Double acting ship

MT Tempera, the first double acting tanker, breaking ice astern.

Double acting ship (DAS) is a type of icebreaking merchant ship designed to run ahead in open water and astern in ice. Such ships can operate independently in severe ice conditions without icebreaker assistance but retain better open water performance than traditional icebreaking vessels.

Double acting ships carrying liquid cargo are generally referred as double acting tankers (DAT). In the early 1990s Kvaerner 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. The concept has been patented by Aker Arctic Technology Inc., the successor company of MARC.

History and development

In the early 1990s studies conducted by Kvaerner Masa-Yards showed that in oil transportation from the Russian Arctic to Europe the ship's open water efficiency is as important factor as its ability to operate in difficult ice conditions as on a direct route 90% of the time will 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 the most 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 charasteristics and is subjected to slamming in heavy weather while a hydrodynamically efficient bulbous bow greatly increases the ice resistance. 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. This was because the forward-facing propellers generated a water flow that lowered the resistance by reducing friction between the ship's hull and ice. 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 bow propellers being damaged is too great. Forward-facing propellers have also a very low propulsion efficiency and the steering ability of a ship is greatly reduced when running astern, so it could not be considered a main operating mode for merchant ships.

Electric podded propulsion

Because of the limitations of traditional propulsion systems the development of double acting ships wasn't seriously considered until the development of electric podded propulsion in the early 1990s. It combines the advantages of the diesel-electric powertrain, already widely used in icebreakers, with the excellent manouverability of azimuth thrusters.

The superiority of Azipod 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 an 1.5 MW Azipod unit she could run astern in level ice as thick as 0.60 meters. The vessel could also easy be steered when running astern in ice. When product tankers MT Uikku and MT Lunni were converted to Azipod propulsion in 1993 and 1994, respectively, the result was similar increase in manouverability 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 only 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. Azipod-equipped river icebreaker Röthelstein, delivered in 1995, was designed to break apart ice ridges deeper than the vessel's draft in such manner. 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.

Icebreaking supply ships Arcticaborg and Antarcticaborg, delivered in 1998, 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, but being capable of breaking level ice up to one meter in thickness when running astern. With the help of two Azipod units the ships can also penetrate ice ridges that in the Caspian Sea sometimes reach to the seabed.

Double acting cargo ships

In 2001 Fortum ordered two 106,000 DWT double acting tankers from Sumitomo Heavy Industries Ltd., Japan. These Aframax-sized crude oil tankers were intended 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. The oil therefore 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, equipped with one pulling-type 16 MW Azipod unit, had the highest Finnish-Swedish ice class, 1A Super, and were designed to be capable of independent navigation and icebreaking in Baltic ice conditions with a possibility to operate in the Pechora Sea as well. The ships followed 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, MT Tempera, was delivered in 2002 and its sister ship, MT 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.

The next generation of double acting ships emerged when the Russian mining company Norilsk Nickel ordered a prototype of a series of 14,500 DWT arctic general cargo/container ships intended to replace its ageing fleet of Norilsk-class (SA-15) vessels that were built in Finland in the 1980s. Unlike the tankers it was designed to have excellent icebreaking capability, 2 knots in level ice with a thickness of 1.5 meters, both ahead and astern. The ship, Norilskiy Nickel, was delivered from Hietalahti shipyard in 2006 and performed beyind expectations during ice trials in the Yenisei Gulf. Four additional ships, with an option for fifth, were ordered in 2007 from Aker Yards's shipyards in Germany, with deliveries in the second half of 2008 (Monchegorsk, Zapolyarny and Talnakh) and early 2009 (Nadezdha).

In 2007 the Russian state-owned shipping company Sovcomflot ordered three 70,000 DWT double acting Arctic shuttle tankers from Samsung Heavy Industries in South Korea and two from Admiralty Shipyard in St. Petersburg, Russia. The first ship, Vasily Dinkov, was delivered in 2007 and its two sister ships, Kapitan Gotsky and Timofey Guzhenko in 2008 and 2009, respectively. Kirill Lavrov, the first double acting tanker built in a Russian shipyard, was delivered in 2009 and the last of the series, Mikhail Ulyanov, in 2010. The tankers, equipped with two Azipod units, are capable of bow loading and independent operation in level ice up to 1.2 meters in thickness.