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A smoother path to air lubrication

The Naval Architect: February 2016

The Finnish naval architect and engineering company, Foreship, oversaw the successful design and installation of its own air lubrication system (ALS) in 2015 on both Quantum of the Seas and Anthem of the Seas, built by Meyer Werft. The system’s installation meant that propulsive power could be reduced, and that when the ships were operating at cruising speeds, a net fuel saving equivalent to 7-8% was achieved, according to Royal Caribbean International (RCI). Foreship says a final evaluation of performance is still ongoing, but the company believes an overall net fuel saving within a defined speed range and set of conditions can be confirmed at roughly 5%.

As the first air lubrication system installed on a cruise vessel, Foreship had to overcome challenges obstructing the uptake of alternative systems, but its success has now meant that ‘several’ retrofit cruise ship installations of its ALS are now also underway for RCI.

Similar in form to this uptake in orders, the company is also looking to expand the range of vessels that could utilise its ALS and has recently undertaken a feasibility study for a large ro-pax ferry with 4,100m freight lane capacity and 900 passengers. According to the company, simulations indicate that the ALS is suitable for ferries operating at less than 20kn which are characterised by high propulsion power. Where the normal service speed is 18kn, net savings of up to 10% (1,500kW) are predicted, depending on the air volume.

However, the company readily admits that there are limits to the current system’s adoption for particular vessel types. One requirement is that the vessel must have easily available electric power for the compressors; at present “roughly half of the gross benefit is consumed by the power for the compressor,” says Foreship.

“As the Foreship ALS is a custom made application that needs to be evaluated from case to case it is not possible to give a generic figure on how much compressor power is needed. Everything depends on the platform that is being evaluated and can also vary between different ship types such as cruise, ferry and general cargo. The most important thing in the end is the net gain.”

For vessels where ALS would be appropriate, initial calculations envisage upfront investment being paid back within a reasonable time. The company concedes that plummeting fuel prices have provided a new variable, but it adds that coming environmental regulations could counter this slump, as they will further increase ship operational costs and drive investment in energy efficiency technologies ­ technologies that will be critical in limiting the bottom line impact. According to the company, the ALS should reduce emissions including CO2 in a directly proportionate way to the reduction of the fuel consumed.

When asked about the efficiency of the system in different weather conditions, Foreship replied that weather conditions, wave heights, side winds, currents etc. will influence the system and that “long term projections have been made for different conditions, but this is still
under investigation. ”

Foreship’s approach has drawn on its core competency in hull form optimisation as well as an extensive database detailing the impact of hull performance optimisation solutions in order to advance the concept of air lubrication. As part of its approach, the company oversees optimisation modelling at the early design stage, but also designs and tracks the performance of supplementary fuel saving devices such as stators, boss cap fins and rudder bulbs.

The company has previously pioneered ‘in-waves optimisation’ of hull forms based on CFD modelling to minimise wave resistance, and has developed RANS-CFD coding that simulate real conditions. Its initial ‘in-waves’ project compared the performance of a bulbous bow with a wave optimised vertical stem bow onboard a cruise vessel, establishing that resistance in waves of a wave optimised ship was better across a wide speed range. (See pages 32-33 July/August The Naval Architect).

This experience led the company to rethink under-hull air lubrication, and resulted in the decision to stream ‘micro bubbles’ (around 1mm in diameter) along the hull bottom. Solutions from other suppliers are available, but Foreship believes its ALS generates a larger surface area than systems where larger bubbles are used, reducing drag more effectively.

In development 
Development of the ALS started in June 2011, initiating full scale CFD simulations to apply the solution to both newbuildings and existing ships. RCI’s commercial application followed four years of design development that also included testing the concept in a vacuum tank and sea trials on a first cruise ship with ALS installation.

The path to market required the development of the distribution principle and the means to produce the micro bubbles in the most efficient way. Initially, Foreship’s proprietary CFD-simulation method was used to model relative resistance reduction at different speeds as a function of the amount of injected air. Detailed box geometry was then optimised by CFD simulations, and verified in a cavitation tunnel at Marintek where pressure losses and noise were measured in addition to visual observations, ensuring the functionality of the bubble distribution boxes. CFD analysis was also used to mitigate the risk of cavitation, vibration or reduced propulsion efficiency caused by air emergence to the propellers.

Each ALS is customised to optimise the performance of an individual ship. Full scale RANS-CFD simulations are used in optimising both air volume and air feed locations, which are based on the drawings of the vessel selected. A complete design project includes the configuration of piping and the distribution principle by full scale CFD simulations and requires engineering documents like steel drawings and system diagrams. The simulation also clarifies the air distribution between the distribution locations and the expected piping system pressure losses, to develop precise recommendations for compressor layout.

Projected net savings are a function of resistance reduction evaluated against the required compressor power. For owners, the projected net saving on fuel is inferred from the percentage decrease in the brake power used for propulsion; the annual fuel saving on normal itineraries can then be estimated.

The range of efficacy
Foreship’s CFD modelling indicates that performance gains begin to be seen above speeds of 14 knots up to 22 knots. Air bubbles tend to disperse sideways below 14knots, the company says, while speeds above 22 knots lessen the effect of the bubbles as they move too fast along the hull to create the required ‘cushion’.

However, it must be stressed that the individual characteristics of a ship’s hull are central to the full impact of the ALS in practice. An initial trial of the technology, undertaken onboard an unspecified cruise ship, showed the system achieving 6% fuel savings at 15 knots, and 8% at 22 knots. In this case, though, the effect was still discernible at lower speeds, and it was only below 12 knots that sideways dispersal of the micro-bubbles occurred.

Foreship stresses, therefore, that the ALS efficiency depends a lot on the vessel in question, and the benefit cannot be estimated without ship specific CFD-simulations.

As well as looking to maximise gains, a main design objective has been for the ALS solution to generate no additional resistance when the system is switched off at lower speeds, or due to, for example, a compressor failure. This means that large recesses or cavities in the ship’s bottom, for example, were avoided.

This lack of ‘frictional penalty’ suggests that the solution could be commercially viable across a range of vessel types. The company advises owners to undertake a feasibility study considering an optimised system’s impact at different speeds before any recommendation is made, and suggests that the performance should also be compared to the itinerary of the vessel.

Making the case for ALS
To better understand the potential for savings on a cruise ship, Foreship made a comparison of 10 real itineraries, deriving average results that show the benefits of ALS once other parameters have been optimised.  

Five typical Caribbean schedules and five typical to the Mediterranean were used in the study, revealing that a larger cruise vessel could be expected to have an annual fuel consumption of between 28,000 and 41,000 tonnes when the vessel is optimised in a ‘traditional’ way for a speed of 22.5 knots. This includes the hotel load. The fuel used for propulsion is typically around 50% of the annual total consumption.

When the same ship is optimised for speeds of 18 knots, the annual fuel consumption decreases by 45 ­ 868 tonnes, or 0.1% - 2.9%, depending on the operational profile, the average being 472 tonnes and 1.3%. The lowest saving is naturally achieved on a few relatively high speed itineraries with longer legs at high speed.

When Foreship adds its ‘in-waves optimisation’ analysis, the additional saving annually is 325 ­ 692 tonnes, or 1.2% - 1.5 %, with the averages now 567 tonnes and 1.4%. Here the assumption is that a significant wave height of 1.4m represents the average conditions. As ‘in-waves optimisation’ helps at every speed, the annual saving is more uniform than on the more itinerary-dependent speed optimisation.

If the air lubrication system is added to the mix, full scale trial savings are assumed, even if the weather conditions may have a significant effect on the saving. In this case, ALS is only used at speeds above 15 knots, meaning that the high speed legs gain the most. According to Foreship, the annual saving is 59 ­ 1,719 tonnes or 0.2% - 3.6%. The average saving of the itineraries is 1,034 tonnes or 2.4%.

When all three ­ speed reduction, in-waves optimisation and ALS ­ are combined, the annual saving is between 1,189 ­ 2,613 tonnes, or 4.3% - 5.4%, with an average saving of 2,074 tonnes or 5%.

Foreship argues that theoretical hull form optimisation based on trial speed and calm seas should be a thing of the past and that today’s ships should be optimised for the correct speed range and the correct conditions. CFD optimisation is possible for both, offering significant reductions in fuel consumption, and an air lubrication system can provide additional savings that should not be overlooked in the right scenario

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