In an attempt to answer some of the questions and difficulties raised when applying these trenchless techniques to longer distances, larger diameter bores in some of the more arduous ground types, particularly in more environmentally sensitive areas, Herrenknecht AG has developed a new method of trenchless installation that combines elements of both the microtunnelling and HDD processes. This new process is known as Easy Pipe.
Current microtunnelling technology
Microtunnelling comprises either the straight-line, or in some cases, curved section, installation of a pipeline between a launch shaft or pit and a target shaft or pit. The ground which the pipe is jacked into is excavated using a suitable and often remote-controlled tunnelling shield, with the final pipeline being installed as a single step operation. The individual pipe sections that make up the final pipeline are pipe jacked into the void created by the tunnelling shield in relatively short lengths whilst also being used to transfer the forward impetus to the shield from the jacking station in the launch pit.
The microtunnelling method offers the installation of tunnel lengths of up to 500 m or more and borehole diameters of up to 2,000 mm or more. The technique can also be deployed in almost all geological conditions from the softest ground to hard rock (up to 300 MPa UCS) and under almost any groundwater condition, with water pressures of up to five bar and higher.
Microtunnelling can be used to install various pipe materials including steel or PE pipes but due to inherent technical problems with these materials, this is rather rare. PE pipes, for example, have a very low resistance to jacking pressure which directly reduces any possible installation length and while steel pipes are designed for high axial loads, each section must be welded in the launch shaft to the pipe already installed.
This leads to several disadvantages regarding their practical use. Such operations are time-consuming as the actual tunnelling works have to be regularly interrupted and depending on the use of the final pipeline, complicated to weld because larger steel pipes need precise orientation and centring. Furthermore, the welded seams cannot be pressure-tested prior to final installation, which is normally obligatory for the installation of high-pressure gas or oil pipelines, because achieving remedial repairs below the obstacle being crossed is practically impossible. Further disadvantages include the difficulties in manoeuvrability of steel pipes and the requirement that they be installed in straight lengths.
Using microtunnelling, this is can only be overcome by installing conventional (concrete, polycrete, etc.) jacking pipes as a carrier or protection pipe into which the actual product pipe is then installed. However, the disadvantages of this method are obvious. The bore has to be over-sized in order to accommodate the carrier pipes and the cost of the carrier pipe has to be absorbed because it is lost in the ground as it serves no useful purpose other than to maintain the void into which the working product pipe is placed. Additional work is also required for the subsequent installation of the product pipe including the deployment of equipment such as winches or similar items.
Current HDD technology
Conventional horizontal drilling technology is a three-phase installation method comprising of pilot drilling, bore expansion and product pullback/installation. The technique is normally used for the installation of non-tensile pipelines such as steel, PE or cast iron. Installation rates are generally higher than those for microtunnelling, with maximum lengths of approximately 2,000 m or more being possible. However, maximum pipe diameters are generally smaller, with 1,400 mm being about the practical maximum.
The major disadvantage of HDD is the relatively high sensitivity to geological conditions. In particular, gravel or stony ground with a low cohesion factor can cause problems in keeping the borehole open prior to the pullback process, particularly when relatively large boreholes (more than 800 mm) have to be drilled.
This is mainly due to the fact that the borehole is only supported by the drilling fluid used in the boring operation with no interim or intermediate pipes being installed as part of the HDD process. In unstable ground conditions, it is often impossible to obtain the required stability of the borehole wall with the drilling fluid alone at larger borehole diameters. Such boreholes might partially collapse after a certain period of time, preventing a successful pullback of the product pipe.
Furthermore, problems can occur in HDD operations where boulders and/or other loose ground come into the bore and become jammed between the borehole wall and pipe during pullback, so damaging the pipe.
In the case of large borehole diameters an extremely high torque, as may be encountered in rock bores for instance, may be transmitted to the cutter head through the relatively thin drilling rods. This can lead to rod fractures. HDD technology also requires a borehole diameter of between 1.3 and 1.5 times that of the product pipe diameter to minimise the risk of jamming due to cave-in and/or sediment in the borehole. This in turn creates technical and economic disadvantages.
Information available on these techniques has shown that the established microtunnelling and HDD methods are not always best suited for the safe and economic installation of large diameter, long distance non-tensile pipelines in difficult ground formations.
Herrenknecht’s new Easy Pipe method has been developed to create a technology that will provide an economic No-Dig solution for the installation of a pre-prepared and tested non-tensile pipeline with relatively large diameters of between 800 and 1,400 mm over comparably long installation lengths of between 500 and 1,000 m in difficult soil types such as gravel, rubble, rock and so on.
The Easy Pipe method distinguishes between two basic situations. The first is where both launch and target points are above ground (or in a shallow pit) with a projected final bore profile similar to a conventional HDD installation, in a curved bore crossing beneath an obstacle.
The second situation is where the launch point is in a shaft with the target point located above ground or in a shallow pit at the surface. In this case the bore profile is rather similar to a conventional pipe jacking installation.
For ease of explanation, the following concentrates on the installation process for the situation with the launch and target points located in pits close to the surface.
Step one of the installation process requires a microtunnelling unit to be prepared and assembled in the launch pit. Consisting of standard components such as the jacking frame, pressure ring, cutter head and jacking pipes, the cutter head is launched and guided in the conventional microtunnelling way along a given planned alignment in accordance with the local technical pipe jacking standards. Cutter head monitoring and position control along the given tunnel alignment is carried out using current pipe jacking technologies.
The jacking pipe sections are added to the advancing pipe string as usual, allowing the thrust forces of the jacking rig to progress the cutter head forward through the bore.
The difference between these jacking pipes and conventional ones is that the special design allows them to be used as jacking pipes in the forward direction whilst allowing them to be retracted from the completed bore to pull in the product pipe. The joints between the jacking pipe sections are such that they bolt together around the circumference of the joint. The joint is designed to withstand the thrust and pullback forces of up to 6,300 kN (630 t) during the initial bore and product pipe pullback operation.
The jacking pipes also stabilise the bore walls as the bore advances and due to the close proximity of the pipe’s outer wall to the bore wall, the potential for collapse of the bore in unstable ground formations is avoided.
After the cutter head has reached the target pit, it is separated from the jacking pipe string. Then, using a specially designed connection pipe, the first jacking pipe is connected to the prepared and tested product pipe.
With the jacking pipe string and the product pipe now coupled together, the jacking pipes are pulled back through the completed borehole using the jacking frame, which has been adapted so that the “÷jacking’ rams can work effectively in either direction. In this way the connection pipe and the product pipes are simultaneously pulled towards the launch shaft along the tunnel alignment.
In the launch pit, the individual jacking pipes are successively removed along with the hydraulic feed lines, slurry pipes, guidance cabling and other equipment utilised as part of the initial boring operation. This process continues until the lead end of the product pipe arrives at the microtunnel launch shaft.
The connection pipe is then separated from the product pipe and removed from the pit. The jacking frame can then also be removed. Finally the product pipe can be connected with the lead-in and lead-out pipelines on either side of the obstacle that has been crossed. Launch and target pits are then back filled or re-constructed to complete the installation.
The advantages and disadvantages of Easy Pipe
The specific advantages and disadvantages of the Easy Pipe method as compared with microtunnelling and HDD are best examined in terms of:
“¢ Drive length;
“¢ Pipe materials;
“¢ Quality standards;
“¢ Ground conditions;
“¢ Drilling fluid;
“¢ Borehole volume;
“¢ Construction risk; and,
“¢ Construction cost.
In terms of drive length, in the small-diameter range, longer maximum drive lengths can generally be achieved with HDD technology, whereas this is not generally the case with microtunnelling or when using the Easy Pipe method. However, microtunnelling and the Easy Pipe method do allow for an extension of the drive length in this smaller diameter range through the installation of intermediate jacking stations within the jacking pipe string and, where necessary in tougher ground or more varied ground, through cutter changes in the borehole, reducing the HDD advantage considerably.
With a launch shaft and an overburden depth requirement that is considerably lower than that needed with HDD, the microtunnel and Easy Pipe methods also allow a considerable corresponding reduction in the total required drive length in comparison with HDD (approximately 30 per cent reduction between the same two launch and target positions).
In terms of pipe materials, principal pipe materials used in HDD can also be used with the Easy Pipe method, i.e. steel, PE and cast iron. However, these materials are either only partially suited or not suitable at all for direct installation using microtunnelling unless the microtunnelled pipe is to be used as a carrier pipe only.
The quality standards achievable with the Easy Pipe method are optimum. This is because the Easy Pipe product pipes can be completely pre-assembled and tested prior to final installation in the same way as with HDD installations. In terms of abrasion of the product pipeline in the borehole with the Easy Pipe method, wear conditions are not as good as when using microtunnelling to install a carrier pipe, but are at least comparable with an HDD installed product pipe.
Ground conditions are one of the key factors that led to the development of the Easy Pipe technique, so Easy Pipe and microtunnelling can be deployed in almost all ground formations (gravel, boulders, rock, etc.) if the cutting head, drive units and other equipment are suitably designed. Where HDD is deployed; however, ground conditions can be a considerable limiting factor, particularly with regard to large diameter pipelines.
Looking at drilling fluids, microtunnelling and Easy Pipe’s quality requirements with regard to the composition and operation of the drilling fluid/slurry are clearly lower than those for HDD. This results from the different soil transport design and operating environment, where there is a considerably higher flow rate in the slurry line of the microtunneller than in the annular space between jacking pipes and the bore wall. The fact that the borehole does not have to be actively supported against cave-in by the drilling fluid is also significant, with higher solid concentration being allowed in the fresh fluid which in turn allows the use of simple pumps (centrifugal instead of piston pumps) as compared with HDD systems.
Easy Pipe and microtunnelling also require considerably lower volumes of drilling fluid overall, giving secondary economic and technical advantages such as reduced storage tank capacity requirements.
With HDD and microtunnelled (carrier pipe) installations requiring oversize bores to safely accommodate the product pipe pull-in, the Easy Pipe method has a clear advantage over both techniques with the diameter of the jacking pipes being optimised to the product pipe diameter.
In general, the construction risk during the trenchless installation of pipelines is directly linked to the geological conditions. Microtunnelling and Easy Pipe installations have clear advantages over HDD given their durability in a wider variety of ground conditions. In addition to this, the option to access the cutter head from within the bore during the jacking work in microtunnelling and when using Easy Pipe is significant in limiting risk.
With the Easy Pipe method minimising the potential for the pipe to become jammed by obstacles or sediment in the bore, as is possible with HDD, again risk is minimised.
Limited applications of the Easy Pipe technique mean that, at present, definitive construction cost data cannot be fully evaluated. However, initial estimates suggest that the Easy Pipe method costs are less than for conventional microtunnelling (with carrier pipe).
Compared to HDD, the specific installation costs depend strongly on the geological conditions. In ground formations suited for both Easy Pipe and HDD methods (sand, clay, etc.) similar costs may be expected. In more difficult ground conditions (gravel, rock, etc.) cost advantages for Easy Pipe are expected.
In connection with the construction cost, investment in the necessary equipment plays a major role. At present microtunnelling and Easy Pipe are expected to require the same amount of investment, both being higher than for HDD.
Costs regarding site implementation and removal must also be considered. While the percentage of machinery required in the microtunnelling and Easy Pipe methods is relatively low, the two methods require additional transport due to the use of jacking pipes, the volume of which depends on the drive length.
In the accompanying table, Easy Pipe, microtunnelling and HDD are compared, for a typical 1,200 mm diameter product pipe installation.
Easy Pipe technology is a new construction method which allows quick, safe and economic installation of large diameter, long distance pipelines in difficult ground conditions. It can be regarded as another viable option when looking at conventional microtunnelling and HDD techniques. Technologically, Easy Pipe is more closely related to microtunnelling, but it can be classified in the HDD category with regard to its application range. The first field tests for Easy Pipe are expected during 2006, and are designed to prove the Easy Pipe method under real conditions.
From the original paper authored by Dr.-Ing. RÌ_diger KÌ¦gler, Engineering Office Dr. KÌ¦gler; Dr.-Ing. Hans-JÌ_rgen John, Meyer & John GmbH & Co. KG; Dipl.-Ing. Uwe Breig, Herrenknecht AG and Dipl.-Ing. Peter SchmÌ_h, Herrenknecht AG.