With all of the proposed LNG terminals for subsidizing natural gas supplies, the major question is: What affect will regasified LNG have on the pipeline, compression facilities and end users? This is a difficult question to answer. It depends greatly on the piping configuration and where the LNG is entering the pipeline:
How far the LNG travels in the pipe from the regasification facilities to the pipeline interconnect?
How quickly the re-gasification facilities can ramp up to full flow?
Where does the gas enter the cross country pipeline?
How much blending can be done within the pipeline?
What size piping is used?
Small generic pipeline systems will be used to illustrate some of the problems that are faced in injecting gasified LNG into piping systems. This would include the effects of Joule-Thompson cooling, mixing, and null point movements. Also, several different LNGs will be used in the discussion to demonstrate that different shipments can cause different issues. Another major issue in looking at LNG injects is getting a common language and set of data established. Marketing people and engineers use the same terminology to mean different things which can lead to severe misunderstandings. Some of these problem areas will be discussed while looking at the results.
The biggest issue normally brought up after the safety of the plants is gas interchangeability. Will the compressor drivers and end user devices be able to affectively combust the gas? Gas interchangeability is normally looked at in term of the Wobbe Index. If the Wobbe Index is within +/- 5%, the gas is interchangeable. In many cases, engineers run steady state analysis and look at the affects of the LNG at steady conditions. In transmission systems this can be acceptable for some pipeline configurations, but not for others. Often you need to look at the rate of change of the Wobbe Index or the HHV to see if a significant issue could exist. The rising cost of natural gas makes supplementing domestic supplies with LNG more likely. The issue then becomes where can a LNG facility be placed and what affect is it going to have on the pipeline gas.
The following terminology is used within this paper:
Wobbe Index - is the heating value of the gas divided by the square root of the specific gravity. Either higher heating value of lower heating value can be used. For this study Wobbe Index is based on the LHV
Higher heating value (HHV) - the ideal heating value of the gas.
Lower heating value (LHV) - the actual heating value at combustion taking into account the affects of water vapour on combustion.