SC450 - A Total Concept Approach Aimed at Divers Individual Equipment

Anders Hägglin, Claes Bengtström
Licentiate of Technology in Underwater Technology
Department of Underwater Technology
Chalmers University of Technology
UHMS Publication No 76 (UNDBR), 10/1/89, Buffaloo 1989

INTRODUCTION
An analysis has been carried out, to determine design constraints, for breathing apparatuses, for ultra deep diving. The result of the analysis showed that the interaction between system components, make it necessary to incorporate certain important parts such as heating system, diving suit and helmet to reach an acceptable result.
A breathing apparatus is not an isolated technical system. It is very much influenced by interactive systems. Thus, it is not possible to design it, without proper notice of these interactive systems.

The diver is an important and expensive tool. Down time due to malfunction with divers personal equipment, may halt operations involving surface vessels, hired at a rate of up to 100 000 USD per day. Any equipment, without a considerable track record will, be regarded with great suspicion, by both contractor and operator.

Re-breathers, introducing electronic sophistication under water, creates fear of physical danger to the diver, but also of economic danger to the client. There is a big mistrust in electronics within the diving community. This mistrust is poorly founded and reflects a lack of understanding, of the principles of electronics on part of the divers. It is therefore necessary, through education, to make the diver community aware of the potential of such equipment.

Operating diving companies and manufactures of diving equipment are compairativly small. This makes them unsuitable for the running of large projects, that are necessary, if the insufficiencies in todays systems are to be overcome.

Until today diver worn equipment have been developed piece by piece, often by companies, interested in special parts of the divers equipment. The adding of components and extrapolating of performance, will not provide the diving community with the technology necessary, to take human diving further. It is now evident, that in order to provide the human diver with means for ultra deep diving, a total concept approach, aimed at divers individual equipment, must be accepted.

The further need for human divers is evident. Underwater vehicles and remotely operated systems will continue to develop and will replace the diver, in many tasks. However, the re-routeing of the pipelines after the Piper Alpha accident, shows a kind of operation, that never could have been performed by any form of remotely operated vehicle system. The only feasible unmanned means, would be a dedicated robotic system, where all possible failure modes have been identified in advance. To have such a system on stand-by, could not be regarded as an optimized solution to the problem.

TOTAL CONCEPT ANALYSIS
In our analysis we have identified some system related insufficiencies when performing ultra deep diving. These are:

-High resistive work of breathing, due to high gas density and apparatus design, causing limited working capacity on behalf of the diver.

- High elastic work of breathing, due to hydrostatic imballance, causing limited working capacity on behalf of the diver.

-Loss of heat from outer cooling, due to break down in, or improper design of, active diver heating system, causing hypothermia and convulsions.

-Loss of heat from inner cooling, due to cold, high density breathing gas, causing hypothermia without early warning convulsive symptoms.

-Risk of hypoxia and hyperoxia when using re-breathers, due to system break down or improper design, causing severe or fatal incidents.

-Risk of CO2 build-up effects when using re-breathers, due to improper function with lime scrubber, causing a variety of symptoms giving restrictions in diver performance.

-Large dead space, due to improper design and poorly fitting of inner mask, causing a variety of symptoms giving restrictions in diver performance.

Our first system approach, were aimed at providing enough bail-out time in an emergency. The re-breather became the obvious answer. If a re-breather is to be used at 450 meters depth, the temperature control of the scrubber unit and breathing circuit is vital. We decided to conserve the heat in the breathing circuit and to expose as little of it as possible to the surrounding water. To solve this problem, we incorporated the breathing apparatus with the active and passive diver worn protective suit system. Thereafter it became logical to try to incorporate more of the diver worn equipment, into our analysis.

The analysis is the base for the design of the SC 450 system and it lead to the following objectives:

-Conserve exhaled heat

-Keep CO2 absorber warm

-Reduce or eliminate need for active emergency heating

-Reduce size

-Improve mobility of diver

-Reduce dead space and breathing resistance

-Improve field of vision

-Make gas supply system rugged and simple

-Reduce size of umbilical


These objectives gave the following concept for the SC 450 breathing apparatus.

1. Suit system, helmet and breathing apparatus is one integrated system.

2. Re-breather

3. Hydrostatic imballance is compensated for in the design

4. Dosage system is of a fail-safe design

5. Active heating combined with passive insulation

The integration of the breathing circuit, with the insulation system and the choise of the re-breather, make it possible to conserve the heat in the breathing circuit and to keep the CO2 absorber warm. Further the bail-out supply will be small and thus contribute, making the total system smaller.

The compensation for the hydrostatic imballance, together with thorough design of all details in the breathing circuit and the solution chosen with the inner mask attached to the diver, will reduce the total breathing resistance and the dead space.

The design of the dosage system will make it possible to keep the pO2 value within tolerable levels, at all times. Further, the combination of active heating and passive insulation, will prevent rapid cooling of the diver in an emergency.

In order to show the possibilities to use a re-breather at large depths, a series of tests were performed at 450 meters depth in the pressure chamber at the department of Underwater Technology at Chalmers University of Technology. There were no problems reaching the objectives, a two hour duration before pCO2 break through at 0.5 kPa, if the temperature of the breathing gas was kept at 28 C.

The detail requirements of the SC 450 concept are divided into the catagories physiological, operational and ergonomic constraints. The physiological restraints are either accepted levels or in some cases lower than previously accepted levels. The operational restraints include e.g. turn around time between lock-outs and bell-runs. The ergonomical restraints will be based on evaluation of the system made by test divers.

The SC 450 diving system is mainly intended for bell-diving operations, in water depths down to 450 meters. By minor alterations, the system can be adapted to shallow water diving. The system comprises the divers personal protective gear including bail-out equipment, breathing apparatus, communication system, diver umbilical, active heating and protective suit.

It is prerequisite, that an appropriate bell diving system is available, which can be used as a basis for the SC 450 outfit. This includes requirements on the size of the bell and bottom hatch, amount of hot water and electricity that can be supplied to the diver, and amount of appropriate breathing mixture, that can be carried by or supplied to the diving bell.

CONCEPT DESCRIPTION

General
The general system concept consists of:

- An integrated system, comprising diving suit, undergarment, helmet, umbilical and breathing apparatus.

- A rebreather type of apparatus.

- The breathing circuit control and gas injection system.

- A bail-out supply, containing enough pre-mixed gas to get the diver back to the bell in an emergency situation.

- A stiff box containing the breathing bellows, in order to minimize the elastic work of breathing. The stiff box also serves as a protection for the soft bladder.

- Active heating system (body heating).

- Combination of active and passive heat insulation and diving suit.

- Active heating of breathing circuit.

- Monitoring system with different function and alarms.

- Continuous reading of partial pressure PO2, PCO2 by bellman and surface control.

- Relief valve from breathing circuit.

- Means to hoist an unconscious diver into the diving bell.

- Communication system.

Breathing gas system
Main supply:

The main supply line feeds the gas injection system in the apparatus with an appropriate mixture from the bell, at an appropriate pressure to overcome flow losses in the hose. At least 10 MPa at the diver end of the umbilical at maximum flow, 75 l/min STPD.

Breathing circuit control system:

The control system continuously monitors the oxygen partial pressure in the counter lung by means of three oxygen sensors. The electronic unit controls the injection valve, so that the partial pressure is

70 kPa < PO2 < 90 kPa during normal operating mode and emergency mode I. During emergency mode II, the PO2 will be maintained within the limits 40 kPa<PO2<195 kPa due to the fail-safe design of the system.

Emergency mode I = Umbilical cut off.

Emergency mode II = Umbilical cut off and battery pack failure.

Bail-out:

If the main supply pressure drops below 9.5 MPa, the bail-out supply is automatically switched into the breathing circuit. The diver automatically receives a warning.

Free flow:

If a fault occurs in the primary breathing circuit it can be disconnected by operating the three way valve handle. The diver then switches over to free flow mode . The breathing gas is then taken from the atmosphere within the suit. In order to reduce the CO2 build-up, the free flow valve is also opened. The excess gas escapes through the suit relief valve. The free flow valve is also used for buoyancy compensation if the diver wishes to become more buoyant and to remove moisture and fog from inside the transparent dome.

By pass:

The by pass valve is used to fill up the breathing circuit when a dive is commenced. It is also used to compensate depth changes. In case of malfunctioning of the scrubber, it can be used to increase the gas flow through the system in order to reduce the CO2 build-up.

Breathing circuit relief valve:

Allows the excess gas to escape to the suit atmosphere. The valve is mounted on the inhalation side of the circuit, which means there is no build-up of CO2 in the suit. In other words, the suit also serves as a breathing gas buffer.

Suit relief valve:

The suit relief valve is adjustable. This makes it possible for the diver to control his buoyancy. The valve can also be operated by pushing it. The gas in the suit then escapes to the water and the diver will become heavy.

Scrubber unit :

The scrubber unit is placed under the protective suit and kept at a temperature which is optimal for the CO2 absorption process. The unit is also protected by an extra layer of passive insulation. There is no possibility for water ingress into the scrubber unit and breathing circuit unless a combination of undesirable events occur simultaneously.

Monitoring system
The SC 450 monitoring system continuously monitors the following technical and physiological parameters:

- Supply line pressure.

- High CO2 content in the breathing bellows.

- One or more oxygen sensors out of function.

- Main electrical power supply is cut.

- Injection system failure.

These are the standard alarms that come with the system. If the customer so desires, the monitoring system can be furnished with alarms for hot water temperatures, low gas temperature etc.

Acknowledgement

The authors wishes to express their gratitude to prof. Gunnar Dahlbäck for his commitment in this project.



Fig.1 Helmet and oral-nasal mask overview

Fig.2 Breathing scrubber and bellows overview