SYSTEM DEVELOPMENT - DEVELOPMENT OF AN INTEGRATED SUBMARINE ESCAPE SYSTEM

DEVELOPMENT OF AN INTEGRATED SUBMARINE ESCAPE SYSTEM

3. SYSTEM DEVELOPMENT

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

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.

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

© 2011: The Royal Institution of Naval Architects On a typical tower, the majority of the escape equipment is mounted within the tower itself. Due to the confined nature of the tower, it is difficult to both inspect and maintain the equipment. Internal mounting also means that the equipment needs to be capable of withstanding the pressures applied during tower evolutions and the potentially corrosive environment present if the tower isn’t kept sufficiently moisture free.

The alternative to this configuration is to mount the equipment on the outside of the tower, wherever possible. This also removes many of the potential snagging points that could hamper an escapee or tear the escape suit. An additional benefit is that the diameter of the tower, and hence floodable volume, can be reduced without compromising the usable space within the tower.

This reduces flood times.

3.1 (b) Tower Geometry

A tower should be tall enough to allow escapees to stand upright with legs and back straight. A hunched position puts strain on the lungs, making breathing more difficult, especially as tower pressure increases. A tower that enables an upright posture therefore reduces the risk of harm.

Tower volume, and hence diameter, should be minimised to reduce flood up times and air consumption. A tower cylinder with varying diameter can reduce tower volume while still enabling adequate hatch sizes and space for equipment. Water bags or syntactic foam can be used to fill void spaces within the tower, if required.

Figure 3, shows an example of a reduced volume tower with full standing room and a clear, unobstructed trunk through the tower.

Figure 3: Profiled Tower Design

3.1 (c) Tower Capacity

The majority of escape towers on small submarines are configured for a single escapee, while larger submarines sometimes utilise towers that are designed for two escapees per cycle. To allow both escapees to stand within the tower, the diameter and consequently the volume of the ‘two man’ tower is larger than that of the

‘single man’ tower. Typical ‘two man’ towers have an unnecessarily large diameter that could, and should, be much reduced.

A spatial analysis of a reduced diameter ‘two man’ tower shows that it still does not provide a particularly efficient use of space, as shown in Figure 4. This results in the floodable volume per escapee being increased, which in turn extends the escape cycle time and exacerbates the pressurisation affects associated with the vent phase.

Figure 4: Tower Volume Required Per Escapee

By marginally increasing the diameter of the ‘two man’

tower, it would be possible to allow a third escapee to be accommodated. This results in a much higher packing density and would allow three men to escape in an almost identical time to two men. This would therefore provide a significant reduction in the crew’s overall escape time, approximately a 30% saving.

The third escapee would be at ‘bottom pressure’ for a longer period than in a two man tower. However, a conservative assumption of a four second escape time per person would increase the risk of DCS by just 0.7% at 180m for the third person [1]. This is considered acceptable, especially when combined with the reduced overall escape time.

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

The flood phase would ideally be completed as quickly as sensibly possible. The flood rate can be assumed to be determined by the flood orifice size and the depth pressure as prescribed by

K ) P P ( A 2

Q ^depth ^ch

(2)

The flood time will therefore increase as the depth pressure decreases - unless the orifice area is able to compensate. A typical tower utilises the same fixed orifice for both the flood and pressurisation phases, the size of which is determined in order to control the pressurisation rate at a depth of 180m. Therefore, at shallow depths the flood time is prolonged while at greater depths flooding will occur rapidly as shown in Figure 5.

Figure 5: Variation of Flood Time With Depth

The flood rates that occur at shallow depths mean that this phase represents a large proportion of the escape cycle time, during which the crew will be consuming valuable HP air resource. Any time reduction in this phase would result in a significant reduction in overall air consumption.

To achieve a reduction, a dedicated Depth Compensating Flood Control Valve can be used to ensure that the flood time is consistent across the entire depth range. Such a valve could, in principle, be manually or automatically operated with the flood time only being limited by the maximum allowable pipe dimensions. A mechanically operated automatic system is considered to offer the best combination of ease of use, reliability and safety, as it requires no human intervention or external power source.

Provided the Depth Compensating Flood Control Valve is capable of opening to approximately 3 times the size of the typical orifice then the flood time could be configured so that it is equivalent to that currently only achieved at 180m across the depth range. This would result in a significant reduction in flood time over the

current 0-180m operational range, with even greater savings becoming achievable if the maximum orifice sixe were to be increased further.

Alternatively, the depth compensated pressure control valve (as described in section 3.3) could provide the flood functionality. As shown in Figure 5 this would result in an even greater saving in flood time at shallow depths but would not achieve the same performance beyond 180m.

3.3 PRESSURISATION 3.3 (a) Typical Tower Performance

To investigate the factors affecting a tower’s pressurisation characteristics a mathematical model was developed from first principles. The model was then validated against test data obtained from an in-service escape tower. The correlation between the results showed that the model provided an excellent description of the towers performance across its operational envelope.

Using the model it was possible to demonstrate that the pressurisation rate of a typical escape tower will vary both throughout the pressurisation cycle and across different operational depths as shown in Figure 6.

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time (s)

Time to double (s)

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Depth (m)

Figure 6: Variation of Pressure Doubling Time With Depth For A Typical Fixed Orifice System Figure 6 shows that the pressure doubling time at depths less than 180m is greater than the 4 second limit. This would cause the escapee to spend unnecessary time at pressure and the escape cycle time will also be increased.

Conversely, at depths greater than 180 the doubling time is less the 4 second limit and would consequently unacceptably increase the risk of barotrauma. It is therefore apparent that the only depth at which a typical system achieves satisfactory performance is at 180m where upon the time to double tracks roughly along but not below the 4 second limit. This result correlates with the principle that the typical system has been optimised of escape at this depth.

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

As an ideal system would achieve the 4 second pressure doubling time across the depth range and throughout the pressurisation cycle, the pressurisation control valve would need to take into account both the pressure within the tower and the depth pressure. Babcock currently provides this type of profile controlled valve as part of its submarine weapon launch system to the Royal Navy’s ASTUTE Class, the Royal Canadian Navy’s VICTORIA Class and the Royal Australian Navy COLLINS Class.

Preliminary investigations have shown that it would be eminently possible to modify this proven equipment to achieve the required functionality. However, achieving this complex control within the current system necessitates the use of some boat services such as electrical power. As this cannot be assumed to be available on the DISSUB, unless a dedicated backup system was implemented, efforts are ongoing to establish whether the same performance can be achieved using purely passive mechanical means.

3.3 (c) Depth Compensation

Based on the analysis of the typical tower configuration, it was possible to deduce that for a given depth an optimal pressurisation orifice size could be identified.

Although only able to account for the changes in depth pressure this system would be capable of mimicking the typical system’s 180m performance across the entire depth range.

Figure 7 shows how the orifice size would need to vary with depth relative to the 180m orifice size.

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Depth (m) Valve Size Relative To fixed Orifice At 180m

Figure 7: How The Optimal Orifice Size Varies With Depth

Utilising these orifice parameters within the model resulted in the pressure doubling time graphs shown in Figure 8. This showed that across the entire depth range the doubling times all approach the 4 second limit but crucially do not cross it.

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time (s)

Time to double (s)

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Depth (m)

Figure 8: Variation of Pressure Doubling Time With Depth For A Depth Compensated System As the performance of the pressure compensated system achieves a doubling time of 4 seconds across the entire pressurisation cycle, the smaller the depth compensated system’s fluctuations the closer its performance is to that of its ideal counterpart. By calculating the difference between typical and depth compensated system relative to the pressure compensated system their relative performance was quantified as shown in Figure 9.

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Fixed Orifice Depth Compensated Pressure Compensated

Figure 9: Variation of Pressurisation Efficiency With Depth

This shows that the performance of the depth compensated valve is significantly better than the fixed orifice at depths below 180m. Although performance reduces as the depth increases, it remains in excess of 85% efficient. This is in stark contrast to the typical system which becomes unviable at depths deeper than 180m due to the excessive pressurisation rate.

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

While the correlation of the doubling time to the 4 second limit is a good indicator of a systems overall performance, it does not describe how it will impact the rest of the escape system. To assess this, the pressurisation cycle time relative to the acceptance tolerances within AS301 [3] was examined. However, AS301 merely ensures consistent performance from one tower to another rather than specifying the optimal performance.

Both the depth and pressure compensating system show significant time savings relative to the typical system over the current operational range while equalisation time steadily increases with depth beyond this point.

As air consumption is a concern, particularly for the retrofit market, the resulting air usage for the depth compensating system needs to be less than or equal to that of the typical system for it to be a viable upgrade.

Although the equations governing the supply of air to the escapee were kept constant across the typical, depth compensating and pressure compensating configurations, the air consumption still varies significantly as illustrated by Figure 10.

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Depth Air usage realtive to fixed orifice at 180m

Fixed orifice Depth Compensated Pressure Compensated

Figure 10: Variation of Air Consumption With Depth This shows that the depth compensating system consumes less air than the fixed orifice at all depths below 180m while at depths greater than 180m it appears to increase almost linearly. As with the pressurisation curves, it is not practical to compare the results with the fixed orifice at depths greater than 180m because its pressurisation rate means that the fixed orifice configuration becomes unsafe.

The proximity of the depth compensating system to the ideal pressure compensating system demonstrates that only a small amount of air can be saved by opting for the ideal pressure compensating system.

Calculating the air consumption profiles has shown that the current air settings appear to provide significantly more air than would be strictly necessary to maintain the volume of the escapee’s hood. It also shows that the air is

contributing to both the temperature regulation within the hood and pressurisation of the tower.

Investigations are ongoing to establish why the excess air is being supplied so that if possible, the air supply rate can be reduced. With less air being supplied, preliminary analysis has shown that the pressurisation valve’s orifice properties have to change not only in scale but also in profile. The impact of this change, along with the possibility of incorporating features of the passive pressure compensating system into the depth compensating system are currently being considered in order to achieve better air performance .

3.3 (e) Escape Cycle

To assess the impact upon the entire escape cycle the flood and pressurisation phases must be examined together. The resulting air consumption relative to the typical system at 180m and cycle times for each of the configurations at a selection of depths are shown in Figure 11.

Fixed Orifice

Depth Compensated

Pressure Compensated Depth

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Cycle Time (s)

Air Usage

Cycle Time (s)

Air Usage

Cycle Time (s)

Air Usage

20 144 170% 39 48% 11 16%

60 96 126% 44 60% 17 28%

120 74 109% 54 80% 25 45%

180 63 100% 63 100% 31 61%

240 N/A N/A 71 119% 35 74%

360 N/A N/A 84 153% 46 104%

500 N/A N/A 96 190% 54 133%

Figure 11: Variation of Cycle Time and Air usage With Depth

This shows that both the depth and pressure compensated systems make significant savings in both parameters even though this does not take into account the further savings that would be possible if the following were implemented:

x Reduced HIS air supply rate x Optimised tower geometry x Rapid tower drainage

Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK

3.4 LOWER HATCH

The primary issue associated with the tower’s lower hatch is that the tower has to be almost completely empty before it can be opened. Although this is an inherent safety feature, it does prevent the possibility of partially opening it so as to facilitate rapid drain down times. The manual nature of the hatch is also undesirable, particularly for the ‘last man’ who will have to perform the lifting and positioning operations unassisted.

Several possible alterative hatch arrangements have been considered:

x Removable Shield

x Upward Opening Hinged Hatch x Internal Arcing Hatch

x Downward Opening Hinged Hatch x Vertical Axis Rotating Hatch x Horizontal Sliding Hatch

All but the first two concepts incorporate features to facilitate rapid drain down. A simple scored assessment methodology has been used to identify the advantages and disadvantages associated with each of the solutions.

This is summarised by Figure 12 which shows how each of the concepts represents a trade off between cost and performance.

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Upward Opening Hinged Hatch Downward Opening Hinged Hatch Horizontal Sliding Hatch Internal Arcing Hatch

Vertical Axis Rotating Hatch Removable Shield

Figure 12: Cost, Performance Trade-off of The Lower Hatch Configurations

As Reflected in the scoring, the Internal Arcing Hatch, Rear Door, Rotating Hatch and Sliding Hatch have the potential to provide significant benefits over the other legacy designs. The similarity of their scores also means that it is likely that the final selection will be made based upon the weighting of a given platforms requirements.

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