The main ship management practice to reduce the transfer of harmful aquatic organisms in ballast water and sediments is for the ship to exchange its ballast water in the open ocean during the voyage (flushing out coastal organisms taken on at the source port). Many potential techniques for treating ballast water are in experimental stages and none have yet been shown to be fully effective in a full-scale shipboard application. As an alternative to ballast treatment systems, a new ship-concept that will essentially eliminate the trans-oceanic transport of ballast water is proposed by researchers at the University of Michigan. The Ballast-Free Ship concept involves a new paradigm that approaches ballast operation as the reduction of buoyancy, rather than the addition of weight to get the vessel to its required ballast drafts.
The traditional ship ballast tanks are replaced by longitudinal structural ballast trunks that surround the cargo hold below the ballast draft. These trunks are connected to an intake plenum near the bow and a discharge plenum near the stern. These ballast trunks are flooded in the ballast condition to decrease the ship’s buoyancy. The pressure differential between the bow and the stern is utilised to drive a slow flow through the ballast trunks to ensure that the trunks always contain “local seawater.” At the end of the ballast voyage, the trunks are isolated and pumped dry using conventional ballast pumps. The lab-based study of this concept at the University of Michigan shows that the Ballast-Free Ship concept may provide an alternative to ballast treatment systems and may possibly provide superior protection from further introductions of non-indigenous aquatic species through ballast water.
This innovative and creative research perhaps constitutes one of the most exciting efforts to solve the ballast water problem undertaken to date. New ship designs that reduce or ideally eliminate the transfer of water from one geographical area to another, are likely to hold far greater promise, than installing highly complex technological devices on board ships to treat massive volumes of water. However, it should be noted that the ballast system would, by structural necessity, still contain transverse frames, and therefore is still likely to entrain and transport at least some water, sediments and organisms. The trunks (and any entrained organisms) would still be emptied upon arrival in port. Any fouling organisms that attach to the inner surfaces of the ballast system may also be transferred. The concept is perhaps better described as a “continuously flushing ballast system than a truly “ballast-free ship”.