Enter the big, workhorse hybrids
Any doubts that times have changed should be resolved by a look at the big commercial hybrids currently shaping up, writes Stevie Knight. And no, for once we are not talking about cruise ships.
Take the latest Offshore Heavy Transport (OHT) vessel. This a truly hefty piece of work, scaled for the latest wind deployments: at 216m long and 56m across, Ulstein’s customised Alfa Lift design crosses a semi-submersible with a foundation installation/heavy lift vessel, with a main deck able to carry 10 large jackets weighing 1,500t or 11 XL monopiles of 2,000t apiece and a similar number of matching transition pieces plus associated equipment on the foredeck.
It’s a neat idea: ducking beneath the surface, it gains increased stability with motion further minimised by the water dampening effect, widening the weather window to allow year-round operation.
Importantly, it has also dispensed with the need for jacking up or mooring in order to gain time savings and deepwater capability.
Operations are therefore underpinned by dynamic positioning, and this is where the energy storage solution comes into play explains Ulstein Design & Solutions BV’s OHT project manager and lead naval architect Dolf Manschot. Firstly, it can stand in for the spinning reserve, enabling one of the engines to be switched off as, given a failure, the batteries take over while another starts up and gets into its stride.
Secondly, DP operation on a vessel this large entails a significant electrical draw. So, while peak shaving is more usually associated with transits, here it’s to be used to lift the top off the demand from all seven thrusters that keep the vessel on the spot during installations.
But the energy storage will have an even wider remit on the second of Ulstein Design’s big hybrids that’s expected to be contracted this year. This will be the 185.4m long Jumbo heavy lift crane vessel, Stella Synergy, which is to have 22MW of installed power onboard (including DF/LNG engines).
Although the fine details are not yet available it’s clear that there are a lot of onboard consumers that could be looking for a peak-demand feed from the energy storage system: first of all, it’s also a DP2 vessel, but the onboard kit also includes a 2,500t main crane, a 400t active heave compensated subsea crane and a 22m-diameter carousel hold. Therefore as Ulstein’s MD Edwin van Leeuwen confirms, “the peak shaving [capability] on the Jumbo vessel will be used for DP and lifting operations”.
However, fully diesel-electric systems like these cope with very varied demands – and sizing is even trickier: “You have quite a wide power range to cover - and you need to determine when to use the battery and what it’s needed for, in advance,” says Manschot.
In fact, dig in a little, and even peak-shaving and spinning reserve requirements point in slightly different directions, says Susanne Lehner of MAN ES.
“With peak shaving, you are generally looking at smaller, shallow cyclesand higher power, but when it comes to spinning reserve, you need deep cycling at full power to buffer the engine start-up for several minutes,” she explains.
It’s a balancing act. On the one hand, peak shaving can flicker in and out, catching any extra demand and if the sizing is done carefully, it might be possible to install a relatively small battery, sized for quick-access power density. On the other, spinning reserve redundancy (sized for overall energy capacity) mostly just sits there, waiting to respond to a drop-out.
Moreover, any guess on how much of the time a ship will need to use this redundant capacity is really just that... a guess. So, what exactly are the parameters for scaling the backup?
That’s a trickier call than it initially appears. It’s technically possible to undersize this element (tempting because you can cycle the batteries for a short time at a higher C-rate than the stamp on the case), but it should be noted that any use will impact the cells’ lifetime. Even slightly deeper discharge cycles than initially dimensioned, will affect the cells’ state of health. Still, cover all the targets, and you’re popping up costs.
Despite this, a very approximate, rule-of-thumb guide comes from Lehner’s colleague, Carina Kern who says, when pressed, “start by considering scaling the spinning reserve to equal one of the engines you are replacing”.
BULK AND CONTAINER SHIPS
While it can be argued that sophisticated, diesel-electric vessels are the natural inheritors of battery systems, it has a far broader appeal. It can also take the load off onboard auxiliary gensets for bulk carriers and containerships.
MAN ES is presently studying the business case for hybridisation of these workhorses. This may not always apply to the super-bloated VLCCs ploughing the Far East trades, “but it should be interesting to feeder-sized vessels that stop off in port every day or a couple of times a week”, explains Kern.
In contrast to the steady transits handled by capable two-stroke engines, harbour operation gives the onboard power a far bumpier ride. To start with, Kern points out that “during thruster manoeuvring there’s a higher risk from a single genset failure so normally you’d want at least two auxiliaries running”.
Moreover, the ship’s cranes can drive considerable fluctuations in the power requirement. While it’s possible to alternate loads with a smart system says Kern, this can slow down operations. As a result, cargo ships generally tend to keep a pair of gensets on the go even at berth.
Interestingly, the potential customer base was spotted early. A DNV project back in 2014 focused on the auxiliary engines of a 50,000dwt open-hatch carrier owned by Grieg Star – though at the time of the original research fuel prices were so high that it seemed like a no-brainer.
Despite the dramatically changed economics, a more recent bulker study by MAN ES has revisited the idea. This investigates the potential for supporting four 30.5t onboard cranes with batteries: as each of them draws up to 228kW, peak demand could sporadically reach a hefty 900kW. It all adds up to good reason to install an energy storage system believes Kern, as that will allow the ship to cater for the various requirements while only running a single auxiliary. More, there’s also the potential for recouping energy from the lowering motion.
However, the focus is slightly different now. Lehner points out that the load profile can be remarkably fast-changing “resulting in very steep load ramps and engines switching on and off quickly”.
That, she explains, has implications for the lifetime of the entire power installation: this kind of treatment stresses both rotating machinery and associated components. Therefore covering the ramp rates and fluctuations, she says, “is not a matter of fuel saving alone, you can reduce the maintenance burden substantially across the whole system”.
For container vessels, there’s also the wild card – the reefers. If a ship is kitted out for a sizeable reefer population, “the auxiliary gensets will be over-dimensioned for everything else”, says Kern. It’s a fairly sizeable demand, she explains: “Since you are typically looking at around 7kW per reefer, slots for 300 of them means over 2MW onboard.”
However, reefer patterns can be wildly erratic: depending on trading season or routes not all the slots may be filled and on top of this, there’s the environment: “Voyages in tropical conditions are going to need more refrigeration than, say, those in Northern Europe,” adds Kern.
Environmental legislation is starting to reshape what’s available at the pump – the impact of the oncoming 2020 sulphur fuel cap has been widely reported, but there are other emissions regulations on the way. What’s less obvious is the potential for energy storage systems to assist in the transition.
For example, Van Leeuwen notes that “bi-fuel gas engines can – under some circumstances - have the tendency to fall back to MGO/MDO given rapidly varying loads”.
However, as Lehner explains, some new fuels could exacerbate these existing issues. Ships utilising gases such as LPG and ethane are just coming into the market, but the further we look into novel fuel types, the more the combustion characteristics vary and this heightens the potential for a missed step when the demand on the engine changes particularly fast.
As she points out, a battery bank might be useful in supporting the transient loads.
Is it critical to have the energy storage element nailed to the last detail before committing to a hybrid construction? The answer seems to be yes... and no.
On the one hand, Lehner says there are “numerous parameters to achieving the right combination of battery capacity and engine size”, the basis for any successful analysis reaching far beyond the normal requirement. It means delving far into the operating profile, “not just the individual time spent on each activity, but also the most likely accompanying weather and environmental conditions”, she explains.
But once you have that outline, perhaps there are reasons to delay. Firstly, vessel construction contract lengths are usually in the order of a couple of years, and that’s a long time in the world of battery development, so as Manschot says, “basically, you make the choice of battery as late as possible in the process in order to take advantage of the most recent developments”.
Secondly, it’s possible to plan for a certain amount of wiggle room: “If you are trialling a few batteries onboard as a starting point but you believe the operation could eventually benefit from a larger installation, it's possible to design-in that option,” says Manschot.
“Although if you are aiming at raising capacity, you still have to make a guess on how much might be required in future, as the equipment, converters, connection points - in fact, all the associated electrical equipment - has to be either sized to handle it... or made modular so everything can be swapped over.”
This entails balancing cost and efficiency against the ability to move the goal-posts. “That’s the puzzle that needs to be solved,” says Van Leeuwen. “We want the vessel to be ready for the future, incorporate the latest battery technology, but we also want to make the system as efficient as possible.”
While having batteries onboard will often yield important advantages, such as noise reduction and redundancy, it’s not a case of ‘one-size-fits-all’: for example peak shaving demands a very different set of characteristics to power regeneration from large cranes.
So, while other heavy installation ships might still make a different choice, the OHT vessel won’t be recouping lifting energy. There’s the matter of finding a crane manufacturer willing to supply the regeneration option on a crane with enough capacity: in the case of the OHT ship this came to 3,000t.
But most importantly, Van Leeuwen points out that you might end up paying dearly for what you get back. “Realistically, crane ships don’t lower stuff very often with the main block, so you need to be careful there: though you might want to take back some of the energy, actually sizing the batteries for that particular reason means you have to ask realistically, how cost efficient is it?”
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