Much has been made worldwide of the difference in performance between new and rebuilt TBMs. Worldwide, a bias exists that seems to favor new machines, but is the bias warranted? The reuse of machines can, if done to exacting standards, reduce costs and time to deliver.
Guaranteeing the quality of TBM rebuilds, however, is another issue. International guidelines have been developed to standardize the process of reusing a TBM for another project. But is standardization possible? And can a used TBM perform to the same standards as a new one?
The Worldwide Marketplace
“There is no reason why a used machine shouldn’t perform as well as a new machine if you know the operational history of the machine and if it’s used in geology for which it’s suitable,” said independent consultant Joe Roby, who has decades of industry experience. “I’d argue that history shows the reverse—rather than new TBMs performing better than used — a used machine is a proven design and more likely to have a successful future.”
Large metro projects worldwide often employ dozens of TBMs working simultaneously, resulting in a glut of secondhand machines on the marketplace at any given time. But contractual constraints often form barriers toward using these machines on subsequent projects. “Consultants employed by project owners often over-specify technical specs — I’ve also seen a lot of cases where they are doing a cut-and-paste job on the specifications without checking on the requirements. Sometimes the consultants don’t have the TBM experience, and it shows in the specifications,” said Roby.
He added, “I think it’s important to continue education — the risks of over-specification may cost contractors and owners in the end. I think what owners ought to be doing, rather than over-specifying or specifying new machines only, is to specify the quality of the rebuild that is necessary. But that requires a certain level of TBM knowledge from the consultants. Certain things could be specified, like that the main bearing is new or has to be certified for a certain number of hours. A cutter load could be specified for hard rock tunneling, but details like thrust and torque should not be specified.”
For Barrie Willis, Manager Tunneling and Civil at iPS who has rebuilt machines worldwide, the bias toward new machines is readily apparent. “For example, over-specification was initially quite prevalent in Singapore. There was no reuse of machines allowed on the early round of metro projects. At one time, there were eight used machines sitting in a storage yard of identical size to what was specified but we couldn’t use them.”
However, over time a more enlightened view did develop to allow reuse of TBMs, though any machine proposed for reuse had to leave Singapore for refurbishment in the original manufacturer’s facilities. Willis continued that on some projects, such as India’s Mumbai Metro, initially there was a similar philosophy but a stipulation was later added to allow refurbished machines to help meet the aggressive delivery schedule.
A Record of Success
So is newer really better? In many cases the record shows that they are equivalent. Changing opinions about rebuilt machines can be done, says Roby: “We have to show evidence where refurbished machines have been used, and show their track record. If you can show that performance and price were good on a given project that is important. Time should also be considered: A refurbished machine can be important if you’re looking to get started quickly. It can save a lot of lead time.”
If the age and number of projects bored by a TBM is seen by some as an issue, a history of record-breaking projects achieved using rebuilt machines does exist. More than one third (36%) of currently standing world records have been broken using a refurbished TBM, some of them in service for decades. Recently a TBM manufactured in 1980 achieved three world records in the 6 to 7 m diameter range at the Deep Rock Tunnel Connector (DRTC) in Indianapolis, Indiana. The 6.2-m diameter Main Beam TBM had previously bored at least five other hard rock tunnels including New York City’s Second Avenue Subway. Design updates for the DRTC included a new back-loading cutterhead with 19-in. disc cutters, variable frequency drive (VFD) motors, and a rescue chamber. The records included “Most Feet Mined in One Day” (409.8 ft/124.9 m), “Most Feet Mined in One Week” (1,690 ft/515.1 m), and “Most Feet Mined in One Month” (5,754.6 ft/1,754 m). The machine is currently boring the next phases of the DigIndy network—a further 28 km in addition to the 12.5 km DRTC already completed.
In cases where it is believed that new TBMs will perform better, there is likely an experience bias at work, says Roby: “I would expect people might have experience of one such job where that was true, and it’s dominating their thoughts. My experience over the years has been that there are a lot of successful secondhand jobs. TBM failures are rare and generally only happen in extreme conditions. A failed project can oftentimes be the result of a TBM employed where it wasn’t suitable or wasn’t rebuilt properly.” He added that, if an older machine was “initially built for sandstone, it will not have enough power to work well in granite 25 years later without modifications.”
A custom design, for a project’s specific requirements and geology, is just as important on a rebuilt machine as a new one. For example, a contractor may wish to save money by purchasing a used TBM and rebuilding it to its original specifications. A 3-m diameter Main Beam TBM, rebuilt to the same diameter and specifications, will cost less than rebuilding the same machine but increasing the size to 4 m and adding custom elements. But are the savings truly obtained if the original TBM specifications do not fit the geology? Cutterhead configurations are a particularly important example, with cutter spacing, cutting tools, cutterhead geometry, and muck openings all coming into play and greatly affecting the rate of penetration.
A good example of this concept can be seen at Mexico City’s Túnel Emisor Oriente (TEO). The geology is highly variable, with the tunnel 62 km in length and up to 150 m deep. One 8.93-m EPB was used to bore two separate lots with very different geology: Lot 1 and Lot 5.
At Lot 1, the machine was used with its original cutterhead and soft ground cutting tools, along with a two-stage screw conveyor, to excavate watery lake clays with great success. The machine bored 4.6 km to complete its section of tunnel on the critical path. The cutterhead was designed as adaptable for both hard rock and soft ground configurations, but before it began excavation of the 8.6 km long Lot 5, it needed some modifications. Personnel added grizzly bars across the muck openings as well as heavy duty abrasion-resistant wear plating. Wear plating was also added to the screw conveyors so they could be used in open mode during excavation in rock. Lot 5 is one of the most challenging on the project, at maximum depth and with geology ranging from lake clays to abrasive full-face basalt. Despite the challenges the machine has been able to achieve a project record of 30 m in 12 hours, showing that an EPB can indeed be used in very different conditions.
The Importance of TBM Maintenance
It is essential to consider the total life cycle of a machine, and in maximizing the efficiency and life of the equipment through good design and maintenance. “I’m a mechanical engineer — if you maintain a car, it will last a very long time. If you don’t maintain it, it breaks down. The same can be said for TBMs where a proper preventative maintenance schedule will pay dividends,” said Willis. “The structure of a TBM is metal — as long as the structure is intact, you can then check on the bearings, conveyor, hydraulics, and other components. You must pay particular attention to components that are hard to reach. The main bearing is one of those parts that is very challenging to replace during tunneling.”
At Bangalore, India’s recently completed Namma Metro, iPS rescued and refurbished a stuck TBM from a European manufacturer, and then operated it for owner Bangalore Metro Rail Corporation (BMRC) alongside the project’s original contractor. “We found severe wear — the cutterhead was essentially bare; the cutters and grill bars had been worn away. We also found a serious crack in the screw conveyor and the flights had been severely worn.” The TBM had been operating for 12 months and had bored 300 m of abrasive ground with insufficiently thorough maintenance. It came to a standstill below the main railway lines at a major Bangalore metro station. iPS were able to build an intermediate shaft and refurbish the TBM to the point that it could advance into that shaft for further rebuild work including a replacement cutterhead.
Once the TBM rebuild was complete, iPS then trained the crew on the importance of maintenance and inspections. “We showed them how to do regular inspections and interventions, what to look for, and how to prevent significant damage. For example, if you do regular inspections of the cutterhead you can detect cutter ring damage early on. With timely replacement one can avoid damage to the hub, put a new disc on, check the seals, and a cutting disc can be reused for a third of the price.”
Maintenance while storing a TBM between projects also affects equipment life and can minimize the costs of a subsequent rebuild—such as storing components indoors, coating the equipment with anticorrosive spray, and making sure the main bearing is filled with oil. Owning and using a new TBM has added hidden benefits including familiarity of machine operation and proven performance for that particular piece of equipment.
Rebuilt, Remanufactured or Refurbished
The rebuilding of TBMs — both the process and the standardization of rebuilds — has become a focus for the industry as more projects with multiple machine requirements and short time frames are being proposed. The focus has been further highlighted by the ITAtech, a technology-focused committee for the International Tunneling Association (ITA-AITES) that produced guidelines on rebuilds of machinery for mechanized tunnel excavation in 2015.
For Robbins, the preferred term is “rebuilt,” which describes any manner of creating a custom TBM from already existing components. The ITAtech guidelines introduce different terminologies depending on the extent to which a TBM is rebuilt. They are, very shortly, described here:
- Remanufacturing – Remanufacturing is a process with the aim to start a new life cycle of the product using its current or modified configuration.
- Refurbishment – Refurbishment can be considered a full maintenance, where defect parts are replaced to extend the life of the product in its original configuration or with small modifications.
The guidelines describe the requirements of each process in order to designate a TBM “refurbished” or a “remanufactured,” but in reality the majority of “rebuilt” TBMs may be somewhere in-between these qualifications.
In terms of the international guidelines, they are certainly necessary and welcomed. However, the strictness of the guidelines can make them hard to adopt for the majority of TBM rebuilds, which are customized based on project needs.
That is not to say that the guidelines are not useful. “I think the guideline, as written, would make a very good standard for owners to reference when allowing the use of a secondhand TBM,” says Roby, but he cautions that certification as to the quality of the rebuild is necessary: “The owners should specify that any secondhand machine be certified by an independent third party engineer. I would suggest that the owner also require the tendering contractors to list in their bid the history and technical specifications any secondhand TBM they intend to use and to include in their bid the name of the company they intend to use to remanufacture the TBM, as well as the third party engineer they intend to employ to certify the remanufacturing. All of these should be in accordance with the ITA guidelines.”
For Willis, rebuild guidelines may be less pressing than changing the culture of TBM operation and maintenance for many contractors worldwide. “I know it can be done — it’s just a matter of convincing contractors to operate the machine properly. This will mean the machine at the end of a project will need less maintenance before it can potentially be reused. However, it’s also important, where possible, to encourage project owners to standardize the sizes of tunnels, thus increasing the possibility of TBM reuse.”
Training should be specific to the machine and the geology, though there are some commonalities in what is required for maintenance. “What I say is that when you first start a machine, inspect it often, until you know what to expect. The cutterhead and screw conveyor may need an intervention once a week, or once a month. Someone has to perform inspections often enough to know that a particular component has damage or is becoming worn out,” said Willis. He added that even when the geology seems predictable, that should not be taken as failsafe, and has some advice for contractors: “Geological surveys are extremely important but can be unreliable. So do interventions until you have a baseline knowledge for how the machine is reacting in specific types of geology. Don’t be complacent — just because a TBM is a big machine with a hard metal cutterhead and tungsten carbide cutters, it’s not true that nothing can damage it.”
Used vs. New
Is a used TBM as good as a new one? In short, the answer is yes, with qualifications. The machine’s rebuilt specifications should fit that project’s geology and unique requirements. With a proper design and rebuild, a used machine has advantages: “The design is proven, the cost is usually lower and there is an advantage in faster delivery times. The risks are only when the TBM is not properly built or when a machine is put into geology where it’s not suitable,” said Roby.
Overall, there are many benefits, both obvious and hidden, to using a rebuilt machine, but the rebuild should be done within certain design restraints to remain economical. “There’s always the possibility to upgrade power and thrust on a machine but there are strict engineering limits. If you increase the cutterhead drive motor power, the gear reducers and final drive ring gear and pinions must have the capacity to take that increase in power. If you’re increasing thrust, you need to check the bearing life and make sure that the bearing can take the increased forces. If you’re exceeding gripper capacity on a hard rock TBM, you have the wrong used machine for the job.” Roby added that the type of TBM and whether it is shielded or not also matters. “If you’re changing the diameter of an EPB such that it requires new shields that may not be the best choice economically. Purchasing a larger EPB would make sense in that situation.”Overall TBM design and usage for the long haul is simply a cost effective, energy efficient, and sustainable way of thinking about tunnel boring. Used machines can and have shown their ability to excavate projects at world-class rates of advance and complete many kilometers of tunnel with success.
This article was contributed by The Robbins Company.