Pushing Polar Explorers
Ten years ago, scientists were claiming that the Arctic would be ice free in 2013 – a little too pessimistic as it has turned out – but the commercial attraction of reducing voyage times between Europe and Asia as well as oil and gas exploitation and expedition cruising, means interest in Arctic operations is still high even if they will take place in the harshest of conditions.
This year marks the entry in to force of the IMO’s Polar Code for newbuildings and how much it is needed was highlighted by two events in January when the USCG reported that the Coast Guard Cutter Polar Star finally arrived at the National Science Foundation’s McMurdo Station in the Antarctic after meeting conditions much worse than usual in high summer. At the same time two Russian cargo ships and their icebreaker escort were trapped in ice after a voyage along the Northern Sea Route.
After last year’s North West Passage cruise by Crystal Serenity, forecasts and recent newbuilding contracts indicate an upward trend in cruise traffic in polar waters. Navigating in such areas must be considered from the start in terms of ship design and the propulsion system selection, as it presents additional risks compared to normal open water shipping. The risks are recognized in the Polar Code, which aims “to increase the safety of ships’ operation and mitigate the impact on the people and environment in the remote, vulnerable and potentially harsh polar waters”. The Code makes direct reference to IACS Polar Class (PC) requirements, which set out in detail structural and machinery requirements for ships “intended for independent navigation in ice-infested polar waters”.
In terms of propulsion systems, ABB’s Azipod is the market leader both in independently navigating ice-going vessels and in cruise ships. This makes it a natural starting point in the design of any cruise vessel intended for independent operation in ice-infested polar waters or nearby sea areas. Azipod units are available with Polar Class notation suitable for the intended operation area, season and ice conditions.
The highest Polar Class vessel built so far is the Azipod-equipped Finnish icebreaker Polaris, accorded PC4 Icebreaker(+) notation by Lloyd’s Register. Polaris can break level ice 1.8m thick at 4kts and is also the world’s first LNG-fueled icebreaker. The next Polar Class icebreaker newbuilding worthy of note will be for the Polar Research Institute of China. With an even higher Polar Class notation, PC3 Icebreaker.
Since the first delivery in 1990, Azipod units have operated in ice without a single instance of structural damage due to ice loads. This includes operation in ice covered Baltic Sea, Caspian Sea, Great Lakes, Okhotsk Sea and Arctic Sea. Currently there are more than 60 very high-ice class Azipod vessels in operation or ordered. Among the latest high-ice class Azipod vessels are a series of LNG carriers being built for the Yamal LNG project in the Russian Arctic. The first, Christophe de Margerie, launched in January 2016, is both the world’s most powerful LNGC and the world’s first ice capable LNGC. She has ice class Arc7 from Russian Maritime Register of Shipping, comparable to PC3.
The world’s first Polar Class passenger vessel will be the Azipod-equipped Scenic Eclipse with PC6 ice class notation granted by Bureau Veritas. Scheduled to launch in August 2018, Scenic Eclipse is a discovery yacht able to navigate in both Arctic and Antarctic waters .
Azipod propulsion has certain benefits that explains why it has almost completely superseded conventional shaftline-rudder propulsion in both cruise and independently ice-going vessels over the last decade. These mainly relate to crew and passenger safety, but also have a bearing on environmental protection, performance in both ice and open water operation and total-cost of ownership.
With Azipod propulsion the full propeller thrust can be directed freely in any direction, whereas in fixed shaftline-rudder arrangements thrust decreases rapidly as helm angle increases. Generally, a conventional rudder can produce only about 40% side thrust compared to maximum ahead bollard pull thrust. The figure for flap rudders is up to 60%. With a 360-degree freely turning Azipod full thrust can be precisely applied in any direction giving 150% more side thrust than a conventional rudder. In addition, it is possible to navigate astern and sideways simultaneously, which is difficult to achieve with a rudder since negative propeller speeds reduce the effectiveness of a rudder considerably. Full thrust in any direction is a great benefit when manoeuvring ships in ice fields and when approaching ports.
Collision avoidance is improved because conventional rudders typically require stern tunnel thrusters to assist weak manoeuvring. However, tunnel thrusters are less effective at higher ship speeds, whereas Azipod units are effective throughout the ship’s speed range and eliminate the need for stern thrusters giving greater flexibility and simplicity in ship design. More effective and safer turning capability of Azipod propulsion have been verified, for example, by full-scale and full-speed turning circle tests between sister-ships MS Fantasy with conventional propulsion and MS Elation with Azipod propulsion which recorded 38% reduction in tactical diameter.
With traditional steering, an emergency crash-stop is accomplished by reversing the propeller rpm from positive to negative. This is time-consuming as the ship’s generators must go from full power to zero power and then ramp up again to full power in the opposite direction. In practice the vessel operating with a rudder will also lose its heading control as the rudder will not work efficiently unless the propeller is producing thrust for it – and at negative propeller rpm there is a little thrust available for the rudder. This means that ship heading and direction during the crash-stop are effectively at the mercy of the elements.
In Azipod vessels the crash-stop can be accomplished in the “pod-way” by steering the Azipod units outwards 180° and keeping positive propeller rpm during the whole crash-stop. This shortens the crash-stop distance considerably – typically by at least 50% and gives enormous side force in any desired direction irrespective of ship speed allowing full control over heading and direction of the vessel during the whole crash-stop even in heavy weather conditions. The combination of 50% shorter crash-stop distance and full heading control is a huge advantage in onboard safety when considering worst-case scenarios – especially in ice infested waters.
One danger of operating in polar waters is the risk of getting strapped in ice. Here Azipod propulsion allows double acting ship (DAS) designs, as pioneered by Aker Arctic which greatly improves a vessel’s manoeuvring capability in ice fields. A DAS ship can navigate stern first in ice conditions which offers several benefits, including:
- Propulsor thrust lubricates the sides of the ship hull, reducing ice friction caused by compressed ice fields
- Azipod unit(s) can be rotated 360 degrees to flush and break ice ridge if impeding vessel progress
- Safer and more efficient manoeuvring in ice fields thanks to the ability to produce full thrust from Azipod units in any direction
- Considerably less installed power (for example 40% reduction) required to achieve ice-going capability compared to a non-DAS ship
In the Azipod, the electric motor is installed directly on the propeller shaft making for a simple and robust drivetrain able to withstand ice loads hitting the propeller. In contrast to mechanical Z- or L-drive azimuthing thrusters, there are no mechanical gears so the Azipod shaftline can withstand both bending and high torque peaks under heavy ice loading.
For extreme ice classes, the electric motor and ship power plant can be configured to provide an over-torque capability that ensures the propeller rotates even in heavy ice preventing ice blocks hitting static propeller blades from an unfavourable direction while a vessel is proceeding under its own inertia. Over-torque ensures that rotation of the propeller is constant and thus the angle of attack is favourable from the perspective of blade strength. In comparison to shaftline propulsion, a further Azipod safety feature derives from its steering system, which is set to yield and absorb extreme impact loads on propeller side blades.
For cruise vessels Comfort Class requirements are easier to fulfil than would be the case with conventional shaftline-rudder propulsion arrangements. There are no noise-generating gears and the pod motor and its shaft are located completely outside the ship hull. More importantly, the pulling propeller receives a steady undisturbed wake field, which gives propeller designers greater scope to optimize propellers for silent operation compared to a conventional pushing propeller with rudder.
In addition, disturbing vibration caused by manoeuvring with high rudder angles are avoided as the Azipod propeller and its housing rotate as a single unit, meaning there is never a high angle of attack between them. Noise and vibration caused by stern tunnel thrusters are avoided as these are no longer necessary.
The Polar Code demands much with regard to environmental protection and Azipods are claimed as the best propulsion system in terms both of risk of oil leakages and overall propulsion energy consumption. The main feature is the US Vessel General Permit approved shaft seal design which has no oil-water interface. The amount of oil used in a gearless Azipod unit is only a fraction of that found in geared mechanical azimuthing thruster or traditional shaftline propulsion.
Furthermore, the fully electric Azipod propulsion with its small footprint for vessel general arrangement makes it easier for ship designer to utilize alternative power sources, such as LNG, batteries or fuel cells – or leave space aside for these for conversion at a later date.
ABB’s recommended starting point for Polar Class expedition cruise vessels is the gearless DO series covering power range from 1.5MW to 7.5MW. In the case of an expedition cruise ship with Polar and Comfort Class notations, expected installed propulsion power saving with twin DO installation is about 10% compared to twin shaftline installation with electric propulsion.