Improvised BWMS for Large Bulkers

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(Image Courtesy: Cambiaso Risso)

Coldharbour Marine’s inert gas based ballast water management system has always targeted bigger ships. Now the company has launched a new version of its gas-lift diffusion (GLD) plant designed specifically for large carriers.

The UK-based manufacturer of inert gas generators and ballast water treatment systems used the Kormarine exhibition in October to launch the new design, which is based on existing treatment systems for tankers and gas carriers but configured differently. In a standard Coldharbour installation, inert gas is sent to GLD units mounted inside the ballast tanks.

There are challenges with this arrangement on bulkers, as the wing tanks and heavy ballast cargo tank arrangements do not lend themselves to this kind of installation. The alternative, developed in conjunction with several ship owners, is to mount a bank of GLD units inside the machinery space and then circulate water from and to the ballast tanks for treatment.  The GLD process is not affected by the change and tanks of any size and configuration can be treated.

As with the standard arrangement, treatment takes place during a section of the ballast voyage, rather than during uptake or discharge and no intake filtration is required. This ensures that the process of taking on ballasting is not hindered by treatment, and that the ballast water discharged at the load port will avoid the problems of organism regrowth during long ballast legs, thus ensuring compliance with IMO and US Coast Guard (USCG) discharge standards.

“A completely new system is essential for big bulk carriers, and in particular one which enables bulker operators to avoid operational delays, financial penalties and, in the worst case, possible off-hire periods due to BWTS issues,” said Andrew Marshall, CEO, Coldharbour Marine.

The majority of the bulk carrier fleet consists of ships ranging from 30,000-60,000dwt. However, on long-haul routes, capesize vessels and even very large ore carriers are often deployed. For these vessels, ballast water plays an essential role in their safe operation when they are not loaded, ensuring a ship’s stability and ultimately guaranteeing the safety of her structure and her crew. Large volumes are required to ensure hydrodynamic efficiency and full propeller immersion.

Iron ore is the single largest dry bulk cargo. It is very dense at approximately 2.5 tonnes/m3 and relatively small volumes soon take a ship down to her maximum permissible draft. Alternate hold loading is frequently used as a technique to minimise longitudinal stress on a bulk carrier’s hull girder, but when a vessel is not loaded, huge volumes of ballast are required for safe operation.

Satisfactory stability is essential in ensuring that a ship rights itself as it rolls in a seaway but it must be carefully controlled to prevent cargo shifting in the holds and excessive accelerations which cause discomfort for passengers and crew. A ship’s metacentric height or GM, the distance between its vertical centre of gravity (G) and its metacenter (M), is the key element in making sure that a ship is stable. A positive GM is required, meaning that the vessel’s centre of gravity lies below her metacentre.

The GM value determines a ship’s seakeeping characteristics. If it is too big, a large righting moment at small angles of heel will make the ship stiff and uncomfortable. Large accelerations can affect safety and cause damage to equipment and cargo. A smaller GM, on the other hand, gives a small righting moment which results in a ship which rolls more slowly and without excessive accelerations.

“These large ships have very specific requirements because of the huge volumes of ballast they require and the nature of their ballast operations makes using other solutions challenging. With the ballast water convention now in force and port state control bodies gearing up to enforce the regulations, the financial penalties of making a poor choice of ballast water treatment installations, particularly for large bulk carriers, could be catastrophic,” Marshall added.

Sea News, November 15