Ballast Water Exchange Methods (D-1 Compliance)

As mentioned in lesson one, there needs to be a successful change of 95% of the water in the tanks to fulfill the convention, regulation D-1, before entering a new territory or state. Three different basic principles can be applied to comply with Regulation D-1 fully. These are: 

Keep in mind that all of these operations, no matter which method your vessel is operating under, need to fulfill Regulation B-4.

The Flow-Through Method

The Flow-through method and the Dilution method are quite similar. Both of them are based upon the principle of never leaving a tank empty.

This method is built upon the principle of continually overfilling the tanks. The tank is filled from the bottom, and the water is then let out at the top. The capacity of the outlets of the tanks needs to, as a minimum, align with the capacity of the ballast water pumps to avoid pressurizing the tanks, which can lead to structural damage. To achieve a 95% exchange, three times the tanks' volume needs to be pumped through the tanks when using this method. However some vessels are not designed to be able to handle this method. 

There should be a cleared space with enough distance from the discharge valve of the Ballast Water tank to ensure that nobody can get hurt during the process. 

This method's perks are that the pumps do not need to vary in speed since stripping and change of tanks are not included in the procedure. This means that there is no need for level control in the tanks, and the stability is not compromised, which provides for trim, list, and free surface effects. Since the stability of the vessel remains constant, this method can be used in varying weather conditions. 

The biggest obstacle to this technique is the required time to complete the procedure. It is also hard to fully control whether there has been a 95% successful water exchange or not. Using this method, the tanks are always a bit pressurized, which could be damaging for the tanks. Although this method can withstand varying weather conditions, there are some limitations at minus degrees due to the risk of the water freezing on the deck or in pipes. This method is also only suitable for full tanks. If otherwise, the sequential process, described further down, should be used. 

The Sequential Method

The sequential method differs from the other two. When using this method, the tanks are first emptied and then refilled. One should achieve a 95% volumetric exchange by performing this method, given that the tanks are emptied until suction is lost. Stripping should be performed. This procedure requires that a large amount of weight is carried away from the ship, and extra caution is needed to be sure that the vessel remains stable. In your BWMP, safe procedures are being described to ensure the safety of the ship. Since the Ballast Tank arrangements and pump capacity differ between vessels, the BWMP is ship-specific and needs to be followed.

When performing the sequential method, much care needs to be taken into consideration regarding the:

• Stability.
• Trim.
• List.
• Effect on manoeuvring.

The decision to continue the BWE after each step should be made by considering the vessel's location, weather prognosis, machinery performance, and crew fatigue. If any circumstances are recognised as unfavourable, the BWE should be delayed or stopped.

This method's perks are that it is more time-effective than the other two, which could be very favourable in cases where the time when water exchange is possible is limited. The exchange is more controlled and has a higher guarantee of 95% successful water exchange. There is also no risk of pressurising the tanks, and the risk of overflowing the deck and freezing is lower. Just as with the dilution method, there is no risk of human contact with the ballast water. 

However, there are some risks with this method, even though it is the most effective of these three. When using the sequential method, there is a risk that the vessel's stability will be compromised, and the ship is also more sensitive to heavy weather. This involves affecting the ship's trim and list. The displacement will change during the voyage, and sloshing may occur; this could jeopardise the vessel's manoeuvring. 

Even if the process itself is more time-effective, stripping can be time-consuming. To perform this operation, the pumps need to be able to operate at varying speeds.

The Dilution Method

The dilution method is very similar to the flow-through method, but instead of pumping water from the bottom, through the tank, and then out to the top, this method is built upon pumping new water from the top and then out. When using this method, approximately three times the tank's volume needs to be pumped through the tank to ensure a 95% successful water exchange. However, most ships that are operating today are not designed to use this method.

Usually, two pumps are used together; one acts as a filling pump and the other as a suction pump. It is crucial to keep the filling level in the tanks constant. The volume of both pumps must be controlled and followed through the whole sequence. 

This method's perks are that the ballast tanks' levels are constant, which does not compromise the ship's stability. Since the two pumps are working together, the risk of pressurising is insignificant. The crew and atmosphere are not, to any extent, exposed to the old ballast water, and much of the sediment is discharged during operation. The system can be operated in many varying weather conditions. The method is also pretty simple and, therefore, relatively cheap to run. 

The cons of this method are similar to the ones with the flow-through method. The tanks need to be full to be able to use this method. Since the operation demands two pumps, more parts can break, and if the suction pump fails, there is a risk of pressurising the tanks. This also means that the filling pump can fail. Low-level alarms are therefore necessary.