by Douglas Clark

Oil spills and man made pollutants leap to mind at the mention of endangered sea life and toppled ecosystems, but unknown to most laypeople, an everyday event in the world of maritime transport is actually among the most significant threats to marine ecology today. Without any of the widespread attention given to a toxic spill, cargo ships and other vessels move from port to port around the world and often quietly introduce non-indigenous species that can wreak havoc in their new habitats. Wesley Wilson, an engineer with the Computational Fluid Dynamics group at the Naval Surface Warfare Center, Carderock Division, in Bethesda, MD, and his colleagues are conducting research that may one day help ocean freighters and other ships to dramatically curb the spread of non-indigenous species.

Biological stowaways threaten environment The unintentional transport of non-indigenous species stems from the age-old sailing technique of using ballast, which is essentially the practice of taking extra weight onboard to stabilize a vessel and prevent capsizing. There are various types of ballast, but a cargo ship’s ballast is usually nothing more than water that is held in tanks at the base of the vessel’s hull. When a typical cargo ship takes on goods at port, it releases ballast water to compensate for the additional weight of the new cargo. When it unloads cargo, it pumps in new ballast water.

This intake and release of water is where problems arise, because the water taken on as ballast contains various forms of ocean life. Ranging from tiny plankton to larger species such as fish, snails, jellyfish, and other plants and animals, this sea life is suddenly introduced into a new environment. If a particular species finds itself free of natural predators in its new surroundings, these uninvited guests thrive to the detriment of other plants and animals, and can quickly tip the delicate balance of the local ecosystem. In addition to environmental damage, related economic consequences often follow as well, such as the decimation of the local fishing industry.

To the credit of government officials, this phenomenon has not gone unnoticed, and regulations require or recommend—depending on the U.S. port—a procedure known as ballast water exchange, which aims to reduce, if not eliminate, the introduction of non-indigenous species.

The procedure consists of flushing ballast water tanks in mid-ocean, following the logic that species living in coastal ports are not likely to survive in mid-ocean environments. While at sea, the ship’s crew pumps water into the tanks until they reach capacity and overflow onto the deck. This is done until a quantity of water equaling three times the tank’s maximum volume has been pumped through the tanks. The assumption is that this will flush the tank of 95% of any coastal water it was carrying. Ships also have the option of modifying the procedure, so long as it exchanges 95% of the tank’s water for mid-ocean water.

Are well-meant standards really effective? The catch is, despite the good intentions of current government regulations, no one really knows whether the ballast water exchange procedure consistently reaches its own standard of 95% exchange in all cases. The standard is based on broad assumptions about tank design, and its efficacy has been widely questioned.

Funded by the U.S. Department of Commerce’s National Oceanic & Atmospheric Administration (NOAA), Wilson’s research seeks to shed light on this issue, since computations to date raise concerns about the ability of the ballast exchange procedure to meet the 95% replacement requirement for all ship types.

In search of the answer, his research has applied computational fluid dynamics (CFD) to examine exactly what goes on during the ballast water exchange procedures and to gauge the procedure’s efficacy. In addition to determining how well the procedure works, he also has his sights on potential ways to improve it. “Another way of potentially using the CFD methods we’re using,” Wilson remarks, “is to look at how you might modify the tanks or change the procedure itself in some way.”

One of the issues that make the ballast water exchange procedure so complex is the varying nature of ballast tank construction. Economy of space is one of the keys to ship design, and that means that ballast tanks must conform to the many space and size constraints of a specific ship. The underpinnings of a ship’s structure affect the design, and the enormous tanks require their own support structures and must provide for maintenance access. As a result, tanks are composed of numerous chambers, bulkheads, access openings, and other geometric irregularities that make for a complex flow pattern.

Visualizations put tanks to the test In the first phase of testing, Wilson and his colleagues used a one-third–scale model of four typical ballast tank compartments to conduct preliminary CFD validation. Using the small-scale model, they tested and refined their CFD methods to make sure they were able to produce accurate results. Once satisfied with the accuracy of their calculations, they moved on to the next phase to produce flow predictions for full-scale tanks.

The researchers used FLUENT, a general purpose commercial CFD application recently acquired by ANSYS, Inc., to carry out CFD computations, and then created visualizations of the results by feeding the data into EnSight, a software program from Computational Engineering International (CEI). EnSight brought real advantages to studying a complex flow problem such as theirs.

“For these types of predictions,” Wilson points out, “we have a fully transient, time-dependent process. EnSight has been our tool of choice for looking at these transient data sets because it has a lot of good features to animate what’s going on. It gives us the flexibility to make surfaces transparent, turn them off, change perspective, and animate the solution—all at the same time. It’s a lot more informative to look at what’s going on in the tank if we can do all those things on the fly, rather than look at a static view of the data.”

Using EnSight visualizations, the researchers are able to easily identify slow-moving pockets of water and dead spots where water fails to circulate sufficiently, despite efforts to flush the tank. These dead spots are exactly where sediment of various plants and creatures can potentially collect, only to be discharged later, once a vessel is in port.

Unfortunately for the environment, the full-scale prediction Wilson and his colleagues carried out did not meet the required 95% replacement of ballast water after three volume exchanges. The good news, however, is that the researchers have formed some hypotheses about how the specific type of tanks they were studying could be modified for greater efficiency.

Wilson and his colleagues are now looking toward continued study of the subject, hoping to extend research to new areas. They would like to investigate different types of tanks to find ways to make ballast water exchange procedures work better, and to identify areas where tank design can be improved.