The practice of remanufacturing or refurbishing tunnel boring machines (TBM) is not new to the tunneling industry. Tunnel project owners and contractors have often turned to TBM manufacturers for “used” machines for many years, and for a variety of reasons.
However, the term “used” can be a bit misleading when it comes to TBMs. Rarely is an entire machine pulled from one project and put to use at another project. The diameter of each tunnel often changes as do the requirements for the TBM. And the ground conditions of each project is always different from other projects.
“It comes down to the ground conditions and unless it is parallel tubes, no two projects experience the exact same ground conditions. That is the nature of our business. Even if the diameter of the tunnel stays the same, which is rare, everything that is related to the ground interface such as the cutter head or mucking system has to be optimized to the ground conditions of the specific project,” Werner Burger, chief engineer at Herrenknecht said.
Because of this, larger and more complex TBMs are rarely used in their entirety; however, the main components of a TBM, such as drive systems, man locks, large hydraulic cylinders, erector, powerpacks, gantry components, or main structural components can be remanufactured and given an entirely new life cycle. And these tried-and-true components often provide as much or better success than a new machine.
Brad Grothen, vice president-engineering of Robbins told T&UC that his company has been in the practice of rebuilding TBMs for decades.
A case in point is a main-beam TBM that was originally built in 1995 and has been used on six tunnels around the world. “At its three most recent tunnels, it has been refurbished as an 8.7 m (28 ft) diameter, single-shield TBM at Theun Hinboun Expansion Project in Laos, start date 2009; as an 8.7 m (28 ft) diameter, crossover XRE TBM at Tunel Emisor Poniente II in Mexico, start date 2015; and as a 7.96 m (26 ft) diameter, single-shield TBM for the Ashbridges Bay Outfall Tunnel in Canada, start date 2019,” said a Robbins spokesperson.
Likewise, Herrenknecht is committed to getting the most out of its machines. The company offers a buy-back program for its machines and has a worldwide unique plant that extends more than 100,000 m2 in Kehl, Germany dedicated to the remanufacturing and refurbishment of components. Olaf Kortz manages the facility and explained to T&UC that Herrenknecht’s process closely follows the International Tunnelling Association’s (ITAAITES) ITAtech group guidelines “ITAtech Guidelines for Rebuilds of Machinery for Mechanized Tunnel Excavation” that were first drafted in 2014-15 and revised in 2019 to include computer and guidance systems. Accordingly, Herrenknecht is quite clear about the distinction between refurbishment and remanufacture: refurbishment allows the part to be used for the rest of its design life, or to extend the original design life to some extent, whereas remanufacturing re-engineers the part to the beginning of a new life cycle. According to the ITAtech guidelines, remanufacturing does not take place on the jobsite due to the required special equipment.
Herrenknecht remanufactures components at its plant to be certified to be as good as new, including updates to the latest technical standards. Over the past decade the company has had numerous machines and components return to action.
Why choose a refurbished machine
In a paper titled “Rebuilding TBMs; Are used TBMs as good as new?” presented at the Rapid Excavation and Tunneling Conference (RETC) in 2019, Doug Harding of Robbins wrote that for years, the advantage of using a refurbished TBM versus a new machine built specifically for a job was based on economics and time. The costs can vary from as much as 75 percent for a simpler machine on a project with tried-and-tested ground conditions to around 20 percent for a project with more complex requirements (Harding, 2019). A refurbished machine can also represent significant savings in terms of time. A refurbished machine can sometimes be ready in a matter of months while it might take more than a year for a new TBM to be manufactured.
“As long as the TBM is well maintained, there will be jobs it can bore economically,” Harding wrote in the 2019 RETC paper. “Optimal TBM refurbishment on a used machine requires a broad knowledge of the project conditions, and there are some limitations: Machine diameter can be decreased within the limits set by free movement of the grippers and side/roof supports; machine diameter can be increased subject to the structural integrity of the machine and the power/thrust capabilities, and propel force can be increased only to the level supported by the grippers’ thrust reaction force.”
For manufacturers such as Robbins and Herrenknecht as well as tunnel project owners and contractors, remanufactured TBMs or TBM components often make good business sense. Recently, the additional factors of sustainability and carbon savings have gained more attention as well.
Tunnels and underground infrastructure, by their very nature, provide solutions to many urban growth challenges, including climate goals, such as reducing carbon emissions from tunnel projects. While mass transit tunnels are one of the most efficient ways to reduce road traffic and the carbon emissions from automobiles and freight trucks, the construction of a tunnel is a carbonintensive process and many owners are facing increasing pressure to reduce the carbon footprint of each tunnel project.
In many public tenders for tunneling projects, environmental criteria are now standard, and it is expected that in many countries this trend will continue and environmental regulations will continue to tighten.
Grothen said that Robbins has seen demand for completely new machines and components wane as project owners realize the sustainability benefits of remanufactured components and machines.
Herrenknecht has calculated that “on average, as a result of the remanufacturing process per ton of components 71.42 percent of the emissions are saved compared to the production of new components.” And in Herrenknecht’s calculation this includes carbon dioxide (CO2) emitted during the transport of components from the jobsite to Kehl.
According to Kortz, with a completely remanufactured tunnel boring machine the energy savings are about 80 percent. In addition, when compared to a new machine, about 99 percent less materials have to be used. This is a huge saving that conserves resources, reduces CO2 emissions and improves the environmental balance of the machine — and thus the entire construction project.
In addition to the sustainable benefits from reused parts, there can be a significant savings in transportation of components.
“Remanufacturing components and TBMs is a big part of our business,” Grothen said of the Robbins process of refurbishing TBMs. “I think the challenge comes as you look at what gets refurbished. It is always tricky — its price and carbon footprint — and when you consider refurbishment, the shipping of big, heavy pieces of equipment is also a huge cost and carbon producer.
“When you do the refurbishment you have to consider where you are going to do the work — onsite or back at the factory,” said Grothen. “The location is important and for shield machines like an open mainbeam machine, you don’t have many pieces touching the tunnel wall so slight changes in diameter are not so detrimental, but when you look at a shield machine it is more difficult to change diameters. They are designed more around the segment of that project so unless there is standardization of the segment it is more difficult to reuse the outer shield of the TBM and all the weight that goes with the shield. So machine type really matters and where you refurbish it.”
Because TBMs are more like a mobile factory than an individual machine, inventory is a challenge.
Some TBMs are moved around the world, while others can stay in the same region. Grothen said that some regions of the world have tried to create a standardized segment lining for certain types of tunnels. This makes it easier to repurpose TBMs for tunnels with the same diameter and helps with the issue of inventory; however, changing ground conditions remain a challenge.
“I have never found two jobs with identical ground conditions,” Burger said. “A project in its early design is maybe 10 years out, so knowing which machine might be available is difficult. When a project is in the design phase there are informal contacts but hardly ever a situation that a project is designed for an existing TBM. What could help in the future would be if there are specs in the bid process that indicate a range of flexibility for the machine or the tunnel. For example, for a water tunnel, if it has a diameter that is 4 inches larger than the specs it doesn’t really change the tunnel, but it could open the door more to bring in existing equipment.”
Much of the programming at the 2022 Cutting Edge Conference presented by UCA and Tunnelling Journal was dedicated to sustainability in the tunneling industry (see page 23). Project owners, contractors and engineers are facing increasing pressure to not only convince outside stakeholders that their tunneling project will be a net positive for the community in which it is built, but to reduce the carbon footprint of building the project.
Innovation and technology will be needed to carry the industry forward.
“We now have more digitization in the machines including artificial intelligence (AI), and this contributes to optimized energy and efficient use of the machines,” said Burger. “These technologies give the operators advice by telling them which motor could be switched off or even having automated systems determine the amount of energy needed. Downsizing of machines could be an option as well, but we would still need to know the ground conditions. There is a compromise of having sufficient power installed to overcome the unexpected but not overstressing.”
Burger said that in the next decade he expects to see more technology incorporated to help TBMs become more efficient in their use of power and fuels.
“For the future I think we need to consider not just one element of the tunneling process but to look at the complete tunnel operation from the project design all the way through equipment and materials,” Burger said. “There is a potential to bring the industry forward in the direction of more sustainable solutions. This will need the cooperation of all involved parties starting with the owners to the equipment supplier.”