Energy in the 15.5 trillion cu ft of marketed natural gas production during 1964 exceeded the energy contained in any other mineral energy resource produced and was nearly 35 per cent of the total energy produced in the United States. The gross investment in natural gas gathering, transmission, and distribution pipelines and related facilities currently approximates $27 billion.

Modern transmission systems are constructed of solidly welded pipe, of 65,000 psi or more minimum yield, 30-42 in. in diameter, for operation at pressures ranging from 1,000 to 1,500 psig.

Many new natural gas distribution systems are utilizing thin-wall steel pipe and plastic pipe, which may have much wider application in the future.

The transmission of natural gas at minimum cost generally is attained by a system in which pipe diameter and operating pressure are maximum for the minimum wall thickness which will comply with safety standards. Future technological improvements probably will involve pipe with larger diameters and thinner walls, made of higher minimum-yield steel; operation at higher pressures; internal coatings; automatic welding of pipe joints; and greater utilization of gas-powered turbines and centrifugal compressors.