Making the case for hybrid propulsion
Kawasaki Heavy Industries believes that its hybrid propulsion solution for offshore vessels offers value to owners even with fuel prices at low levels. Stevie Knight reports.
A few years ago hybridised offshore support craft were popular, driven by the powerful winds of high oil prices. But now the picture has changed and fuel efficiency is no longer enough. So, what can these designs offer the industry now? “Everyone wants to save on operating costs, but we think these days you have to do more than that – you have to save on capex too,” says Tatsu Ohno of Kawasaki Heavy Industries (KHI).
He points out that while hybrid systems are known for their ability to raise efficiency, most people believe this technology is associated with a higher initial investment. And, he admits that with the oil price so low until recently, “we were having a harder time convincing customers to buy on the basis of lower opex alone”.
Therefore KHI set to work on a solution to this conundrum, and found it in the form of an efficient, rugged AHTS propulsion package with a lower price tag than a conventional system. This turns on its head the age-old idea that new technology only comes in at a premium. In fact, most fleet owners are used to a sliding scale where more investment up front (with luck) results in lower running costs and if a system is cheap, then by its very nature it’s less efficient and more expensive to run. So, how has it been done?
Tatsu Ohno explains that since “real data” can’t be beaten, the company’s calculation tool was used to dig into customers’ AHTS operating profiles to unearth potential gains. He explains that three sizes of DP2 class vessel were winnowed out for a detailed look: one big AHTS with 160 tonnes of bollard pull, one of 90 tonnes, and a smaller vessel with a 60-tonne capability.
He explains this last is important to KHI: “Although the Asian market hasn’t got so many large-scale AHTS vessels, there is a far greater number of this type of smaller vessel than you’d find in the North Sea”. While it might have been guessed that the high power operations were outweighed by the rest, the year-long analysis clearly showed that none of these vessels spent more than 6% of their time on anchor handling operations.
The same went for high bollard pull work. In fact, it was the low-DP standby or transit modes took the lion’s share. Operational realities The results, he says, underlined the central issue: while AHTS’s engines are “very powerful and dominate the investment cost” in reality these vessels spend the greater proportion of their time on low propeller loads.
Therefore standby or manoeuvring would be best carried out using gensets; after all, low loading the engines and thrashing the water with propellers at almost zero pitch doesn’t do much for emissions, fuel efficiency or maintenance.
On the other hand, Ohno explains that the overriding concern during DP operation is continuity of the propeller thrust and the feed has to be seamless – which, by itself, necessitates capable gensets.
Given both these elements, the obvious solution is to drop the scale of the main engines and when extra grunt is needed, divert the energy flow from the gensets and shunt it to the propellers instead.
Of course, it’s easier said than done and necessitates KHI’s hybrid controller to balance the propulsion and power management systems. It does make a convincing argument. When the figures are interrogated, they show it’s possible to lose 444kW each (888kW in total) from the main engines of a 60-tonne AHTS, and although it means swapping the shaft motors for a more expensive bidirectional motor-generators, their size is reduced by two-thirds.
On the larger, 160-tonne bollard pull AHTS the potential savings are even more dramatic: the engine sizes drop from 4,500kW to 3,030kW and while the genset increases by 615kW, the motor-generator is half the original size. “This way you reduce the running costs by 20% to 25%,” says Ohno.
Further, when it comes to the combined capex/opex picture after five years, the difference is even clearer: the hybrid propulsion package comes in at 79% of the cost of the conventional mechanical AHTS. However, he says there are other issues to overcome. “You have to realise, these vessels often work in very harsh conditions and sometimes the propellers or side thrusters rise out of the water; as a result, they end up racing.
The fluctuating loads can propagate back through the system - even causing a blackout.” The usual method of dealing with these surge-and-fall patterns is to make sure the genset is scaled up to absorb it all. But this brings consequences of its own: “You end up with a genset that’s sized for the extremes... so most of the time it’s running at nothing more than half load.” Not ideal, and a long way from the 85% optimum for which these steady-rpm units are designed.
According to Ohno, putting in a battery would offer the additional benefit of stabilising the power. It doesn’t have to be huge, just big enough to blunt the edge of the wild fluctuations that result from heavy seas. Given that the oil price is unlikely to reach the dizzy heights it occupied back in 2014 any time soon - as well as the oil majors’ continued crimping of costs and the falling price of batteries - this solution might even grow in attractiveness.
Source: The Motorship