Efficient Pressurization for Efficient Excavation

There is a time and cost impact of keeping a TBM (tunnel boring machine) moving forward. For example, the added upfront cost and efficiency of mobilizing a saturation diving system is considered and justified in the context of the cost of TBM downtime. The tunnel depth, amount of conventional air or mixed gas decompression time and impact of substrate complexity are but a few of the variables that must be analyzed. Efficient pressurization becomes a cost-effective financial analysis that the hyperbaric intervention provider can judge collaboratively with the TBM contractor. Deep tunneling, for example 200 m below a body of water where expected pressures are above 10 bar, dictates that saturation diving is the most efficient and safest pressurization solution.

Why hyperbaric support? TBM, tunnel equipment and tunneling procedures should be designed to enable reliable application of a pressurized environment at all times during excavation. Considerations of this need include counteracting the level of groundwater pressure, abrasiveness of the substrate and the length of tunnel sections. The TBM should include provisions for hyperbaric intervention using compressed air, mixed gases or saturation diving techniques, depending on pressure level and duration of expected intervention time (see Table 1 for comparative estimates). The support pressure in the excavation chamber is regulated via an air cushion between the submerged wall and bulkhead of the TBM.

Pressurization for efficient excavation assumes provision of expertise in equipment, staffing, training and consulting. Equipment specifications for hyperbaric intervention include the custom design and manufacturing of TBM manlocks (“airlocks”), medical hyperbaric oxygen therapy systems, saturation systems design and fabrication, deep/shallow transfer-under-pressure (TUP) shuttles (Figure 1), and TBM cutter wheel tool replacement. Staffing for efficient on-time results in hyperbaric interventions includes compressed gas workers, certified welders, teams experienced in heavy rigging in confined spaces, medical and safety oversight, and decompression medical attendants/confined space rescue team. Training beyond the level of compressed gas worker includes manlock operator training and hyperbaric operation supervisor training. Consulting expertise needed is in regulatory compliance, variance service, and mixed gas table development including adaptations for altitude settings.

Challenges for future trends in tunnel construction include consideration of longer and deeper tunnel designs. Tunnel depths below 40 m render routine compressed air work inefficient because of the physiological requirement for compressed air workers to fulfill a decompression obligation or risk the probability of decompression sickness. This paradigm shift will require a change in the operational concept for the repair and replacement of EPB TBM tools, transitioning from current employment of compressed air workers to mixed gas divers working in hyperbaric environments using saturation diving methods that span operational times of days to several weeks.

Experience and unique skill sets needed to support the performance of hyperbaric intervention and working in extreme pressure environments for efficient access and cutterhead maintenance include the use of mixed gases and saturation systems, physiologists and medical doctors specializing in the development of custom dive tables to protect the health and safety of compressed gas workers, and higher qualifications of compressed gas workers vs. compressed air workers. Benefits incurred by the customer include minimized TBM downtime, reduction of construction time, and hence a reduced operating budget. Whether performing EPB TBM maintenance or repair, compressed air or gas workers must at a minimum be able to weld and cut under water in bentonite suspensions, perform inspection and weld surveys, and remove and replace tools on the cutter wheel. The efficiency of various methods of air and saturation diving is shown in Table 1.

Equipment required for hyperbaric intervention includes design and manufacture of all primary pressure vessels and integration of components, mechanisms and ancillary items. The hyperbaric chamber is a pressure vessel for human occupancy (PVHO) or habitat where compressed gas workers complete their decompression schedule. Transfer under pressure (TUP) refers to the PVHO used to transfer compressed gas workers into a hyperbaric habitat via shuttle and is designed with a mating flange compatible with both the TBM and the hyperbaric chamber. Compressors, gas racks, lighting and communications are components of the support system.

Hyperbaric oxygen therapy increases tissue oxygen levels and has been accepted as the prophylaxis and definitive treatment for decompression sickness for over 50 years. Oxygen breathing improves the speed and reliability of decompression and reduces the probability of long-term decompression disorders such as dysbaric osteonecrosis. Helium is a lighter gas thannitrogen and easier to breathe at greater depths. Trimix is a mixed gas containing the appropriate fraction of oxygen to not exceed a maximum oxygen partial pressure for the depth of the exposure, nitrogen and sufficient helium to reduce the inert gas narcosis to an acceptable level.

A medical team should be integral to the tunneling effort and consist of experienced submarine disaster management experts, decompression profilers, saturation decompression experts, physician and medical technician experts, and hyperbaric researchers able to implement novel approaches to TBM support. Systems should be containerized and mobilized by land, sea, or air in classification society approved transportation systems using lift and transfer mechanisms that interface with both commercial and military transport carriers. Full life support and operational reliability, including spare parts, gas requirements and staffing flexibility increases the effectiveness of the operation. Use of gas reclaim concepts, rebreathers, and regeneration systems can also be employed to reduce hyperbaric intervention operational cost. Commissioning, a full training spectrum in hyperbarics that covers the scope of TBM operations, documentation through full design, operating and maintenance manuals for all hyperbaric systems with operating and emergency procedures, class certification and life cycle system recertification, spares provisioning, system reliability and life cycle support are clearly some of the essential components of an efficient pressurization program.

Hyperbaric intervention cost should take into consideration adequate primary components and backup systems installed on the TBM to eliminate major problems including cost overruns and time delays. The HBI provider must have the engineering capability and approach for innovative solutions specific to TBM hyperbaric intervention. For efficiency and safety of compressed gas workers ergonomic helmet designs specific to TBM confined space needs should be considered, cooling suits for warm confined compartments used, and reliable, effective accelerated decompression procedures developed.
Finally, the optimal solution suggested for efficient pressurization is therefore mixed gas saturation designed as a containerized system for a maximum pressure of 20 bar that can be located on the TBM, in the shaft, or on the surface. The transfer-under-pressure shuttle transports the divers from the habitat outside the tunnel zone to dock at the TBM. Efficient excavation can then proceed unencumbered once inspection, maintenance and repair work to the cutterhead has been performed through hyperbaric intervention. Therefore, a General Contractor need only engage one company that manufactures different styles and types of diving saturation systems, is supported by an experienced decompression medical team that can develop specific advanced decompression tables, and provides a professional team of mixed gas saturation divers.

Dr. Michael A. Lang is Vice President-International Sales for OxyHeal Tunneling Group Inc., A Member of the OxyHeal Health Group. He can be reached at mlang@oxyheal.com.