Ecological impact of pipeline installation and the installation methods, trenchless or otherwise, are inextricably linked. Environmentally sensitive areas such as palustrine wetlands, estuaries, rivers, lakes, gardens and forests are important features that need to be protected from human interference.
Often, gas pipelines pass through such areas, and it has become necessary that contractors utilize trenchless construction methods such as horizontal directional drilling that cause these features little to no disturbance. In addition, the convenience and ease of installation, and the reduced risk of damaging other utility lines passing through that area are some of the advantages associated with trenchless methods.
While not completely risk-free, the benefits of trenchless technology for installing gas pipelines outweigh the drawbacks. Research shows that the ecological advantages of using trenchless methods can reduce the emission of greenhouse gasses by up to 90% and save cost up to 30% when compared to traditional trenching methods.
The Transport of Gas through Pipelines
The production of gas, fit for use by the consumer is a long and tedious process involving many processes and miles of pipelines. Natural gas is the principal source of energy to run our day to day activities and hence is in high demand. In the infancy of the natural gas industry, its main use was in the lighting of streetlamps and heating homes. Today natural gas is used for a variety of needs right from homes to industries. The challenge lies in transporting natural gas safely and effectively right to the end user. This is accomplished by means of a complex network of pipelines.
The transportation route consists of three types of pipeline systems namely, the gathering system, the interstate pipeline system and the distribution system. The second system is of two types – the interstate pipeline and the intrastate pipeline. The interstate pipelines transport gas across countries or state boundaries while the intrastate pipelines transport the gas within the state limit to distribution centers across the state. The various pipe systems use different pipe sizes based on the requirement and function.
Installation of Gas Pipelines
Depending on the route of the gas pipeline and the terrain it will cross, various trenchless pipeline installation techniques can be used. The most popular and widely used method is horizontal directional drilling (HDD) because of its versatility.
It is estimated that about 70% of subsurface installation is carried out using HDD. It can be used under roads, river crossings, sea, or under environmentally sensitive locations without disturbing the delicate ecological balance. HDD can be used to install pipelines of diameter up to 1200 mm, and for installation runs up to 2000 meter. The gas industry has been innovative in the development of new technologies to make their business more efficient.
For smaller pipe diameters (1/2” to 6”) ideal for distribution pipelines, piercing tools such as pipe ramming are used. Horizontal auger boring (HAB) is also used for pipeline installation, however; it can only be used effectively in some areas. It proves to be costly and difficult while augering under rail embankments and roads because it can cause settlement or upheaval. Below is a brief discussion of the main methods of gas pipeline installation.
Horizontal Directional Drilling (HDD)
HDD is a three-step process – drilling of the pilot bore, the reaming operation and the pullback process. Once the launching location is established, a pit is dug for aligning the drill rig. Drilling fluids are mixed and made ready for pumping into the pilot hole during the drilling process.
The pilot bore is a small diameter hole that is drilled using the appropriate drill bit selected based on the geologic soil strata encountered in the area the pipeline is to pass through. The drilling is carried out along a preplanned route that has been mapped out based on the geotechnical investigation with the help of a steerable drill head. When the pilot bore is completed, the reaming process is carried out to upsize the hole to the desired pipe diameter. Once the desired diameter is obtained, the product pipeline is pulled through.
Pipe ramming is carried out with the help of a pneumatic or hydraulic hammer that is attached to the bore pipe. The repeated percussive blows drive the pipe into the ground. A cutting shoe ahead of the lead casing helps cut through the soil and reduces friction; bentonite or a polymer lubricant can be used for the same. For shorter lengths, an entire pipe section can be rammed in at once or for longer sections, one section at a time.
This is carried out by removing the ramming tool after each ramming operation and welding the next section to the installed section. The ramming tool is then connected to the new section and the process is repeated. The spoil collected inside the pipe can be removed by pigging, compressed air, water or by an auger system.
Trenchless methods are a great boon for cities because it provides a way for installing pipelines for gas with minimum disturbance to the surface. Often roads in excellent condition have to be dug up to install or rehabilitate pipelines. This is not only time consuming, but also an economic drawback for the city in addition to being a nuisance and an eyesore.
Trenchless methods like HDD are equipped with technology to maneuver through restricted spaces, sometimes in between two pipelines, without damaging either. The other up sides of trenchless installation methods are that roads are not blocked, traffic is not affected and noise and air pollution is significantly minimized. Now that natural gas is in high demand, distribution centers are extensively broadening their network using trenchless technology to install, upgrade and rehabilitate their pipelines throughout the city to effectively serve the population.