A quarter of the energy needs of the United States are met by natural gas. One can only imagine the extensive network of pipelines that crisscross the country bringing natural gas from production sites to gathering systems, but trenchless technology allows for the installation of these pipes with minimal surface damage.
Transporting Natural Gas to Homes
Processing equipment removes corrosive elements such as sour gas and other impurities at the gathering stations, making the gas usable. Thereafter, the gas is transported to interstate pipelines that deliver the gas to distribution companies.
This extensive system for natural gas is known as the ‘interstate highway’ and consists of high-strength steel pipes varying in diameter from 30-inches to 36 inches in diameter, totaling about 180 thousand miles. The natural gas travels across the interstate pipeline at high pressures ranging from 200 to 1500-pounds per square inch (psi). This reduces the volume of the gas about 600 times, helping in propelling the gas while allowing more amounts of gas to be transported.
The component parts of interstate pipelines consist of transmission pipes, compressor stations, metering stations, valves, control stations and supervisory control and data acquisition (SCADA) systems. As the need for natural gas increases so does the need for transmission pipes to transport them.
Some of the methods used for installing natural gas pipelines are horizontal directional drilling (HDD), auger boring, pneumatic pipe ramming, microtunneling and impact moling depending on the subsurface conditions.
Trenchless Installation of Natural Gas Pipelines
Horizontal directional drilling involves setting up a launching and receiving pit on either side of the installation and setting up the drill rig. Depending on the diameter of the pipe to be installed, a pilot bore is drilled through from the entry to the exit point. The drill string is then fitted with a back reamer and the bore is enlarged until the desired diameter of pipe can be installed.
The HDD process is facilitated with the help of lubricant slurry consisting of bentonite that lubricates the drill bit, stabilizes the borehole and transports the soil cuttings back to the surface. The strung-out pipes are welded and tested for leaks hydrostatically and by x-ray. The joined pipes are then aligned to be pulled through the borehole.
Microtunneling is a laser-guided, steerable method of pipeline installation allowing for the accuracy of up to 1-inch to designed line and grade. It is similar to pipe jacking but is done remotely with the help of a tunnel boring machine (TBM).
The casing pipe is positioned and jacked into the ground using jacking force from the launching shaft. Another casing pipe or product pipe is inserted between the jacking machine and the TBM and the process is repeated till the pipe exits at the reception shaft.
Auger boring utilizes a steel casing fitted with augers and mounted on tracks in the launching shaft. Thrust from the boring machine is applied using hydraulic rams at the back of the pit, advancing the casing with the augers forward.
The rotating augers help remove the spoil from the casing. Once the first casing is inserted, the next casing or product pipe is attached to the previous one including another section of the auger to the previous auger. The process is continued till the pipe exits at the reception shaft.
Pneumatic Pipe Ramming
Pneumatic pipe ramming is most favored when installing pipes under road and railway embankments. The leading edge of the casing pipe used is either open-ended or closed. A pneumatic tool is used to hammer the pipe into the ground using percussion force. The excavated soil enters the casing in the case of an open-ended casing and is removed either by compressed air, water jetting or by using an auger. The product pipe is installed simultaneously.
Impact moling uses a pneumatically powered, self-propelled mole generally of torpedo shape fitted with a reciprocating hammer at its nose. When the hammer is activated, it propels the mole forward at a rate of 3-inches to 4-feet per minute, depending on the soil condition. Once the mole exits, the product pipe is pulled or pushed through. Only short lengths of installation are done using this method because of the inability to control the direction of the mole.
Trenchless Rehabilitation of Natural Gas Pipelines
Leaking gas pipelines are a major cause of concern and should be addressed immediately. With trenchless rehabilitation methods, it is now possible to detect major damage or minor leaks and repair such pipes without having to replace them. Methods such as cured-in-place pipe, pipe bursting, close-fit sliplining and robotic pipe repair are suitable to rehabilitate gas pipelines because they cause little to no surface disruption.
Cured-in-place pipe (CIPP) utilizes a liner made from polyester yarn with a surface layer of polyurethane. The host pipe is cleaned of debris and the liner is soaked with a polyurethane resin-based adhesive just before insertion.
Once the liner is in place it is inflated until it bonds firmly onto the internal surface of the host pipe.
Pipe bursting/splitting is used for replacing undersized or worn out gas pipelines. This method works by hydraulic or pneumatic expansion, or by pulling a pipe behind a bursting head with a diameter slightly larger than the diameter of the host pipe. The broken fragments of the old pipe are pushed into the surrounding soil while pulling in the replacement pipe.
The close fit lining is a reduction method in which the diameter of the pipe-liner is slightly larger than the diameter of the host pipe. A mechanical device exerting a pulling force with the help of a reduction dye slightly reduces the outer diameter of the new polyethylene (PE) pipe while it is being inserted.
Once the PE pipe is correctly placed within the host pipe, the pulling force is released allowing the PE pipe to return to its original diameter making a close fit against the host pipe. This method eliminates the need for filling the annular space with grout.
Robotic Pipe Repair
Robotic pipe repair method utilizes robots fitted with a control module that enables an operator to navigate the device within the pipe. The data transmitted helps the operator to access the condition of the pipe whether it needs to be repaired or replaced. Robots that are used for repairing pipes are fitted with a rotating nozzle capable of applying sealant. Once a crack is detected, the robot is positioned in place and a servo rotates the nozzle applying sealant directly on the crack. (Learn more in "An Overview of Robotic Pipe Repair.")