MARINE ENVIRONMENT
Ballast Free Ship: Will it Work?
Recent Testing Indicates the Ballast-Free Ship Concept Can Also Provide Fuel Savings
By Michael G. Parsons, Arthur F. Thurnau Professor, University of Michigan & Miltiadis Kotinis, Assistant Professor, SUNY Maritime College
Model-scale testing of a Seaway-size bulk carrier has shown recently that when optimally designed the BallastFree Ship concept can also yield a significant reduction in the propulsion power required in the ballast condition. The Ballast-Free Ship concept was invented at the University of Michigan in 2001 to provide a new way to address the problem of the introduction of Nonindigenous Aquatic Species (NIS) through ballast water transfer across the globe. Professor Michael Parsons and then Ph.D. student Miltiadis Kotinis developed the concept, with assistance from other department colleagues, and this initial work was reported at the Annual Meeting of the Society of Naval
Architects and Marine Engineers (SNAME) in 2004. In the Ballast-Free Ship concept the traditional ballast tanks are replaced by longitudinal trunks that run from bow to stern below the ballast waterline. Pairs of these trunks are flooded in the ballast condition and are left open to the sea. This can be viewed at reducing the buoyancy of the vessel rather than adding weight as occurs in traditional ballasting. There would be three trunks per side on a Seaway-size bulk carrier. The natural pressure distribution that develops around a hull at speed produces a positive relative pressure at the bow and a negative pressure at the stern. This pressure differential is used to drive a slow flow through the trunks so that they always contain "local water." The trunks and openings are sized so that the water in these trunks is swept out every hour or two to meet the environmental
objective, but not increase the resistance of the ship significantly in the ballast condition. This prevents the ballast water transfer of NIS across the globe. In order to place sufficient volume in the ballast trunks to reach a safe storm ballast draft, the innerbottom must be raised and the hull must be made deeper to provide an equivalent bale capacity for the ship. Thus, the concept is really only practical for new construction. A Seaway-size bulk carrier would have its innerbottom at about 2.4 m above baseline and be 1 m greater in depth. The added steel weight and the buoyancy lost in the flooded inlet and outlet plena at the bow and stern, respectively, can be easily be offset by a slight increase in the fullness of the hull. This ensures that the operating draft is not increased. When the vessel completes the ballast voyage, the trunks are isolated using sluice gates or large disk valves and then
pumped dry using conventional ballast pumps. The outboard two trunks on each side would also contain isolation valves at each cargo hold bulkhead. These would be closed in the full load (trunk empty) condition to provide adequate protection in the case of grounding or collision. The build-up of sediment in the trunks results in lost cargo capacity and IMO Regulations require that this be removed from ballast tanks to reduce the possibility of transporting benthic NIS within the sediment. With the higher innerbottom it will be easier for the crew or contractors to keep the ballast trunks free of sediment. To further facilitate this cleaning, the transverse web frames are cut away from the outer shell plating except for the narrow clip that is used to provide tipping stability for the bottom longitudinals (see accompanying midship section).
In the Ballast-Free Ship Concept there is a slow flow through trunks extending the length of the ship below the ballast waterline. 24 Maritime Reporter & Engineering News