DEVELOPMENT OF AN INTEGRATED SUBMARINE ESCAPE SYSTEM
2. TOWER ESCAPE SYSTEM REVIEW 1 REQUIREMENTS
The key stakeholder requirements for an escape system can be summarised as follows.
The system must:
x Operate safely and successfully at all depths down to submarine collapse depth or the limits of human capability
x Reduce the risk of harm to escapees to ALARP x Consume air efficiently to ensure there is
adequate supply for all escape scenarios x Provide a reliable and consistent outcome x Ensure a de-risked and confident acquisition
programme
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects x Minimise disruption to the submarine programme
during installation
x Provide a simple service solution with assured system availability
2.1 (a) Operate Safely at all Submarine Depths
The escape system should operate successfully at the shallowest of depths and continue beyond the limits of human capability and submarine collapse depth. It is essential that an inoperable system does prevent an otherwise viable escape.
Rescue vehicles can find it difficult to operate at shallow depths, due to low hydrostatic pressures, so escape may be the only means of evacuation from a DISSUB.
2.1 (b) Reduce the Risk of Harm to ALARP
The risk of injury and illness to an escapee should be reduced to As Low As Reasonably Practicable.
Submarine escape is an inherently hazardous exercise, but steps must be undertaken to reduce the probability of occurrence.
Causes of injury include physical knocks whilst accessing/exiting the escape tower, prolonged exposure to pressure (resulting in DCS) and over-rapid pressurisation (resulting in barotraumas).
It is therefore vital that the escape process happens in a controlled manner, minimising the time exposed to elevated pressure whilst not pressurising the tower so quickly as to cause barotrauma.
The safe limit for pressurisation is that the pressure within the escape tower should never double in less than four seconds [1].
In fact, this pressurisation profile, which equates to:
4 t
ch 2
P (1)
can be taken as the optimum pressurisation rate.
Doubling the pressure in less than four seconds leads to unacceptable risk of barotrauma. Any slower and the escape process is unnecessarily slow, subjecting the escapee to elevated pressure for longer than required and increasing the consumption of air from the dedicated supply .
2.1 (c) Consume Air Efficiently
Once inside the tower, the escapee is connected to breathable quality air supplied via dedicated HP Air bottles.
It is fundamental that there should be sufficient supply to cover the air demand in any escape scenario. There is generally limited space available for the HP Air bottles, so using the air in an efficient manner is preferable to installing additional bottles.
2.1 (d) Prove Reliable and Consistent
To confidently predict air consumption and pressurisation rates, the system hardware must operate in a repeatable manner. Valves and mechanisms should perform reliably and consistently. The system must continue to operate after a shock event and with all main power supplies unavailable.
2.1 (e) De-Risk Acquisition
The acquisition of new equipment is simpler and cheaper if there is confidence that the equipment will work as intended. The use of COTS and MOTS components, coupled with an incremental test and acceptance plan de- risks the development and acquisition process. The entire escape system should be at Technology Readiness Level (TRL) 6 before it is installed on the submarine [2].
This involves demonstrating the system at whole-system level in a representative environment.
2.1 (f) Minimise Impact on Submarine
Installation and through life maintenance of the escape system should have minimal impact on the platform, to ease scheduling and reduce cost. A fully tested modular system would simplify the system integration.
2.1 (g) Assure System Availability
There is a clear benefit to submarine operators having a support provider taking responsibility for assuring the availability of the system. Approaches such as
‘Contracting for Availability’ incentivise increased availability and reduced through life maintenance costs.
Such contracts are impractical in piecemeal systems where there is no clear design authority.
2.2 PRINCIPLES OF OPERATION
Towers can be sized to accommodate one, two or more persons. Regardless of size the basic principles of tower escape are the same, as shown diagrammatically in Figure 2.
.
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects Figure 2: Typical Tower Escape Sequence
In its simplest form, the process requires completing the following actions in the shortest possible safe time:
x Enter tower
x Pressurise to depth pressure x Escape
x Drain tower 2.2 (a) Enter Tower
Factors that affect the ease, speed and safety of entry include the size of the access hatch, the height of the tower above the deck and the hatch operating and sealing arrangement. Hatches that sit on the inside face of the tower are more difficult to open if there is a deadweight inside the tower, for example an Emergency Life Support Supply (ELSS) posted pod, and require more internal tower volume to manoeuvre the hatch. However, they are simpler to seal as the tower pressure forces the hatch closed.
Once within the tower the hatch must be secured. As with all elements of escape process, the ‘last man’ must be able to do this unassisted. The escapee then inserts a
‘stole’, an integral part of the escape suit, into an air supply regulator valve, and is ready to receive air.
Initially the air is only needed for breathing purposes.
However, during the escape cycle the air serves four purposes:
x To provide air for the escapee to breathe x To inflate the hood of the escape suit against the
ever increasing tower pressure x To inflate the escapee life jacket
x To contribute to the pressurisation of the tower 2.2 (b) Pressurisation
Once in the sea, the escapee will be subject to hydrostatic pressure equivalent to approximately 1 bar for every 10m
of depth. The escapee can not be instantaneously subjected to this pressure; instead he must be brought up to this pressure in a controlled manner. The optimum rate of pressurisation is to double the pressure of the air trapped within the tower every four seconds.
It is not necessary to pressurise the entire tower volume.
‘Vented’ escapes introduce a flood phase prior to the pressurisation phase. A vent valve initially allows the air to escape into the submarine as the water level rises. At a prescribed water level, the vent closes and pressurisation commences on a smaller volume of air.
This can result in lower tower temperatures and, depending on how pressurisation is achieved, reduced air consumption. For escape towers situated in engine rooms there is a benefit in the tower temperature not exceeding the auto-ignition temperature of diesel and lubricant oils.
The flooding phase is distinct from the pressurisation phase and has different control requirements. Flooding does not cause physiological stress to the escapee and so should happen as quickly as possible, within sensible limits.
‘Unvented’ escape systems eliminate the flood phase entirely and therefore reduce the escape evolution time, but at the expense of elevated temperatures. Unvented escape can be considered as equivalent to having the vent at the very bottom of the tower. At the other extreme, the vent could be at the top of the tower leaving a tiny bubble of air to pressurise. However, it is difficult to control the pressurisation rate of very small air volumes as they are more sensitive to changes in air and water supply. The position of the vent could lie anywhere between these two extremes.
With the vent shut, pressurisation begins. The air can be pressurised either by flooding the tower with water, thus reducing the volume of the air bubble, or by adding extra air, thus increasing the mass of air. As air is a limited resource but sea water is plentiful, there is a clear
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects preference to use water. In practise, pressurisation
results from a combination of both air and water as it is necessary for air supplied for breathing and hood inflation to vent out into the tower. It is therefore important that the air and water supply are regulated to act in harmony. However, typically at low pressures (for example, at shallow depths or early in the pressurisation phase) it is the addition of air that is the dominant pressurisation method. This is an inefficient use of air.
2.2 (c) Escape
The top hatch is sprung loaded and opens when the net pressure load acting upon it approaches zero, due to the equalising pressure. Once the escapee exits the tower the outer hatch is then closed, via a mechanical linkage from within the submarine. To minimise ‘bottom time’ (time at maximum pressure) it is important that the hatch opens quickly and that a clear, unobstructed path is presented to the escapee. In the case of a multi-person tower it is imperative that the order of escape is understood, to prevent bottlenecking or unnecessary delays.
2.2 (d) Drain Tower
With the escapee now ascending to the surface the upper hatch is shut and the tower drained of water as rapidly as possible. If the system is a vented system the vent valve should be opened to prevent a vacuum forming in the tower, which would increase the drain time.
Lower hatches that seal against the inner surface of the tower can be difficult to raise against the head of water and hence require the water to be drained through a drain valve, with the hatch shut. An outward opening hatch can be opened with the tower filled with water, allowing faster drain times, but care must be taken when releasing a tower’s worth of water into the occupied escape compartment.
2.3 ISSUES
A typical tower escape system suffers from a number of issues.
2.3 (a) Lack of Control
Many tower escape systems have very limited control over the water ingress rate, which affects both the flood and pressurisation phases. Water enters through a fixed diameter orifice and flow rate is then simply a function of submarine depth.
2.3 (b) Depth Limited Escape
At one specific depth, usually co-incident with the continental shelf, this results in a pressurisation rate that approximates the optimum doubling every four seconds rate. At any deeper depth the rate of pressurisation is too fast and so the system can not operate safely. This
means the system fails to operate before the limits of human capability are reached and whilst the submarine structure is still sound.
2.3 (c) Shallow Water Capability
At depths shallower than the critical depth, the tower floods and pressurises unnecessarily slowly. This prolongs the escapees time under pressure, thereby increasing the risk of DCS. Air consumption is also increased. It is believed that some classes of submarine have insufficient air to supply all escapees in extreme scenarios.
2.3 (d) System Reliability
Submarines have evolved significantly in both size and role since tower escape was first introduced. Many system components have remained relatively unaltered in this time and now operate outside the parameters they were originally designed for. As a result, some components have frequent and costly maintenance programmes and perform inconsistently.
2.3 (e) System Ownership
Typical escape systems comprise of hardware supplied by a number of OEMs, with no single system owner to take overall responsibility. This can lead to delays in identifying the cause of issues and resolving them.
2.3 (f) Unproven System Performance
Escape systems are not generally demonstrated at the whole-system level prior to installation on the submarine.
Frequently they are only been proven to TRL 5 (laboratory testing at component and sub-system level) before undergoing acceptance tests on the submarine, TRL 8. There is therefore an undesirable leap of faith involved in commissioning the system, with associated risk.