline not available) could also be used as feed gas to an LNG plant for on-delivery either for export shipping or heavy-vehicle and power plant fuel distribution.
If storage is an issue in a CNG delivery or supply project, then an LNG system may be the alternative solution.
