A double acting ship is a type of icebreaking ship designed to travel forwards in open water and thin ice, but turn around and proceed astern (backwards) in heavy ice conditions. In this way, the ship can operate independently in severe ice conditions without icebreaker assistance but retain better open water performance than traditional icebreaking vessels.Juurmaa, K et al.: The development of the new double acting ships for ice operation. Kvaerner Masa-Yards Arctic Technology, 2001 and 2002 .
Double acting ships carrying liquid cargo are generally referred as double acting tankers. In the early 1990s Kværner Masa-Yards Arctic Technology Centre (MARC) developed the concept for oil transportation between the Russian Arctic and Europe and the first double acting tanker, Finnish crude oil tanker Tempera, was delivered in 2002.Mastera/Tempera , Neste Oil.
History In the early 1990s, studies conducted by Kvaerner Masa-Yards showed that the ship's open water efficiency is as important a factor as its ability to operate in difficult ice conditions in oil transportation from the Russian Arctic to Europe. This was due to the fact that on a direct route 90% of the time would be spent in open water. Direct independent transportation with a vessel capable of navigating in both ice and open water was also found out to be a more economical alternative in comparison with transshipment, i.e. the use of different vessels for different parts of the journey, or normal ships relying on icebreaker assistance.
Although icebreaking cargo ships had been built in the past, their hull forms were always compromises between open water performance and icebreaking capability. A good icebreaking bow, designed to break the ice by bending it under the ship's weight, has very poor open water characteristics and is subjected to slamming in heavy weather. However, a hydrodynamically efficient bulbous bow greatly increases the ice resistance, making it unsuitable for icebreakers.Juurmaa, K et al.: New ice breaking tanker concept for the arctic (DAT) . Kvaerner Masa-Yards Arctic Technology, 1995.Kujala, P and Riska, K: Talvimerenkulku (TKK-AM-13). Department of Applied Mechanics, Helsinki University of Technology, 2010. As a result, the total efficiency of icebreaking ships is 20–40% less than that of good open water vessels of similar size mainly due to the bow form.
In the late 1800s, captains operating ships in icebound waters discovered that sometimes it was easier to break through ice by running their vessels astern. Although not known at the time, this was because the forward-facing propellers generated a lubricating water flow that lowered the ice resistance by reducing friction between the ship's hull and ice.Aker Arctic Technology Inc.: Icebreaking Supply Vessels Arcticaborg and Antarcticaborg . However, as the steering ability of a ship is greatly reduced when running astern, it could not be considered a main operating mode. These findings resulted in the adoption of bow propellers in older icebreakers operating in the Great Lakes and the Baltic Sea, but in the more severe Arctic ice conditions they could not be used because the risk of the bow propellers being damaged by multi-year ice floes was too great. Furthermore, forward-facing propellers have a very low propulsion efficiency and they considerably increase the ship's open water resistance, making them unsuitable for merchant ships.
Development Because of the limitations of traditional propulsion systems, the double acting ship concept wasn't seriously considered until the development of electric podded propulsion units which combine the advantages of the diesel–electric powertrain, already widely used in icebreakers, with the excellent manoeuvrability of azimuth thrusters. Initially developed as a co-operation between the multinational electrical equipment corporation ABB Group and the Finnish shipbuilder Masa-Yards in the late 1980s, the new propulsion unit became known as Azipod (a portmanteau of "azimuth thruster" and "podded propulsion unit") which is today a trademark of ABB Group.
The superiority of electric podded propulsion in icebreaking ships, especially when running astern, was proved when the first propulsion pod was installed on fairway maintenance vessel Seili, owned by the Finnish Maritime Administration, in 1990. Before the conversion the ship could not break ice astern at all, but after the propeller and rudder were replaced with a 1.5 MW Azipod unit she could run astern in level ice as thick as . The vessel could also easily be steered when running astern in ice. When product tankers Uikku and Lunni were converted to Azipod propulsion in 1993 and 1994, respectively, the result was similar increase in manoeuvrability and icebreaking capability. Even though the ships were originally designed with icebreaking capability in mind, after the conversion ice resistance in level ice when running astern was 40% of that when breaking ice ahead despite the ships being equipped with an icebreaking bow and not designed to break ice astern.
Model tests conducted by MARC in 1994 showed that a double acting ship equipped with an Azipod propulsion unit could break through ice ridges in continuous motion instead of ramming like conventional icebreakers. It also required less power for running in level ice than traditional designs, resulting in 40–50% reduction in ice resistance due to lubricating effect of propeller-induced water flow, more open stern design and the propellers being allowed to mill (crush) the ice. The icebreaking capability of an Azipod-equipped icebreaker operating astern in level ice was also found out to be superior to traditional icebreakers regardless of propulsion arrangement.
Double acting ships
Icebreakers and other special vessels
The first double acting icebreaking vessel, Azipod-equipped river icebreaker Röthelstein, was delivered in 1995. The vessel was designed to break apart ice ridges deeper than the vessel's draft when moving astern. It was followed by icebreaking platform supply vessels Arcticaborg and Antarcticaborg, delivered in 1998, that were the first vessels to fully utilize the double acting ship concept by having the bow designed for operation in open water and light ice conditions. The vessels were designed to be capable of breaking level ice up to one meter in thickness when running astern and, with the help of two Azipod units, penetrate ice ridges that in the Caspian Sea sometimes reach to the seabed.
The Norwegian Coast Guard operates a double acting offshore patrol vessel KV Svalbard, built in 2002 and equipped with two 5 MW Azipod propulsion units. On 9 July 2007, the Canadian Broadcasting Corporation reported that Canadian Prime Minister Stephen Harper had announced that Canada would be building six to eight patrol vessels based on Svalbard. However, since the announcement, the design has progressed from an Azipod-equipped double acting ship to a more traditional shaft-driven version.
Icebreaking supply and standby vessel SCF Sakhalin, delivered in 2005 as FESCO Sakhalin, was the first large-scale double acting icebreaker. The vessel, propelled by two 6.5MW Azipod propulsion units, is designed to break level ice up to thick and ice ridges with a consolidated layer.. Aker Finnyards. She has since been followed by six vessels based on the same hull form and built at the same shipyard for the offshore operations in the Sea of Okhotsk: platform supply vessels Vitus Bering in 2012, Aleksey Chirikov in 2013 and Gennadiy Nevelskoy in 2017, and standby vessels Stepan Makarov, Fedor Ushakov, and Mikhail Lazarev in 2017. The owner of these vessels, Sovcomflot, also operates three icebreaking platform supply vessels of slightly different design that are capable of breaking ice in astern direction. The 2006-built ships, SCF Endeavour, SCF Enterprise and SCF Endurance, are propelled by two 7MW Z-drive thrusters and capable of breaking first year ice up to thick as well as multi-year ridges with a thickness of .E-Class Ice Breaking Supply Vessels (IBSVs). Swire Group.
The 3,500-ton United States Coast Guard multipurpose icebreaker and buoy tender USCGC Mackinaw, delivered in 2005, also incorporates some of the features typical for double acting icebreakers, such as podded propulsion and astern icebreaking capability.Wilkman, G. New Ship Designs Under Development at Aker Yards. Powerpoint presentation. Aker Arctic, 2006. Slide 26.
The 25MW diesel-electric line icebreaker ordered by Rosmorport from Baltic Shipyard under the project name LK-25, Viktor Chernomyrdin, will be capable of proceeding continuously both ahead and astern in compact ice field up to thick with a snow cover at . The hybrid propulsion system of the new icebreaker is a further development of the traditional double acting concept and will consist of two 7.5MW ABB Azipod units and a fixed 10MW centerline shaft. The construction of the new polar icebreaker began in 2013 and it is expected to enter service in the Northern Sea Route in 2018.ABB Wins $35 Million Order For Icebreaker in Russia's Arctic. 4-traders, 1 October 2012. New possibilities in Arctic offshore through new technologies. Mikko Niini, Aker Arctic, 4 February 2010.
Gazprom Neft has ordered two 22MW icebreakers, Aleksandr Sannikov and Andrey Vilkitskiy, from Vyborg Shipyard. The vessels, currently under construction and scheduled to be delivered in 2017, represent a further development of the Finnish icebreaker Polaris with capability of breaking ice in both ahead and astern directions.Russia invests in Arctic icebreakers. IHS Fairplay, 3 February 2017.
The new Chinese polar research vessel, currently under construction, will utilize the double acting concept. The Polar Class 3 vessel, fitted with two azimuth thrusters, will be able to break level ice with a thickness of and a snow cover of .Laukia, K. and Weijia, Q.: Chinese Polar Research Vessel . Arctic Passion Seminar, 2013. . World Maritime News, 31 July 2012.
Cargo ships
Tankers
In 2001 Fortum ordered two double acting Aframax tankers from Sumitomo Heavy Industries Ltd., Japan, to replace the company's older tankers that, because of their lower ice class, had traffic restrictions during the worst part of the winter and could not deliver their cargo all the way to the refineries in Porvoo and Naantali because they were not given icebreaker assistance. When this happened, the oil had to be transported to smaller ships of higher ice class at the edge of the ice — a practice that was both uneconomical and hazardous. The new ships are equipped with one pulling-type 16 MW Azipod unit and have the highest Finnish-Swedish ice class, 1A Super. They are designed to be capable of independent navigation and icebreaking in Baltic ice conditions with a possibility to operate also in the Pechora Sea. The ships follow the double acting principle with a bulbous bow for open water performance and stern designed with icebreaking performance in mind. The first double acting tanker, Tempera, was delivered in 2002 and its sister ship, Mastera, in 2003. The icebreaking capability of the vessels proved to be superior to other ships — in shuttle service between Primorsk, Russia, and the Finnish refineries the tankers required no icebreaker assistance and even acted as icebreakers for other merchant ships.Anatoly Gorshkovskij & Göran Wilkman: "Norilskiy Nickel" a Breakthrough in Cost Efficient Arctic Transports . Aker Arctic Technology Inc, 2007. The ships can break level ice up to thick at when operating astern.Fortum ́s Double Acting Tankers . ABB.