Roy Slocombe introduced the revised guide (‘Guidance on good practice for work in compressed air’, ISBN 9780954610654) on behalf of the Compressed Air Working Group (CAWG). He noted that since the ‘Work in Compressed Air Regulations’ came into force in 1996, the nature of compressed air work had changed because of developments in tunneling techniques and mechanization. In the past it had been common for the whole tunnel to be pressurized and excavation carried out by gangs of miners or sometimes by open-face tunneling machines operating wholly within the pressurized workings. Now, compressed air working is used predominantly for periodic short excursions into the head of a closed-face tunneling machine for inspection and maintenance.

Developments in machine technology have allowed the construction of larger, deeper tunnels with much higher water pressures. These higher water pressures have required the use of compressed air at correspondingly higher pressures for face access. High exposure pressure techniques, extensively developed in the offshore commercial diving industry, have been adapted for high-pressure compressed air tunneling. This has included the use of non-air breathing mixtures comprising oxygen, nitrogen and/or helium, and saturation exposure techniques.

‘Low pressure’ in tunneling is still work at pressures not requiring stage decompression (<0.7 bar(g) in the UK); ‘intermediate pressure’ is work requiring stage decompression but below the statutory limit (0.7 to 3.5 bar(g)) and now the Guide includes the new category of ‘high pressure’ being work above the statutory limit (>3.5 bar(g) in the UK). Guidance is also provided on procedures for obtaining an exemption from this limit for high-pressure work.

The Guide is a revision and update of the 2012 edition of the BTS guidance, expanded to incorporate work at high pressures. It also includes changes arising from the current revision of various British and European standards. Although primarily drafted for use in the UK, it can be applied to work in compressed air in the majority of countries.

The Guide forms a holistic package of guidance on work in compressed air when read in conjunction with BS 6164 ‘Health and safety in tunnelling in the construction industry – Code of practice’; BS EN 16191 ‘Tunnelling machinery – safety requirements’; BS EN 12110 Tunnelling machinery – airlocks safety requirements’; ITA/BTS CAWG Report 10 ‘Guidelines for good working practice in high pressure compressed air’; HSE publication ‘Guidance for appointed doctors in the Work in Compressed Air Regulations 1996’; and ITA/BTS CAWG Report 20 ‘Client guide to high pressure compressed air work’.

Important aspects of the revised guide include the focus on the role of compressed air contractors and the many duties placed upon them, including the management and control of the health and safety risks of compressed air work at any pressure. The compressed air contractor is usually the principal contractor who should be competent to discharge the duties under these regulations.

NEW REQUIREMENTS

HSE no longer approves decompression procedures under regulation 11(1), which contractors are required to use. It is now up to compressed air contractors to select appropriate procedures for decompression. An exemption from the statutory limit is still needed from HSE for work at pressures above 3.5bar.

The guide is predominantly concerned with airlocks on TBMs but it is also applicable to boiler locks on bulkheads in tunnels.

A new requirement is for the air supply system to incorporate sufficient storage to allow the safe abandonment of the intervention along with a full decompression and the capability to compress the locks once from atmospheric pressure.

Another new requirement is for an intermediate chamber between the main compartment of the personnel lock and the TBM bulkhead when saturation exposures are being undertaken, to provide a relatively safe but dirty working space between the personnel lock and the working chamber from which entry into the cutterhead could be made and controlled.

Interim technical guidance in the guide acts as a stand-in until the revision of BS EN 12110 has been published as a prEN.

The oxygen system should be capable of supplying oxygen at a maximum pressure of 1.8bar(g). This will allow therapeutic recompression treatment using US Navy Table 6 to be carried out in the personnel lock in an emergency, such as in the case of a person who has collapsed in the vicinity of the lock immediately after being decompressed.

Completely new guidance has been provided on surface habitats which provide living and sleeping accommodation, along with hyperbaric toilet/washing/ showering facilities for those in saturation. There are no British or European standards covering habitats for use in diving or as part of HPCA work in tunneling.

Similarly, the new document contains interim technical guidance until the publication of the revised EN 12110, on ‘Transfer Under Pressure’ shuttles. For UK use, they should comprise two compartments as for personnel locks. The requirements for a shuttle path through the TBM are documented in the revision of the EN 16191:2014.

The guide clarifies the role of the contract medical adviser in providing health surveillance. This differs from the role of the appointed doctor who will certify medical fitness for work, but both roles can be fulfilled by the same person.

An important topic that is covered in the guide is the use of physiological monitoring of individuals post-decompression for determining the effectiveness of new or unproven decompression tables, or investigating anomalies in otherwise proven tables. Doppler is one such accepted method of undertaking this monitoring. The incidence of DCI events can be an unreliable measure in this application because of the subjectivity in response by workers.

Medical surveillance is one of the measures which limits the risk to health of workers who are, or will be, exposed to compressed air. It is intended to limit risks to health by ensuring that only individuals who are considered fit to work in compressed air do so. All people who work in compressed air are therefore required to be subject to medical surveillance provided by an appointed doctor or employment medical adviser.

DONALD LAMONT

Donald Lamont explained that he did not intend to describe in detail the guidance on high-pressure exposures, leaving the audience to read the guide along with ITA/BTS CAWG Reports 10 and 20. Instead, he would address three currently contentious topics:

? Saturation is a natural phenomenon not to be feared or to be associated only with diving.

? The significant hyperbaric-related differences between work in compressed air and diving operations.

? Why 3.5 bar(g) is the appropriate pressure to switch to mixed gas.

PHENOMENON OF SATURATION

The audience was reminded of the importance in high pressure work of recognising the difference between absolute pressure (gauge pressure + 1bar), gauge pressure, and partial pressure (volume fraction x absolute pressure).

Saturation is a natural state of dynamic equilibrium in which the partial pressure of the gases in body tissues equals those of the gas mixture being breathed. It is not specific to diving. Any change in the partial pressure of a gas being breathed i.e. a change in its pressure and/or composition, results in a corresponding change in the partial pressures of gases in the body tissues. That change results in either partial saturation or supersaturation until saturation has been re-established. Re-establishing saturation takes less than between 30mins–1 hour for vital organs, and up to 12–18 hours for limbs. Importantly, once the body has become saturated, no more inert gas is taken on, no matter how long the exposure. (It was something of a surprise to some in the audience to discover that without knowing it, they were all experiencing saturation in air at atmospheric pressure in the BTS lecture theatre).

The differences between non-saturation (short duration) exposures and saturation (long duration) exposures were then discussed. Aside from the high costs associated with saturation, it is nevertheless the safer and more productive way to undertake high pressure work.

SIGNIFICANT HYPERBARIC-RELATED DIFFERENCES BETWEEN WORK IN COMPRESSED AIR COMPARED WITH DIVING OPERATIONS

Relating to buoyancy and orientation, workers in compressed air do not benefit from buoyancy and tend to work upright. This requires physical work to be done by the lower limbs supporting the worker’s self-weight. On the other hand, a diver is roughly horizontal and buoyant in the water. These differences lead to fatigue as well as altering gas uptake and blood distribution in the body. With divers, blood drains away from the lower limbs to the torso. This means that diving decompression tables are not necessarily appropriate for use in compressed air work.

IMMERSION

While on a TBM, workers in compressed air breathing mixed gas tend to alternate between being in a mixed-gas pressurised environment (lock) and an air-pressurised environment (intermediate chamber and excavation chamber). This can lead to local supersaturation due to helium (being the lighter gas) diffusing in through the skin more rapidly than nitrogen (the heavier gas) can diffuse out, with the possibility of skin irritation similar to a minor skin bend occurring in extreme cases.

CONTROL OF PRESSURE

In tunneling, a supervisor controls pressure in the excavation chamber to ensure face stability, restrict water ingress and prevent blow out, as well as monitoring air inflow against loss into ground. In addition, the supervisor controls the mixed gas breathing supply. In diving, there is no control over pressure – it is simply a function of water depth. However, as in tunneling, the supervisor controls the mixed-gas breathing supply.

Periods of work away from the habitat are defined either as interventions when working pressure in the excavation chamber equals habitat storage pressure, or as excursions when working pressure in the excavation chamber is different to storage pressure and preferably above it. Interventions are the saturation equivalent of free-air exposures but at an elevated ambient atmospheric pressure, with excursions being the equivalent of no-decompression stop exposures of around six hours in duration but at an elevated ambient atmospheric pressure. Limits on excursion pressure differences from storage are fairly restrictive and are set out in the relevant guidance.

There is an important fundamental difference in hyperbaric terms between having an air-pressurized excavation chamber where the pressure does not vary between crown and invert, compared with a fluid-pressurized excavation chamber where there is a hydrostatic pressure distribution increasing with depth between crown and invert. The result is that any entry into the excavation chamber when air pressurised, effectively becomes an intervention, whereas entry into the excavation chamber when fluid pressurized becomes an excursion with the accompanying restrictions on pressure changes. Typically, excursion limits are around 0.7bar–0.8bar for saturation storage at 3bar–5bar(g). Excursions in European practice may only be undertaken in one direction per exposure – either higher than or lower than storage pressure, which is restrictive when the hydrostatic pressure differential across a 15m-diameter cutterhead is 1.5bar.

WHY MIXED GAS ABOVE 3.5BAR(g)

There are three breathing gases of interest – oxygen, nitrogen and helium. A fourth gas, carbon dioxide, is always present but seldom mentioned.

It should be remembered that it is a partial pressure of 0.2bar of oxygen which sustains life, however it just so happens that measuring volume concentration is easier, hence the 20% figure used in confined-space work at atmospheric pressure.

A number of recommendations on oxygen partial pressure for storage and working in saturation exist, along with upper limits in various circumstances. The upper limits are to reduce the risk of oxygen toxicity which manifests itself in both acute form as convulsions and muscle spasms, as well as in a chronic form as loss of lung capacity and tracheitis. Acute oxygen toxicity is characterised by the lack of warning of onset and in the variability of response by an individual and between individuals. Chronic toxicity, which is more predictable, is based on the loss of lung vital capacity. Oxygen toxicity does not occur below a partial oxygen pressure of 0.5bar.

Exposure to high-pressure nitrogen results in three well-known risks – nitrogen narcosis, increased work of breathing and decompression illness as measured by Doppler score, not decompression illness (DCI) events. Of these, narcosis and work of breathing are the most critical in determining when to change to mixed gas. This is a time of stringent on-site drug and alcohol limits, and an equivalence can be determined between the level of narcosis due to nitrogen exposure and that from alcohol consumption. A level of impairment equivalent to being at the UK blood/alcohol driving limit equated to breathing air at around 3bar(g).

Helium is a low density, non-narcotic inert gas which is used as a diluent/replacement for oxygen and nitrogen to keep partial pressures within the required limits. Although its use leads to problems of high-pressure nervous syndrome, this does not occur until pressures of around 15bar are reached. A significant commercial disadvantage with helium is its cost and hence helium reclaim was frequently undertaken in diving operations.

The fourth gas discussed was carbon dioxide (CO2) – a gas always present but seldom spoken about. Carbon dioxide is a narcotic gas with anaesthetic properties and which is denser than air. It arises from two sources – contamination in the inspired gas but, much more importantly, it arises as waste from the human metabolism from the work of breathing and physical exertion. Carbon dioxide is known to exacerbate the narcotic effects of nitrogen and the toxic effects of oxygen, with at least one source linking it to an increased risk of decompression illness.

WORK OF BREATHING

Nitrogen is a heavy gas and as its partial pressure increases, the mass density of nitrogen also increases, forcing the lungs to inhale/exhale a greater mass of gas with each breath. This is the work of breathing. Increased work of breathing leads to fatigue and reduced tidal flow in lungs, which in turn leads to reduced flushing of the gas from the lungs and increased carbon dioxide retention. Symptoms of hypercapnia (excess exposure to carbon dioxide) become apparent in air above 3.5bar(g), however for saturation exposures a lower limit is appropriate to counter any long-term effects of fatigue.

Helium, used to reduce the narcotic potential, also has the benefit of reducing the density of the breathing gas. While the density of air at 3.5bar(g) was 5.4gm/lit, the density of a 20/80 oxygen/helium mix at the same pressure was only 1.8 gm/lit. A breathing gas density of 6 gm/lit is considered the threshold at which the work of breathing is considered to be excessive. Hence again, 3.5bar(g) is an appropriate pressure to change from air to mixed gas.

Although inspired gas through masks has a very low level of carbon dioxide contamination, levels in habitats, shuttles and locks can be high due to exhaled gas in the chamber atmosphere. This has to be removed by ventilation or chemical scrubbing to maintain levels that are comparable with those existing in the normal free-air atmosphere.

A comparison was made between decompression from the maximum permissible exposure at 3.5bar(g) on the Blackpool tables with a typical decompression from saturation storage at 3.5bar(g). The former was a 355-minute-long stage decompression with rapid pressure drops, resulting in supersaturation to generate bubbles, followed by increasingly long soak periods to off-gas the bubbles. The saturation decompression was a reasonably linear reduction in pressure over a period of around two and a half days, ideally slow enough to off-gas by diffusion and prevent bubbles forming. The risk of decompression illness was therefore much lower following decompression on the saturation profile.

While decompression profiles can be developed for non-saturation exposures at pressures up to 7bar(g) on air, the effects of narcosis and work of breathing at such pressures cannot be overcome.

DISCUSSION OF BREATHING MIXTURES

Trimix – an oxygen/helium/nitrogen mixture – reduces narcotic risk and fatigue from enhanced work of breathing. Heliox – an oxygen/helium mixture – removes all narcotic risk and further reduces fatigue from work of breathing. However, the current cost of helium for a team of four workers at 6bar(g) can be as high as £2,500 (US$3,300) per hour.

Trimix is preferred in tunneling as it is cheaper due to its lower helium content. A further benefit of trimix is the relative ease of blending it in habitats. Depending on storage pressure required, compressing the habitat on air from atmospheric pressure to a pressure of 1bar– 1.5bar(g) then compressing further to storage pressure using helium with final fine tuning as required could be undertaken to achieve the required trimix mixture.

Non-saturation exposures require a high oxygen mix to enable their use from atmospheric pressure to 6bar(g). Typically, a 20/30/50 O2/He/N2 could be used over this pressure range and remain within relevant partial pressure limits. A more expensive alternative could be 20/80 heliox.

Saturation exposures require low oxygen mixes to keep down partial pressure of oxygen (PO2) but consequently are irrespirable at atmospheric pressure. A typical mix for storage at 6bar(g) could be a 6/65/29 O2/ He/N2 trimix and a working mix for that pressure could be 10/61/29 O2/He/N2 trimix.

Fire is a major risk in hyperbaric work at low and intermediate pressures and for this reason oxygen concentrations should always be kept below 23% by volume. However, with low oxygen mixes, flammability decreases with reducing oxygen content, such that below 6% the oxygen volume concentration is sufficiently low for total inflammability to exist. This could be the case for storage at pressures which are above around 5.5bar(g).