Coldharbour Marine, a UK-based manufacturer of inert gas generators and ballast water treatment systems, is to launch a new version of its inert-gas based GLDTM treatment plant, designed specifically for large bulk carriers, at Kormarine, in October. The company’s unique technology for bulk carriers is based on existing treatment systems for tankers and gas carriers but will be configured differently.
In a standard type Coldharbour BWT installation inert gas is sent to GLD (gas lift diffusion) units mounted inside the ballast tanks. There are challenges with this arrangement on bulkers insofar as the wing tanks and heavy ballast cargo tank arrangements do not lend themselves to this kind of installation.
The alternative, developed by Coldharbour in conjunction with several ship owners is to mount a bank of GLD units inside the machinery space and to then circulate water from/to the ballast tanks for treatment. The GLD process is unaffected by this change, but it means that tanks of any size and configuration can now be treated using the proven, reliable GLD process. As with the standard arrangement, treatment takes place during a section of the ballast voyage, rather than during uptake or discharge. No intake filtration is required. This guarantees that not only will the ballasting process be unaffected but also that the ballast water discharged at the load port will avoid the problems of organism regrowth during long ballast legs, thereby ensuring compliance with IMO/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,” declared Andrew Marshall, Coldharbour Marine CEO. “That is why we have tailored our technology to meet the demands of these vessels. We are delighted to be unveiling this exciting development at Kormarine where we will be talking both to dry bulk operators and shipyards about our new technology which we believe breaks new ground.”
“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.” Marshall explained. “With the ballast water convention coming into 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.”
During ballast voyages, bulk carriers need ballast water for stability but also to make sure that ships’ propellers are properly immersed. Upper wing tanks are used, as well as lower wing tanks, because carrying ballast higher in the ship raises its centre of gravity for safe seakeeping (see ‘The ship science’, below).
Since the IMO’s Ballast Water 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 US Coast Guard 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 much vaunted USCG TA actually only tests for one day! 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,” he commented. “If ballast water fails to meet the discharge standard, there will be delays and penalties, and possible long-term reputational damage.
“That’s why we sat down to think about the particular challenges faced by the operators of these vessels, and we feel that the Kormarine arena will provide an ideal opportunity to talk about it. 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.”
The ship science
Bulk carriers are designed and built principally for the carriage of dry commodities including iron ore, coal, grain, phosphates and bauxite. The majority of the fleet consists of ships in the so-called “handy-size”, “handy-max” and “supra-max” categories ranging from 30-60,000 dwt. However, on long-haul routes, capesize vessels and even larger very large ore carriers are often deployed to move large volumes of dense cargoes, including iron ore and coal.
For these vessels, ballast water plays an essential role in their safe operation when they are not loaded. This is because ballast water is vital in 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 GM, or metacentric height as it is known – the distance between its vertical centre of gravity and its metacentre – is the key element in making sure that a ship is stable. For this, a positive GM is required – in other words, a vessel’s centre of gravity must always lie below her metacentre.
However, the size of the GM 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 “tender” ship – one which rolls more slowly without excessive accelerations.
Bulk carrier operators, therefore, like to use the upper wing tanks (see diagram), 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. Now, the treatment, pumping, piping and power systems required to treat large volumes of ballast to comply with the IMO’s Ballast Water 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.