Some System Analytic Views on Hyperbaric Evacuation

Anders Hägglin
Dept. of Underwater Technology, Chalmers University of Technology
Intervention 87, San Diego

Abstract
An analysis of the possibilities to meet the operational objectivs of a hyperbaric rescue operation has been performed. It includes an analysis of the operational suitability of different existing methods for hyperbaric evacuation during probable emergencies. This paper will discuss hyperbaric evacuation from a system analysis point of view.It will show some of the pros and cons with today's systems and also discuss the risks normally involved in saturation diving compared with the added risks of hyperbaric evacuation. One purpose is to discuss how a seemingly just authority demand produces a system which cannot fulfill its objectives and which under certain circumstances exposes the users to great danger.

Introduction
During operations that involve manned underwater activity, prepared actions in case of an emergency are taken. As diving operations in the North Sea have grown safer, the discussion concerning diving safety has become wider and now concerns things far from breathing resistance, proper diver heating, functional diving communication and bail-out time. With the increasing number of saturation diving operations, the question of effective and operationally sound methods for the evacuation of divers under pressure, from a surface facility in serious distress, has aroused. During the past decade, legislative demands have been put on the diving operators to equip their diving support vessels with a hyperbaric evacuation system (1-3). It has always been a discussion among sailors when to abandon ship. In many cases, it has been shown that it is far more dangerous to abandon ship than to stay and try to stabilize the situation. Hyperbaric evacuation represents in itself a substantial risk to the divers and obviously one does not want a situation were the cure is worse than the cause.

A Few Aspects on Hyperbaric Evacuation
Main physiological aspects are decompression, gas consumption, thermal balance, water supply, food, sea sickness and sanitation. Of these aspects, food and decompression are of lesser importance due to the short time required for a hyperbaric lifeboat to be salvaged. Decompression is a critical operation which demands perfect surveillence and accurate instruments (4). This is impossible to achieve under extraordinary circumstances. Thermal problems during a hyperbaric evacuation can be a very complex question. Depending on the saturation depth, the number of divers in the system, the insulation and the surrounding temperature of air and water, cooling or heating of the chamber atmosphere may be necessary (5).

Dehydration of the divers may occur if the temperature rises due to failure in the thermal control system or if the divers vomit due to seasickness. Plenty of water must therefore be provided. In the event of a hyperbaric evacuation it is wise to remember that seasickness can kill quite rapidly (6). The seaworthiness of any hyperbaric rescue system must be questioned and taken into account before its operational use.

Technical aspects mentioned here are energy supply, gas supply, stability, propulsion and the running of the system during an emergency. The necessary energy supply for the life support system and propulsion is 72 hours (3). This can be provided by a conventional marine diesel if the hyperbaric rescue system is equipped with such. If not, necessary heating and cooling will be hard to manage.

The gas supply consists of two parts, oxygen for breathing and a heliox mixture to compensate for any loss of pressure. The oxygen storage for 16 divers during 72 hours is not a problem but compensation for pressure losses may be, depending on saturation depth and leakage. The propulsion system of a rescue system must be able to keep it clear of its mother vessel in high sea states and to manoeuver it favourably against the sea to reduce the roll.

The life support system is run by the HRL crew from the outside of the chamber. During an emergency, seamen who knows nothing about the life support system may end up being the crew of the HRL. Therefore it is suggested that the system should be designed to be run from both the outside and the inside of the chamber (7, 8).

Main operational aspects are duration of emergency situation, number of divers in saturation, split level saturation, launching and recovery situation and location of the diving site worldwide. The depth on the diving site is perhaps not a genuine operational aspect, but it does significantly influence most of the operational aspects. Maximum duration is 72 hours. That is more than sufficient in the North Sea or the Mexican Gulf, but probably not enough offshore northern Australia. In the busy offshore areas, a rescue within 24 hours is plausible and in certain situations, such as high sea states necessary. It is argued whether the HRL should leave the area of the accident or if it should stay and await assistance from other ships. However, with a possible speed of 6 knots, it may cover the distance to a safe haven (7, 8).

Number of divers in saturation normally vary from four to 16. There are, however, saturation systems which can house up to 28 divers. Since almost every DSV is a unique design, the hyperbaric rescue system must be designed to satisfy the demands of the specific vessel which it serves.

A most possible case of split level saturation is one team working and one under decompression. The standard procedure is to pressurize the divers to the deepest saturation level. This can prove too difficult if diving is taking place at a very deep spot,i.e. there will not be sufficient time to both pressurize the divers and to launch the rescue system (9).

The delicacy of the launch of a hyperbaric rescue device makes it hard to envision a successful operation in higher seastates or with a badly listing surface vessel. The recovery of a hyperbaric rescue device without a dedicated lifting system is impossible in higher seastates and alredy difficult in calmer seas (8, 10).

Available Methods for Hyperbaric Evacuation
There are different systems available for hyperbaric evacuation. Some of these are purpose designed while others, such as an SDC, are compiled of standard equipment. This paper deals with two purpose designed concepts for hyperbaric evacuation. They are, the hyperbaric rescue lifeboat and the hyperbaric rescue chamber. Further the wet transfer is considered and different lift-off techniques are briefly discussed.

The hyperbaric rescue lifeboat (HRL) consists of a compression chamber placed inside the hull of a lifeboat. It has room to carry the maximum number of divers for which the saturation system is designed. The system is equipped with considerable life support for the divers and with conventional engine power (8, 10-11). The life support system is depending on the engine power for its energy supply. This makes the system vulnerable if the engine breaks down.

The HRL is launched in the same way as a conventional lifeboat, using davits. The weight of the HRL is substantial, between 12 and 16.5 tonnes, which of course in itself is a problem, specially during gale conditions or with a listing mother vessel. Due to the weight and accessible propulsion the seaworthiness is not good.
The HRL is supposed to have a crew consisting of the diving supervisor and tenders with experience from saturation life support systems. This experienced crew is a necessity for a successful operation.

Finally, the recovery and mating of a HRL to a saturation system onboard another vessel may very well prove to be the most difficult part of the entire operation. The size and weight of it makes it difficult or even impossible to do it unless weather conditions are perfect. The HRL is not designed to be lifted at one point, which will make it hard to attach the lifting aids. If improperly lifted the HRL may break up due to its own weight.

Lift-off techniques means that a pressure vessel of any kind is lifted from the DSV to a ship alongside or launched into the sea. In most cases, the pressure vessel will be the diving bell (SDC) or preferably a hyperbaric rescue chamber (HRC).

A HRC is not equipped with a propulsion system. It is basically a compression chamber with a buffer frame, bouyancy system and a basic life support package. This concept has been used for the largest hyperbaric rescue device built, which can accomodate 28 divers. The HRCs are also large and bulky (8, 10-12).

If a HRC is launched some insufficiencies are obvious. The life support system is somewhat rudimentary and no crew will be present to take care of possible malfunctions. Also, the seaworthiness of the HRC is very poor since it has no propulsion. The recovery of the HRC in heavy weather will be a very difficult operation.

A method for recovering HRCs over the rolling stern of a supply vessel has been tested. With special arrangements, the recovery operation was successfully carried out with a 1.5 meter swell. If the pressure vessel lifted off is a SDC or adeck decompression chamber(DDC), normally an operational part of the diving system, neither will be equipped with a self-contained life support system of any significance. However, if the meeting ship is equipped with a saturation system, means for connecting all life support functions are available. If it is not a DSV, then there is still a possibility that a saturation system can be reached within hours, at least in the busy parts of the offshore world (8, 10-11).

If a small number of divers are involved the SDC can be used in the lift-off situation. A SDC is so small that it can be placed on most surface vessels and handled by most cranes. A standard SDC will not take more than four to six divers. The SDC is, on the other hand, familiar by both divers and surface crew.

A special form of lift-off technique is the wet transfer. This means that the divers to be evacuated are lowered to their saturation depth and then transferred through the water to the SDC of another diving vessel (8, 11). This method has actually been used in some emergencies and is therefore considered both realistic and proven. The divers stay in a familiar environment during the entire operation and when they reach the meeting SDC they are comparably safe. The poten- tial dangers of transfer through water decreases the closer the SDCs are, and the risks can be lessened with small means. It is vital that the divers are in good condition if they are wet transferred. There will always be a discussion about the possibility to abort a decompression at lower depths (10 meters) in an emergency. If the decompression is aborted and the divers can reach a safe chamber complex within minutes, they will probably not sustain any lasting injuries. To improve the propability of a successful evacuation the divers should breathe pure oxygen for as long as possible before, during and after the abortion.
At depths less than 10 meters, it may be possible to establish emergency procedures for the abortion of a decompression, if a safe chamber complex is minutes away. A suggestion from Norwegian specialists is to abort at depths lesser then 5 meters, to judge the prevailing situation at depths between 5 and 10 meters and to evacuate using the HRL at greater depths (13).

Analysis of the Operational Suitability of Different Methods for Hyperbaric Evacuation During Probable Emergencies
In this chapter, the different hyperbaric evacuation systems will be compared through a sequence of events which are feasible in an emergency. Some emergencies represents situations where a fast decision of evacuation may be necessary while others allow some time for mobilization. Different weather situations have an influence on the possible use of the different launching systems, the transfer period, and the possibilities to recover a hyperbaric evacuation system from the surface of the sea.

The sequence of events can be listed as follows:

-Mobilization

-Launching

-Transfer

-Recovery

-Re-establishment of safe conditions

Below, these events are commented upon.
The mobilization of the hyperbaric evacuation system is defined as the time which elapses between the decision to evacuate and when divers inside the hyperbaric evacuation system are ready to be launched.

A HRL and a HRC are both constantly mobilized and it only takes a few minutes to transfer the divers, if they are all on the same saturation depth (8). There is no or little difference in mobilization time due to emergency situation.

A lift-off operation with a pressure vessel other than a dedicated rescue pressure vessel, takes time to prepare. A SDC must be equipped and re-arranged for the situation. A DDC must be mobilized and separated from the diving system. The time to perform those operations is measured in hours rather than minutes.

A wet transfer is performed when another DSV is close and alongside the DSV in distress. Depending on the initial distance between the ships, the mobilization time of such an operation can be substantial. Further mobilization takes place when the ships are in position (11).

The launching operation is defined as the actual physical lifting or launching of the rescue system from the mother vessel. This definition gives a different operation depending on which rescue system is used.

The launching of a HRL and a HRC is done with a dedicated launching systems which is equipped with separate emergency energy systems. This makes them suitable even if an accident has damaged the main energy supply (8). It is, however, very hard to launch these systems in heavy weather if the mother vessel is listing and due to the davit capacity in relation to the weight of the rescue systems. In a lift-off situation, the ordinary crane system is used. This means that if the hydraulic system of the cranes is damaged, the lift-off is impossible to perform. A lift-off operation to another ship that is a simple operation in good weather can, however, be hard to perform in higher sea states. Not only does the pressure vessel need to be removed off the deck, it must also be kept from swinging to finely be placed on the rescue vessel without being smashed.

The launching of the SDC in a wet transfer operation is standard procedure as long as all systems are operational. If the SDC is cross-hauled to the side of the DSV in order to reduce the distance between the two SDCs, that is an operation with possibilities to go wrong, especially in bad weather.

The transfer period for the HRL and the HRC is defined as the time when they are in the water. For a lift-off pressure vessel that is placed on another ship, it is the time it is hanging in the crane. For the wet transfer operation it is the time from launching until the last diver is inside the meeting SDC. In a resonably calm sea, the HRL and the HRC perform satisfactory during the transfer period. However, in a higher sea state, the conditions inside will be very bad (8, 10, 11).

During the transfer period of a wet transfer, the divers are constantly in a well known environment. The SDC is, however, affected by a heaving mother vessel and the actual transfer by strong currents and poor visibility (11).
The recovery operation is defined as the finding, hooking and lifting of a hyperbaric rescue system from the sea. For the wet transfer, it is the recovery of the SDC.

To locate a hyperbaric rescue system floating in the sea can be difficult in bad weather. In higher sea states, it is even harder to keep contact with it since it floats very low in the sea.

To attach a lifting device to a HRL is difficult because it is launched with special equipment. It may even be advisable for a ship with small crane capacity not to try a lifting operation but rather be standby and wait for a ship with a larger crane. Lifting such large and heavy objects involves an obvious risk of making them break due to incorrect lifting procedures.

The recovery of the meeting SDC in a wet transfer is standard procedure. The SDC will be crowded and heavy but that should not create any substantial problems.

The re-establishment of safe conditions for the rescued divers means that external heat, breathing gas and gas for the pressure keeping are connected. This must be done before decompression starts. All SDCs and dedicated hyperbaric rescue systems are fitted with standard couplings for gas and hot water. This makes it easy to establish relatively safe conditions if the rescuing ship is a DSV (10, 11).

If the pressure vessel cannot be mated to a saturation system or temporary lifesupport the rescuing ship must head for a rendezvous with a DSV or head for port where appropriate pressure chamber facilities are available.
In the case of a wet transfer, the divers will be inside a safe saturation system immediately after recovery.

Analysis of the Possibilities to Meet the Operational Objectives of a Hyperbaric Rescue Operation
This chapter will present the objectives and restraints that influence the result of a hyperbaric rescue operation. Together with the operational suitability discussed in chapter four, these objectives and restraints are the basis for this analysis.

The main objectives of a hyperbaric rescue system are:

-To safely evacuate divers from a surface vessel in distress

-To take the divers to a safe haven during acceptable conditions

-To re-establish safe conditions for the decompression of the divers

These objectives are set by international and national laws and regulations, classification rules, insurance demands, union demands etc (1-3). The industry of shipbuilders and diving contractors try to meet these demands.

The main operational restraint is, what kind of diving is performed: Surface decompression diving, bounce diving, saturation diving or split level saturation diving (10-11). Further operational restraints are: Depth on the diving site, number of divers involved, number of saturation levels and location of the diving site.

During surface decompression and bounce diving it is not likely that a dedicated hyperbaric rescue system is available. These operations are small and available equipment such as a SDC or a DDC will be used in the event of a hyperbaric evacuation.

Saturation diving operations are complex and extensive. They are large enough to justify the presence of a dedicated hyperbaric rescue system. The size of that rescue system is determined from the maximun number of divers in the system at any time.

Depth has a very strong influence on the possibilities of using conventional hyperbaric rescue techniques in the case of split level saturation diving. From a diving depth of 190 meters and deeper, it is not advisable to evacuate two diving teams by decompression and compression to a fellow-depth during an emergency that demands an immediate abandoning. In a worst case of difference in saturation depth, it will take between 37 and 38 minutes to reach that fellow-depth if the starting depth is 190 meters (9). The status of the divers will be questionable after this.

The number of saturation levels has a corresponding influence on the mobilization time as the operational diving depthis.

The location of the diving site has great influence on the possibilities to reach a safe haven with the hyperbaric rescue system within the given time.

Discussion
Depending on what type of diving is performed, what operational restraints are in force, and what hyperbaric rescue system is available, a different operation will take place to fulfill the objectives given. In order to get a full picture of the situation the sequence of events must be run through for each type of diving, each operational restraint and each hyperbaric rescue system. Of course, a lot of these can be left out of the analysis, but a substantial number must be penetrated if an acceptable result shall be reached.

A diving contractor does not design a DSV for only one part of the world. Given a certain period of time, most DSVs will show up in the North-Sea as well as the Mexican Gulf and offshore Brazil. Every location has its special considerations to take into account, especially the distance to a safe haven for a HRL.

This means that one code of practice for hyperbaric evacuation that covers all areas world wide will be hard to live by. If a very high level of safety is to be maintained, certain points must be discussed.

Consider the following examples of diving operations:

Type of diving Bounce Diving
No. of divers 2
Duration 16 hours
Depth 70 meters
Saturation levels - Location 60 nm from safe haven

Type of diving; Saturation Diving
No. of divers 8
Duration 2 weeks
Depth 180 meters
Saturation levels 180, 3 meters
Location 200 nm from a safe haven

Type of diving Split level Sat. Diving
No. of divers 18
Duration 5 weeks
Depth 200 meters
Saturation levels 200, 160, 3 meters
Location 600 nm from a safe haven

The first operation is, in all probability, an inshore surface decompression dive with small resources where a hyperbaric rescue system will not be required and not provided. In an emergency the SDC or DDC will be used for evacuation. The second operation may very well be a North Sea operation where today's legislative demands can be met with existing systems. The third operation can meet the legislative demands but it cannot maintain the intended level of safety if only one DSV is contracted for the mission.

It is possible to achieve a level of safety for the saturation divers to correspond to that of ordinary seamen, but it will be very expensive and operationally hard to handle (1). In remote parts of the world and during extraordinary circumstances elsewhere, two DSVs must be mobilized for one operation in order to reach the necessary level of safety.

Today Norwegian legislation demands one HRL on every DSV (2). If the DSV is listing the wrong way in an emergency situation, the HRL will be impossible to launch. To fulfill international maritime regulations for conventional life-boats, there has to be two HRLs (14). The maritime codes also give details on regular emergency drills, something which is not being performed on HRLs.

The objectives state that divers must be safely evacuated from their surface vessel. If the evacuation system is a HRL, a safe evacuation cannot be made if the vessel is listing in the wrong direction, if the sea state is severe or if the launching system is damaged.

Further, the objectives state that the divers should be taken to a safe haven during acceptable conditions. A HRL of today's standards can hardly be regarded as a vessel capable of bringing divers to shore under severe weather conditions, in an acceptable manner.

The re-establishment of safe conditions for the decompression of divers can be performed if the conditions are stable. Such conditions will only be reached onshore or onboard a DSV.

Conclusions
Emergency situations due to fire, heavy weather, collision or running aground are events which can occur to any ship. If a DSV is affected, it can lead to a hyperbaric evacuation. Hydrocarbon production related emergencies are more rare.

If an assisting DSV can be brought close enough to allow the performance of a wet transfer, experience shows that the method is feasible during good weather conditions. It must be pointed out that if the weather is suitable for a ship to ship transfer, it is suitable for a wet transfer.

If the divers in saturation are few, the SDC is a suitable means for hyperbaric evacuation in a lift-off situation. If the meeting vessel is a DSV it is possible to connect the SDC to the life support system. It is even better if it is possible to mate the SDC to the saturation system.

Actual emergencies have shown that the surface crew of a DSV can pass the limits of what can be expected of them in order to save divers. It is also shown that a threatening emergency situation sometimes can be controlled and stabilized and thus, a hyperbaric evacuation will represent a far greater danger to the divers than staying onboard (6). In really bad situations such as e.g. the Ocean Ranger disaster, divers under pressure would have perished no matter what system for hyperbaric evacuation had been available. The psychological impact from having means of rescue should however not be disregarded.

The legislative demands are to provide a hyperbaric rescue system on every DSV. The demands are met in the North Sea area. However, the function of the system is not discussed. This analysis reveals an authority demand that is improperly prepared and analyzed no matter how just.

The improper function with today's hyperbaric rescue systems should not be blamed on those who have provided them, since they only tried to fulfill a given specification. In order to give the saturation divers a proper means of evacuation, we must return to the top of the pyramid and discuss our objectives.

Perhaps it is more realistic to look over the objectives and to discuss an operational level of safety corresponding to other risks run by the saturation divers. Such risks are, e.g. hypothermia due to improper heating, unconsciousness due to improper breathing equipment and insufficient bail-out supply if the umbilical is cut.

Today it is easier to analyze the dangers faced by the DSVs and how to avoid them rather than to create a system to evacuate the divers. To generally meet the demands on hyperbaric evacuation today, two DSVs must be mobilized. Such a decree is impossible to present today. Still, the demands are there and it would not be appropriate to reduce the level of safety. This contradiction and its solution is a key to the problem.

It is hard to envision how a further development of existing hyperbaric rescue systems can fulfill the objectives. HRLs will be large and heavy. New concepts must be based on operational programs that take the entire diving operation into account. Such programs must incorporate safe and effective diving in a manner that shows that safe diving is effective diving.

Acknowledgement
The author expresses his thanks to his two pupils Johan Nerlund and Stefan Jarlheim for their enthusiastic if not sometimes unorthodox methods of collecting information on this subject.

Refrences
1. "Code of Safety for Diving Systems", International Maritime Organization, Resolution AS.536, Chapter 3, 1974.

2. "Provisional Regulations for Diving on the Norwegian Continental Shelf", Norwegian Petroleum Directorate, Section 2.2, 1980.

3. "Rules for Certification of Diving Systems", Det norske Veritas, Section 9, 1982.

4. "US Navy Diving Manual", Best Publishing Co., 1980, Carson.

5. Furevik, D. M., et. al. "Manned Test of Rescue Mate", NUTEC, rapport nr. 34-82, 1982, Bergen.

6. James, P. B., "Transfer Under Pressure: A Re-evaluation", Developments in Diving Technology, Chapter 14, Graham & Trotman, 1985, London.

7. Morgan, R., "The Design and Development of a New Class of Self-propelled Hyperbaric Lifeboat for Diver Rescue", Submersible Technology, Chapter 26, Graham & Trotman, 1986, London.

8. Logan, C., "Hyperbaric Evacuation", Submersible Technology, Chapter 27, Graham & Trotman, 1986, London.

9. Jacobsen, E., et. al.,"Tidsmarginer ved hyperbar evakuering", NUTEC, rapport nr. 34-84, 1984, Bergen.

10. Haux, G., "Subsea Manned Engineering", Balliere Tindall, London , 1982.

11. Sisman, D., "The Professional Divers Handbook", Submex Ltd., London , 1982.

12. Young, M. N., "Marine Recovery of a Hyperbaric Rescue Vehicle", Submersible Technology, Chapter 25, Graham & Trotman, 1986, London.

13. Hjelle, J., "Evakuering av metningsdykkere naer overflaten i en krisesituasjon", Letter 88-12-22.

14. "International Convention for the Safety of Life at Sea", International Maritime Organization, Chapter 3, 1974.