Press digest

2021 April 28

Air lubrication systems and propulsion improvement solutions for LNG carriers

In this tech talk, Jeremy Crossman, General Manager, Sales Excellence & Coaching at Wärtsilä Marine Power, talks to two leading experts about solutions to increase the propulsion energy efficiency of LNG carriers. Piet van Mierlo, Team Leader, Product Team Propellers at Wärtsilä Propulsion, and Sharad Sinha, Product Sales Manager at Wärtsilä Propulsion, discuss air lubrication, Wärtsilä EnergoProFin, Gate Rudder technology and other solutions, sharing their expert insights into propeller systems and energy-saving technologies.

“Decarbonisation is the single biggest challenge facing the shipping industry today, and we have developed many technologies and solutions to overcome it,” begins Sinha. “This not only helps our customers to operate their vessels in a more sustainable way, it also improves their operational efficiencies. If you think about which vessels are causing the largest emissions in shipping, cruise ships, container vessels and LNG carriers come to mind, but smaller ships with different operating profiles need their own solutions too, so we develop for both ends of the market.”

Wärtsilä EnergoProFin and air lubrication

“Two solutions that are fully compliant and proven in the field are Wärtsilä EnergoProFin (EPF) and Silverstream Technologies’ Air Lubrication System (ALS),” explains van Mierlo. “EPF is an energy-saving propeller cap with fins that rotate together with the propeller. This improves propulsive efficiency by weakening the propeller hub vortex, providing average fuel savings of 2% with a payback time of less than one year. The solution can also be used to reduce propeller-induced noise and vibrations.

“ALS focuses on a different critical area of the ship, fundamentally changing the interaction between the water and the vessel’s hull. Air is released and creates a carpet of bubbles that coat the flat bottom on the vessel. As a result, the frictional resistance of the ship is lowered, significantly reducing fuel consumption and associated emissions. The technology has already been implemented in a variety of vessels of different sizes and types, both new build and retrofit, with great success.”

The right solution depends on LNG carrier size and type

“The right efficiency solution very much depends on the size and type of the LNG carrier in question,” points out Sinha. “For larger carriers of more than 150,000 cubic meters, ALS’s performance has been evaluated and measured, and it is already bringing a lot of value to customers. In addition to that, EPF goes hand-in-hand with the propellers that are designed with it. For smaller LNG carriers, there is another innovative technology that we have recently introduced to the industry: Gate Rudder. Traditionally there is a rudder behind the propeller, but this creates drag, meaning more thrust is needed to achieve the desired speed. With Gate Rudder you have two rudders on the sides of the propeller, which creates thrust instead of drag, helping the propeller to overcome the resistance of the vessel and achieve the desired speed with an efficiency improvement of around 9–10%. Gate Rudder also improves manoeuvrability and therefore port operations.”

“Wind-assisted technologies are also very much in focus right now, but we must consider the type of environment the vessel is operating in and its operating profile,” Sinha continues. “There is obviously more wind in the North Atlantic in January compared to May in the Middle East. But the studies we’ve carried out have shown that wind-assisted technologies will bring a lot of value for LNG carriers in the future, and we intend to develop them further over the coming months and years.”
 The studies we’ve carried out have shown that wind-assisted technologies will bring a lot of value for LNG carriers in the future, and we intend to develop them further over the coming months and years.
 - Sharad Sinha, Product Sales Manager, Wärtsilä Propulsion

What makes EPF so well-suited to LNG carriers?

“The largest LNG carriers are typically equipped with fixed pitch propellers in a twin-screw setup with a twin-skeg layout, so they have quite a full body upstream of the propeller,” explains van Mierlo. “The propeller is also loaded quite heavily – for some well-known large LNG carriers it can be up to 750 kilowatts per square metre, which is quite a lot compared to other merchant applications. The disturbance of the flow towards the propeller, together with this high load, leads to the formation of a hub vortex. The EPF propeller cap increases efficiency by avoiding this unnecessary drag.”

“Not only does EPF deliver excellent energy-saving capabilities, but there are also knock-on effects that benefit the environment,” van Mierlo continues. “The hub vortex downstream of the propeller can have a very low pressure at its core, and when this reaches vapor pressure, vapor is formed – also known as cavitation. When this happens – or when it implodes because a higher pressure is reached – it makes underwater radiated noise (URN). As EPF stops the vortex being formed, there is no low-pressure zone, and the noise caused by cavitation will not be generated. URN emissions are becoming more and more important, with various research projects looking at regulating URN levels in the near future. In the Vancouver area of Canada there is already a local incentive to reduce URN levels to protect whales. As part of the scheme, operators that have the EPF propeller cap installed get a reduction in port fees of 23%.”

Air lubrication and LNG carriers – a perfect match

“LNG carriers are also a perfect match for air lubrication technology,” points out van Mierlo. “Their size, low draft variation, high speed compared to other merchant applications, long operational time, and large flat-bottomed area are all ideal for air lubrication. The expected savings are in the order of 6–10%; for a 174,000-cubic-meter LNG carrier consuming 115 tons of fuel daily, that’s approximately 10 tons of fuel savings a day. The system is relatively simple and has been designed for a high utilisation rate with low maintenance requirements. It also has the added benefit that the air bubbles keep the flat-bottomed area clean, stopping marine growth from creating drag. LNG carriers are among the biggest polluters, and air lubrication can be applied alongside other technologies such as batteries, hybrid systems and wind energy to help designers or operators more easily comply with emission regulations while keeping the operational flexibility of the ship.”
 Air lubrication can be applied alongside other technologies such as batteries, hybrid systems and wind energy to help designers or operators more easily comply with emission regulations while keeping the operational flexibility of the ship.
 - Piet van Mierlo, Team Leader, Product Team Propellers, Wärtsilä Propulsion

Existing vessels can benefit too

“EPFs are a pretty simple solution for retrofits – you have a cap behind the propeller so all you need to know is the geometry of the propeller and how it has been designed and then you can design the EPF according to that and the vessel’s operating profile,” points out Sinha. “We have even installed these solutions while the vessel was afloat by sending divers into the sea. More studies need to be done on retrofitting air lubrication systems onto complex ships like LNG carriers, but one has just been added to a 135-cubic-metre carrier so we should soon see concrete data on how well it’s working. Gate Rudder is also applicable to retrofits, especially for smaller vessels. Wind is probably the easiest to retrofit because it doesn't really interfere with many other systems on board.”

The importance of integration and optimisation

“Traditionally, when an owner wants to build a ship, he goes to a shipyard and says how much cargo he needs to carry and the speed the vessel needs to reach,” observes Sinha. “The shipyard then proves the vessel’s capabilities during a sea trial. But the sea trial might be done in calm conditions when the vessel is going to be operated in a very different environment. For the best results we’ve learned to sit together with the owners to understand how they intend to operate the vessel, where they will operate, for how long, at what speed, in what conditions, and so on. By doing this we can optimise the vessel with the right engine complemented by the right size of shaft generator and the right type of propeller, finding integrated solutions that give the best benefits over time. The synergy between different solutions brings a lot of added value over the lifetime of the ship.”