The DP2-equipped light construction / ROV vessel Dina Star was delivered last year by Kleven Shipyard to Fonnes-based shipowner, Myklebusthaug Offshore AS and from the following images (shot with a 50 megapixel Hasselblad) she looks like a pretty standard offshore vessel. The power grid inside that drives all her high tech systems however, is far from normal.
Unlike pretty much every other ship in the world, the Dina Star uses a DC power grid (vice AC) to supply electrical power to the onboard systems. It’s not really a new concept, however new technology developed by ABB has enabled it to be used safely on board a marine vessel for the first time – and with extremely impressive results.
In tests conducted by Denmark-based Caterpillar dealer Pon Power, in collaboration with ABB, fuel consumption rates were found to be up to 27 percent lower on this vessel as compared to similar vessels with conventional AC power grids installed.
In a conversation with ABB’s Head of Marine Design, Mr. Eero Lehtovaara, he notes that the key to this system is the fault protection. In conventional AC systems, circuit breakers are naturally easier to break due to their zero crossing every half cycle, this is not the case with DC however, as the voltage is constant.
Lehtovaara notes that their OnBoard DC system involves distributing the DC current through a new DC link, a protection system that is used in order to interrupt faults. Semi-conductive, controllable thyristor rectifier devices are used in combination with isolators in order to make it work, however the specific technical details are not being made public.
When it comes to the reliability of the system, Mr. Ismir Faglazic, Onboard DC Grid Product Manager notes that it was very difficult to actually bring the system down on board the vessel, that is, create a blackout. “The automation system of the DC is quite quick as you no longer have to wait for syncro or magentization. You get instant power with the DC grid.”
In the event of a major fault on the DC bus itself, ABB notes the system is protected by thyristor rectifiers which also double as protection devices for the generators.
The lack of switchboards on board the vessel is not the source of the power savings however, it allows the vessel to operate its power system far more efficiently.
The following is a diagram of what the power system looks like.
In this new system, the main AC switchboard as well as all the thruster transformers have been deleted, instead the AC power from the generators is immediately rectified at the generator and fed to a common DC bus that distributes the power throughout the ship. Each consumer of the power is then fed by separate inverter units which in some cases power variable frequency drives.
What this means, and most importantly, is that the engines can be run at optimum load in order to feed the DC bus with a steady source of current while using battery banks (or any other source of DC power) to provide the immediate power needed when loads increase unexpectedly. In addition, the main switchboard room and additional weight is eliminated.
For a dynamically-positioned vessels with major fluctuating power requirements like drillships, this seems like an ideal setup, however ABB notes the current technology is slightly limited in that the max installed power of a vessel using this system is 20MW.
Lehtovaara is still very encouraged with the results however, “our feeling is that in the very short time frame, tugs, ferries of different sizes, OSVs, OSCVs, and PSVs will be significant benefactors to this technology,” he adds.