A Conceptual Design of an Underwater Hotel, namely ‘Hydro-season’
Carl T.F. Ross & Mark El-Hajj
(Department of Mechanical & Design Engineering, University of Portsmouth, Portsmouth, United Kingdom)
The paper presents a conceptual design in Pro-Engineer of an underwater hotel off the coast of Cyprus. This area was chosen for two reasons, namely because it is a beautiful holiday island with a pleasant climate and also because it is the country of origin of the junior author. The paper reports on three other underwater hotels, one of which is currently operable and two of which are being built. The underwater hotel which is currently operating is called “Jules” and the other two which are being built are called “Hydro-polis” and “Poisedon”. Jules is situated in Key Largo, Florida, it is an underwater lodge with overnight accommodation. The other two hotels, which are under construction, are being built in Dubai (Hydro-polis) and Fiji (Poseidon).
The hotel of the current project is called “Hydro-season”; its circular shape is ideal for housing three different types of customers in a comfortable interior of “one atmosphere” internal pressure; also providing easy access and a relatively low submerged depth. One group of customers’ rooms will be totally submerged, while a second group of customers’ rooms will be partially submerged. The third group’s rooms will be above water level.
It is suggested that the entire structure is made buoyant with detachable rooms, and supported by adjustable legs in order to ease maintenance and for relocating and emergency procedures.
Keywords: Underwater hotel, Cyprus, pressure vessel, PMMA, acrylic, FGRP, Pro-Engineer.
As the wealth of people is increasing, together with their leisure time, more and more people are searching for an unusual holiday. For example in the UK, in the recent past, many holidaymakers were content with taking their holidays in the ‘Mediterranean’, but as the wealth of Britons has grown they have become more and more adventurous with their chosen holiday destinations. Destinations such as the Far East, together with the west coast of the USA and Canada and also Latin America are quite common holiday destinations. Thus, what could be more adventurous than taking a comfortable holiday underwater in a luxury hotel?
1.2 Benefits of taking a holiday underwater.
2.1 A potential market
If we take a look at the current market, the success of a similar project is seen in Jules , which is currently the world’s only operating underwater hotel; it is situated in Key Largo, Florida and despite being accessible only to trained divers, it is still booked months in advance! Having only three well-established underwater hotel projects (Jules, Hydro-polis and Poseidon) in the world, it is reasonable to say that the present design itself can be considered as a relatively new concept with respect to the current generation. This concept being relatively new, provides an advantage, especially knowing that most individuals are looking for something new and exciting to experience, and what is better in offering such a sensation than an underwater hotel? Nevertheless, since it is the only underwater hotel with respect to its region, namely Cyprus, the project itself has no local competition. Therefore it is an opportunity to position this project at the top end of the market with no apparent market threat and at the same time capturing worldwide attention.
Another market that may have an interest in similar structures could be the military world, because it is known that heat-seeking missiles and radar do not ‘work’ underwater.
2.2 Hotel standard
Considering the regional (Cyprus) current market for hotels, the majority of the successful hotels range from 3-5 star, and looking at the current market for underwater hotels, this includes 5 and 10 star hotels; this places the 5 star hotel as an ideal entry to the market and therefore stands a better chance with the local and international competition.
Since the hotel is ranked as a five star one, it should include all the standard facilities that a similar hotel contains. A comparative hotel would be the Poseidon five star underwater hotel , that includes facilities such as a restaurant, a café, bar, swimming pool, sports’ room, meeting lounge, Jacuzzi, spa, shops, etc. Note: addition or subtraction of any extra facilities should be justified.
Since the project itself would be considered as the first of its kind with respect to its location, it would be appropriate that only one construction is to take place; this project can be considered as a test of the current market, and thus based on the level of its success, the decision for conducting similar projects can be made.
Cyprus has more than 300 days of sunshine a year; warm water temperatures ranging from 18 to 28 C or (64 to 82oF) and great underwater visibility ranging from 50 to 130 ft (15.24 to 40 m); together with interesting marine life .
3.2 Underwater Depth of Hotel.
The maximum underwater depth of the hotel was determined as follows;
o The submerged depth of a totally submerged room, was determined by the ceiling height of the room (2.65 m is usually the minimum regional value), as well as the height allowance (a maximum 1.5 m is thought to be appropriate) needed for installing pipes, ventilation, wiring and other similar systems.
o The height for tidal allowance, whereby according to the region, 1m was thought to be appropriate to keep the room submerged at all times.
o The height for mud line allowance, whereby a value of 1m was thought to be appropriate to keep the bottom part away from the corrosive sand.
From the above parameters an approximation to the total submerged depth was obtained, as follows: 2.65 + 1.5 + 1 + 1 = 6.15 m. A maximum 6.15 m submerged depth was said to be sufficient to ensure a clear visible underwater marine life, as this would not have been the case in shallow waters, where waves could cause a water sand mixture and as result of this, a muddy like view.
3.3 Underwater visibility
Underwater visibility is of great importance, since it is one of the elements of attraction that adds value to the hotel. As mentioned earlier the location (Cyprus) is ideal for providing a good underwater visibility ranging from 50 to 130 ft (15.24 to 39.6 m) .
3.4 Marine life
According to research there are proximately 33 types of native sea fish around Cyprus.
3.5 Distance from mainland and site allocation
The greater the distance that the hotel is from the mainland, the longer it would take the customers to reach their destination and this to some extent is not favoured; increasing the distance increases the cost and necessitates for a greater construction. Therefore it would be ideal to allocate a site that can provide the required depth, at a relatively short distance from the mainland.
3.6 Some important seawater properties .
o Density = 1020
o Salinity ~3.5%
o Ph value ranges from 7.5 to 8.4
o The speed of sound in seawater is about 1500 m·s-1, and varies with water temperature and pressure.
o Table 1 shows the composition of seawater:
Total Molar Composition of sea water (Salinity = 35)
Concentration (mol . kg-1)
Table 1 Showing seawater composition
The cost should be as low as possible, but at the same time meeting the customers’ requirements and expectations; this can be achieved with the aid of a well-engineered and efficient design (low manufacturing cost, and a careful material selection).
Since the project relates to a five star hotel, the outcomes, such as relative size (the size needed to accommodate the facilities required in a five star hotel) and the quality, could be similar to that in the Poseidon underwater hotel; the latter’s cost could be used to demonstrate an overall cost distribution for similar projects i.e. Hydro-season.
4.1 Cost breakdown for the Poseidon underwater resort.
= Total cost = $ 40 million .
= Construction cost per unit area = ($19380/m2) .
= Room floor size = 550 = 51 .
= Number of rooms = 20 .
= Frisbee-shaped module floor size (contains other facilities)
= 3000 (279 m2) 
= Number for Frisbee-shaped module = 2 .
=Total size of hotel = = (550 20) + (3000 2) = 17000 (1579 m2) (1)
= Total construction cost = = 17000 1800 = $ 30.6 million (2)
= Total Fittings and equipment costs = - = 40 – 30.6 = $ 9.42 million (3)
Although there can be a great age difference between the customers, all will expect a comfortable environment. For a five star hotel a comfortable environment is defined as follows:
“A comfortable environment is one in which there is a freedom from annoyance and disturbance. The designer has to consider the organisation of space, background noise, lighting and colour schemes, psycho-sociological interactions between the occupants, and the temperatures of air and surfaces as well as the amount and distribution of ventilation and the humidity of air”.
In normal ambient conditions a comfortable situation refers to temperatures between 18 and 22, and relative humidity of between 50 and 65, therefore an appropriate air conditioning system is required to ensure the temperature and lie within these levels.
Excitement should be part of what the hotel offers; it should take into account all age levels. This can be achieved by conducting different activities to which customers can join. Examples of such activities include: swimming courses, diving lessons, games, tours, marine life expeditions, etc.
5.3 The ‘target customers’
Unlike ‘Jules’, the project will be constructed solely alongside the assumption that the hotel itself can host any type of customer with no particular diving or swimming skills or other strenuous requirements. In other words an easy (convenient) access will be provided to a hospitable environment. The ‘target customers’ will be middle to upper class with respect to the current economy. The hotel will have to meet in addition to excitement, the customer’s standard expectations (i.e. what is expected of a five star hotel), such as luxury and services. The project should also consider the possibility of having a number of customers that are interested in the idea of being so close to the sea and the marine life, but unlike some of the others, are not so keen to live under water. Thus, the structure of the hotel should be such that there is a choice of accommodation above sea level, semi submerged and totally submerged accommodation. This approach will enhance the attraction of potential customers.
5.4 Customer transport
Once the maximum depth of the hotel is specified, then depending on the location, the distance of the hotel from the main land can be fixed to ensure customers’ length of their journey to the hotel. The following options should be considered:
5.4.1 Transport via a boat
o Construction wise, this can eliminate the high costs and risks that arise when building any stationary passages (for example bridges and tunnels).
o All types of customers can be transported.
o On windy days this type of transport can turn out to be somewhat uncomfortable for some customers.
o If the wind is immense transport cannot take place.
o Fuel consumption may be significant.
o Emission gases may be a concern when considering environmental aspects.
o In the case of an accident (for example collision) there is a great risk for harmful fluids (such as fuel oil) to be spilled into the sea and cause a thereat to the environment.
o This type of transport can create noise pollution in busy seasons and therefore can disturb customers who seek a relaxing environment.
o In some cases the process can be time consuming.
o The need to hire an expert such as a captain or an experienced sailor.
o Rain and fog can limit visibility, which may be a concern when dealing with safety, and in extreme cases it can also be the cause for delays.
5.4.2 Transport via an underwater tunnel
o Can provide comfort as one can be easily transported from one well known region (land), to a totally different environment (underwater).
o Introductions to the marine life and the foundation of the hotel can be implemented on the way to the underwater resort, where a direct view of the underwater world can be seen.
o The hotel’s construction can be viewed as the customers approach their destination; this can add value to the hotel, as well as a sense of appreciation to the construction itself.
o A fully automated electric train can be implemented in the tunnel, to transfer customers and their goods upon request as well as the transfer of staff and stocks.
o Wind does not pose any major threats.
o Rain and fog do not pose threats and therefore delays.
o Customers can be at great risk, if any collapse in the structure is to occur.
o Construction costs can be great depending on the complexity of the structure.
o There is a great possibility that maintenance costs would be high, unless maintenance is well thought of in the design stage.
5.4.3 Transport via a floating bridge
o Construction wise, this approach can be relatively cheap, and easy to conduct.
o The cost of maintenance can be relatively low and easy to conduct.
o On windy days this type of transport can turn out to be somewhat uncomfortable for customers, as waves might cause a sense of movement within the structure.
o If the wind is immense, transport cannot take place because of safety considerations.
o The construction does not add any value to the hotel, i.e. there is no direct unique scenery, in relation to the underwater life.
o Since the floating bridge would be mobile, it can present a challenge to some customers, for example old and disabled people.
o Rain and fog might be a problem unless the whole bridge is shielded.
5.4.4 Transport via a fixed bridge/above sea level tunnel.
o The cost can be relatively low, if the length is kept to a minimum.
o The bridge can be used by customers for jogging, or for taking a relaxing walk after dinner.
o Unlike a floating bridge, the structure is firmly fixed and therefore there is no sense of movement within the structure that can cause an uncomfortable transportation.
o Electric cars can be used to transport customers at request.
o Heavy rain strong winds and waves might pose a threat to both customers and the bridge itself, unless the bridge is designed to withstand such hazards and is properly sealed.
o There is a great possibility that maintenance costs would be high, unless maintenance is well thought of in the design stage.
5.4.5 The requirements for two passages.
o To ensure transport is viable at all times, especially when one form of transport is not available due to maintenance requirements.
o To prevent overcrowding, which may occur if customers and staff used the same passage.
Looking at the transport options listed above, it can be seen that the two most reliable options, are via an underwater tunnel or a fixed bridge, the latter would be considered as a paramount option with respect to the associated cost with having underwater passages as well as the safety and maintenance aspects, although the only disadvantage is the lack of an underwater view. This can be argued in the sense that customers have enough time for an underwater view during their stay, and therefore it would be unnecessary (unprofitable) to include underwater tunnels.
6.1 General shapes for underwater structures
Since the project is mainly of an underwater construction and it is subjected to an external fluid pressure, the most likely shapes to be considered for the structure are circular cylinders and spheres, which tend to be most efficient (resistance wise) when dealing with such loads . It should be emphasised that although spherically shaped shells are better at withstanding underwater fluid pressure as compared to circular cylindrical shaped pressure vessels (i.e. spherical shells require less wall thickness), the latter can offer much more internal volume by making the cylinder longer. This is important when dealing with storage and even more importantly with the customers’ comfort. Other advantages that circular cylindrical shells have over spherical shells is that they tend to be easier to transport and store . In the case where less material usage is preferred to comfort, spherical shaped shells can come in handy; for example as safety domes.
6.2 Established underwater hotels
6.2.1 Hydro-polis (Building to be commenced in 2007)
A view of this hotel complex is shown in Fig. 1.
Figure 1 shows a section of Hydro-polis hotel
o One atmosphere internal pressure.
o ‘Protected’ with missile-detecting Radar
o Exterior has underwater lights.
o Appears to have more facilities than the other underwater hotels, namely Poseidon and Jules.
o Has more bedrooms than Poseidon and Jules.
o The published nightly rates ($ 1 500) appear to be less than that of Poseidon ($ 2 140), .
o Train to the main area of the hotel transports customers.
o It has important facilities as well as technical centres. Goods’ storage and loading are based on the land station to avoid disturbing the calm and relaxing atmosphere enjoyed by the hotel guests. This also minimises the risk of sea contamination, as well as the cost needed to maintain the required air conditioning.
o No in-room Fish Feeding system.
6.2.2 Jules (Already built & running)
A view of this lodge is shown in Fig. 2.
Figure 2 shows the layout of Jules underwater hotel
o Nightly rates are less than that of Poseidon and Hydro-polis.
o The structure is relatively small, thus the hotel can easily be maintained.
o Staff employment is low, with respect to Poseidon and Hydro-polis.
o The lodge includes independent support systems as well as redundant backup systems.
o The lodge is currently operable.
o No in-room Fish Feeding system.
o The hotel itself, can only offer customers accommodation that is totally submerged in water.
o Accessible to only trained divers and not made suitable for the average person.
o A 42” view-port window in each bedroom is relatively small as compared to Poseidon and Hydro-polis, therefore it can only provide a limited view of the sea.
o The hotel is relatively small as there are only two bedrooms available that can only accommodate 6 people.
o The fact that Jules contains compressed air, mainly to prevent the water from rising and flooding the rooms; it is not one –atmosphere. As a result this might cause decompression sickness.
o Contains the least facilities with respect to the other underwater hotels, namely Poseidon and Hydro-polis.
o The power needed to maintain the pressure can be costly.
6.2.3 Poseidon (Building to commence in 2007)
Figure 3a Shows a general view of Poseidon underwater hotel Figure 3b Shows a top view of Poseidon underwater hotel
Views of this hotel are shown in Figs. 3a & 3b.
o One atmosphere interior pressure, i.e. surface pressure .
o In-room fish feeding system .
o Each room consists of 70 % Acrylic, offering a great viewing angle of 270 degrees .
o Exterior underwater lights can be controlled by guests .
o Customers are transported through two elevators .
o Only five different acrylic moulds are needed to construct the entire complex .
o Each individual room is neutrally buoyant making it possible and easier for being removed from the water and from the complex .
o Automated view-port cleaning systems are used to replace the laborious and expensive need for divers to clean the view-port surfaces of marine growth .
o All important systems are shore-based or inside the complex, where they are easy to maintain .
o Customers in flats positioned nearer to the centre of the corridor are required to walk a longer distance to get to their suites.
o Only two exit doors are available through the corridor.
6.3 Hydro-season’s (Current Design) general shape
The following shape (Fig 4) was obtained by providing a solution to the listed disadvantages of the established underwater hotels especially for Poseidon, as this is a comparative five star hotel. The following design carries in addition to some of the above listed advantages the following:
Figure 4 Shows a top view of the general shape of Hydro-season
o All customers are required to walk relatively the same distance to get to their suites.
o Unlike Poseidon there are more than two exit doors in the corridors.
o Access is made easier due to the circular shape.
o There is less possibility that the corridor might get crowded.
o The design of the underwater hotel, offers customers the flexibility to select what accommodation is suitable for them, e.g. such as totally submerged, semi- submerged or non submerged rooms, and at the same time still being in the sea environment.
o The design also offers the ability for customers to seek fresh air without having the need to exit the entire underwater resort. This is possible as the design allows the top part of the hotel not to be sealed, thereby allowing a direct contact between the inner part of the hotel and the external atmosphere. This is useful, as the opening allows natural ventilation, and therefore less air conditioning (power savings) is required to contain the maximum level of contaminants and increase the percentage contents of natural air.
o The opening also provides a “one atmospheric” internal pressure (i.e. same pressure as onshore) and therefore prevents customers and the members of staff from having decompression sickness.
o Since part of the hotel is above sea level, there is a possibility for full-size waves to strike the upper part of the hotel and cause damage to the structure. This generally can be avoided by building artificial islands around the complex, which act as barriers to such threatening waves.
6.4 Various suggested shapes for suite construction
6.4.1 Shape1 (Spherical shell; see Fig. 5)
Figure 5 Shows a side view of a spherical shaped room
o The most structurally efficient i.e. less wall thickness is required to withstand a given pressure .
o Not particularly suitable for housing
o Offers the minimum amount of space for a given diameter, unlike (say) a circular cylindrical vessel, which can be made long.
o Not suitable for storage
o Not suitable for transport
o High manufacturing cost
o Loss of space, as shown in Figure5
6.4.2 Shape 2 (Circular cylindrical shell blocked of by two hemispherical domes and placed horizontally with respect to the sea bed surface; see Fig. 6)
Figure 6 Shows a side view of a cylindrical shaped room, blocked by two hemispherical domes and placed horizontally
o Can offer a greater amount of space than a spherical shell with the same diameter .
o Less manufacturing costs than the spherical shell.
o Simply making the cylinder longer can increase the space.
o A greater thickness is required than a spherical shell in order to withstand the same pressure .
o Loss of space, as shown in Figure 6
o If the circular cylinder is very long, the buckling resistance could be small .
6.4.3 Shape 3 (Circular cylindrical shell blocked of by two hemispherical domes and placed vertically with respect to the sea bed surface; see Fig. 7).
Figure 7 Shows a side view of a cylindrical shaped room, blocked by two hemispherical domes and placed vertically
o No loss of headspace
o In order to reach the space requirements, the internal radius is made larger than in shape 2, which in turn requires a greater submerged depth.
o Simply making the cylinder longer cannot increase useful space.
By comparing shapes 1, 2 and 3, it can be seen that the advantages of the last two shapes (2 and 3) outweigh shape 1 and therefore only shapes 2 and 3 will be taken into consideration in this paper.
6.4.4 Analysing ‘horizontal’ shape 2 and ‘vertical ‘shape 3
As discussed earlier shape 3 requires a greater submerged depth than shape 2 in order to fulfil its requirements as a totally submerged unit; this greater depth has its disadvantage in the sense that a greater distance from shore is required to supply this increase in depth, and therefore a longer bridge is needed. This is in addition to the fact that a thicker cylindrical wall is needed to withstand the increase in external pressure (assuming that the safety factor is maintained constant), another important disadvantage is that the minimum submerged depth of the hotel is reduced.
A proposed solution could be to have the top end of shape 3 (that is the end closer to the surface) blocked by a circular plate end and the bottom end by a hemispherical dome shell, thus providing a lower submerged depth. The only problem here is that if the unit (room) is required to withstand a tsunami, the wall thickness of the circular plate end will be relatively large.
Although space loss will remain a disadvantage in the horizontally placed shape 2, it can be argued that its advantages outweigh the advantages shape 3, as follows:
o The two hemispherical shell domes can still be used and not affect the submerged depth.
o In the case where more space is required in the future, this can be simply achieved by making the cylindrical part longer; this is considered to be a premium advantage. This advantage cannot be achieved in a shape 3, since making the cylinder longer will simply make the room higher.
6.5 Hydro-season (Current Hotel) size
Since it has been decided that the project should deliver a five star hotel based mainly on a circular shape, the number of rooms in the present design have been made equal to that found in the Poseidon underwater resort. The latter’s size accommodates and provides a guide for all the required features and space requirements needed for such a five star underwater hotel. Thus, the dimensions can be obtained by using the floor space specified for the Poseidon resort. The size of Hydro-season’s cylindrical pod is of similar value to the combined two Frisbee shaped modules (Pods) in the Poseidon underwater resort.
Thus the main dimensions of the parts that form Hydro-season can be obtained as follows:
Hydro-season suite floor space = Poseidon suite floor space = 550 = 51
= Room internal radius = 2.5 m.
A 2.5 m radius ensures that the room has a comfortable shape and size. A smaller diameter will require a longer length to compensate for the loss in floor space, causing the room to be too long and uncomfortable. Although a greater diameter will make the room shorter, the ceiling height would be greater and can somewhat be uncomfortable to the human eye. In other words 2.5 m radius is thought to be ideal for a cylindrically shaped room.
Ideally to provide the maximum floor space, the floor would be placed in such way that it passes through the centre of the cylinder. Doing so will provide approximately a 2.5 m gap at the bottom of the room which is more than enough (a maximum of 1.5 m) for what is needed to install pipes, ventilation, wiring and other similar systems.
Therefore placing the floor through the centre will cause a loss in volume, hence the floor is lowered a magnitude of 1 m from the centre. Although lowering the position of the floor, will increase the room height from the minimum regional value (2.65 m) by approximately 1 m. This would be acceptable, since the room has no windows for natural ventilation and thus a greater height than a normal room with windows would be required to aid in maintaining a good air quality.
Having the floor placed 1m below the centre, the length (L) of the cylinder is 8 m in order to provide the 51 of floor space.
6.5.2 Cylindrical Pod
Poseidon consists of two Frisbee shaped pods containing the hotel’s facilities such as the restaurant and conference room etc. Each having 3 000 (278.7 m2) of floor space, therefore, the Hydro-season cylindrical shaped pod should contain a floor space of 6 000 (1828.8) as this replaces the two pods in Poseidon.
= 1828.8 m2 (4 )
= Pod internal radius = 24 m
The total height of the vessel is around 8 m; this is obtained by including the following factors:
o 2 x 2.65 m representing the height of two floors
o 1 m of mud-line allowance.
o 1 m allowance on each floor needed for installing pipes, ventilation, wiring and other similar systems.
7. Material selection
7.1 Material and design options
Although there exists a great deal of options when dealing with material selection, only the ones that meet the requirements with the lowest costs are to be chosen. This step will lower the total cost. When designing the suites, consideration for ensuring the buoyancy are important, and therefore when selecting the material for the appropriate suite size, it is important to ensure that the weight limit is not exceeded.
Material selection is made easier by considering each part of the hotel separately and at the same time dividing each part into sub-parts, listing their requirements/constraint and functions.
Note: Other than being extremely expensive, metal matrix composites are not considered in the following material selection, as their properties are not fully established at present.
7.2 The required properties for the material used:
o Clear, where appropriate, to allow for a clear view of the surrounding marine life.
o Low creep.
o A good impact resistance.
o A good insulator of noise i.e. Minimise noise transition.
o Lightweight i.e. high strength to weight ratio, to ensure where appropriate a reserve in buoyancy.
o High mechanical strength
o Low moisture absorption
o Neither food retardant nor toxic, especially in places where it is likely that direct contact is made with individuals or the surrounding marine life.
o Fully recyclable, this is important when dealing with the decommissioning part of the hotel.
o Easy to clean and maintain.
o Easy to shape into creative designs i.e. manufacturing is made easier and cheaper.
o Excellent resistance to chemical attack and high corrosion resistance, prolonging the part’s life.
o Low cost.
o Available on request.
It is worth noting that for an existing similar project , the main materials used for construction were an acrylic and a steel, the former was used mainly to provide the required visibility, whereby steel was used for reinforcements (supports), and for flooring .
7.3 Material options for the non-visible areas
Looking at some habitable underwater marine constructions such as submarines, the main materials used for construction are :
7.3.1 High strength steels such as HY80
o Relatively cheap
o Higher yield strength than aluminium alloy.
Steel (metal) is a good electrical and heat conductive substance; this can be a disadvantage in some cases where heat and electric loss is undesired (see sections 8.2.15). These factors can generally be avoided by proper use of insulation followed by routinely inspections, or simply using steel in areas where heat loss and electrical current are insignificant. Note this disadvantage applies also to aluminium and titanium alloys as well.
Steel has a relatively low strength to weight ratio and if not used where appropriate (efficiently), this might prove to be a problem where reserve buoyancy is desired. Example of areas where steel is best used are; ring stiffeners, supports, and frames.
As the structure is submerged in salt water (corrosive substance), and if it is not carefully manufactured at the building stages or has not the use of proper insulation, it can corrode. High strength steels can be most vulnerable to corrosion especially where direct contact with the external environment is made. Corrosion in metal-based materials can take place in the following form or a combination;
o General corrosion such as rust (oxidation) along with the presence of some marine organisms, where by material loss is about 3 to 6 mm per year for steel .
o Galvanic corrosion
o Stress corrosion
7.3.2 Aluminium alloy
o A better strength to weight ratio than steel.
o Low cost and easy fabric-ability.
o Anodic to most other structural alloys
o Difficult to weld, due to the non-matching strength in weld metal and base metal. This results in making the welds thicker than the surrounding base metal, and in most cases welding is only applied in areas where stress is low.
7.3.3 Titanium alloys
o A greater strength to weight ratio than aluminium alloy and steel.
Very expensive (5.5 times more expensive than aluminium alloy)
7.3.4 Composites (the composite considered below is GFRP, as it is more commonly used in marine structures than other form of composites).
o Very high strength to weight ratio.
o Low cost as compared to other composites.
o Although the cost of HY80 per unit weight is similar to that for GFRP, the density of the former is much greater than the latter, thus more volume of GFRP is bought for the same price.
o Good sound absorption.
o Does not corrode in seawater.
The role of the above materials in constructing the underwater hotel is to reinforce the structure and used in places where visibility is not a desired property.
Since cost, weight, heat and electrical loss and maintenance are significant factors in designing the underwater hotel, it can be seen that the advantages of using composites favour the requirements, and therefore composites are considered the ideal materials used for construction.
Some mechanical properties of GFRP are shown in Table 2.
Fibre volume fraction
Tensile modulus (GPa)
Compressive strength (MPa)
GFRP (Epoxy/S-glass unidirectional)
7.4 Material selection for the visible areas
7.4.1 Why use acrylic and not glass for the transparent part of the underwater hotel.
Acrylic is used in preference to glass for the following reasons:
o Acrylic is less dense than glass; acrylic being 1150-1190, glass being 2400-2800 .
o Acrylic has higher impact strength than glass and does not shatter .
o Acrylic has similar refracting index as water, therefore the natural size and colour of the surrounding marine life is maintained .
o Acrylic is a good transmitter of light; 92% of light is transmitted 
o Acrylic is a good electrical insulator (for low frequency work), this is important when considering the health and safety of both individuals and the surrounding marine life .
o A good resistance to chemical attack. Chemicals such as alkalis, water and most aqueous salt solutions .
o Acrylic has better insulation properties than glass .
o Acrylic sheets are said to be easier to handle than glass.
Disadvantages of using acrylic
o Although acrylic is easier to scratch than glass, protective layers can be used such as scratch-resistant coatings , or simply removing the scratch (shallow scratch) by polishing 
o Higher costs than glass .
o Acrylic will turn yellow over time when exposed to direct sunlight, this effect is minimised since the acrylic part is submerged underwater .
Some mechanical properties of acrylic are shown in Table 3 [11,14,15].
Young’s modulus (GPa)
Compressive strength (MPa)
8. Health and Safety
The safety of the customer as well as the member of staff is extremely important, and particularly when dealing with such a project, the risks and the circumstances can be devastating if any collapse of the structure is to occur. Therefore an overall high safety factor needs to be considered at the design stage of the hotel.
From research, it was found that a safety factor of 6 is used [2,16], resulting in 4 inch (10.2 cm) thick acrylic walls of the underwater rooms .
8.2. Hydro-season safety factor
The following are safety factor and wall thickness calculations for visible (acrylic) and non-visible parts (GFRP) of the hotel. Note: In the following sample calculations only acrylic is used, the later being the weakest material in the main construction of the hotel.
a) External pressure (used for guide)
P = pressure = (5)
= Water depth = 40 m (the hotel rooms are required to withstand one of the biggest recorded tsunamis in1883 Krakatau, Indonesia  ).
= gravitational acceleration = 9.81
= sea water density = 1020
P =400248 Pa
b) Failure due to circumferential or Hoop stress (used for guide)
= acrylic compressive yield stress = 100 M pa
R = Room radius = 2.5 m
= 0.010 m = 10 mm
This equation dose not take into account that the structure may buckle at a pressure ( ) which is less than that to cause axisymmetric yield, therefore this equation cannot be used to determine the wall thickness of the structure, instead it is used for guidance.
c) Buckling pressure consideration
The buckling pressure for the circular cylindrical shell can be obtained by using an interactive codes computer program , written by Ross, whereby the required input is as follows:
L = unsupported length of the cylinder
a = mean shell radius
t = wall thickness of shell
E = young’s modulus of elasticity
Nu = Poisson’s ratio
The program gives the theoretical buckling pressure and the thinness ratio. From the appropriate chart of this book, (a plot of against ), the actual buckling pressure, namely can be obtained and the corresponding (water depth) can be found using equation 5. This value is then compared with the desired height of value 40 m, made to withstand the biggest recorded tsunami in the 1883 Krakatau, Indonesia . If the values are not equal, the procedure is carried on for different values of ‘t’ until a height of approximately 40 m is obtained; in other words a trial and error approach is needed.
The safety factor is obtained by dividing the theoretical (= 40 m) height by the actual height ( = 7 m) i.e. S = safety factor = / = 40/7 = 6 (7).
i) Sample calculation for buckling pressure consideration
· Inserted values
L = unsupported length of the cylinder = 8 m
t = wall thickness = 10 cm = 0.1 m (inserted value)
a = mean shell radius = m (8)
E = 2,5 G pa = 2.5Pa (Young’s modulus of elasticity for acrylic)
Nu = Poisson’s ratio for acrylic = 0.37
= yield stress = 14500 psi = 100 Pa
· Obtained results
= Buckling pressure (Von mises) = 254680 Pa
· Calculated values
PKD = = 1.3 (from chart) (9)
= = = 195908 Pa
Knowing that P (pressure) =
= gravitational acceleration = 9.81
= seawater density = 1020
Making h the subject and replacing P with in equation (5);
= = 19.57 = 20 m
It can be seen that therefore a greater thickness is required another.
After having done a series of such calculations; the corresponding wall thickness for the visible parts (acrylic) and the non-visible parts (GFRP) corresponding to a depth of 40 m are 140 mm (5.51inch) and 40 mm (1.58 inch) respectively.
d) Suddenly applied load , where by the calculation are carried out on acrylic material as the later is the weakest material used in construction.
a = Bullet acceleration = 4.4 e+5 .
= Suddenly applied impact load (10)
a = bullet acceleration
A = Bullet tip area = = = 3.14 (11)
= bullet radius
M = Bullet mass = 175 gr = 0.175 kg
= 49 GPa
e) Cylindrical shell with hemispherical ends .
Since the selected shape of the room is in the form of a circular cylindrical shell with hemispherical ends made of the same material, the cylindrical part will experience a larger deflection than that in the hemispherical shell, and if these shapes have the same wall thickness there would be a bending stress accumulated at the joints. To reduce this stress, the wall thickness of the hemispherical should be less than that of the cylindrical shell. The ratio between these thicknesses is in the form of
1: 0.412 that is = 0.412 (Equation 12) whereby is the wall thickness for the hemispherical shell, andis the wall thickness for the cylindrical part. Thus for the values of obtained earlier (140 mm and 40 mm) the corresponding values for would be 57.68 mm and 16.48 mm respectively.
8.2.2 Cylindrical pod
Similar calculations to that for the suite were carried out for the cylindrical pod. The wall thickness obtained for the acrylic and GFRP parts were 300 and 85 mm (0.3 and 0.085 m) respectively. The safety factor corresponding to this wall thickness was only about 2; adding ring-stiffeners in turn can increase this.
8.2.3 Safety domes
Each unit of the hotel should include a safety dome, containing an entrance hatch, made only accessible to emergency divers. The size of the safety dome should be such to accommodate the maximum allowable number of individuals in the room.
The safety dome need not be too spacious, as its purpose is not to provide luxury and comfort, but instead to provide a shelter for a short time, until the rescue team reaches the hatch. The shape of the safety dome should be spherical, as this shape provides the maximum volume with the minimum wall thickness to withstand the external pressure. The safety dome should also contain essentials to sustain the well being of an individual. During an emergency the safety domes should automatically be supplied with fresh air.
8.2.4 Internal breathing valves
Fitting an internal breathing valve system into each submerged room that is automatically activated when the room is being flooded with water, increases the chances of survival by providing oxygen to individuals. The valves should glow when activated, so that the escapees can easily find them.
In the case of a structural breach or any accident, the damaged part of the hotel can be isolated to prevent any threat from being spread to the rest of the hotel. This can be achieved by the use of internal watertight bulkheads.
Any foundations used to supports the hotel as well as bridges that contribute to the passage of customers, should account for high currents (wind and water) that might distort the original position of the hotel and therefore cause high structural stress due to structural instability.
8.2.7 Safety utilities.
The hotel should take into account the possibility for an emergency, and therefore be prepared to provide the abundant essential utilities needed.
In addition to proper training, a well-advanced communication system is required among the members of staff, which are responsible for the daily running of the hotel. This is essential not only to ensure the safety of customers but the quality of service.
8.2.9 Detectors and warning signals
An advanced detecting and monitoring system is extremely important for ensuring the well being of customers and members of staff. The system can provide a warning signal and therefore appropriate actions could be taken before a serious damage threat is done. The system should account for detecting and warning the following:
o Approaching water and air borne objects, such as boats, ships, seaplanes, and submarines, large enough to pose a threat if it were to collide with the hotels structure.
o A leakage in any part of the structure.
o A dramatic pressure change. The pressure at different parts of the hotel should be monitored at all times, thus in the case of a dramatic pressure change is detected, appropriate actions could be taken.
o A dramatic change in heat and air content levels. Heat and air content levels at different parts of the hotel, should be monitored at all times, thus in the case of a dramatic temperature or air content change is detected, appropriate actions could be taken.
o The weather should be monitored at all times, and forecasting made possible and accessible at all times for all members of staff as well as customers. This will prevent any harm to both customers and the members of staff that would result if unexpected bad weather (storm) were on to come without warning.
As it would be expected, most of the customers would be unfamiliar with such an environment (underwater habitat), and therefore would require guidance. Thus, an efficient communication system is essential between the member of staff and customers; this is vital especially in the case of a real emergency situation, where the safety of customers depends greatly on communication.
The maximum brightness (light level is high) should be at the entrance of the hotel, this ensures that customers can easily adapt to lighting inside the resort especially during day time .
It is important to ensure that there is no sudden change in lighting between one area and another, as this will cause accidents since the human eye has little time to adapt to such quick changes, therefore in order to ensure that the eye has sufficient time to accommodate itself, the light changes should be made gradually . In an urgent situation, a system is required to allow for emergency lights that guide individuals to the nearest exit.
Communication and warning lights are required, especially on the exterior parts of the hotel, as these will warn any large approaching objects such as ships and prevent collision.
As it has been mentioned earlier, most of the customers are unfamiliar with such an environment (underwater habitat), therefore a good system for guidance is required to aid the customer in finding there way into the hotel. This can be done by having: guiding members of staff, reception desks, signs and maps.
8.2.13 Power backup
In the case of a power failure, a back up system is required to ensure the vital components of the hotel are running.
8.2.14 Electric current
Knowing that water is a moderate electrical conductive substance, electric current should not be allowed to leak (escape), as this might pose a threat to nearby swimmers and divers.
8.2.15 Health and safety standards
Since the region (Cyprus) belongs to the European Union, the latter’s a correct health and safety procedures should be in place and well managed.
8.2.16 Outward opening doors
The doors in each room are said to open outwards, this maintains the room’s water integrity especially during installation or removal. A similar opposing door is found on the complex, this ensures the latter’s water integrity.
The design should include stairs that lead to the outer part of the hotel, these stairs are to be used in the case where the elevator is out of action.
9. Environmental impact considerations
9.1 Environmental impact by product
To minimise the effect that the underwater hotel will have on the surrounding environment, consideration should be taken on the following:
9.1.1 Material surface
The surface of the hotel would be considered as a direct contact with the external environment and hence any regional marine life, as a result of the latter is not viable to any threat (contamination) produced by the material used at the surface. Therefore during material selection, the chemical properties should be carefully considered with concurrence of the well being of the marine life.
The design itself should facilitate for waste disposal and the well being of the environment; waste generated should be carefully monitored. There should not be in any circumstances, any discharge of waste material that would harm the surrounding marine life directly or indirectly.
The construction itself should be designed in such a way, that there is minimum interference with the natural habitat hence, the marine life.
9.1.3 Noise pollution
It should be emphasised, that the noise transfer between both of the environments i.e. inner and the outer part of the hotel, should be reduced as low as possible. This would be essential when considering the customers need for relaxation, as well as the marine life that inhabit the surrounding environment, where noise can disturb their natural habitat. Noise minimisation can be achieved by using materials, which offer a good insulation to sound.
9.1.4 Environmental management system (EMS)
An environmental management system is essential in order to sustain in an efficient manner a healthy surrounding marine life.
9.1.5 Staff training
All Staff members should be well trained in order to understand and appreciate the importance of having a healthy environment and they should be acquainted with maintaining the well being of the surrounding environment.
9.1.6 Electricity conduction
Since water is known to conduct electricity (although a poor conductor), any leakage in areas of high voltage can pose a threat not only to the surrounding marine life, but also to nearby divers. In order to minimise the possibility of electric leakage, the minimum amount of conductive materials should be used in construction, as well as the use of low current demanding devices and where possible good cable insulators.
9.1.7 Waste products
A well-designed waste disposal system should be implemented to prevent any harmful wastes from being discharged into the surrounding environment.
The product life cycle should be well verified, and a planned procedure for decommissioning is essential and should be as such that minimum environmental impact is achieved.
The design of the hotel itself should be as environmental friendly as possible. Therefore recyclable materials should be considered at the design stage, to assure a well being environment, especially when dealing with replaced parts and also at the decommissioning stages of the hotel.
9.1.10 Heat transfer
As the temperature of the inner part of the hotel is more like to be higher than the temperature of the surrounding environment (see section 3.1 on “Attraction” for water temperature), heat transfer will occur mostly from the inner part of the hotel to the adjacent surroundings, and thus would be accounted as heat loss. The latter should occur at the lowest rate possible in order to minimise the associated running costs.
An extensive heat transfer can cause a change in the temperature of natural habitat and would affect the equilibrium system of the surrounding marine life.
9.2 Environmental impact on product
Before selecting the appropriate material, it is important to acknowledge the property and type of corrosive substances present that are likely to interfere with the role of the component considered.
9.2.1 Corrosion consideration
Corrosion plays an important role in predicting the life cycle as well as predicting the number of maintenance procedures required. In some cases it can be the cause of damage and therefore, it is important to ensure that the minimum amount of corrosion occurs. This can be achieved by selecting the appropriate materials with respect to the corrosive environment, in addition to the selection of the correct manufacturing process, and the use of protective coatings.
10.1 Unique Moulds
Since the project is one of a kind and therefore unique in its own right, this implies that a unique shape of a moulds are required. It is worth noting that due to the simple shape of the hotel, the number of different type of moulds needed for constructing the various parts is lowered, this in turn will reduce the manufacturing cost.
For the main parts of the hotel such as the cylindrical shaped pod and the cylindrical suite with hemispherical ends, there are mainly three different types of moulds needed for construction;
10.1.1 Cylindrical suite with Hemispherical ends
a) Mould 1
This mould has a width of 4 m and is part of the cylindrical section of the room and represents a quarter of a circle of diameter 5 m. The corresponding component (unit) will take form as shown in Figure 8.
b) Mould 2
This mould is part of the spherical section of the room and forms a quarter of a sphere of diameter 5 m. The corresponding component will take form as shown in Figure 9.
10.1.2 Cylindrical shaped Pod
a) Mould 3
This mould is similar in shape to mould 1, except that it represents a sixth of a circular cylinder of diameter 48 m. The corresponding component will take form as shown in Figure 10.
10.2 Corresponding units
Figure 8 Figure 9 Figure 10
The following table shows the main parts of the hotel along with the type of moulds and the number of components (units) needed:
Type of mould needed
Number of corresponding units needed to form the entire part
Suite (cylindrical part)
Suite (spherical part)
11.1 Sea or land construction
Although it is known that the wages for underwater construction would be grater than on land construction, due to the difficulty encountered when working underwater, this should be carefully thought of. That is, if the structure is large in size, a full construction on land would bring about a problem and the risk of damage that might arise during transport to the main location, therefore a sensible way overcoming this dilemma is to construct different parts of the hotel on land, which later on, can be transported separately to the main site where they should be assembled.
Another approach was that taken when constructing Hydro-polis i.e. “Floating caissons towed to the site enable the construction of the facility in dry surroundings. As the building gradually increases in height, together with weight during the construction process, it will be gradually lowered until it reaches its final position and it will then be firmly anchored” .
As maintenance is costly, especially when dealing with an underwater construction, the need for having a minimum amount of maintenance is required. The ease for implementing this process should be carefully thought of at the design stage of the hotel and when dealing with the material selection. One way for achieving such a requirement is to design, if possible, the appropriate parts (such as the suites) of the hotel, where maintenance is important, to be detachable and buoyant; this enables the component to be transported to land (bearing in mind that the distance form land is small), where maintenance cost is cheaper and easier to conduct. This approach is seen in Poseidon underwater resort as shown in Figure 11 .
Figure 11 Showing a maintenance procedure taking place at Poseidon underwater hotel
The decisions for having the entire hotel as buoyant will enable to conduct major maintenance on sight, only this time most of the structure is above sea level.
The transparent parts of the hotel, should be at all time clean, to ensure that the quality for observing the marine life is maintained; this is achieved by an automated system, which acts as a replacement for high costs when hiring divers as well as the use of a self cleaning coating known as smart materials.
The source for the main power supply should be shore-based, where maintenance is easier to implement as this removes any inconvenience due to the presence of customers.
Not only buoyancy is used to ease maintenance procedures, but can also be used to relieve the loads on the structure, as well as serving as a means for evacuation procedures. Buoyancy is a property that is experienced through out the entire structure.
This buoyancy is achieved and regulated by using ballast tanks.
13. Power supply
13.1 Main power supply
In order to calculate the rating for the main power supply, the following parameters are considered:
Knowing that the power needed to supply all necessary electricity for an aquatic pod suite with 150 ft2 (13.9 m2) interior is 2.5 kW  and assuming that the power supply is proportional to the area of the room, the total power rating for Hydro-season can be calculated as follows:
Total power rating = 320 kW (13)
Recalling that = total floor size of Hydro-season hotel
Note: this calculation is not accurate; its purpose is to serve merely as a guide.
14.1 Final design (See Figure 12)
Figure 12 showing the final suggested shape for Hydro-season underwater hotel
14.2 Submerging depth
Figure 13 Shows a totally submerged room and its total minimum submerged depth
It can be seen from Fig. 13 that the total submerged depth for Hydro-season depends on the submerged depth of its totally submerged room, which can be seen to be no less than 7 m.
14.3 Overall height
If necessary, additional space required, can be achieved by increasing the height of the hotel, as to include extra levels. As this project takes into account only two levels of floor the overall height can be obtained as follows (See Figure 14):
Figure 14 Showing a side view of the Cylindrical Pod along with its height dimensions
Therefore as it can be seen from Figure 14, the overall height of the hotel is approximately 8.3 m.
14.4 Adjustable legs
Adjustable legs are used to position the structure horizontally on the seabed, and keep the structure above the mud line; these adjustable legs are ideal to account for neither a flat nor a horizontal seabed.
The total weight of the hotel is sufficient enough to firmly hold the structure in place. Note the adjustable legs stretch to cover a greater diameter than that of the main hall; this approach insures the stability.
14.5.1 Weight of Poseidon underwater resort
The mass of Poseidon underwater resort is around 7 600 tonnes ; this corresponds to a weight of 74.56 MN (7 600 000 9.81) where by the construction consists of 80% Acrylic and 20 % steel .
14.5.2 Weight of Hydro-season underwater resort
The total mass of Hydro-season is around 4 000 tonnes and is obtained from a computer model of 1:1 scale using the Pro-Engineer package; the mass corresponds to a weight of 40 MN (4 000 000 9.81).
14.6 Hydro-season’s volume
The total volume () of Hydro-season and surface area is around 2 567 and is obtained from a computer model of 1:1 scale using the Pro-Engineer package.
Using this value the weight of the displaced water can be obtained as follows:
= weight of displaced water = = 2 567 1 020 9.91 = 25.68 M N
This can be seen to be less than the total weight of Hydro-season, therefore ballast tanks should be capable of providing the difference of 48.88 M N, in order for the structure to be maintain buoyancy.
14.6 Customer journey to the underwater hotel takes place through the following stages (See below):
14.7 Recalling Hydro-season special features
o Adjustable legs.
o The entire hotel is buoyant with detachable rooms, easing maintenance, repositioning and emergency procedures.
o Accessibility made easier due to the circular shape.
o Natural ventilation is made possible by having an opening in the ceiling, connecting the inner part of the hotel to the external atmosphere.
o The hotel’s shape offers a choice of accommodation above sea level; semi submerged and totally submerged accommodation. This approach enhances the attraction of potential customers.
o Underwater hotels appear to fill a need in the holiday trade for an unusual and exciting holiday!
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