The biggest challenge—after initial gas field discovery—is transporting the gas from the field to the consumer. The natural gas chain, as shown in below, follows the produced gas through its processing, transportation, and delivery to the consumers. At the gas processing plant, NGLs are separated for direct sale to the industrial and petrochemical markets. Methane is then transported by pipeline or LNG tanker to gas markets. Gas markets are generally residential and commercial users (utilizing gas for space heating), industry (directly burning natural gas), transport (for natural gas–powered vehicles), and electrical power generation.
The first step in transporting gas occurs once gas reaches the surface via the tubing pipe. Produced gas flows through surface valves and flanges collectively known as a wellhead or Christmas tree assembly. Pressure gauges and emergency valves are part of the assembly. From the wellhead, the fluids may be separated into phases (liquid and gas) using simple gravity separators and sent via in-field gathering pipelines to the central processing plant.
Gas Transmission Pipelines
The first pipelines were built in the late 1800s to transport low-Btu coal gas through cast iron and lead pipes for street lighting. Long-distance, high-pressure pipelines began operating in the United States in 1891. Pipelines are the most common, and usually the most economic, delivery system to transport gas from the field to the consumer. Pipelines are a fixed, long-term investment that can be uneconomic for smaller and more remote gas fields.
The volume of gas that can be transported in a pipeline depends on two main factors: the pipeline operating pressure and pipe diameter. The maximum diameter of pipelines continues to increase every few years. As diameters of 48 in. (121 cm) become common, the industry may be approaching the practical limit to onshore pipelines. To handle the increasing demand, it is likely that operating pressures will increase rather than the size of the pipe.
Most transmission pipelines operate at pressures of more than 60 bar, and some operate as high as 125 bar. To maintain a high operating pressure, compressors maintain the pressure of gas, and depending on the length of the pipeline and the topography, may be installed at intervals of 150 km to 200 km.
Increasing pressure requires larger and thicker pipes, larger compressors, and higher safety standards, all of which substantially increase the capital and operating expenses of a system. The gas industry uses an interesting unit to measure pipeline costs, dollars per inch per kilometer ($/in.-km), measuring the cost of 1-in. diameter per kilometer length. This cost has come down, more substantially in offshore pipes where larger diameter and longer distance pipelines are proposed. By some estimates, the cost of offshore lines has reduced from more than $100,000/in.-km to around $25,000 to $40,000/in.-km. Thus, a 400-km, 48-in. line would cost around $480 million to $770 million today, versus double that amount 20 years ago. The rising cost of steel, accounting for 45% of the cost of a typical pipeline, has offset some of the gains in pipe construction and fabrication costs.
More information on gas pipeline construction can be found in the videos below:
Onshore pipeline construction:
Offshore pipeline construction: