Azipod® M propulsion for ferries and RoPax vessels: faster, safer, cleaner

Kimmo Kokkila, M.Sc.
Product Manager, Compact Azipod®, ABB Marine & Ports
kimmo.kokkila@fi.abb.com

The Azipod® M series is designed to help ferry and RoPax operators respond to the systematic tightening of the Energy Efficiency Design Index (EEDI) and ever-stricter emission controls. The latest addition to the Azipod® propulsor family, the Azipod® M series, can help shipowners reduce emissions, lower the total cost of ownership, and provide improved operational safety and flexibility.

Figure 1. Azipod® M propulsor.
Figure 1. Azipod® M propulsor.

The new Azipod® M podded propulsor series covers three frame sizes in the power range from 7.6 MW to 14.5 MW per propeller, and is based on the Azipod® C and D series. Azipod® C technology has been on the market since 2000, and Azipod® D – since 2015, giving them a proven track record. The compact dimensions of the Azipod® M allow it to be installed under the car deck of RoPax vessels.

A ferry or a RoPax design with the inclusion of the new Azipod® M series offers several advantages compared to conventional shaftline-rudder propulsion:

  • Faster port approaches and departures
  • Improved on-time performance of sailings
  • Better resilience to weather
  • More payload and more room for alternative energy sources
  • Lower energy and fuel consumption
  • Competitive vessel newbuilding price
  • Improved passenger comfort
  • Improved operational safety

Faster port approaches and departures

Ships equipped with Azipod® propulsion have superior manoeuvrability with the 360° steerable main propellers. Turning of the ship, crabbing, steering while decelerating and stopping are more effective, accurate and faster compared to conventional shaftline-rudder propulsion. Figure 2 shows an example from a simulator run, where turning in a dock with a 200-meter RoPax was six minutes faster with Azipod® propulsion.

For newbuilding projects, port and vessel specific time savings in manoeuvring can be estimated in ABB’s deck simulator facility in Helsinki, Finland. Customer representatives can make runs in a variety of different ports and see the differences between Azipod® and shaftline vessels for themselves.

Figure 2. Example berthing tracks of RoPax dockings by professional captains in a bridge simulator. On average, the left maneuvre with Azipod® propulsion was 6 min faster compared to right one with conventional shaftline propulsion.
Figure 2. Example berthing tracks of RoPax dockings by professional captains in a bridge simulator. On average, the left maneuvre with Azipod® propulsion was 6 min faster compared to right one with conventional shaftline propulsion.

Resilience to weather

Extreme weather conditions can pose challenges during docking or approaches in tight channels. The ability to use full thrust from the main propellers in any direction improves control of the ship in extreme wind conditions, as well as the crabbing capability of the vessel.

Better resilience to weather improves on-time performance and allows the schedule buffer to be reduced. Time saved can be further used to decrease the maximum ship speed in transit or to increase number of sailings per day. Decreasing maximum ship speed reduces fuel costs (OPEX) and enables a lower installed power requirement and cost for newbuilds (CAPEX).

Precise manoeuvring with 150 percent more side thrust

Generally, a conventional rudder can produce only about 40 percent side thrust compared to maximum ahead bollard pull thrust. The figure for flap rudders is up to 60 percent[endnoteRef:2]. The 360-degree rotating Azipod® delivers 150 percent more side thrust than a conventional rudder. Full thrust in any direction is a significant benefit when manoeuvring ships in tight and busy channels. [2: Mehldau, J., Station Keeping with High Performance Rudders, presentation at Dynamic Positioning Conference 9-10 Oct 2012, slide 5, http://dynamic-positioning.com/proceedings/dp2012/design_control_mehldau_pp.pdf, visited 4 Nov 2018]

57 percent better crabbing capability

One of the best-known Ferry and RoPax designers, Deltamarin Ltd., have performed a detailed case study of an Azipod® M-equipped RoPax ship compared to conventional shaftline-rudder design, including crabbing performance. The propulsion and vessel details of the comparison are given in Table 1. According to the study, Azipod® propulsion improves the crabbing capability of a 225 m long RoPax by as much as 57 percent, as shown in Table 2. Tailwind conditions are especially challenging for conventional shaftline propulsion, whereas Azipod® propulsion excel in tail winds.3

Table 1. Thruster set up of propulsion alternatives.3

LenghtOA 225 m, breadth 34 m, draught 6.7 m

Azipod® ship thruster set up

Shaftline ship thruster set up

 

Power

Pcs

 

Power

Pcs

Azipod® units (FPP)

10.6 MW

2

CPP with flap-rudder

10.6 MW

2

Fwd tunnel thursters

2 MW

3

Fwd tunnel thrusters

2 MW

3

Table 2. Maximum allowed wind speed for crabbing for the propulsion alternatives. 3

Wind direction

Max. Allowed wind speed [m/s]

Improvement with Azipod

Azipod

Shaftline

15°

34

33

3%

30°

24

23

4%

45°

21

20

5%

60°

22

21

5%

75°

22

20

10%

90°

23

20

15%

105°

24

19

26%

120°

25

18

39%

135°

26

17

53%

150°

33

21

57%

165°

41

27

52%

More payload, more room for alternative energy sources

Azipod® propulsion enables a flexible machinery arrangement that is easy to design for the vessel’s specific requirements and priorities. In the case study, Azipod® propulsion motors installed outside the vessel hull, without long shaftlines, saved 255 m2 of machinery footprint compared to conventional diesel-mechanical shaftline propulsion, see Figure 4. Lack of fixed shaftlines gives more freedom for locating propulsion and power plant machinery, enabling re-arrangement for higher payload, and clearing additional space needed for alternative energy sources such as LNG tanks, batteries or fuel cells. Table 3 demonstrates some benefits of re-arranging the general arrangement for the case vessel.[endnoteRef:3] [3: Deltamarin Ltd., Report for Project 7107: Marine Study on Azipod M® – Comparison of Azipod and diesel-mechanical shaftline propulsion systems, 5 June 2018, Confidential]

Figure 3. Re-arrangement of the case vessel’s tank top plan with Azipod® propulsion freed total 255 m2 footprint.
Figure 3. Re-arrangement of the case vessel’s tank top plan with Azipod® propulsion freed total 255 m2 footprint.
Figure 4. Azipod® arrangement allows more valuable space in front of engine rooms.4
Figure 4. Azipod® arrangement allows more valuable space in front of engine rooms.4

Similar space savings were also achieved by ship designer Foreship Ltd., who concluded that Azipod® M propulsion would enable main engine rooms to be located in one watertight compartment aft, saving at least 10 m compared to mechanical propulsion for Safe Return to Port (SRtP) designs, as seen in Figure 5. This would leave more space in the forward part of the vessel for additional stowage, LNG tank rooms or lower trailer holds.[endnoteRef:4] [4: Foreship Ltd., FS2939: Ferry Pod Feasibility, 8 Nov 2017, Confidential]

Figure 5. Due to lack of shaftline, supporting brackets and tunnel thrusters, the pulling Azipod® propeller re-ceives steady incoming water flow, resulting in less noise and vibration, and better efficiency.
Figure 5. Due to lack of shaftline, supporting brackets and tunnel thrusters, the pulling Azipod® propeller re-ceives steady incoming water flow, resulting in less noise and vibration, and better efficiency.

Table 3. Examples of utilization of freed space.3

$1,700,000 annual savings in fuel and energy consumption

Operational costs of conventional and Azipod® M propulsion were also included in the case study. The main fuel consumption advantages with a twin Azipod® vessel stem from lower vessel resistance and better propulsion efficiency. As shown in Table 4, the difference in resistance was only 10 percent because the shaftline alternative was not equipped with stern tunnel thrusters, which typically increase resistance.

Table 4. Effect of appendages on hull resistance.3

Savings with podded propulsion increase further compared to shaftline propulsion due to undisturbed water flow to the propeller and optimum propeller angle towards the inflow which both increase propeller efficiency, Figure 6. According to the case study, the savings on delivered power (PD) at 22 kts was 12.0 percent with Azipod® propulsion, see Table 5.

Table 5. Delivered power requirement for the case vessel at 22 kts.3

Figure 6. Full-speed steering tests from MS Elation show the superiority of Azipod® steering compared to her Fantasy class rudder-equipped sister vessel.
Figure 6. Full-speed steering tests from MS Elation show the superiority of Azipod® steering compared to her Fantasy class rudder-equipped sister vessel.

Taking into account mechanical losses (3.5 percent) for shaftline propulsion employing 10 bearings and a gearbox on a mechanical drive train, and electrical losses of Azipod® propulsion drive train (9 percent) including propulsion motor, transformer, frequency converter and generators, the savings in engine power (PB) with Azipod® propulsion is 6.6 percent.

These savings were further simulated for seven existing ferry routes relevant for this size of vessel. The simulation also considered the fuel oil consumption advantage of electrical power plant in partial loads. The resulting fuel oil cost saving with Azipod® M propulsion is on average $1.7 M per year for the seven routes presented on Table 6. The monetary values are based on prices for LNG of $355/ton and for MGO $555/ton.

Table 6. Fuel oil cost saving with Azipod® M propulsion compared to conventional shaftline propulsion.3

In addition to savings in energy consumption, Azipod® M propulsion saves on other operational expenses. For example, lower installed power on main engines requires less engine maintenance and lower lubrication oil consumption. Estimated savings from these are listed in Table 7.

Table 7. Savings due to less ME maintenance and lower lubrication oil costs.3

Superior safety with 38 percent smaller turning circle

In collision avoidance manoeuvres, an Azipod®-equipped vessel is more likely to avoid collision than a vessel with conventional shaftline-rudder arrangement. This is because conventional rudders typically require stern tunnel thrusters to assist in manoeuvring. However, tunnel thrusters do not work effectively at higher ship speeds, whereas the superior steering capability of Azipod® units is effective throughout the ship’s speed range.

The more effective and safer turning capabilities of Azipod® propulsion have been verified by full-scale and full-speed turning circle tests on sister ships MS Fantasy with conventional propulsion, and MS Elation with Azipod® propulsion. A 38 percent reduction in tactical diameter[endnoteRef:5] was recorded, see Figure 7. Model experiments with a wider set of ships have shown similar results, see Figure 8. [5: Kurimo, R., Sea Trial Experience of the First Passenger Cruiser with Podded Propulsion, Practical Design of Ships and Mobile Units, 1998, page 743]

Figure 7. Turning circle data of ship models equipped with podded or conventional propulsion units.
Figure 7. Turning circle data of ship models equipped with podded or conventional propulsion units.
Figure 8. Full-scale comparison between “pod-way” and conventional “negative rpm” crash-stops from full speed.
Figure 8. Full-scale comparison between “pod-way” and conventional “negative rpm” crash-stops from full speed.

Shorter crash-stop distance with full heading control

With traditional rudder steering, an emergency crash-stop is accomplished by reversing the propeller pitch or rpm from positive to negative. Especially changing rpm from positive to negative direction is time-consuming, as the ship’s power machinery must go from full to zero power and then ramp up again to full power in the opposite direction. In practice, any vessel operating with a rudder will also lose control of heading during the crash-stop, as the rudder does not work efficiently unless the propeller is producing thrust, and negative propeller pitch or rpm generates very little thrust for the rudder. This means that ship heading and direction during the crash-stop are effectively at the mercy of current, wind and waves, a condition exacerbated in heavy seas.

In Azipod® vessels, crash-stop can be accomplished by steering the Azipod® units 180° and keeping positive propeller rpm during the entire procedure. This shortens crash-stop distance considerably – typically by about 50 percent (see Figure 9). Moreover, during the crash-stop, Azipod® units can generate enormous side force in any desired direction irrespective of the vessel’s speed. This gives the captain full control over the heading and direction of the vessel during the entire crash-stop, even in heavy weather conditions. The combination of short crash-stop distance and full heading control is an extreme advantage in onboard safety when considering worst-case scenarios.

Robustness suitable for ice classes

Azipod® M products are also available with ice class up to 1A Super and PC 6 – or even higher if power is de-rated. Inside the Azipod®, the electric motor is installed directly on the propeller shaft, making the drivetrain extremely simple and robust against any 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.

Figure 9. The Azipod® propulsion motor is integrated onto propeller shaft, making the drive train gearless and robust.
Figure 9. The Azipod® propulsion motor is integrated onto propeller shaft, making the drive train gearless and robust.

The world’s best passenger comfort

Most modern Azipod-equipped cruise ships are classified according to strict Comfort Class 1 requirements governing onboard noise and vibrations levels. There are no noise-generating gears and the pod motor and shaft are located outside the ship’s hull. More importantly, the Azipod® unit’s pulling propeller receives an undisturbed wake field, as shown in Figure 6, giving propeller designers greater scope to optimize propellers for silent operation compared to a conventional pushing propeller with rudder.

Vibration caused by manoeuvring in ports with high rudder angles is also avoided, as the Azipod® propeller and motor housing rotate as a single unit, meaning there is never a high angle of attack between them. Stern tunnel thrusters are not needed with Azipod® propulsion, thus eliminating associated noise and vibration.

Environmental protection

All Azipod® designs are best-in-class propulsion products in terms of both risk of oil leakages and overall propulsion energy consumption. The main feature is the U.S. Vessel General Permit (VGP)[endnoteRef:7] approved shaft seal design, eliminating any oil-water interface. The amount of oil used in a gearless Azipod® unit is only a fraction of that in geared mechanical azimuthing thrusters or traditional shaftline propulsion. Furthermore, fully electric Azipod® propulsion, with its small footprint for vessel general arrangement, makes it easier for ship designers to utilize alternative power sources such as LNG, batteries or fuel cells, or leave space aside for conversion at a later date. [7: U.S. Environmental Protection Agency, National Pollutant Discharge Elimination System (NPDES), Vessel General Permit (VGP), https://www.epa.gov/npdes/vessels-vgp, visited 4 Nov 2018]

Azipod® M series

At the core of the Azipod® M product line are the latest 4th generation permanent magnet (PM) motors developed by ABB. These motors are structurally as sound as the well-proven Azipod® C and Azipod® D series PM motors, but are optimized further with today’s mass-computing capacity and evolutionary algorithms to a) maximize electrical efficiency and b) minimize the use of expensive rare-earth elements needed to build strong permanent magnets. For the ship owner this means that Azipod® M with 4th generation PM motor will have extremely high electrical efficiency, typically 98 percent, at a competitive price.

The Azipod® M series features additional technical solutions that provide benefits for ferry and RoPax owners and operators. These include:

  • Low onboard height. The Azipod® M unit, including its auxiliary units, have been designed for low onboard height to allow placement under the car deck of RoPax vessels, ensuring more intact loading and unloading, as well as enabling the maximum number of lanemeters.
  • Tailorability. The strut height of the underwater propulsion module can be selected for each project to achieve the best possible propeller diameter, efficiency, and tip clearance. The location of auxiliary units on board (in the pod room) is easily adjustable in order to get the best fit for tight aft-ship designs.
  • Simplicity. Designed to be as simple as possible, ensuring robustness, reliability and easy maintenance for the crew, with all the active auxiliary components easily accessible in the pod room.

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