The use of tunneling across the world is at its highest level, and the trend is for its growth to continue. With all of these instances of tunnel boring machines (TBMs) making what are called “drives,” the opportunity for a fire-related incident is also increasing. And while TBM fires are thankfully not terribly common, there have been enough incidents reported to allow fire professionals to start identifying trends. In this paper, we will examine what is driving the increased use of TBMs, causes of incidents and recommended precautions, and fire protection measures.

The Demand for Tunnels

The world has developed a seemingly insatiable appetite for tunnels to fulfill increasingly varied needs. One reason so many new projects are coming to fruition is because the tunnel boring industry has gotten better at its craft. For starters, today’s TBMs are faster, which saves considerable expense. They also handle challenging geological conditions better; which in turn identifies new  tunnelling opportunities previously thought impassable.

Tunneling projects are undertaken to satisfy one of the following needs:

With this increase in demand, the risks of a TBM fire-related incident also increase as discussed below.

Fire Risks During TBM Drives

All of these processes are driven by complex electric and hydraulic systems. With electrical power being the primary source of power, a TBM’s consumption of electricity is substantial. A mid-sized TBM can use three to five thousand kWs on a job, with large TBMs operating in difficult geological conditions consuming up to twenty MWs.

Electricity

With all of this electricity coursing through the machine, it represents a significant fire risk. The major electrical fire risks include:

The key to electrical safety is a solid maintenance and monitoring system and effective power management. The problem with electrical fires is they can occur in equipment that has worked with no issues for years. Because of this risk, effective fire detection and suppression systems should be installed in each location where an electrical fire risk exists.

Hydraulic Fluid

The hydraulic systems on TBMs operate at a very high pressure. If there is a leak, the hydraulic fluid emits as a fine mist which, should it contact a sufficiently hot surface, is very flammable. There have been accidents where flammable hydraulic fluid has caused fires during TBM drives. Hydraulic fluid contacting a hot surface was also the cause of the Kaprun Disaster which was created by hydraulic fluid leaking onto a heater. The conveyance was in a tunnel at the time of the fire, which led to the deaths of 155 people.
The most effective response is to use non-flammable hydraulic fluid. There are standards that require this in some regions, but combustible hydraulic fluid is still often used on TBMs. Aside from this, regular inspections and maintenance are imperative. If flammable hydraulic fluid is being used, a fire detection and suppression system should definitely be present.

Conveyor System

In many countries, the use of a fire-resistant conveyor is mandatory. But there are loopholes, and it is possible to encounter a combustible conveyor. Like using non-flammable hydraulic fluid, the key here is also to use non-flammable conveyor material. Conveyor systems take a hard beating, so regular inspections and maintenance are a must. If a combustible belt is used, it should be protected by a fire detection and suppression system.

Terrain

The medium in which the TBM operates can hold plenty of risks. In addition to running into unforeseen geological conditions and unmapped structures, there are often fire risks too such as:

The key to mitigating this hazard is performing research, conducting tests, and surveying. In spite of solid due diligence, a gassy underground cannot always be predicted. For this reason, the air quality should be continuously monitored for unsafe conditions. Each TBM will have its own protocol if an unsafe atmosphere is detected, but these steps typically include:

Fire Suppression in TBMs

Because a TBM operates in a remote area and help is long way off, any emergency must be dealt with by the crew and/or onboard systems. Given the value of the machines ($80 million+) and their ability to generate $100 million a mile, every TBM should be a highly protected asset.

Protecting a TBM from fire can be a challenge for several reasons. First of all, it must be disassembled/reassembled at jobsites. Further, it must carry the fire protection system with it. And lastly, It operates in a modular fashion with different TBM areas performing different tasks.

The aforementioned conditions effectively rule out a water-based system because the TBM would have to also transport the water which is heavy and bulky. Laying a temporary water source into the tunnel is expensive and runs the risk of flooding the tunnel.

Any system that requires piping is also problematic. For one, connecting the various sections of the TBM with piping for the fire suppression agent is difficult because the sections often move at different speeds. Piping also implies that there is a supply of agent located somewhere on the TBM. Big cylinders of gaseous agent, CO2, or dry chemical take up valuable real estate on the TBM, plus adds additional weight.

With all the problem areas on a TBM requiring fire protection, a fire suppression system must be able to discharge in a wide range of locations. Because these areas are located all around the TBM, this too makes a connected, central system difficult.

The best answer is to protect each fire-risk location independently using a self-contained fire suppression system. Such a system may act as a single unit or be connected to other units in its location. The units should be able to act independently at a set temperature and also capable of being linked together to form a total flooding system. The agent should be rated for Class A, B, & C fires.

The ideal units are simple to install, require no piping, and can easily be retrofitted to an existing TBM. The units should also stand up to harsh environments, be minimally affected by set up and tear down of the machine, and require no complicated testing or maintenance. Most importantly, the agent should not be harmful to personnel, the environment, or the machine.

The Stat-X Solution

The good news for the tunneling industry is that this agent exists. Stat-X® is an innovative, condensed aerosol, fire suppression agent that discharges to interrupt the chemical chain reaction of a fire and cause extinguishment.
Stat-X units tick all of the boxes described above for the ideal fire suppression solution for TBMs. They can be installed as a single unit to protect a single piece of equipment, or they can be linked together for total flooding of an area. The compact units mount in out-of-the-way places and are built with harsh environments in mind. There are minimal testing and inspection requirements, and the units are virtually maintenance-free.
From a safety and environmental standpoint, the units are completely safe to use around personnel and equipment and have none of the environmental issues facing other types of fire suppression agents. Stat-X has zero global-warming or ozone-depletion potential, and also has zero atmospheric life.

Conclusion

The prevalence of tunneling is increasing at a rapid pace. With more TBM drives taking place and the capabilities of TBMs constantly pushing the envelope, the risk of fire during tunneling operations is very real. Because the design and operation of a TBM poses challenges, it requires a specialized approach with fire suppression systems. Stat-X provides the ideal fire protection solution for TBMs, and for the reasons discussed herein, is especially well suited for the task.

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