Gas pipelines - More efficient full or half-full?

In those big honkin’ natural gas pipelines (or any gas pipe, I suppose) is it more efficient to run them really full or less full?

By efficient, I mean which method uses less energy to move one unit of volume a certain distance.

Really full argument:

As you put more gas in the pipe, it becomes more like a liquid and you have more inertia in the gas. You aren’t fighting surface friction as much (per unit of volume) and if you’re going to run a compressor, it’s best to fully utilize it, rather than run it at half power.

Half full argument:

When the pipe is half full, you need much less compression to make it move. You can basically let the gas flow at a leisurely pace and run the compressors at a very low power. You aren’t getting as much volume through per unit of time, but you’d be using much less power per unit of volume.

Anyone know the answer? Need some clarification?

Full disclosure - I’m wondering about the amount of carbon dioxide that would be released by compressor stations. Would a company that runs their pipes half full be in a better position regarding carbon trading or would a company with full pipes be doing better?

Although I’m not sure on the efficiency energy wise of moving a full vs 1/2 full pipeline of gas, I can tell you a few things about the pipeline business. The company I work for deals with selling gas, so we are in constant communication with the pipeline companies.

The most efficeint use of pipelines is when temperatures are constant. The worst thing for pipelines are wide swings in temperature from one day to the next. This causes “backups” on the lines, caused by pressure/volume waves and the major pipeline companies issue what is called an OFO (Operational Flow Order). When an OFO order is issued, they curtail the “cross flow” of gas to and from the pipeline “offloading” sites, called “City Gates.” This results in somtimes mandatory curtailment of gas delivery to large consumers.

Ironically, we have no problems in periods of extended cold, but rather have HUGE issues when the temps move from 60F to 20F in a 24 hour period. It’s also very difficult for the planners to put the proper gas into place, which is done on a daily basis, 24 hours before delivery is expected (Monday plans for tuesday consumption)

Planning for gas pipelines, and city gate usage come from a combination of “Seasonal norms” (long term planning), and forcasted conditions (short term planning).