Safety is of prime importance in today's society. In fact, we all want to know how safe our streets are after dark, or whether it is safe to swim in the ocean when we know we share it with sharks. However, when it comes to the safety of those using sophisticated, modern-day equipment, we rely on a false security, believing that the equipment designer has considered all safety issues and has resolved all critical design flaws before the equipment is delivered to the user. If this were true, we would not have manufacturer safety recalls, which occur on a regular basis. In fact, most designers in America are only concerned with equipment performance, producibility, and cost to manufacture, not safety. Safety considerations at the design level are usually limited to issues mandated by law or government regulations (e.g., federal seat-belt laws, Occupational Safety and Health Administration regulations).
Pieces of tunneling equipment in general, and TBMs in particular, are not mass-produced items, but rather are one-of-a-kind prototypes, designed for a specific user with a specific application. As such, these designs are not field-proven to be raft-safe. While TBM manufacturers have been around for decades, the engineering design effort still remains largely void of engineering analysis techniques that would provide safety insight into the design on an a priori basis.
Some government agencies (e.g., the National Aeronautics and Space Administration, the Department of Defense, and the Federal Aviation Agency) have been instrumental in developing engineering analysis techniques due to the catastrophic nature of equipment failure in their environments. Techniques such as FMECA and FTA provide great insight into the design before the item is even built. Failure of the design team to perform such analyses leaves the eventual user or operator with equipment for which safety is by chance, not by design. Failure of government agencies and their management and operations contractors to understand fully the benefit of and to require contractually these types of analyses during the design of sophisticated electrical and mechanical systems and equipment can result in preventable, fatal accidents.
This paper will describe two types of analysis, FMECA and FTA, as they apply to typical tunneling equipment and TBMs. Most designers are not even aware that these safety engineering design analyses have been developed to proactively understand equipment modes of failure while the design is still on the drawing board. This stage is the most cost-effective timeframe in which to understand basic failure modes so they can be designed out, rather than being discovered later after a catastrophic event, such as a major injury or loss of life, during in-service use.