Stenersen prepares for future tanker trade

Stenersen's newbuilds represent a first tanker application for shaft generator and lithium-ion battery arrangements

Stenersen's newbuilds represent a first tanker application for shaft generator and lithium-ion battery arrangements

Bergen-based Rederiet Stenersen is bolstering its fleet with the addition of two 17,500dwt chemical tankers next year, with propulsion and machinery choices revealing a keen eye on the future of regional tanker trade. Gavin Lipsith reports.

For Rederiet Stenersen, owner of 16 product/chemical tankers active in the Baltic Sea and North Sea, trading has changed a lot in the past two years. From 2015 its whole sailing area – barring the recently opened route to St Lawrence in the US, serving the Great Lakes – became a sulphur control area under MARPOL Annex VI. Ports too have introduced more stringent measures governing emissions for ships manoeuvring and quayside.

Emissions advances will continue over the coming years. Even the transatlantic route will have a sulphur cap from 2020, when the global 0.5% limit is applied, and in 2021 the second round of NOx Emission Control Areas will encompass the Baltic Sea and North Sea. Considering those circumstances, it is perhaps no surprise that Stenersen’s forthcoming newbuilds, its first since 2010, will be its most environmentally friendly vessels to date.

Steel cutting for the first of two sister ships (an option for two more will be decided upon by April) took place at Taizhou Kouan Shipbuilding Co in November, with construction on the second to start next month. Burning low-sulphur diesel oil through a modern, ultra-long stroke MAN B&W G45-ME C9.5 engine represents a return to two-stroke engines for the company: while Stenersen had traditionally used low-speed engines, its current fleet is powered almost exclusively by Wärtsilä four-stroke engines due to the scarcity of two-strokes during the Chinese building boom of 2000-2008, when you had to have a crankshaft already to place an order.

If the ships’ prime movers and auxiliary engines (two Yanmar 6EY22ALW and one 6EY18ALW generators) are a traditional choice, the supplementary power configuration is anything but. Aligned to the main engine is a shaft generator powered by permanent magnet motors and an energy storage system supplied by WE Tech, with Corvus Energy to provide a. The vessels will be the first chemical tankers to feature such an arrangement.

BATTERY OPERATIONS

John Stenersen, director ship management, Rederiet Stenersen, explains: “As a tanker you need two auxiliaries running when you are manoeuvring - one in standby. With the battery, we can close down one of those engines, and then we also have a frequency converter on the shaft generator. So in and out of port we will run only a shaft generator and the battery. If the shaft generator trips the battery kicks in immediately. That will hold for ten minutes and in the meantime, we will have started two auxiliary engines.”

There is potential to employ the battery and shaft generator set-up in other ways. The company hopes to use battery power during discharge operations, running one auxiliary on full power with the battery taking the remaining power demand instead of having two engines running on 60% load. Stenersen also believes there could be potential to use the battery for peak shaving from the main engine, allowing the engine to be run at relatively constant speed while the battery is engaged for any higher demand. But this function has yet to be fully explored.

The permanent magnet motor driving the shaft generator is slightly more expensive than traditional reduction gear that would be needed to couple the device to the main engine. But the absence of gears will have a significant impact on fuel efficiency, with 2-3% less mechanical loss to the propeller. A frequency converter on the main engine means that the speed can be kept low at around 80 rpm, but not fixed as shaft generators traditionally require, meaning that the engine can always be operated close to the load curve.

There are also safety benefits. The shaft generator has a ‘take me home’ function whereby if the main engine the auxiliary engines can power the shaftline. And the fast-responding battery to eliminate the possibility of a blackout.

A 400kW shore power connection means that auxiliaries can be switched off when the ships are idling quayside or in drydock, with the battery meaning that there is no interruption in power between turning off the engines and connecting to shore electricity. Stenersen is investigating the possibility of enlarging the shore power connection in future so that it can be used during loading and unloading, although at present this is not allowed at most tanker terminals and would need to be regulated for.

EFFICIENT DESIGN

With 80-100 port calls a year expected for these vessels, the battery and shaft generator arrangement is expected to save Stenersen a significant amount of fuel when going in and out of port, with a corresponding positive effect on emissions.

“These vessels are 1,800 tonnes bigger than the series we have and we expect to run them on less fuel,” says Stenersen. “At design draught and 13 knots we will use 13.7 tonnes of diesel a day. That’s two to two-and-a-half tonnes less than the older ships.”

The battery and shaft generator play a large part in that saving. So too does the efficient MAN main engine and controllable pitch propeller, along with a hubcap reducing the wake field over the Becker rudder. The hull shape, an original ‘semi-V’ design, also ensures that fuel consumption remains low in ballast: 10.9 tonnes of diesel a day as opposed to 13 tonnes for the older vessels.

The ship was designed with Bergen-based naval architect Marinform, the company created by William Bland, former managing director of Shipskonsulent (SK), when SK was bought by Wärtsilä and relocated to Singapore. Many of the 10-20,000dwt tankers trading in the North Sea were designed by SK, including those belonging to Stenersen.

“Marinform came up with the semi-V hull concept and we worked closely on cargo and engine requirements,” notes Stenersen. “All our vessels are SK design so we know him very well and he’s been in close contact with our company for 30-35 years.”

Despite the novelty of the battery installation, Stenersen notes that rather than challenging the designer it gave greater flexibility. There are of course extra costs for the battery, and the need for additional breakers and other equipment associated with the shaft generator, but the inclusion of the battery – at a relatively late stage in the design, Stenersen reveals - provided more options for the machinery arrangement.

“The big challenge is that this is a first for us, and in seeing whether what we think is an opportunity actually is,” says Stenersen. “This is a new field – it’s not a ferry sailing from A to B – but we believe very strongly that it will work for us.”

GAS READY

Sharp-eyed readers will have noticed that, when the tankers’ propulsion arrangement was originally reported on The Motorship’s website, accompanying the report was a drawing of the vessels equipped with LNG fuel tanks. Stenersen explains that the vessels will in fact be run initially on a diesel main engine, but has been designed (though not classed) as gas-ready.

“The process started with the idea that we should have a dual-fuel set up, but the challenge was that commercially there doesn’t seem to be any great interest,” Stenersen explains. “A year ago we had an exercise with various charters around and our feeling was that there wasn’t a willingness to give higher rates for LNG-fuelled vessels. Building for gas fuel would have been US$4.5 million extra and that is too much without an addition to the charter rate.

“The dilemma today is that we have still not come out of the poor market and there is a lot of low-cost competition around. So you can understand why the charters would not pick a Rolls-Royce when there is a cheaper alternative.”

The gas-ready design – including strengthening of the deck for future installation gas fuel tanks, and the consideration of space for piping, gas handling at midship and a gas valvetrain unit near the engine room – did not add significantly to building costs. Stenersen notes that some additional steel was required for deck strengthening. Nor would the LNG tanks have a drastic impact on cargo capacity, as less fuel oil would then be carried, he adds.

The prospect of future conversion to gas operation, when the market is ready for it, did influence the choice of main engine, Stenersen reports. While both MAN and WinGD engines can be converted for dual-fuel option, the company decided that a MAN engine would an easier conversion, as well as more efficient in diesel mode than WinGD’s Otto Cycle counterpart.

ENGINE CONVERSION

“It is easy to say you just need to change the cylinder heads, but of course there is more to it than that,” says Stenersen. “But you don’t have to refit the whole engine. Without knowing when we will convert, running in diesel mode for five years on an Otto cycle engine would also mean higher fuel costs.”

With an eye on future environmental regulations, the vessels have also been designed in preparation for NOx abatement. While the Yanmar auxiliaries will be installed with selective catalytic reduction (SCR), the main engine will not, although space has been allocated for a future installation. The vessels will also feature UV ballast water management systems from GEA Westfalia. Stenersen notes that the company believes in UV rather than electro chlorination or chloride injection ballast water treatment due to the ease of use, low power requirements and the lack of chemicals added to water – something he believes could face regulatory challenges at later date. The company will also be looking to retrofit UV treatment systems on its existing vessels in due course.

Continuing Stenersen’s newbuilding strategy since 1997, the vessels are being built in China, although with a new shipyard partner. The company looked at five yards before settling on Taizhou Kouan Shipbuilding Co and signing a contract in November 2015. “We felt they were the company that could fulfil our needs while allowing us to come with our own design,” says Stenersen.

Anticipating future fuel use is not easy for any ship operator, Stenersen admits. While LNG has advantages, it has challenges too – it does not eliminate carbon emissions and (in most engines) also leaves some NOx abatement required. The company is testing hybrid fuel oils but is cautious given that, unlike with conventional fuels, the varying brands cannot necessarily be mixed in the same tanks. And the future price of fuels will be a key factor in deciding what operators eventually use.

The new Stenersen tankers are ready to convert to LNG when it becomes economically feasible. And by employing a combination of battery and shaft generator they are not only reducing emissions and fuel costs, but also providing a valuable first reference for the technology in a new vessel segment.

SELECTED MACHINERY

Main engine

MAN B&W 6G45ME-C9.5

Auxiliary engines

2 x Yanmar 6EY22ALW (1,050kW), 1 x

6EY18ALW (480kW)

Rudder

Becker twisted leading edge flap rudder

CPP and controls

MAN Alpha VBS1350-ODS; Alphatronic 3000

Shaft generator

WE Tech permanent magnet motor

Battery pack

Corvus Orca Energy lithium-ion

Clutch

Renk PSC (propeller shaft clutch)

Boilers

Alfa Laval oil-fired and composite

Cargo pump system

FRAMO (Alfa Laval)

Cargo and engine room automation

Kongsberg K-Chief 600

Ballast water management

GEA Westfalia BWT 500 Ex