Introducing the truly innovative Edda Ferd
The latest addition to the Ostenso Rederij fleet claims that it’s capable of cutting the typical PSV energy draw by between 25% and 30% - while still running plain ordinary diesel, writes Stevie Knight.
So, it’s worth asking exactly how the 96.2m Edda Ferd does it. Ketil Aagesen of Siemens explains that some of it is down to the use of a cleverly managed battery bank: “To start with, the batteries cut in with instant power to take the edge off some of the sudden spikes in any peak power demands,” he says, adding that the hybrid installation is connected to two AC asynchronous water-cooled motors that can push out 2,700kW each.
This adds a whole new layer of flexibility, with the batteries acting as the sole energy source for the low load conditions and handling peak demands without starting up standby engines explains Mr Aagesen.
However, it’s the way batteries can act as an energy buffer that is possibly the most important point for the vessel’s DP2 systems; it means the bank can step in as a bridge, allowing different kinds of energy configuration to provide the necessary spinning reserve.
This is linked to another trick up the Edda Ferd’s sleeve. Normally you’d see the main engines, in this case a pair of 3,000kW MaK 9M25Cs and two, 2,000kW MaK 6M25Cs simply acting as gensets, running at a fixed speed engine to produce a solid block of AC current to be used across most of the ship. However, to avoid simply thrashing the water with zero-pitch propellers new builds often now include a variable speed drive, with a rectifier to change the current to the ‘straight’ DC this needs - normally.
Mr Aagesen points out that despite the customary set up, its long been known that running the diesel engines at a constant pace with control of power taking place only after it has been delivered by the engines, fuel efficiency is being consistently compromised. This is simply because as far as offshore support vessels are concerned, a large slice of the power is being wasted on the changeable appetites of the propulsion and thrusters. “In fact, it could be likened to controlling a car by keeping the gas pedal down but letting the clutch in and out,” he points out.
So, the Edda Ferd has a DC main grid, not AC. This gives it the ability to use variable engine speeds that ‘get the message’ that there’s a rise in demand via Siemens' box of tricks, working harder at keeping the voltage up when needed, letting it drop when demand falls instead of the usual constant running. In other words, bringing control back to the ‘gas pedal’.
Since it’s also matched with different engine sizes, the result is much, much better fuel efficiency, “not just in one particular area but across a whole range of speeds” adds Mr Aagesen. It is also where the battery bank adds especial value, as those batteries allow the engines a bit more latitude to meet the draw.
Moreover, this set up makes it easier to integrate auxiliary motors or even regenerative power from, say, the lowering of the onboard cranes, feeding the recaptured power back into the onboard grid.
You can’t ignore the part the hydrodynamics play in the design’s efficiency. The hull itself has taken more than a little innovation, explains Bjorn Oskar of designer Skipsteknisk, since the aft part of the vessel has been specially designed to take a pair of 2700kW Voith Schneider propellers.
“Although Voith propellers aren’t uncommon in construction and survey vessels, PSV operations are a little different,” he explains. “You have to retain that deep-V hull on a PSV. On the front you need to make the lines as slender as possible, keeping down the flare in the upper wave area to make the ride comfortable and decrease resistance.” In fact, it’s been so central to the development that while the design keeps its 7.4m draught and 20.6m beam, the forecastle has been raised by a deck just to enhance the entry.
However, this slender bow has an impact further back. Getting a PSV’s stern to make the most of Voith units has taken a lot of work, and Skipsteknisk has been running tests on just this kind of application since 2000. “Voith propellers require a different kind of thinking from that of normal propulsion units. Usually PSVs are rounded out at the rear end, but we have found that the propeller installation actually needs a clean-cut stern in order to stop any interference with the water flow,” explains Mr Oskar.
Beneath the 1,038m² capacity cargo deck the internal arrangement is also rather different from a standard PSV. There have been a couple of issues; one has been transporting drill cuttings, not easy since they are very, very abrasive and need careful handling, but another is what the rigs call ‘returnment’. “The problem with returnment is it could be anything. You never know exactly what is coming back under this label, so, the only way to transport it is in a system designed for whatever might come along,” he says.
The solution has been four specially designed tanks in the centre of the ship that can deal with low flashpoint liquids; for safety reasons these have their pumping system running into the void space instead of being submerged in the tank. Still, with a flick of a few switches these tanks can change mode and be swap over to taking on liquid mud or other kinds of residue.
Interestingly, the Edda Ferd possess an SPS 2008 notation that allows it to carry special purpose personnel, resulting in enough capacity for 60 people. In other words, if the PSV market dries up, it can switch to the construction field. “This particular aspect meant working around the stability of the vessel and has taken a lot of designing in,” admits Mr Oskar.
However, he points out, what Ostenso Rederij holds at the end of the process is a very adaptable vessel. “We thought very long and hard about the additional capabilities; of course they have a financial impact but the flexibility means you can put accommodation modules or cranes onboard and change the operation to one of subsea construction,” he says, a move that makes sense in markets which frankly need to have some kind of ‘intelligent management’ to help negotiate the ups and downs.
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