Fundamental of Marine Surveying and Durability Specifications DWT Product Tanker - Term Paper Example

Construction, structural and strength requirements for 37000mt DWT product tanker A Report Introduction The paper provides an overview of the use of technology to develop a proposed 37000mt DWT product tanker with double hull. A design study would be undertaken to find out the feasibility of using technology for building the tanker and the double hull of the proposed product tanker. The study would also analyse the advantages and disadvantages of using technology, primarily SPS, for building the tanker. In the recent past, various specialised tankers have been developed to protect the tankers from accidents and oil spills. As most of these tankers are involved in various polluting oil spills, the shipbuilding companies are required to follow various stringent operational and design policies. Therefore, through this comprehensive report, I aim to provide an outline for the construction, structural and strength requirements for the proposed 37000mt dwt product tanker. Constructing product tankers Special ships designed to transport bulk petroleum products are known as oil tankers. These tankers are divided into two types, namely, product tanker and crude oil tanker (Hayler & Keever 2003). While product tankers, which are smaller in size, are used to carry petrochemicals to the markets from the refineries, crude tankers are designed to carry huge volume of unrefined crude oil to the refineries from the oil extraction sites (Hayler & Keever 2003). These tankers are generally divided as per their size and occupation. Most of the oil tankers come in the sizes starting from thousand metric tons of DWT or deadweight to huge tankers of around 550,000 DWT (Huber 2001). These tankers are an efficient way of transporting oil from one place to another and are as efficient as oil pipelines. Further, these tankers also save costs in oil transportation (Huber 2001). Therefore, due to the various advantages of using oil tankers to ferry oil from the refineries to the market, our shipping company intends in the near future to construct a new 37000mt dwt product tanker with the following particulars: Draft-11.5m LOA-180m beam-26m tanker-14 (12+2 slops) GRT-20300 NRT-10150 double hull construction In the recent past, various specialised oil tankers have been developed to protect the tankers from accidents and oil spills. As most oil tankers are involved in various polluting oil spills, the shipbuilding companies are required to follow various stringent operational and design policies. Thus, my company decided to appoint me to create a comprehensive report on the construction, structural and strength requirements for the 37000mt dwt product tanker. Existing structural design and engineering However, even before outlining the proposed structure for the product tanker, it is important to understand the current structural design requirements for a product tanker. Most product tankers usually consists of around 8-12 tanks, with every tank being divided into 2-3 separate compartments (Turpin & McEwen 1980). These compartments are separated with bulkheads. A small space between two bulkheads is often left open, which is known as cofferdam, to protect the tanks from fire, heat and accidents. Most of the tankers usually have these cofferdams at the beginning of the tanks and sometimes even between the separate tanks (Turpin & McEwen 1980). Hull designs One of the most important components in constructing a product tanker is designing the outer structure or the hull of the ship. The product tankers usually consist of hulls that are either single, double bottomed or double hulled. A single hulled tanker has just one outer shell between the oil products and the ocean. However, most contemporary product tankers come with double hull, which consists of extra gap between the tanks and the hull (Hayler & Keever 2003). There are various hybrid designs in the market as well. Some of them come with the combination of single and double-hull designs, whereas, some has double-bottom hulls. As per the International Convention for the Prevention of Pollution from Ships, 1973 (MARPOL), the tankers with single hulls would be taken off service by 2026 (Hayler & Keever 2003). The United Nations has also decided to phase out tankers with single hull by 2010 (“Single Hull Oil Tankers Banned” 2003). Due to the stringent regulation policies of the UN and MARPOL, the company decided to construct a product tanker with double hull. Therefore, it is also necessary to understand the advantages and disadvantages of double-hull design. The Marine Board of the National Academy of Science had undertaken a survey in 1998 to find out about the advantages and disadvantages of double-hull design. Given below are some of the advantages listed by the experts who were interviewed for the survey (Marine Board 1998). Decrease in corrosion due to decline in saltwater ballasting in the tanks Easier and faster discharge of the cargo Effortless ballasting incase of emergency events Protection of cargo in a better manner in situations of collisions of low impact and grounding Efficiency in washing the tank Ability to protect the environment in a better manner However, the survey also listed various disadvantages of double-hull designs, which includes (Marine Board 1998): More expensive to build a double-hull tanker Operating costs are also higher Surface area of such a tanker is also more and therefore, maintenance of the surface is higher as well Difficult to provide ventilation in the ballast tank Greater risk of accidents and explosions in case the vapour detection system is not fitted or malfunctions Ballast tanks of double-hull tanker requires constant maintenance and monitoring Difficulty in cleaning the ballast tanks of double hull ships Although, it has been established that tankers with double hull offer better safety than tankers with single hull during grounding incidents, it is however, not clear whether double hull ships offer similar benefits in case of high speed collisions (Marine Board 1998). Further, it has also been found that although most tankers with double hull prevent oil spills in smaller accidents, during bigger accidents, when both the hulls are damaged, the oil spill would be evident. In fact, experts believe that oil spills from double hull tankers are considerably higher than the tankers such as Coulombi Egg Tanker and Mid-Deck Tanker (Devanney 2006). Inert gas system Just as the hull design, the inert gas system of a product tanker is also an important component of the overall design of the tanker. Although, it is hard to ignite fuel oil, the hydrocarbon vapours present in the oil are explosive and if mingled with air, it can catch fire. Therefore, it is essential to create a system that would control the atmosphere inside these tanks in such a manner that hydrocarbon vapours are not allowed to mix with air (Hayler & Keever 2003). Therefore, I suggest that an inert gas system should be made in such a manner that when the gas is mixed into the hydrocarbon vapours, it lowers the combustible limit at which the vapour may catch fire. As soon as the oxygen concentration in the product tank would reach around 11 per cent, the lower and the upper combustible limits would merge together and then combustible range would disappear. Most inert gas systems carry air that has less than 5 per cent concentration of oxygen. Once the oil from the tank is taken out at the time of delivery, these tanks are filled with inert gas to protect it from fire and heat till the time of next cargo loading. Thus, investing into a good inert gas system and constant monitoring and maintenance of such a system is an important component of this product tanker design. Structural strength After studying the various existing structural design and engineering processes, it was decided that to create a product tanker of such a huge capacity should follow renowned technologies such as the Sandwich Plate System (SPS). This system, which has evolved considerably over the past decade, offers various benefits such as decrease in complex structures and development costs, improved environmental protection and safety features, better utilisation of space and provision of enhanced comfort for the passengers as well as crew members. The efficacy of this new technology has been established through various comprehensive testing and analysis, as well as through constant innovation and introduction of new applications (Welch 2005). The report establishes that to construct a 37000mt DWT product tanker, SPS technology should be used by the organisation. This paper would also describe the latest developments in SPS technology and how such advancements can be used to create the proposed product tanker. Using SPS technology has various benefits over that of other traditional steel structures. These benefits include decrease in weight, easier structural design, improved fatigue resistance and reduction in vulnerability to corrosion, which finally leads to fewer maintenance and simplified inspection process. The technology also offers various other unique benefits such as in-built fire resistance system and improved resistance from blast, puncture, ballistic and impact. Due to its inherent characteristic of structural damping, the technology offers the advantage of controlling vibration and minimising the spread of noise borne by a structure. Finally, all these benefits translate to not only creating a better and safer tanker, but also reduce the production cost (Brooking & Kennedy 2004). The technology of SPS was developed initially in order to offer plating with impact resistance to various offshore structures at the Canadian Beaufort Sea. This technology has seen various marine engineering related research and developments in the last decade. These developments especially focused on the behaviour of the structure and its performance, fire resistance technique and engineering processes, designing principles, characterisation of the material, design theories based on energy absorption and the creation of correlation details especially related to structures using sandwich plate. Further, in the recent past it has been seen that many civil as well as marine projects are being undertaken to develop or repair existing structures using SPS technology (Brooking & Kennedy 2004). Using SPS for creating proposed product tanker As a part of the assignment given by the organisation, I undertook a design study for the tank of the 37000mt DWT double-hulled product tanker. Firstly, as a reference point a traditional steel design for the tanker was used, this was then used to describe the SPS design study. The main aim of this study was to find out whether use of SPS technology would be feasible for constructing the proposed double hull tanker. For the scantling design, ShipRight Direct Calculation Procedures (2006) and Lloyd’s Register’s Rules (2002) were used. It was found that the SPS technology could be used to replace the conventional stiffened plate used for the inner and outer shell, main deck, bulkhead and the double bottom of the tanker. As no secondary stiffening was reported, it became easier to eliminate most of the problems arising from corrosion and fatigue. Due to the use of sandwich plate, the bending strains are scattered throughout the primary structure, thereby, reducing the formation of hard spots which are mostly associated with breakdown of cots and stiffening intersections (Vinson 2005). While comparing the SPS structure with the steel one, it was found that the former structure needed lesser labour input, decreased surfaces to coat, shortened length and volume for the weld, better application of coating and decreased weight of the hull. Operationally as well, the SPS plate provided improved drainage and cleaning of the ballast tank (Vinson 2005). For the proposed product tank, two steel plates were bonded together to form a compact polyurethane elastomer core. As the sandwich plate offers the flexibility to tailor the plate strength as per the requirement, we could select the thickness of these plates appropriately to make its strength equivalent to that of stiffened steel plate. This bonded elastomer gives constant hold to the steel plates, thus preventing local collapsing and reducing the need to provide narrowly spaced detached stiffeners, and reassigns the shear between the steel plates. Comparison between SPS and Conventional Structures Further, the two-part liquid, known as the elastomer, is infused in the space that was formed between the steel plates and the perimeter bars, which were welded with both the plates. In order to strengthen the structure and gain strength more than 6 MPa, the SPS plates were grit blasted and cleaned properly. The cleaning was done to ensure that the structure is free from any contamination and is completely dry so that elastomer can be infused. Description of SPS design At the designing stage, both the traditional as well as the SPS designs were studied to find out the feasibility of using SPS design. FE analyses of both the design results were carried out by using the design assessment procedures of ShipRight. While conducting the design study, it was found that the SPS design may use a space of 3.5m between the web frames. Further, the secondary longitudinal stiffeners were also removed from the SPS structure. Along with the stringer on the side of the structure and the central bulkhead, double bottom girders were also used. This increase in the floor spacing with the use of extra stringers and girders ensured that the panel sizes of the SPS plates that were provided on the internal as well as the external hull were maintained optimally (Kujala et al 2004). Further, due to the elimination of the secondary stiffeners and the improvement in accessing the inner bottom, it became easier to reduce the height of the double bottom by as much as 500mm. This would give added advantage to the company for carrying more cargo in its tank. The below figure provides an inner view of the SPS hull section. Structural layout of the SPS Tanker It has been seen that in the recent years, many accidents have been involved with product tankers, especially the one with single hulls. Therefore, it is important to focus on this issue and to ensure that the tanker should be built keeping in mind all the safety requirements (Noury et al., 2002). However, together with safety, one also needs to ensure that the tanker provides the below mentioned features as well: Creation of a structure that is simple and easy to construct Does not require comprehensive maintenance Does not suffer any corrosion Does not have much breaks in coatings Does not suffer any cracking or other structural issues Is resistant to vibrations Creating double hulled tankers using SPS Through my study of the SPS technology, it was decided that the same technology would be used to create the double hull of the tanker as well. It is already established that the SPS technology provide greater safety benefits which include impact and collision resistance, explosion and fire resistance etc. Further, the use of SPS structure is commercially viable as it ensure reduction in production costs and decrease in maintenance of the tanks. Also, due to the removal of the secondary stiffeners, the risk of breaking of the coating, corrosion and fatigue are also decreased. Thus, it was decided that the SPS technology would be used to create the double hull of the tanker as well. The use of SPS technology provided a fresh insight into the ship designing, especially in the structural intersections. The assessment of the basic longitudinal strength was calculated using the prevailing methods such as the classic naval architecture theory. Although the calculations took into consideration the outer and inner face plates of the SPS sandwich plates, the elastomer core was not incorporated (Brooking & Kennedy 2004). It was further found that the steel area around the netural axis can be redistributed to the outer hull or the deck area to offer more strength to the structure. This will also help in maintaining the section structure in spite of decreasing the overall weight. It was also found that the SPS panels were steady as the elastomer used in the core prevents the formation of buckling effects. Further, it was also found that such SPS structures are resistant to fatigue and the bond formed between face plates and the elastomer is also resistant to fatigue. These joints between the SPS panels and the connecting panels which are supporting the structure would have good resistance power. The fatigue assessment for these joints can be done through the use of existing assessment techniques of the marine industry (Brooking & Kennedy 2004). Thus, it can be said that the design used for the SPS technology may be used for making the double hull of the proposed product tanker. Further, the study also demonstrated that creating double hull using SPS technology offer various benefits, which includes: As compared to the weight of the traditional steel designs, the SPS design was lighter. This SPS design weight also included the weight of the elastomer core. The design offered various savings especially painting costs due to the removal of secondary stiffeners. This reduced the internal surface by around 20 per cent. The design also saved considerable production cost and time as the removal of stiffeners ensured the reduction in weld volume by as much as 50 per cent. Due to the uncluttered nature of the SPS design, the structure can be created by reducing the height of the double bottom by as much as 500mm. This in turn would provide the company to accommodate cargo with an additional capacity of around 7 per cent. The SPS design also ensured that the maintenance and the inspection of the structure could be carried out easily. This means undertaking faster inspection. Also the design ensured that the structure has very little coating breakdown or structural faults. Production methodologies The SPS design structures ensured that this kind of structure would provide equivalent strength as compared to traditional steel ships structures. The production of the SPS structures, however, requires certain unique SPS technologies. Some of this includes: The steel cavities need to prepared, especially the internal surfaces, which should have the surface roughness of around 55 microns and the surface cleanliness of around SA 2. The process also uses the current steel fabrication practices. In order to prepare the steel cavities, it is advisable to use steel grits as they consist of hard particles and have angular elements. Before injecting elastomer, it is important to ensure that the internal cavities are clean and dried properly. Further, it should also be checked that the cavities are airtight to prevent any leakages. During the process of curing and injecting elastomer, the SPS panels are kept restrained. This is done to make sure that the face plates of the SPS panels should become flat. The injection of elastomer takes around 7-13 minutes. This should be done at the temperature of around 20 degrees Celsius. However, in order to derive the maximum benefit out of SPS technology, it is essential to integrate the technology with the production line at the shipyards as well. At present the shipbuilding industry uses two options for producing SPS structures (Huber 2001). These are: Using pre-fabricated SPS panels that are transported to the production unit Integrating of the SPS panel that are fabricated in the production unit at the shipyard In case the company decides to use the first option, the SPS panels would be prepared as per the specifications given by the company in a separate location. This option provides the flexibility to prepare various SPS panels at a single location and then distribute it to various sites. The biggest benefit of this option is that a single facility with all the modern technologies can produce the panels with various specifications and more numbers. Due to the economies of scale involved in the production of the SPS panels, the cost for producing these units would also be lower. This option is especially useful for small shipyards were the SPS panels are needed to be used in a new vessel (Huber 2001). The second option however, requires large space in the shipyard to construct the SPS panels. This can be a costly affair and requires resources to supervise the operation as well. Therefore, I recommend that the company should use pre-fabricated SPS panels that can be later transported to the production unit. This report has only provided an outline of the processes that the company may use to create the proposed product tanker. However, through this outline itself, it can be seen that the SPS technology can help in not only saving production cost and time, but also provide added strength to the overall structure through the injection of elastomer. Conclusions The paper provided an overview of the use of SPS technology to develop a proposed 37000mt DWT product tanker with double hull. A design study was undertaken to find out the feasibility of using the SPS technology for building the tanker and the double hull of the proposed product tanker. The study found that the use of SPS technology can have several benefits for the company that are listed below: Improved safety benefits and protection for the environment The SPS technology provides excellent resistance from collisions and impacts, as well as grounding due to its characteristic of energy absorption. This can be of particular interest for tankers with double hulls as it reduces the risk of hull puncture and flooding into the hulls. This in turn can help in recovering the ship in a faster manner. As the SPS technology provides fire resistance inherently, many SPS panels do not require installation of extra fire insulators. Thus, it saves on the cost for investing into fire insulating the tanks. Further, the SPS also offer resistance from ballistics and blasts. Therefore, the ship can also be protected from damages due to accidents or attacks from pirates or terrorists. Reduction in costs The SPS design enables construction of simple structures and also removes the necessity for using secondary stiffeners or other connectors. The removal of stiffeners ensures reducing welding volume. This would translate into decrease in construction and painting costs. Due to the use of pre-fabricated SPS panels, companies can further save on production costs. Further, due to the reduction in maintenance and inspection costs, it is simpler and faster to carry out inspection of a SPS structure. Also, most SPS structure have little coating breakdown or fatigues. Maximum utilisation of space and provision for crew comfort SPS designs have simpler structures. The elimination of stiffeners free up a lot of space. This provides additional spaces, especially in the lodging areas of the ship for the crew. Also, the SPS design has in-built ability to manage the vibration and structure borne noise, which in turn would provide greater comfort to the crew members and passengers. Reference: Welch, D. “The Sandwich Plate System, Presentation, Glasgow College of Nautical Studies.” 2005: 1-4. Brooking, M.A. and Kennedy, S.J. “The performance, safety and production benefits of SPS structures for double hull tankers”. In: Proceedings of the RINA Conference on Double Hull Tankers, 2004, February 25-26:1-2. Provisional Rules for the Application of Sandwich Plate Construction to Ship Structure, Lloyd Register, London, 2006. Rules and Regulations for the Classification of Inland Waterways Ships, Lloyd Register, London, 2002. Vinson, J.R. “Sandwich structures: past, present, and future.” In: Thomsen, O.T., Bozhevolnaya, E. and Lyckegaard, A. (Eds.): Sandwich Structures 7: Advancing with Sandwich Structures and Materials. 2005: 3-12. Kujala, P., Romanoff, J., Tabri, K. and Ehlers, S. “All steel sandwich panels – design challenges for practical application on ships”. In: Proceedings of the 9th International Symposium on Practical Design of Ships and Other Floating Structures, September12-17, 2004: 915-922. Noury, P., Hayman, B., McGeorge, D. and Weitzenböck, J. “Lightweight construction for advanced shipbuilding – recent development”. In: Proceedings of the 37th WEGEMT Summer School, November 11-15, 2002: 11. Hayler, William B.; Keever, John M. American Merchant Seaman's Manual. Cornell Maritime Pr. 2003: 14. Huber, Mark. Tanker operations: a handbook for the person-in-charge (PIC). Cambridge, MD: Cornell Maritime Press. 2001: 211 Turpin, Edward A.; McEwen, William A. Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. 1980: 8-25 ‘Single Hull Oil Tankers Banned Worldwide from 2005,’ Environmental News Service, December 5, 2003. Marine Board. Double-Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990. Marine Board Commission on Engineering and Technical Systems. Washington, D.C.: National Academy Press. 1998: 259-261. Devanney, Jack. The Tankship Tromedy: The Impending Disasters in Tankers. Tavernier, FL: The CTX Press. 2006: 381-383. Read More

One of the most important components in constructing a product tanker is designing the outer structure or the hull of the ship. The product tankers usually consist of hulls that are either single, double-bottomed or double-hulled. A single-hulled tanker has just one outer shell between the oil products and the ocean. However, most contemporary product tankers come with a double hull, which consists of the extra gap between the tanks and the hull (Hayler & Keever 2003). There are various hybrid designs in the market as well.

Some of them come with a combination of single and double-hull designs, whereas, some have double-bottom hulls. As per the International Convention for the Prevention of Pollution from Ships, 1973 (MARPOL), the tankers with single hulls would be taken off service by 2026 (Hayler & Keever 2003). The United Nations has also decided to phase out tankers with single hulls by 2010 (“Single Hull Oil Tankers Banned” 2003). Due to the stringent regulation policies of the UN and MARPOL, the company decided to construct a product tanker with a double hull.

Therefore, it is also necessary to understand the advantages and disadvantages of double-hull design. The Marine Board of the National Academy of Science had surveyed in 1998 to find out about the advantages and disadvantages of double-hull design. Given below are some of the advantages listed by the experts who were interviewed for the survey (Marine Board 1998).Although it has been established that tankers with double hulls offer better safety than tankers with single hulls during grounding incidents, it is, however, not clear whether double hull ships offer similar benefits in case of high-speed collisions (Marine Board 1998).

Further, it has also been found that although most tankers with double hull prevent oil spills in smaller accidents, during bigger accidents when both the hulls are damaged, the oil spill would be evident. In fact, experts believe that oil spills from double-hull tankers are considerably higher than the tankers such as Coulombi Egg Tanker and Mid-Deck Tanker (Devanney 2006). Just as the hull design, the inert gas system of a product tanker is also an important component of the overall design of the tanker.

Although it is hard to ignite fuel oil, the hydrocarbon vapors present in the oil are explosive and if mingled with air, it can catch fire. Therefore, it is essential to create a system that would control the atmosphere inside these tanks in such a manner that hydrocarbon vapors are not allowed to mix with air (Hayler & Keever 2003). Therefore, I suggest that an inert gas system should be made in such a manner that when the gas is mixed into the hydrocarbon vapors, it lowers the combustible limit at which the vapor may catch fire.

As soon as the oxygen concentration in the product tank would reach around 11 percent, the lower and the upper combustible limits would merge, and then the combustible range would disappear. Most inert gas systems carry air that has less than a 5 percent concentration of oxygen. Once the oil from the tank is taken out at the time of delivery, these tanks are filled with inert gas to protect it from fire and heat till the time of the next cargo loading. Thus, investing in a good inert gas system and constant monitoring and maintenance of such a system is an important component of this product tanker design.

After studying the various existing structural design and engineering processes, it was decided that to create a product tanker of such a huge capacity should follow renowned technologies such as the Sandwich Plate System (SPS). This system, which has evolved considerably over the past decade, offers various benefits such as a decrease in complex structures and development costs, improved environmental protection and safety features, better utilization of space, and provision of enhanced comfort for the passengers as well as crew members.

The efficacy of this new technology has been established through various comprehensive testing and analysis, as well as through constant innovation and introduction of new applications (Welch 2005).

Read More