A ship with control issues
While control automation is desirable in ordinary vessels, its importance can’t be underestimated when it comes to innovative designs such as the asymmetric ‘Baltika’, says Stevie Knight.
A novel Aker Arctic design, Baltika, besides being able to transit pack ice up to 1.0 m thick at speeds of 3 knots, features something that’s been described as a ‘third bow’. That is to say, the icebreaking supply vessel can not only go forwards and backwards but sideways too, a trick that enables the vessel to turn and cut through ice at oblique angles of up to 45°, leaving wide channels up to 50m across.
This odd formation also makes it extremely flexible: however, it also results in a ship with control issues to overcome.
Ekaterina Kucher of Navis, which supplied both DP and autopilot, explains that not only are the three thrusters placed around the hull in a pattern that reflects the hull’s irregularity, but the Baltika’s shape translates the vessel’s ‘push’ into asymmetric reactive forces.
So, its control systems had to be extremely precisely tailored. A very sophisticated mathematical model was created to cope with the uneven forces along with customized algorithms for thruster control. “Special settings were made to reduce these effects and to make position and course keeping more stable - without oscillation,” she adds.
However, the challenges didn’t stop there. “Due to the asymmetrical hull, relative wind coming from starboard and port side results in different forces, the same applying to the effect of the current.”
She admits that compared to a conventional system it meant a lot more work; beyond “elaboration of an unusual model” it also resulted in longer trials and onboard adjustments.
Happily, it has all paid off: sea trials confirmed that dynamic positioning and heading control systems both met their declared performance giving a positioning accuracy of 1m and a 1.00° heading accuracy at Beaufort Sea State 6 and winds of 14m/s.
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