Improved rolling motions, increased safety, and quicker loading and discharge times have resulted from retrofitting raised tweendecks purpose-built to accommodate steel coils on a general cargo ship. Working in close collaboration with Finnish shipowner Oy Langh Ship Ab, MacGREGOR has provided a solution to improve seakeeping when carrying steel plates, tubes and coils on board the 4,400 dwt general cargo vessels Christina and Sofia.
Traditionally the cargo has been stowed on the tank-top, but this has caused an excessively large metacentric height (GM), resulting in a very stiff ship with an unpleasant rolling period. To solve this problem, Christina, was converted with the addition of four new tweendeck covers specially designed to carry steel coils. Each tweendeck panel incorporates special V-shaped grooves, which can secure different sizes of coils without adjustment. The grooves can support coils ranging in diameter from 1,200mm to 2,200mm.
The maximum coil load for each panel is 350 tonnes, stowed in five 70-tonne grooves. The coils are lashed with belts, with lashing points at 200mm intervals to suit all available coil lengths. In addition, the covers are dimensioned to carry uniformly distributed loads of 4.54 tonnes/m˛ (steel plates) and loads of five 30-tonne containers.All four panels are held in position by removable sockets, which sit in pockets high up in the sides of the existing holds. The areas around the hold pockets have been suitably strengthened to support the forces generated by the tweendeck loads.
This conversion has proved very successful and the concept is currently subject to a patent application. When fully loaded using the new tweendecks, Christina’s GM is reduced from 2.9m to 1.4m, and this has significantly improved the ship’s motions. Benefits include much improved crew comfort, easier and quicker loading and discharging operations, no cargo movement, etc.