A BWMS for big bulkers

Capesize and very large ore carriers are in need of a tailored ballast water solution, according to Coldharbour Marine Capesize and very large ore carriers are in need of a tailored ballast water solution, according to Coldharbour Marine

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,” says Andrew Marshall, CEO, Coldharbour Marine. “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.

CHOOSE WISELY

“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.”

The majority of the bulk carrier fleet consists of ships in the so-called “handy-size”, “handy-max” and “supra-max” categories 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.

OPERATIONAL CHALLENGES

Bulk carrier operators like to use the upper wing tanks, sometimes also known as upper hopper tanks, for ballasting purposes because this raises a ship’s centre of gravity and reduces the GM. Traditionally, ballast water from these large tanks is released directly overboard without treatment. But now the treatment, pumping, piping and power systems required to treat large volumes of ballast to comply with the BWM Convention discharge standard pose a major economic and operational challenge for ship operators.

The issue is further complicated by the fact that most system technologies treat ballast water as it is pumped on board. No single system is completely effective, however, and large bulk carriers deployed on long-haul routes may be subject to “re-growth” during a ballast voyage. This could well mean that discharge standards at the next loading port cannot be met.

Since the IMO’s Ballast Water Management Convention lays down strict discharge standards, ballast water now shipped in upper wing tanks must be treated before it can be discharged. However, Marshall points out that there is no technology available today that can treat large volumes of ballast water as it is taken on board during and after cargo discharge whilst still guaranteeing that ballast water discharge standards will be met when the vessel arrives at the load port, probably at the end of a long voyage.

Marshall does not believe that the re-growth issue has been properly addressed either by the IMO or USCG type approval processes. No treatment system is completely effective, he argues, so regrowth on longer voyages is inevitable.  This is supported by the overwhelming body of scientific data.  Therefore, Coldharbour’s technology, optimised as it is for the large long-haul vessels, employs a treatment process that takes place during a part of the voyage, rather than during uptake or discharge.

“The regrowth test for IMO is only five days after treatment, whilst the vaunted USCG type approval actually only tests for one day,” Marshall notes. “Some of these large bulkers have ballast legs more than 10 days and in extremis as long as 42 days. Even if a relatively small number of marine organisms survive the initial treatment process, they will have plenty of dead organisms to feed on over a long ballast voyage. If ballast water fails to meet the discharge standard, there will be delays and penalties, and possible long-term reputational damage.”

For Marshall, launching a large bulk carrier system at the Kormarine exhibition made perfect sense. “After all, most large bulkers are built either in South Korea or China, and Asia is the world’s largest consumer of bulk cargoes, especially iron ore,” he argues. With the installation timeframe for the BWM Convention about to kick in, operators of these ships now have another option specifically tailored to their particular design challenges.

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