Double acting ship

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.

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.

In the early 1990s, studies conducted by Kvaerner Masa-Yards showed that the ship's open water efficiency is as important 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. 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. 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.

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 0.6 metres (2 ft). 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.