A conceptual design of an underwater vehicle

A conceptual design of an underwater vehicle

ARTICLE IN PRESS Ocean Engineering 33 (2006) 2087–2104 www.elsevier.com/locate/oceaneng A conceptual design of an underwater vehicle Carl T.F. Ross...

254KB Sizes 4 Downloads 61 Views

ARTICLE IN PRESS

Ocean Engineering 33 (2006) 2087–2104 www.elsevier.com/locate/oceaneng

A conceptual design of an underwater vehicle Carl T.F. Ross University of Portsmouth, Portsmouth PO1 3DJ, UK Received 27 July 2005; accepted 23 November 2005 Available online 3 March 2006

Abstract The paper presents a conceptual design of an underwater star wars’ system, which will be more difficult to detect by the enemy than a recently proposed ‘surface’ star wars’ system. The paper suggests that for the proposed structures needed for the underwater star wars’ system, the material of construction should be a composite and not a metal, as use of the latter for large deep diving underwater vessels will result in such structures sinking to the bottom of the ocean like stones, due to the fact that they will have no reserve buoyancy. The paper also shows that composites have better sound absorption characteristics, thereby making the underwater structures difficult to detect through sonar equipment. It is proposed that these underwater structures should operate up to a depth of 7.16 miles (11.52 km), as at this depth, all of the oceans’ bottoms can be reached. The author shows that current technology can be used to construct and operate such vessels, but more progress needs to be made with metal matrix and ceramic composites, so that the hulls of underwater missiles and torpedoes can be constructed in these materials. r 2006 Elsevier Ltd. All rights reserved. Keywords: Submarines; Pressure hulls; Composites; Deep oceans; Star wars’

1. Introduction Some three-quarters of the Earth’s surface is covered by water and only about 0.1% of the oceans’, bottoms have been explored. Indeed the surface area of the Tel.: +44 23 9284 8484; fax: +44 23 9284 3082.

E-mail address: [email protected] 0029-8018/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2005.11.005

ARTICLE IN PRESS 2088

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

Earth’s surface covered by water is 10 times larger than the surface area of the moon, and the Earth’s surface covered by water is about three times larger than the Earth’s land area. Thus, an interest in underwater star wars is probably more important than an interest in surface star wars. The average depth of the oceans is somewhere between 4000 and 5000 m and the greatest ocean depth of the oceans is found in the Mariana’s trench, which is some 7.16 miles (11.52 km) deep. This distance is about 30% larger than the height of Mount Everest and this depth has been conquered by man, just once; in 1960 by Auguste Picard! In the case of that particular vessel, the vessel was a thick-walled spherical shell of diameter 6 ft (1.83 m). Even though this vessel was so small, it had no reserve buoyancy and could only achieve reserve buoyancy by attaching the vessel to an overhead float, filled with gasoline. Undersea technology is already used for military purposes, but most large submarines can only dive to a depth of about 1312 ft (400 m). It is the author’s belief that as the maximum depth of the oceans is 7.16 miles (11.52 km), or nearly 29 times the diving depth of a large conventional submarine, the potential of the oceans for military purposes is not being fully exploited. It is the author’s belief that an underwater star wars’ system will prove far superior to a surface star wars’ system. The advantages of using an underwater star wars’ system are as follows:

    

Radar does not work underwater. Heat-seeking missiles do not ‘work’ underwater. Satellite spy cameras for the filming of submarines, operating at depths of 11.52 km, will be ineffective. The surface area of the Earth’s ocean bottoms is about three times larger than the Earth’s land area. The underwater vessels can move around the ocean bottoms without detection more easily than surface ‘vehicles’. Disadvantages of the underwater vessels are as follows:

  

The underwater vessels can be detected by sonar. It will be necessary to supply the underwater vessels with food and other provisions. The discharge of refuse from the underwater vessels can be detected by the enemy.

These disadvantages can, however, be overcome to some extent. For example, to decrease detection by sonar, its hull can be constructed with a material such as ‘S’ glass which has a sound absorption coefficient as high as the material used for acoustic tiles; this idea was presented by the present author (Ross, 1992), when he proposed a conceptual design for a stealth submarine. In the case of the shortage of provisions, these can be overcome by supplying the underwater vessels with provisions (say) every month with the aid of special mini and larger submarines. These supply submarines should have propulsion systems that will allow them to hover above the underwater vessels, so that their hatches can mate and form a watertight seal. Perhaps the propulsion system for these submarines should include

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2089

water jet propulsion. After the underwater vessels have been supplied with their monthly provisions, they can stealthily move away just above the oceans’ bottoms. Similarly, the discharge of refuse can also take place at monthly intervals. The above arguments appear to show that the advantages of operating an underwater star wars’ system clearly outweigh the disadvantages.

2. The design 2.1. Hull form The usual shape of a submarine pressure hull is in the form of a ring-stiffened circular cylinder, blocked by end caps, as shown by Fig. 1. The pressure hull is sometimes surrounded by a hydrodynamic hull, which is in a state of free-flood and is therefore unlikely to fail due to hydrostatic pressure. The advantages of using a circular cylinder are as follows:

   

A ring-stiffened circular cylinder is a good structure to resist the effects of external hydrostatic pressure. Extra space inside the pressure hull can be achieved by making the cylinder longer, so that a circular cylinder of relatively small diameter can have a much larger volume than (say) a sphere of twice the cylinder’s diameter. A circular cylinder is a good hydrodynamic form; better than (say) a spherical form of the same volume. A circular cylindrical shape can be easily docked; better than (say) a spherical shape of the same volume.

Disadvantages of using a circular cylindrical shell for a submarine pressure hull are as follows:



A cylinder has two ends and if crew members are required to move from the forward end to the aft end, or vice versa, it may prove difficult because of congestion.

Fig. 1. Submarine hulls.

ARTICLE IN PRESS 2090

 

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

Hydrostatic and hydrodynamic stability can be a problem for a hull of cylindrical form underwater. The conventional submarine cannot move easily in three dimensions.

For the underwater vessels proposed here, for most of their time underwater, they will be stationary and when they move, they will move slowly. Therefore a very good hydrodynamic form is not a pre-requisite. Thus, to incorporate the advantages of using a circular cylinder, and remove the disadvantage that a cylinder has two ends, it is proposed to manufacture the main hulls of these vessels in the forms, shown in Figs. 2 and 3. The plan views of the main hull need not necessarily be pure ellipses; they can be more oval, like a racetrack, or of similar forms. If these forms are used, the hull can be constructed in sections, which can then be bolted together (Fig. 4), as suggested by NCRE (Smith, 1990) and by the present author for corrugated pressure hulls (Ross, 1992, 2001). The proposed design is similar to the design of an underwater drilling rig, as proposed by the author and his former student (Ross and Laffoley-Lane, 1998), and also to a underwater missile launcher (Ross, 2005). The present structures have a better hydrodynamic form than those previously suggested by the author; this new form may also allow better manoeuvrability than the previous forms. They will also yield much larger internal volumes of the submarines, so that they can store more. If water jets are used for manoeuvring and propulsion, the underwater space stations can move three-dimensionally, like a helicopter, except that they can also move ‘backwards’. Hydrostatically, stability will prove less of a problem, both on the surface and underwater, because of the shape of the space stations. It is suggested that at the centre of the ellipsoids (plan views), either spherical shells of twice the

Fig. 2. Underwater space station 1.

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2091

Fig. 3. Underwater space station 2.

Fig. 4. Method of construction of corrugated pressure hulls.

diameter of the circular cylindrical form are attached, as shown in Fig. 2, or another larger circular cylinder, as shown in Fig. 3, is attached via walkways. The main purpose of the designs of Figs. 2 and 3 is to give these vessels sufficient volume, so that they can be used as storage devices to supply friendly submarines. Thus, in

ARTICLE IN PRESS 2092

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

time of a global war, the friendly submarines can be resupplied without having to return to a land base. The supply vessels can store consumables of every description. Additionally the supply vessels will contain a hospital, so that sick or injured personnel can be attended to without returning to a land base. In the case of the design of Fig. 3, the large central circular cylinder can be used as a dry dock to repair damaged friendly submarines. This will put them back into action quickly and avoid the danger of returning to the land base; remember Pearl Harbour! Land bases can be more easily targeted today than they were in the days of Pearl Harbour, as mankind now has better radar and satellite spy cameras, and missiles than in those days, including heat-seeking missiles! The above design appears to indicate that if underwater star wars’ system is used in preference to a surface star wars’ system, we are in a ‘win-win’ situation! 2.2. Manpower and living conditions It is suggested that the required manpower should be about 200 personnel; this is slightly more than the number that are currently used to operate a large military submarine. Since, in peace times, personnel in the underwater space stations will be inside them for about 3 months at a time, it will be advisable to give each individual a reasonable amount of space and a good headroom allowance. In current ocean vessels, the average volume allowed per person is about 5 m3. Since personnel on the vessel are required to carry out their work without it causing them any undue stress, a minimum volume of 10 m3 is proposed. This gives a total living quarter requirement of 2000 m3. Canteen and recreation facilities are also required and it is proposed that these are 3000 m3. It is proposed that the hospital space should be about 1000 m3; this makes a total of manpower space requirements of 6000 m3. 2.3. Power requirements It is suggested that a maximum power rating of 30 MW should be more than adequate to support life and allow the vessel to be operable. The power will be required at two fundamental levels; a normal high power level (30 MW) and at an emergency level (several kilowatts), in the event of failure in the primary power system. The selection of the power system will not only be determined by the power level, but also by the endurance time and in the case of this vessel, it will have to be in the region of several years. For the main power requirements, the existing generating sets used on surface vessels will be unsuitable, partly because they require large quantities of oxidiser and partly because their exhaust disposal facilities will leave a trace for the enemy to detect. The only suitable source of power generation for this vessel is nuclear power. There are several different types of nuclear generators and these include the following:

 

Radioisotopic generators. Pressurised water reactors (PWR).

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

  

2093

Boiling water reactors. Liquid metal fast reactors. Thermal system.

Each of these systems have their advantages and disadvantages, but the most suitable reactors are radioisotopic generators, PWRs and liquid metal reactors, since these have the smallest cores. Radioisotopic generators are small, but they will have difficulty in generating 30 MW of power. Although liquid metal reactors have the smallest cores, they need to keep the metal molten at all times, even during periods of shutdown. This renders them hazardous, and because of this, they will be unsuitable to power the new underwater vessels. This leaves the PWR as the most suitable for powering the vessel, as it can generate the power, is small, and has been proven safe for submarine usage. Additionally, suitable designs are readily available. A suitable PWR, in terms of size and weight (Haux, 1981), including generating sets, etc., is as follows: size ¼ 3000 m3 . weight ¼ 2000 tonnes. 2.3.1. Emergency power supply This must be a non-atmospheric system which is normally independent of the main supply. It must be sufficiently large to run emergency life support systems, lighting, rescue and escape operations. Additionally, it must be able to operate some control systems to allow sufficient time for the crew to survive and be rescued, or for the main generator to be repaired. Due to the fact that the ambient water temperature at 11,520 m is likely to be between about 0 and 2 1C, there will also be a heating requirement and this may lead to a significant power demand. The emergency power level must be such that it will make the vessel safe in an emergency situation. For example, in addition to providing lighting, etc., the emergency power level must be sufficient to blow out the ballast tanks at a depth of 11.52 km, so that the vessel may return to the surface. The power level for an emergency level will probably be about 60 kW for a period of 5–7 days; this can be met by using a large number of batteries, similar to those used on conventional submarines. It must also be remembered that rechargeable batteries will give off hazardous gases and this must be accounted for as well. 2.4. Environmental control and life support systems The atmosphere and other factors, such as noise and vibration, within the subsea system must be considered, so that no physiological or psychological performance degradation occurs. Noise should prove less of a problem than a conventional vessel, because in the present case, a good sound absorbing material such as ‘S’ glass is likely to be used. Although under emergency conditions, limits can be set to which personnel are exposed for short periods to these unwanted conditions, without suffering any adverse effects.

ARTICLE IN PRESS 2094

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

Environmental control systems are required to sustain a breathable atmosphere and to maintain the internal climate within a ‘comfort zone’. Logistic support by specialist mini and larger submarines will be required for provision of food, goods, etc., and for the transfer of personnel. The expected crew change will be around 3 months and it is proposed that supplies will come every month. 2.4.1. Atmospheric control The critical aspects of atmosphere are oxygen supply, carbon dioxide removal and trace containment control with atmospheric analysis to ensure the safety of the environment. Oxygen consumption is dependent on work load and dietary balance (Haux, 1981) (see Table 1). This equates to an average of 30 l of oxygen per hour per man; this figure is based on extensive data collected from submarines. Conversely, this results in the generation of 25 l of CO2 per hour per man, and this carbon dioxide must be removed from the atmosphere and the oxygen replaced. Several oxygen replacement systems may be considered and these include the following:

   

High pressure (gas) oxygen storage. Liquid (cryogenic) oxygen storage. Electrolytic oxygen generation. Chemical oxygen sources.

The use of electrolytic oxygen generators from water is probably the best method, since a supply of water is not a problem and it is a well proven technology resulting in high reliability (Haux, 1981). This system does not have the resupply problems of other systems, such as in high pressure or liquid oxygen storage nor does it have the safety and operational problems. The only drawback of electrolytic oxygen generators is that they require high electrical power, but since we have a nuclear reactor on board, we do not have this problem. In the event of an emergency, it is suggested that an emergency back-up system of bottled oxygen is kept on board. 2.4.2. Carbon dioxide control The air we breathe contains about 0.03% of carbon dioxide (equivalent to a partial pressure of about 30 Pa) (Haux, 1981). Such a level will be difficult to maintain and the required effort to so do, will not be justified. If the level of CO2 Table 1 Oxygen consumption Oxygen consumption

Ideal

Normal maximum

Normal minimum

kg/man day

0.9

1.6

0.5

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2095

reaches 4%, the atmosphere will prove lethal to humans. Therefore, the system should be capable of maintaining the carbon dioxide level well below that which will impair mental and physical performance. This results in the requirement for maintaining a maximum partial pressure for carbon dioxide of 1500 Pa (Haux, 1981), or 1.5% of CO2. There are many systems currently in existence on both spacecraft and submarines and these depend on absorption and adsorption. Such systems include the following:

    

Metallic absorbents. Molecular sieves. Monoethanolamine scrubbers. Bosch reaction. Sabatier reaction.

Metallic absorbents are currently widely in use, but for large manning levels and long submergence times, they become restrictive, although they would be suitable as an emergency back-up system. The Monoethanolamine scrubber is also regenerative and it is currently used in nuclear submarines, although it does require large power requirements and deteriorates with time. The Bosch system can be operated in the Sabatier mode and although it is expensive and complex, it could make an excellent system for a ‘permanent’ system, since it also gives off oxygen. 2.4.3. Contaminant control Since the environment within the new underwater vessels will be sealed, it will be contaminated over a period of time with trace quantities of gaseous and particulate matter emerging from the crew and from the materials and processes within the enclosure. An internal system will therefore be required to control the level of the contaminants, dependent of their type and toxity. Table 2 (Haux, 1981) shows a few possible contaminants and their exposure limit, in an enclosed vessel such as the new underwater vessels. There may be many other contaminants due to operations such as food preparation and submarine refurbishment. Some of the contaminants will be difficult to detect and remove and therefore it is suggested that the structure is partitioned so that the atmosphere from one section does not contaminate another. The only way for the total removal of all the contaminants within the vessel is to purge the vessel from time to time. 2.4.4. Climate of the atmosphere The climate of the enclosure is very important and for crew comfort, it must be set so that it does not induce any physiological stresses into the crew. In normal ambient conditions, it is generally accepted that the temperature should be between 18 and 22 1C. Similarly, a relative humidity of between 50% and 65% is pleasant (Haux, 1981). It is therefore proposed that the temperature and humidity in the vessel

ARTICLE IN PRESS 2096

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

Table 2 Typical contaminant exposure limits Substance

8-h weighted average limit (ppm)

Ammonia Carbon dioxide Carbon monoxide Freon-12 Hydrogen chloride Hydrogen fluoride Mercury Nitric acid Nitrogen dioxide Oil mist Ozone Phosgene Stibene Sulphur dioxide

50 5000 50 1000

Ceiling concentration (ppm)

5 3 0.1 mg/m3 25 5 5 mg/m3 0.1 0.1 0.1 5

should be maintained at these levels. Due to the heat generated by all the process equipment, there will be a requirement for a suitable air condition system. It is also a good idea for the crew to control the ‘local’ climate in their cabins, etc.

3. External requirements 3.1. Support legs The main external requirement of the structure is that of a system of legs or base to position the structure in a horizontal position on the seabed. Additionally, it will be ideal if the maximum length of the adjustable legs is such that they are sufficient to keep the submarine’s hull above the mud line. Therefore, any system developed here must be able to take account of the state of the seabed. If the seabed is not flat and horizontal, it will definitely be necessary to have adjustable legs. 3.2. Other external requirements There are many other external requirements that are needed for the underwater vessels; these include:

    

sonar system; lighting cameras; remote operated vehicles (ROVs); docking system; escape system. This list is by no means complete and it will need further investigation.

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2097

3.3. Size of the elliptical structure From Fig. 2, it can be seen that the structure consists of an outer elliptical structure and several inner spheres, these being joined by connecting tunnels. It is suggested that the internal diameter of the cross-section of the elliptical structure should be 10 m. It is suggested that the diameters of the inner spheres are about 20 m; this will correspond to a similar strength as the circular cylinder of similar thickness. This structure should be dockable at many ports. The cross-section of the elliptical structure can be separated by three levels; each about 3.33 m apart, on average. The outer major axis of the elliptical structure should be about 100 m in length and the outer minor axis should be about 50 m in width. The internal required volume of the elliptical structure that we will be seeking is likely to be in the region of 18,000 cubic metres. 3.4. Central spherical shells Based on the structural strength of thick-walled pressure vessels (Case et al., 1999; Ross, 1999, 2001) and assuming that the wall thickness of the space station’s circular cylindrical shell is to be the same as that of the sphere, then the allowable internal diameter of the sphere can be approximately 20 m. Such a sphere will yield an internal volume of approximately 4190 cubic metres. If a sphere of this diameter is likely to cause a docking problem, then spheres of smaller diameters can be used. Naturally, of course, spheres of smaller diameters can have smaller wall thicknesses. To protect the underwater space station, torpedoes will be required. It is suggested that these torpedoes can be launched at any angle, so that they can strike a ship or submarine above, in the same way as surface launched missiles are aimed at aircraft and rockets. It must be emphasised that as the missiles and torpedoes are being launched from great depths, they must be stiffened by rings or corrugations (Ross, 2001) to prevent their hulls from collapsing. Additionally, as these missiles have to be as light as possible, it will be necessary to construct their pressure hulls from composites, as these materials have better strength: weight ratios than metals and low weight is a high premium for missiles. Perhaps, as metal matrix composites (MMC) have a strength: weight ratio some 30 times greater than high strength steels, they can be used for the hulls of torpedoes and missiles. Ceramic composites may also prove suitable. It should be emphasised that if it is required to destroy a building of the enemy, then simply by typing in the Zip Code of the building into the missile’s internal computer, the building can be ‘taken out’ from the oceans’ bottoms. 3.5. Connecting walkways It is suggested that there should be many inter-connecting walkways. From the calculations regarding volumes, there appears to be enough space in the main structure of the underwater space station for the spheres and the peripheral tubing to house all the equipment and storage goods, and because of this, the walkways need only be a means of connecting the various components of the structure together. The

ARTICLE IN PRESS 2098

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

walkways will simply be passageways for personnel and for moving equipment; they may also be used for exercise. It is suggested that these walkways should be of internal diameter 7 m. 4. Material property requirements Since the structure is to be designed for use up to depths of 11,520 m, then the successful development of such a system will depend on the availability of suitable materials of construction. From previous work on submarines, it is already known that advanced materials with diverse properties will be required. Whereas composites first come to mind, complex alloys may also be used. The materials for underwater pressure vessels must not only be capable of withstanding very high external pressures, but must also have other suitable properties that can withstand the environment. Some of the required properties are as follows:

       

good resistance to corrosion; high strength:weight ratio: if the wall thickness is too large, the vessel will sink like a stone; good sound absorption qualities; material costs; fabrication properties: can the vessel be manufactured ‘easily’ in the chosen material? pressure hull design? susceptibility to temperature: fire protection? operating life span of the material.

Unfortunately, like most projects, there is not one material that is best for all the above requirements. 4.1. Choice of material The main materials for the design of submarine pressure hulls are:

   

high strength steels; aluminium alloys; titanium alloys; composites.

4.1.1. General corrosion In the marine environment, corrosion has been extensively studied and a lot of data generated regarding corrosion rates. Hence, it is relatively easy to predict and to compensate for. The attack of submerged surfaces is governed principally by the rate of diffusion of oxygen through layers of rust and marine organisms. With reference to steel, this

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2099

amounts to a loss of between 3 and 6 mm per year; it is substantially independent of water temperature and tidal velocity, except that industrial pollution leads to higher rates of corrosion. Certain marine organisms can also generate concentrated cell and sulphur effects (Haux, 1981). 4.1.2. Stress corrosion cracking This is a form of localised failure, which is more severe under the combined action of stress and corrosion than would be expected if the two individual effects were added together. There are many variables affecting the instigation of stress corrosion cracking and amongst these are alloy composition, tensile stress (internal or applied), corrosive environment, temperature and time. There are methods of relieving the internal stress and it is possible to solve the susceptibility of materials to stress corrosion cracking by using fracture mechanics. Thus, although this is a problem, it is one that can be reasonably well predicted. 4.1.3. Other factors These include:

  

brittle fracture; fatigue fracture; problems induced through fabrication: for example, stresses induced by welding together with the detrimental effects of heat-affected zones, etc.

4.1.4. High strength steels Table 3 shows the properties of some high strength steels that are popular with submarine construction. HY80 is the most commonly used of the high strength steels shown in Table 3; it is also commonly used for commercial applications including pressure vessels, storage tanks and merchant ships. 4.1.5. Aluminium alloys From Table 4, it can be seen that aluminium alloys have a better strength:weight ratio than high strength steels. Aluminium alloys are attractive as a construction material because of their availability, low cost and fabricability, apart from their

Table 3 Strength of high strength steels Material

Specific density

Young’s modulus (GPa)

Compressive yield strength (MPa)

Heat treatment

HY80 HY100 HY130 HY180

7.8 7.8 7.8 7.8

207 207 207 207

550 690 890 1240

Q Q Q Q

& & & &

T T T T

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2100

high strength:weight ratios. They have the disadvantage of being anodic to most other structural alloys and are therefore vulnerable to corrosion when used in mixed structures, however, these problems can be avoided by special design modifications (SNAME, 1990). It is also difficult or impossible to obtain matching strength in weld metal and base metal and it is therefore necessary for the welds to be thicker than the surrounding base metal or for the welds to be located in light stress areas. 4.1.6. Titanium alloys From Table 5, it can be seen that titanium alloys have an even greater strength:weight ratio that aluminium alloys. As titanium alloys have such a large strength:weight ratio, they are an ideal material to be used for the pressure hulls of large submarines. Their big disadvantage is that there are very expensive; they are about 5.5 times more expensive than aluminium alloys. 4.1.7. Composites Table 6 shows the strength and relative costs of various composites. The most commonly used composite for marine structures such as ships, etc., is glass-fibre reinforced plastic (GFRP) based. The reason for this is partly that GFRP has a very high strength:weight ratio and its cost is relatively small when compared with other composites MMC have many advantages over GFRP’s and carbon fibre reinforced composites (CFRP), but at the moment they are still in the development stages and their costs are very high. If a structure is likely to suffer from structural

Table 4 Strength of aluminium alloys Material

Specific density

Tensile strength (MPa)

Proof stress 0.2% (MPa)

5086-H1116 6061-T6 7075-T6 7075-T73 L65

2.8 2.8 2.9 2.9 2.8

290 310 572 434 —

207 276 503 400 390

Table 5 Strength of titanium alloys Material

Specific density

UTS (MPa)

Yield strength (MPa)

6-4 alloy (annealed) 6-2-1-1 alloy 6-4 STOA alloy CP Grade 2

4.5 4.5 4.5 4.5

896 869 870 345

827 724 830 276

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2101

Table 6 Strength and relative costs of composites Material

Specific density

Fibre volume fraction

Tensile modulus (GPa)

Compressive strength (MPa)

Relative cost

GFRP (Epoxy/S-glass unidirectional) GFRP (Epoxy/S-glass filament wound) CFRP (Epoxy/HS unidirectional) CFRP (Epoxy/HS filament wound) MMC (6061 Al/SiC fibreUD) MMC (6061 AL/Alumina fibre UD)

2.1

0.67

65

1200

1

2.1

0.67

50

1000

3.2

1.7

0.67

210

1200

3.0

1.7

0.67

170

1000

5.1

2.7

0.5

140

3000

11

3.1

0.5

190

3100

15

buckling, it is better to use CFRP’s than GFRPs, because the former has a much higher tensile modulus than the latter. However, CFRP’s are expensive. The cost of HY80 steel per unit weight is in the same region as ‘S’ glass, but as its density is much greater than that of ‘S’ glass, you will get a smaller volume of it than ‘S’ glass for the same price! 4.2. Pressure hull designs The general shape of the structure is a circular section cylinder, which is elliptical in plan view, surrounding several spheres, the various structures being connected by inter-connecting circular cylindrical walkways. As the structure is of large diameter and it is intended to dive deep, it will be necessary to make the wall thicknesses very large. This will mean that the vessel will not suffer structural buckling. Hence, it will not be necessary to ring stiffen or corrugate the vessel. The vessel will be unstiffened. 4.3. Required wall thicknesses 4.3.1. Wall thickness calculations Since the wall thicknesses are large in comparison with the diameters, it will be sufficiently precise to calculate the wall thicknesses of the outer circular cylindrical section, which is elliptical in plan view and the walkways’ by the Lame’ line (Case et al., 1999; Ross, 1999, 2001). Similarly the wall thickness of the spherical shells can be calculated by standard thick shell theory (Ross, 1999, 2001). According to these theories, the calculated wall thicknesses of the circular cylindrical shell of the elliptical structure are given in Table 7 for high-strength steel, aluminium alloy, titanium alloy and GFRP, together with the weight/unit length (W) and the buoyancy/unit length (B), where the weight/unit length does not include the

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2102

Table 7 Wall thickness (t) of the circular section of the elliptical structure External diameter (m)

Wall thick (t) (m)

W (kg/m)

B (kg/m)

550 503

14.6 15.2

2.301 2.6

0.7E6 0.27E6

0.17E6 0.19E6

4.5

830

13.78

1.39

0.22E6

0.15E6

2.1

1200

11.8

0.91

0.066E6

0.112E6

Material

Specific density

HY80 steel Aluminium alloy 7075-T6 Titanium alloy 6-4 STOA GFRP composite Epoxy/S-glass

7.86 2.9

‘Yield’ strength (MPa)

Table 8 Some sound absorption coefficients Material

500 Hz

2000 Hz

Acoustic tiles on solid wall Glass fibre 50 mm resin bonded Marble on solid backing Water, as in swimming pool

0.85 0.70 0.01 0.01

0.65 0.75 0.02 0.02

weight/unit length of the equipment, goods, etc. From Table 7, it can be seen that the only material that possesses reserve buoyancy is ‘S’ glass. And that if any of the other materials are used the vessel will sink like a stone, as their strength:weight ratios are much too low to be used for this vessel. The wall thickness for the sphere is not given, because if the internal diameter is twice that of the circular cylindrical shell, its wall thickness will be of the same order as the cylindrical section. The wall thickness of the walkways will be much smaller than that of the elliptical structure. From Table 7, it can be seen that it is virtually impossible to construct this structure in a metal, as the wall thicknesses are much too large. Additionally, even if it were possible to construct the structure in a metal, the structure will have no reserve buoyancy and it will sink like a stone down to the ocean’s bottom. In contrast to these arguments, the structure can be built in GFRP by laying layer upon layer. Construction can be aided by building the structure in smaller components, which will later be bolted together as described in Section 2. Additionally, buoyancy calculations on this structure show that it will have adequate reserve buoyancy, so that by the use of buoyancy tanks, it will be possible to raise and lower the structure in the water. Additionally, GFRP has good sound absorption coefficients so that the vessel will be difficult to detect by the enemy and make the noise levels within the vessel tolerable (see Table 8). It should be emphasised that it is possible to use CFRP, but this was not considered as its cost was some five times more than GFRP.

ARTICLE IN PRESS C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

2103

From Table 8, it can be seen that glass fibre has a sound absorption coefficient as good as an acoustic tile.

5. Conclusions Star wars underwater should prove a more formidable form of defence than star wars on the surface, as the latter system can be detected by radar and satellite spy cameras. Additionally, surface structures can be attacked by missiles, including heatseeking missiles. The hulls of currently available missiles and torpedoes will crush, due to the water pressure, at comparatively shallow depths of water; this will enhance the case for ‘star wars underwater.’ Previous design studies carried in the 1960s and 1970s show that the present concept can be built with present day technology, except for the hulls of the missiles and torpedoes, as more advances are needed in metal-matrix and ceramic composites to construct the hulls of torpedoes and missiles from these materials. Problems may occur with the slow build up of contaminants in the atmosphere and from time to time the vessel may need purging. This will be lessened to some extent, as the crew will probably work in two or three rota shifts. Considerations must be made so that the crew does not suffer from physiological and psychological problems. Outside support of the vehicle from specialist mini and other submarines will not be easy. These submarines should have the capability of hovering above the underwater vessels, so that their hatches can engage. This hovering facility can be achieved through water jet propulsion. The use of universally adopted hatch covers for submarines and submersibles should be given much consideration to aid rescue missions. However, such vessels should prove suitable for defence purposes, as the enemy will find the vessels very difficult to trace, as their signatures will be miniscule. The plan view of the main hull need not be an elliptical structure, but can be of oval shape, like a racetrack, or of similar form. The use of nuclear power to produce electricity for the vessel should prove quite satisfactory. Emergency battery power should also be available. If a GFRP composite is used, the vessel will have sufficient reserve buoyancy to be raised and lowered in the water. Additionally, the good sound absorption qualities of a GFRP composite will make the vessel very difficult to detect by the enemy and should also make noise levels inside the vessel tolerable. In general, GFRP will not corrode in salt water. The building of these new underwater vessels will not be easy.

Acknowledgements The author would like to thank his colleagues, namely, Terry Johns and Grant Waterman for their contributions to this paper.

ARTICLE IN PRESS 2104

C.T.F. Ross / Ocean Engineering 33 (2006) 2087–2104

References Case, J., Lord A.E.H. Chilver, Ross, C.T.F., 1999. Strength of Materials and Structures. ButterworthHeinemann, Oxford, UK. Haux, G., 1981. Subsea Manned Engineering. Balliere Tindall. Ross, C.T.F., 1992. The silent submarine, Inaugural lecture, University of Portsmouth, Portsmouth, UK. Also from the web site with the following URL: http://www.mech.port.ac.uk/CTFR/ Ross, C.T.F., 1999. Mechanics of Solids. Horwood Publishing Ltd., Chichester, UK. Ross, C.T.F., 2001. Pressure Vessels : External Pressure Technology. Horwood Publishing Ltd., Chichester, UK. Ross, C.T.F., 2005. A conceptual design of an underwater missile launcher. Ocean Engineering January (1), 85–99. Ross, C.T.F., Laffoley-Lane, G., 1998. A conceptual design of an underwater drilling rig. SNAME Journal of Marine Technology 35, 99–113. SNAME, 1990. Submersible vehicles design, USA. Smith, C.S., 1990. Design of Marine Structures in Composite Materials. Elsevier Science Publishers Ltd., UK.