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Back to 16/2008

The Ballast-Free Ship concept:
An alternative solution to ballast water treatment

Ballast-free bulker
Figure 1. The Ballast-Free Ship concept bulk carrier utilizes longitudinal ballast trunks open to the sea rather than conventional ballast tanks.
Photo: .

Rather than treat ballast water with onboard treatment equipment, The Ballast-Free Ship concept reconfigures the vessel to create a slow, continuous flow of local seawater through longitudinal ballast water trunks, essentially eliminating the movement of ballast water from one part of the world to another.

Evolving IMO ballast water requirements will require the installation of UV treatment, filtration, chemical treatment, and other types of equipment to treat ballast water to reduce the risk of nonindigenous aquatic species (NIS) and pathogen introductions around the world. These installations have already been introduced on some vessels and will be required in general when ballast water exchange is phased out due to its limited biological effectiveness.
  Since 1995, the author has conducted research on various aspects of ballast water exchange, screen and disk filtration, UV treatment, and chemical treatment; all motivated by the need to reduce the introductions of NIS into the North American Great Lakes. The zebra mussel and ruffe, in particular, have had major impact on the Great Lakes aquatic environment.
  In 2002, the author introduced the concept of the Ballast-Free Ship to provide a paradigm shift in ship ballasting that would essentially eliminate the movement of ballast water across the globe. Collaborating with then Ph D student Miltiadis Kotinis and others, this concept was studied in Kotinis’ dissertation and reported at the 2004 Annual Meeting of the Society of Naval Architects and Marine Engineers (SNAME) and included in the 2004 edition of Transactions of the Society.
  This initial work was sponsored by the National Oceanic and Atmospheric Administration (NOAA) National Sea Grant program. This investigation considered the overall design, arrangement, hydrodynamics, structural design, stability, probabilistic damage assessment, seakeeping, and economics of the new concept.
  Parsons and Kotinis, now Assistant Professor of Mechanical Engineering at Old Dominion University, Norfolk, Virginia, have further investigated the hydrodynamic aspects of the concept, in particular the optimum placement of the trunk discharges. This subsequent research has been sponsored by the Great Lakes Maritime Research Institute (GLMRI), a cooperative activity of the University of Wisconsin, Superior, and University of Minnesota, Duluth, funded by the U S Maritime Administration. This additional research was reported at the 2007 Annual Meeting of SNAME and included in the 2007 edition of Transactions of the Society. Additional testing will take place this summer. In the Ballast-Free Ship concept the traditional ballast tanks are replaced by longitudinal ballast trunks that run from the bow to the stern of the vessel below the ballast condition waterline.
  The basic concept is shown in Figure 1. The design requires that a typical bulk carrier have a slightly larger depth to provide the needed ballast capacity and also maintain grain capacity. The resulting inner bottom is higher, which will facilitate the cleaning of the ballast trunks to minimize the accumulation of sediments that could also contain NIS. For a seaway-size bulk carrier, three trunks would be installed port and starboard on the vessel as shown in the attached comparison of the midship sections of a typical single-hull conventional bulk carrier and a comparable speed and capacity Ballast-Free Ship concept bulk carrier (see Figure 2 on the previous page). The trunks are flooded in the ballast condition and then isolated and pumped dry using conventional ballast pumps when the vessel is ready to load cargo. The detailed design also features the cutaway of most of the floors between the longitudinals at the bottom shell to further facilitate the cleaning of the ballast trunks.
  When a ship moves through the water there is a region of relative positive pressure created near the bow and a region of relative negative pressure created at the stern. This pressure differential is utilized to drive the slow flow through the trunks without the use of pumps. Thus, the trunks will always be filled with ‘local sea water’ and there will be no transport of ballast in the usual sense. The system is sized so that the trunk water is changed about once each hour as needed to accomplish the environmental protection goal, without adding excessive resistance to the ship. Computational Fluid Dynamics (CFD) studies using Fluent 6.1 have been used to establish the pressure differential expected in the ballast condition. They have also been used to show that the flow will initiate and be sustained at the ballast speed.

Figure 2. Comparison of midship sections of seaway-size typical and Ballast-Free Concept bulk carriers of the same capacity and speed.

Higher propeller efficiency
To maximize the ballast trunk pressure differential, the trunk intake is taken at the center of the bulbous bow at about the 25 per cent design waterline. Hydrodynamic studies have tested discharge locations at approximately Station 17 at the 45 per cent waterline, just aft of the forward engine room bulkhead; and at Station 19 at the 30 per cent waterline, just forward of the after engine room bulkhead. The location at Station 17 was found to be near optimum from a propulsion standpoint.
  Even though the use of the ballast trunks would result in the slight increase in the resistance of the ship, the discharge of the trunk flow into the upper half of the propeller disc tends to smooth out the inflow to the propeller circumferentially, allowing the propeller to operate at the higher propeller efficiency. In tests with a five-meter scale model in the University of Michigan Marine Hydrodynamics Laboratory (MHL), the required developed power declined by 7.3 per cent from the no trunk case to the case with the scaled ballast trunk flow being discharged near Station 17. This situation is similar to the use of Mitsui ducts, which reduces ship required power by diverting flow into the upper half of the propeller disc in order to provide a more circumferentially uniform propeller inflow and improve propeller efficiency.

Two duct discharge locations tested in Marine Hydrodynamics Laboratory.

Location of duct intake in the bulbous bow.

Further tests
One of the confounding issues involved in the hydrodynamic testing reported in the 2007 edition of SNAME Transactions was that, as is typical of early stage hydrodynamic testing, a stock propeller available in the MHL was used instead of a custom propeller optimized for use in the full load condition of the seaway-sized bulk carrier model tested.
  This leaves the question of whether or not the improvement with the discharge at Station 17 resulted from the choice of a stock propeller that was not ideally selected for optimum operation in the full load condition. This question is being further investigated with the use of a second propeller better matched for the load condition. Further tests were performed in July 2008 with this new propeller to clarify this issue and establish if the particular choice of the stock propeller resulted in at least part of the 7.3 per cent reduction of required delivered power found in the most recent testing. These results will be reported before a meeting of the Great Lakes and Great Rivers Section of SNAME. These results will be submitted for publication in SNAME’s Marine Technology following presentation in November 2008.
  Full-scale confirmation of the concept will most likely have to await the construction of the vessel using the Ballast-Free Ship concept.

Required Delivered Power in the Ballast Condition (15.5 knots) without Ballast Trunks Baseline and with Ballast Trunk Discharges near Station 17 and Station 19.

Fluent Computations showing flow initiation 500 seconds after trunks are opened in the ballast condition.

Latest update 29-08-2008


No 16/2008
SST Yearbook of Maritime Technology 2008

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