Heavyweight proof for hybrids
If anyone wanted hard-metal proof that battery array vessels are beginning to be taken seriously, it’s to be found in the 150m long, 27m wide ‘Edda Freya’, writes Stevie Knight.
This SALT OCV304 is a whole step up in scale from Ostenso Rederi’s initial foray into hybrid engineering, heralded by the 2013 launch of the Edda Ferd. However, there are some surprises: at 1,050VDC with a capacity of 546kWh, the battery system is actually rather smaller than you’d think for a vessel this size.
Despite this, Sean Puchalski of Corvus Energy is quietly confident of the scale, given a worst-case engine failure “it’s still enough to provide for the propulsion system, the pumps, all the necessary equipment until the auxiliaries start up”. Certainly it would be a lot more of a concern if the groundwork hadn’t already been laid by the first vessel.
Egil Arne Skare of naval architects Carl J Amundsen – the company handling the power system – admits: “When we designed the Edda Ferd, nobody could tell us how big the battery array should be: there were no guidelines, rules or anything... we couldn’t find an answer. So we decided to turn the question upside down and ask instead how small could the batteries be - we worked out we could actually use just one battery string with a capacity of 136kWh on each of the two switchboards.”
He admits he was nervous during onboard testing: “The first time we tried out what would really happen with a complete engine loss on the Edda Ferd, I was really quite uncertain whether it would work with such small batteries. But it was fine, no blackout, the batteries took over running everything, even if the engines were shut off.”
Altogether the Ferd has been a success and the vessel is now working happily and intensively in the UK’s North Sea waters – although it’s not its expected arena in the Norwegian segment. This highlights a particular issue, says Skare: “You can build for a specific market, but actually these vessels hardly ever stay there.” He points to the necessity of using a calculation tool, in this case MindSET, which can take a peek into a vessel’s future by looking at its demographic profile, then bringing the relevant technology to bear on the overall picture.
It still leaves room for skill, and with this latest vessel, as always, there are some tricky balances: “If we design for speed, then we are in danger of losing the big deck, but good deck load performance is critical to the role,” explains Tor Vestabostad of Salt, the design house behind the OCV304 pattern. He adds: “There’s also a lot of work done by the handling equipment when the vessel is in dynamic positioning mode, but there’s is no point in putting a huge crane onboard if the vessel’s seakeeping won’t actually allow you to use it.”
Therefore, some thought went into tailoring the Edda Freya to its offshore lift, construction and subsea installation future, while not forgetting it’s ROV survey and IMR roles. To this end, Freya has a 150 tonne vertical lay system, fed from a below-deck carousel plus a huge working deck of 2,250m2 with an available length of 92.8m – strengthened at the aft to 15 tonnes per m2 and 10 tonnes at the rear.
But placing modules on the seabed at depths of up to 3,000m necessitates a stable working platform: “While the best form is always a brick, if you’ve got to get to Brazil, a brick isn’t very useful,” says Skare. The solution “is not a fancy signature hull” but quite a conventional bulbous bow, thoroughly tested out against the expected operations; unlike Ferd, “this vessel will probably be spending around 70% of its time on DP or getting into position, not in transit”, he explains.
However, the vessel still has to make good progress – around 12 knots - so Freya is to be fitted with a pair of 4.3m diameter contrarotating propellers, two 4,200kW tunnel thrusters aft and three 2,050 versions plus a pair of retractable 1,500kW azumuthing thrusters.
To answer the demands of both deck gear and propulsion, the main Caterpillar engines have to be big. In total there are six engines on the Edda Freya, three in each engine room: one MaK 6M20C with a 1,140kW capacity, MaK 6M32E, giving 3,300kW and a big MaK 8M32E giving 4,400kW. All together these yield around 17,000kW.
Running these engines could be wasteful given the proportion of the time that the Freya will be involved in low-power trolling and DP work, but once again these are variable speed engines that will respond to the different levels of draw from propulsion and thrusters - a big step away from the traditional AC set up which simply runs the engines at a constant speed. “The engine speed will adapt to the power need and simply step up and down depending on what’s required,” says Skare.
Like Ferd it’s tying the power together with the Siemens’ BlueDrive PlusC system. It’s interesting that DC distribution, despite some perceived safety challenges (answered by very swift microelectronic circuit-breakers) is coming out of the shadows: DC now seems set to establish itself as the way to integrate variable speed engines with batteries and other, possibly more intermittent power sources, and, unlike AC, there’s no need to wait and synchronise the output at start up.
There’s another element: easy shore-power adaptation. Emissions are a very ‘visible’ issue in Norway at least, the pollution cap in Bergen can already be seen by the naked eye from the local hillside and the drop in oil price has meant even more vessels laid up at berth.
However, this new vessel could make even more use of the DC system than its PSV stablemate. Puchalski explains that this hybrid system “should be able to yield larger returns” as the whole vessel, from propulsion to equipment is simply on a much bigger scale.
Freya is fitted out with construction-sized deck gear including a huge 400 tonne capacity crane, so the possible regenerated energy available from lowering a subsea module 3,000m is potentially vast, well overtaking the amount needed for control. Skare explains: “Rather than burn it off using resistors, we are putting this back through the DC grid and running it to the other consumers.” It’s an important point: the batteries don’t have to mop all the regenerated energy up so while they will be taking what they can in order to get a top up, the propeller and hotel loads will effectively be acting as the crane’s braking system.
Despite the battery banks’ modest size, Skare says the array is very good at peak load shaving: “The way we use the batteries is not actually very hard on them. In fact we found that on Ferd we were operating within 5% of the battery total capacity,” he says, so the array on Freya will take advantage of the same pattern although there will be four switchboard segments instead of just two.
When it comes to emissions, keeping the strain off the engines is useful in more ways than one, says Skare, and it goes further than appropriate loading: “The issue is that urea has to be provided as a chemical catalyst for NOx reduction, so if the engine loads keep shifting up and down, so does the temperature and then it’s very hard to dose the selective catalytic reduction (SCR) unit properly: if you have too much urea you find it goes straight through and out the exhaust, but if there isn’t enough you run the risk of a NOx emission increase. So by using the batteries to dampen the transient loads we can better lower NOx emissions.”
Puchalski concludes that hybrid systems are now ready to go out and grab a share of the market: “There have been two things that have held hybrids back so far, the perceived technical risk and the costs. However, there’s been a lot of work on both fronts so as time goes by people are beginning to understand this is a durable technology.”
He adds that the limiting oil-revenue downturn won't stop the trend: “While there’s been a drop in oil price and this has slowed down offshore interest, the applications are quite economical even at present levels. Plus, though there’s a ‘holding pattern’ in new technology for the offshore sector, the long cycle time means that a base level of activity will probably still continue underneath the surface.”
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