ABS Approves New CNG Carrier: The CNG Optimum 200

David Stenning's picture
David Stenning, Chief Operating Officer, GEV Canada Ltd.
John Fitzpatrick's picture
John Fitzpatrick, Chief Operating Officer, GEV Canada Ltd.
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Figure 1: The new CNG Optimum Ship (200 MMSCFD capacity)

The quest to develop a low-cost, volume efficient CNG ship that could unlock the full potential of Marine CNG has been pursued for two decades. The optimum way to store compressed gas in a ship is to stack long lengths of hexagonal close-packed pipe horizontally in the holds. Past attempts to use this approach failed because as the ship flexes at sea, the pipes rub against each other which is clearly unacceptable. A novel solution overcoming this limitation has been developed, tested and verified by ABS resulting in a new class of CNG ships: the ‘Optimum’. Based on this novel principle, a CNG ship with a capacity of 200 million standard cubic feet of natural gas was recently Approved for Construction by the American Bureau of Shipping.

Global Energy Venture’s (GEV) new CNG Optimum 200 ship design has overcome previous marine CNG limitations and is now ready for commercialization.

The CNG Optimum ship has two significant competitive advantages over other CNG ships.

  • It is the lowest cost CNG ship by a large margin.
  • It can be fully constructed by a shipyard without the need for a specialized CNG tank fabrication facility. This facilitates both shipbuilding and financing, thus reducing the “first-mover risk” which has been a major impediment for marine CNG projects.

The basic idea behind this invention (patents pending) is that long hexagonally packed CNG cylinders, laid horizontally and self-supporting, provide the most efficient and practical geometry for carrying CNG in a ship. This geometry is further enhanced if the ship is designed without transverse bulkheads, as it allows the pipes to stretch the full length of the cargo space. This configuration maximizes the amount of CNG containment that can fit in a ship and minimizes end connections; thus, it is the optimum configuration. 

Past attempts at this optimum geometry failed because as the ship flexes at sea the close-packed pipes will rub together. Design codes and common sense do not allow pressure vessels to rub against each other.  The innovation behind the CNG Optimum design is that the pipes are strongly forced together creating enough friction between the pipes so that the shear force mobilized prevents the pipes from moving relative to each other or relative to the ship.  Figure 2 shows the general concept and principal particulars of the CNG-O-200 ship.  The cut-away shows the pipes close-packed and running the full length of the ship’s cargo space.

Figure 2: CNG Optimum Ship with a capacity of 200 MMscf (‘CNG-O-200’): Approved by ABS

Several shipyards competed to build the first fleet of CNG Optimum ships. CIMC Raffles was chosen by GEV to build the first series of ships.  The cost of each ship is between US$135 and US$140 million and the time to construct the first ship is up to 30 months with the rest of the ships in the fleet following in 4-month periods. 

The economic application for the Optimum ship is to connect markets with regional gas sources, including offshore gas fields. For short transportation distances, it is simpler and less costly to compress the gas than liquify it.  This means that significant upfront capital investment in fixed process facilities are not required. Offsetting this lower infrastructure cost is the higher cost of CNG ships compared to LNG ships on a volume transported basis.  Marine CNG is a regional transportation system, somewhat like subsea pipelines, but unaffected by water depth, sea bed conditions, environmental issues or political boundaries. The economic niche of marine CNG has been described by many and is illustrated in Figure 3.

Figure 3: The Marine CNG Market

Marine CNG will play an increasingly important role in connecting gas supply to market demand as industry works towards supplying more markets with competitively priced natural gas.

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