Hybrids – an answer to inefficiency
True hybrid solutions could, with the right push, go much further than their diesel electric forebears in answering some of the inefficiencies which lie at the heart of offshore support operations.
The step change is possibly not as far away as you’d think. There are several advantages to having electrical power storage onboard, the biggest possibly being the potential for Dynamic Positioning (DP) redundancy to be taken over by the battery bank instead of by the more usual group of water-churning gensets that are usually kept online to act as spinning reserve.
The basis for this, Alasdair Anderson of Lloyd’s Register explains, has been around for a while in the form of peak-load shaving by which engines are helped to meet a sudden rise in demand by feeding in another form of energy to fill the gap: batteries are the obvious choice, being able to provide an instant, millisecond power response. Further, he adds that different engines have different reactions and although low pressure LNG varieties are growing in popularity especially in the offshore industry, they do tend toward some “sensitivity” when it comes to steep load steps.
Interestingly, the recent order for a 90m Havyard 833 WE ICE hybrid design PSV has just been announced, with the yard saying: "Response time in relation to variation in power demands is also a lot shorter, so there is a quicker transfer of power to the ship's thrusters, which ensures more precise and secure positioning, for example in operations close to a rig."
However as Mr Anderson says, handing over the DP operation entirely to batteries has not been previously explored – not something actually being proposed by the Havyard vessel. To actually shut off certain engines entirely is “an idea worth exploring” says Mr Anderson. It’s now the case that batteries can now be adequately sized to have enough power both initially and also for long enough duration to allow the engine to get back online again. However, he points out, “while it looks an interesting possibility, we don’t yet really know the drawbacks”.
Recent developments, therefore, are proving very interesting. The Fellowship test boat Viking Lady is, at time of writing, being put through its paces with a battery bank, installed at the very end of last year, taking up the redundancy role for the DP systems.
Brent Perry of Corvus, the company supplying Viking Lady’s battery pack, is convinced that recent advances “are about to change the way ship systems are designed”, adding that offshore support, involving long periods on DP mode, is “one of the best possible applications for the new technology”.
Despite the bank’s relatively recent installation, it has already been well backed up by thorough testing: “Wärtsilä pretty much replicated Viking Lady in a warehouse, a full demonstration unit with real pumps and motors, so we had the battery bank running in that for nine or 10 months before installing on the vessel.”
It takes some care to scale the banks correctly as each vessel needs a different sized bank for its operation but unlike an engine, the scale of the batteries isn’t in a linear relation to the power “unlike an engine that has to be sized to match the greatest demand” points out Grant Brown of Corvus. Instead, you are looking at the ‘peaks and troughs’ of the energy, the draw from the application in relation to how much is being put back in. So, says Mr Brown, a battery can supply or augment power demand at the upper end of the duty cycle, then recharge while demand is lower.
Linked to this is a very useful characteristic: batteries can put out considerably more power than what’s printed on the box, at least for limited periods. “If necessary, batteries can discharge five or even 10 times their rated capacity in short bursts” he says, giving an almost unlimited flexibility around how the power is used.
Further, there is an inbuilt redundancy within the battery pack. Mr Perry explains: “Even if a module cracks out the bank can still go on with almost full functionality; you’d have to have half the batteries fail to really make a dent in ordinary, short term performance.”
There is yet another argument for hybrid systems’ entry into the support market: subsea construction vessels such as pipe layers use a lot of energy which at the moment is simply lost to resistors or friction. Mr Anderson concludes that if it can be recovered this could feed in a very large power resource.
Mr Brown explains that the biggest stumbling block in the past has been the relatively short, very high bursts of power generated by the lowering motion and the battery pack’s lack of ability in capturing a worthwhile part of it.
However, new battery technology from Corvus that can handle the high charge and discharge cycling rate means that hybrid shore-side cranes are already in place landside, one application being on RTGs in the port of Vancouver. Here the regenerative battery solution “can supply up to 25% of the energy that the system uses” says Mr Brown and so the newer subsea market may well be fertile ground for development.
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