Hendra Saputra
Hendrasaputra_utm@ymail.com
Postgraduate student of Mechanical Engineering (Marine Technology), Universiti Teknologi Malaysia (UTM)
2012
(this paper is draft only and use for personal only)
I. INTRODUCTION
An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters, the term usually refers to ice-breaking ships. For a ship to be considered an icebreaker, it requires three traits most normal ships lack: a strengthened hull, an ice-clearing shape, and the power to push through ice-covered waters.
Icebreaking ships have become a specialized niche within naval architecture and shipbuilding over the last one hundred years. These ships have typically fallen into the categories of either transportation or support for navigation and offshore oil operation. Within the support category, the ships typically have large propulsion systems in relation to their overall dimensions, similar to a tug. Manoeuvrability for this type of ship is an important criterion, as they have to operate in close proximity to other ships or fixed installations, but their transportation capacity is limited. Ships in this category are typically government-operated icebreakers or commercial offshore supply ships (Kim, 2004).
Icebreakers are needed to keep trade routes open where there are either seasonal or permanent ice conditions. According to the globalsecurity (2011), an icebreaker is intended to break ice in order to escort merchant vessels, do ice management or carry out some other special task in ice. Usually these kinds of vessels are pure icebreakers, ice breaking supply vessels or cruise ships (modified usually from icebreakers). The operation of icebreakers to support maritime trade is usually included in the infrastructure given by the port state (i.e. Finlandia, Swedia, Latvia, Denmark, USA, Canada, Japan, Rusia and other several port state). Icebreaker ships are used to assit merchant ships operating in ice area must pay the cost of assists is quite expensive. The icebreaker which is assist merchant ship in Northen Sea Route (NSR), Niini et al (2006) mention that traditionally the costs of icebreakers and icebreaking have been funded by governments, and shipowners have been charged only on the actual assistance, when their ships have been in troubles with ice, and icebreaker has been called for help. That resulted in situation where ice icebreaker assistance was expensive, but use of the fairways was free or cheap. In addition, Ardev (1999) also mentioned that cost for icebreaker support (assist) more than 20% of total transport costs. Due to the inherent cost of the practice (icebreakers used to assist ships navigation), ice breaking tankers and other concepts were developed (Kumar, 2008).
II. ICE BREAKING TANKER
The other category of ships requires the modification of typical bulk carriers (oil or ore) for operation in ice-covered water. In North America, development of this type of ship has focused on Arctic and sub-Arctic ice conditions, since northern Canada has extensive mineral reserves. The first example of this type of ship in North America was the oil tanker ‘Manhattan’, which made two attempts at sailing through the Northwest Passage in 1969 and 1970 (Gray and Maybourne, 1981). At the time it was built ‘Manhattan’ was the most powerful tanker in the world, with twin steam turbine powered propellers. The original bow was replaced with a long raked bow, which was radical at the time, but has since become a feature typical of icebreakers. The modifications were carried out to ensure that the ship could navigate the Northwest Passage, and no attention was paid to the changes that increased power in open water. Many of the features incorporated into the ‘Manhattan’ are still considered necessary in modern designs (Hyun-Soo, 2004).
The conventional ice breaking tankers had a bow somewhat similar to that of an icebreaker. The principle for breaking ice was to sit on the ice and break it by its own weight. However due to the modified bow form the efficiency of such tankers were vastly reduced in the open water regions. Thus another engineering solution was developed in the concept of Double Acting Tankers.
II.1. Hull Form Design
Aframax Size, Moderate to Heavy Ice Conditions
The first designs developed by SHI to be tested at IOT were for Aframax sized tankers, operating on a hypothetical route between the Pechora Sea (northern Russia) and Western Europe. The ship was required to move steadily forward in first year ice, with a thickness between one and two metres. The maximum flexural strength of the ice was expected to be approximately 500 kPa. The ship was required to progress at 4 knots in level ice, 1.0m thick, and maintain forward motion in ice 2.0m thick. The design must also be able to transit ridges and move forward in snow-covered ice. The design speed in open water was 16 knots. This mission profile imposed constraints on length, beam and draft. The resulting design had a displacement of approximately 100K tonnes, which was comparable to a typical Aframax tanker. The bow and stern for the second of the two designs are shown in Figures 1(a) and (b) respectively (Hyun-Soo, 2004).
Suezmax Size, Moderate to Heavy Ice Conditions
Aframax size tankers are relatively small and as a result can operate on flexible trading patterns. However, the restricted displacement is a limitation for some ship owners. The next design development was to produce an icebreaking tanker of a similar size to a Suezmax design (150K tonnes). The length and beam for the Suezmax design were similar to the initial designs described above, but the draft restriction was relaxed, to a design load draft of 16.5m. The resulting displacement was approximately 160K tonnes.
As a result of the increased draft, the propellers could be placed further away from the broken ice pieces by lowering the line of the propeller shaft. This had the hydrodynamic advantages of increasing the propeller diameter, resulting in a more efficient propeller in open water, and reducing the need for protecting the propellers from the broken ice pieces, which in turn lowered the wetted surface area of the hull and the appendages. The resulting stern design was twin screw/twin rudder, but with simple bossings supporting the propeller shafts, rather than the more complex skegs used for the Aframax size tanker. The stern for the Suezmax size tanker is shown in Figure 2(a). The bow shape was radically different from the Suezmax size tanker. For this design a sloping spoon bow was chosen, similar to the bows developed for the icebreaking supply boats used in the Beaufort Sea. The bow shape is shown in Figure 2(b) (Hyun-Soo, 2004).
Suezmax Size, Light Ice Conditions
To be competitive with conventional tanker design, the stern of this hull was to be fitted with a single screw and a single rudder. The stern arrangement for the ice capable tanker is shown in Figure 3(a). The bow for this design was a modified bulb, which at the design draft was much lower in the water than a conventional bulbous bow. The hull above the bow was heavily flared, so that the resulting shape was similar to the spoon bow discussed above. The underside of the bulb was also flared to provide some ice breaking capability at the ballast draft. A picture of the bow is given in Figure 3(b) (Hyun-Soo, 2004).
Latest Hull Form Design Ice Going Tanker
American Association for Respiratory Care (AARC) has extensive knowledge of ice going vessels and Daewoo Shipbuilding & Marine Engineering Co., Ltd. (DSME) of open water vessels. By combining their knowledge the companies managed to develop a new hull form that can efficiently be used both in ice and in open water. They design an Aframax size tanker with a completely new type of ice breaking stern. The aft body has twin POD propulsion, twin gondola type hull and cut-off transom. The twin gondola stern results in high propulsion efficiency. In spite of a slightly higher open water resistance of the twin gondola a fuel saving of around 10% in open water can be reached compared to a conventional open pram type stern with twin pod propulsion. This improved performance in open water is possible because of a very favourable flow interaction between the stern gondolas and the propulsion units. The successful fine-tuning of the gondola form means no sacrifices of ice going capability were made. Running astern the vessel can penetrate consolidated ridges almost continuously with creeping speed and by turning the PODs from side to side (Risto, 2011),
II.2. Construction
According to the Kumar (2008) on the project report of MT SUSHMA, design of a 150,000 t double acting ice class tanker, the different rules and regulations governing double hull ice tanker construction are :
a) Classification Society Rules
b) IMO Regulations
c) International Convention for the Prevention of Pollution from Ships
d) International Convention for the Safety of Life at Sea (SOLAS), 1974
e) International Convention on Load Lines, 1966
Double hull construction makes use of wing tanks and double bottom spaces throughout the cargo region, so that even if the outer hull is damaged, oil out flow will not occur. Double hull construction is the modern trend. The vessel is to be classed under LRS. All steel for hull construction is of ship building quality High tensile steel (DH32 or DH36) and grade of steel is in accordance with FSICR as par Ice Navigation requirements. The ship is made of Higher tensile steel (DH32 and DH36) and is of all welded construction. The wing tanks and double bottom constitute the double hull of the ship (Kumar, 2008)
II.3. Propulsion System
The idea of running an icebreaking vessel with the end with propellers first is over 100 years old. The early development utilizing the new technology is described by Juurmaa et al (1995). Since 1995 the operability of Azipod propulsion in severe ice conditions has been overwhelming. The tankers Uikku and Lunni have made several voyages in the Northern Sea Route. Sometimes the operation has required them to turn the ship around and break ice running astern (Kimmo, 2001). According to the Hyun-soo (2004), the propulsion system that use for the ship was a single screw, single rudder stern with an innovative bulbous bow. Pod propulsion system without any rudder and shafting is normally employed for double acting tanker. It can generate thrust to arbitrary directions of 360 degrees. Utilizing this characteristic, double acting tanker (DAT) was built at Sumitomo Heavy Industries, Ltd. DAT is a double-bow tanker, which one bow is a bulbous bow and another is an ice breaking bow, Bulbous bow can reduce resistance of the ship by about 15% from ordinary ice breaking ship with ice breaking bow (fuel economy 20%), and in addition during navigation on ice sea area, broken pieces of ice can be separated from hull by propeller flow and thus high ice breaking efficiency is expected.
According to the Evgeny, et al (2011), vessels with an azimuth propulsion system (APS), including double-acting tankers (DATs) designed for autonomous ice navigation in astern-running mode, which makes it possible to improve their icebreaking capability and operation performance in open water by means of bow shape optimisation, are considered to be promising ship types for hydrocarbon and ore material transportation along Arctic Passage routes.
The double-acting-tanker (DAT) concept works by allowing the vessel to proceed forward in thin ice and astern in heavy ice. Based on an Azipod propulsion configuration, the DATvessel can turn through 180 deg. As a result, the forward region of the hull is optimised for open water, retaining a bulbous bow, while the stern is ice strengthened to optimise ice-breaking and efficient and safe navigation. Operation astern in level ice is possible due to the combined effects of a decrease in hull ice resistance, due to lubrication of the hull by the water flow from behind the propeller and a decrease in buoyancy of the ice sheet ahead of the ship due to the water flow into the propeller, making the ice easier to break. Ice ridges can also be navigated as the milling effect of the propeller cuts a path for the vessel (Tanker Operator, 2006).
II.4. Performance
Manoeuvrability for common ice breaking type of ship is an important criterion, as they have to operate in close proximity to other ships or fixed installations, but their transportation capacity is limited (Hyun-Soo, et al, 2004). Ice tanker ship must have same Manoeuver characteristic as common ice breaking. The vessels are powered by the Azipod propulsion system enables the vessels to turn around without moving forward. The double-acting-tanker (DAT) concept works by allowing the vessel to proceed forward in thin ice and astern in heavy ice (Tanker Operator, 2006). Ice tanker that going astern have slow speed but normal speed in open water. Hyun-Soo, et al (2004) mentioned that ice tanker ship for Aframax size required to progress at 4 knots in level ice, 1.0m thick, and maintain forward motion in ice 2.0m thick. The design must also be able to transit ridges and move forward in snow-covered ice. The design speed in open water was 16 knots and type of ice tanker ship that was introduced by Niini (2004), the vessel can cut through a 70-cm-thick layer of ice and ice ridges 13 meters thick.
III. CONCLUSION
1. 1. Tankers are designed to pass through the sea ice area without using icebreaker ship has many advantages and benefits for shipowners
2. Ships can operate independently in severe ice conditions without icebreaker assistance but retain better open water performance than traditional icebreaking vessels
3. 3. Operate independently (without using icebreaker vessel) is save money for more than 20% of total transport costs
4. 4. The capability of breaking ice is measured in uniform ice conditions (level ice, brash ice) by the speed at which certain ice thickness can be broken. Ice ridges and multi-year ice floes are distinct ice features and the capability in these is measured by the ability to penetrate these. The speed that the ship makes in ice is determined by the ice resistance determined by ice properties, and the hull shape and main dimensions as well as the thrust provided by the propulsion.
References
Arcdev. 1999. Public Summary Report of the ARCDEV Project. European Commission Under The Transport Rtd Programme Of The 4 Th Framework Programme.
Globalsecurity. 2011. World Wide Icebreakers. http://www.globalsecurity.org/ military/ world/ icebreakers.htm
Kumar, Vimal. 2008. Project Report MT SUSHMA, Design Of A 150,000 T Double Acting Ice Class Tanker Of Service Speed 15.0 Knots In Open Water And 5.0 Knots In Severe Ice Condition. Department Of Ship Technology Cochin, University Of Science & Technology.
Kim, Hyun-Soo, et al. 2004. Hull Form Designs For Icebreaking Tankers. Samsung Heavy Industries, Marine Research Institute, Koje Shipyard, Korea & David Molyneux Institute for Ocean Technology, National Research Council, Canada.
Kurimo, Risto. 2011. Improved Double-Acting Ship. Arctic Passion News, March 2011, Page 3.
Kimmo. Juurmaa, Wilkman G., Bäckström M., 1995 New Icebreaking Tanker Concept for the Arctic (DAT), POAC 95, Murmansk, Russia.
Kimmo. Juurmaa. 2001. The Development Of The New Double Acting Ships For Ice Operation. POAC 2001, OTTAWA 12-17.8.2001.
M. Appolonov, Evgeny. 2011. Regulation of extreme ice loads acting on hulls of azimuth propulsion systems for ice ships. pages 239-247 on book Ships and Offshore Structures.
Niin, M et al, 2006. Arctic Shuttle Container Link from Alaska, US to Europe, AARC Report K-63 , Mach 2006. Aker Arctic Technology Inc.
Tanker Operator. 2006. Ice Class Shipping Review. Ice Class TankerShipping Supplement included with this issue. Tanker Operator, September 2006, page VII
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