gas lec2

• CNG


  Natural gas compressed at pressures of 2,000 to 3,000 psi (130 to 200 atm) and sometimes chilled (but not liquefied) to temperatures down to (– 40°C) for even higher reduction of its volume
• CNG is a technology proven in many applications, including transport by ship, truck, and barge
• Factors preventing CNG marine transport


  • Most investment have been on LNG, for understandable reasons
  • The use of CNG was envisioned to take market share away from LNG, which, as was explained above, is not necessarily a good approach, because CNG and LNG are suitable for different transportation scenarios
  • Innovative low-cost and high-efficiency designs for CNG vessels have become available in the 2000s
• There are several areas where population centers are separated from natural gas sources by 2,000 km (or 1,200 miles) or less across water
• For each of these areas, there exist multiple scenarios for CNG distribution, in terms of number of vessels, vessel capacities, and itineraries
• Identification of promising scenarios is necessary to determine project economics, and possibly guide future technological developments, particularly as new CNG vessel technologies become available
• CNG technology


  Quite simple and can be easily brought into practical applications, assuming the economics are attractive
• CNG ships under consideration


  • Optimized to transport large quantities of gas
  • Can carry approximately one-third the amount of an LNG carrier of the same size
• CNG technology variant 1


  1. High-pressure gas storage and transportation system based on a coil of relatively small diameter pipe (6 to 8 inches, about 15 to 20 cm) sitting in a steel-girder carousel

  2. Considering natural gas compressed at 3,000 psi and at ambient temperature, a typical CNG carrier assembled with 108 carousels can offer up to 330 MMscf (about 10 MMscm) capacity
• CNG technology variant 2


  1. High-pressure gas storage and transportation system, is based on horizontal or vertical arrays of 36-meter, long large diameter pipes (about 1 m), segregated, and manifolded into a common pressure and flow system in groups of 24, called modules

  2. These modules are then arranged in holds, whose count determines the CNG carrier capacity

  3. The largest model of such a vessel can offer up to 800 MMscf (about 22 MMscm) of capacity
• Chilling helps reduce the volume of CNG


  The volume taken by an amount of gas n, is proportional to ZT/p. Consequently, if gas pressure needs to be raised to a certain value, for gas volume to be reduced to a certain amount at ambient temperature, lowering the temperature (chilling) can reduce the compression requirement for the gas to occupy the same volume
• At typical CNG pressure levels (2,000–3,000 psi), the Z-factor may differ significantly from 1 when the temperature varies
• The Z-factor must be taken into account in related calculations
• Calculating the natural gas capacity (volume) of each vessel


  1. The total capacity Gtotal for a fleet of n vessels per cycle

  2. The (theoretically) minimum total capacity for given tconnect, L, v, qc, and qload

  3. The lower bound on the number of vessels, n, required to implement a schedule

  
• The smallest number of vessels needed for each cycle

  Must be greater than or equal to the minimum number of vessels required
• Using 3 vessels per cycle would reduce the vessel size requirement to 430 MMscf each, and the total fleet size for 3 vessels per cycle would be 5,160 MMscf
• Compared to the results in Example 2-1, there is a clear reduction (by 25% of the reduced values) in the total fleet volume that would be required to service consumption at the actual (lower) capacity