Microtunneling and horizontal directional drilling (HDD) are trenchless pipeline installation methods that are used extensively for laying pipelines. When two such tried and tested methods are combined, the result is usually a hybrid that incorporates the advantages of both methods. The direct pipe method is the trenchless installation method that results from the combination of microtunneling and HDD to give rise to a method that is versatile, quick, efficient and cost effective.
It’s a one-pass method that does not require back reaming as in HDD, and does not require a shaft for pipe thrusting as in microtunneling. The direct pipe method involves simultaneous excavation of the borehole and installation of the pipeline – all in one step – and is suitable for any ground/soil condition. (For more on microtunneling and HDD, see Microtunneling Vs. Horizontal Directional Drilling: Understanding the Differences Between These Key Trenchless Methods.)
Installation using direct pipe was first carried out in Worms, Germany in 2007, at the Rhine river crossing. It was used in the United States for the first time in Florida, in August 2010, for crossing Highway 70. The longest direct pipe method installation was done with a 48” diameter pipe for a length of 1,400 m.
The Direct Pipe Method
The direct pipe cutter machine is mounted and welded onto the front of the pipeline. Similar to the pipe jacking process, the tunnel face is excavated using a cutter head. Depending on the prevalent soil conditions, the cutter head can be equipped with cutting tools. Unlike HDD, direct pipe allows for installation through hard and soft rock, unstable soil and large boulders. This is because the pipe is simultaneously installed along with the borehole excavation, eliminating the need for borehole wall support.
A slurry circuit is installed from within the pipe, allowing the excavated material to be transported back to a separation plant at the surface. The slurry and soil are separated in the separation unit and the cleaned slurry is fed back into the lubrication system.
The direct pipe machine is controlled by an operator on the surface near the rig. The machine is constantly monitored to keep it in designed line and grade using a gyro compass and hydrostatic water leveling system. The trailing end of the direct pipe machine has a lubrication ring that acts as a transition point between the machine and the product pipe. The annular gap in the direct pipe method is larger than that provided in the microtunneling machine and is filled with bentonite to facilitate easy pipe installation.
The product pipeline is welded into a string, inspected and positioned on rollers. The cutting machine is installed at the leading end of the pipe string, and the pipe thruster is set up to drive the pipe into the ground. (To learn more about bentonite, see Bentonite and the Use of Drilling Mud in Trenchless Projects.)
Instead of a jacking machine thrusting the pipe into the borehole as in a pipe jacking operation, a pipe thruster clamps the pipeline from the side and pushes the pipe and the cutting machine using its two thrust cylinders in strokes of 5 meters. The thruster is well anchored to a structure and uses clamps to transfer its leverage without damaging the pipe coating. Typical diameters of pipes that can be installed using the direct pipe method are 26”, 42”, 48”, 56” and 60”; maximum distance installable is a function of diameter and geological conditions at site, and varies from 300 mm to 1,500 mm.
Advantages of the Direct Pipe Method
The direct pipe method can be used in various soil types, including unstable soil, hard and soft rock, and soils with large boulders. Because of this, it reduces failure risks due to ground conditions. Additionally, high accuracy of installation allows a large bending radius.
Site setup is required only on the rig side, and no elaborate launching and reception shafts are required as in microtunneling – only a moderate-sized area is required for retrieval of the direct pipe cutting shield on the receiving side. Furthermore, the pipe thruster can be used to retrieve the pipe and tunneling machine in case a cutting retooling operation is required.
The direct pipe method requires less time than either microtunneling or HDD, as no time wasted in coupling pipes (microtunneling) or drill rods (HDD). It also uses a one-time pass with no need for back reaming. No extra support is needed for boreholes due to the presence of pipe as the borehole progresses, and large pipe diameters can be installed without casing pipes.
Slurry lines are installed within the pipe that transports the soil cutting directly from the cone into the spoil lines, which eliminates the possibility of cuttings left out in the borehole. The use of bentonite slurry is minimized, and there is minimal risk of lubrication fluid frac-out. And finally, the method has only a small environmental footprint.
Direct pipe is a hybrid method incorporating the benefits of HDD and microtunneling, while setting aside tedious, time-consuming processes associated with them. The results achieved with the direct pipe method are impressive, and practically advantageous. This method is able to achieve good results where other methods may fail.
Since HDD is extensively used for pipeline installation through long distances and under environmentally sensitive locations such as wetlands, rivers and swamps, using the direct pipe method with the benefit of microtunneling, and simultaneous pipeline installation, can be a more economical option to reduce time and cost associated with large projects.