Fuel consumption and emissions can be reduced without impacting bollard pull by involving propulsion suppliers during early design phases
Tugs are built to a required length, beam, draught and bollard pull. They have more than adequate power from engines and azimuth thrusters to meet the towage pull needs of ship operators. But they seldom operate at these top power needs, meaning their propulsion systems are overproportioned for most of a tug’s operations.
Berg Propulsion managing director for the western hemisphere Jonas Nyberg says vessels, particularly towing ones, can be designed with more optimal propulsion if manufacturers are involved in an earlier phase.
“We are trying to get into this process as early as we can,” he said at Riviera’s Alternative propulsion systems: the selection conundrum webinar on 1 October, “and to understand more about the operational profile and how this is going to be used with the contractual requirements.”
These operational requirements can include towing forces, vessel functions, speed, cargo transporting and other capacities. The growing regulatory requirements come from IMO, flag, class, ports and regional authorities concerning functionality and emissions control.
“All of these are ingredients to ultimately coming up with the vessel technical specification, so a yard can bid on this and you can get the vessel,” said Mr Nyberg. He said owners “are trying to match a power plant with the propulsion concept to meet contractual requirements”. But this may not be the best solution for the overall operational profile and minimising emissions.
“There is a contractual requirement on a towing vessel to meet a certain bollard pull, but most of the operational time tugs are not even close to this point,” said Mr Nyberg. He questioned how tug owners could ensure their vessels are configured for meeting contractual, regulatory and optimised requirements. “How are you actually going to operate this vessel with the lowest possible operating cost?”
Mr Nyberg said designers, owners and shipyards should work in partnership with propulsion suppliers to configure a compromise that suits both contractual and environmental requirements “so that you get to the leanest vessel system you can ever get to”.
Tugs may only need the maximum bollard pull for about 10% of their operating time, and two thirds for another 15-20%. The rest of the time they could be idle, in transit or providing other ship handling services.
“Our approach with this is to work much more at the earlier phase of the vessel design,” said Mr Nyberg, “working with the propulsion concept, in concert with the operational profile and the vessel design to make this as optimal as possible to minimise energy consumption.
At this stage, it is easier to understand the propulsion design, to reduce the power and revolutions of the engine, but increase the size of the propeller and enable variable speed operations.
“Then you are in a better position to really pin down the vessel technical specification, and the power requirements,” said Mr Nyberg. “Because the other way around you might typically start with selecting a main engine that is too big, and there is a knockdown effect from that,” he said.
Tug owners and shipyards can therefore configure the whole propulsion, engine to propeller, as one system and introduce hybrid and electrical units as required to reduce emissions further.
Propulsion for tugboats will be discussed in depth during Riviera’s Smart Tug Operations Virtual Conference on 1 December. Use this link for more details, programme and to register