All along the ship’s length, the ballast system utilizes water tanks positioned in the double bottom, along the sidewalls, and beneath the ship’s main deck.
In order to keep the ship’s draught and trim proper, to provide maximum stability, and to keep the ship’s stress and bending moments within acceptable limits, water is added to or removed from the ballast tanks. One tank or pair of tanks should never be partially filled (slack) at any given moment, as slack tanks provide a free surface effect that is detrimental to stability.
Ballast systems typically refer to tanks with pipes and pumps that may be filled or emptied in order to alter the ship’s mass and hence its draft, but this is not always the case. Heeling systems on the other hand simply recirculate the ship’s ballast water to maintain a ship’s heel or trim. Heeling tanks are partially filled with water and are used to adjust the heel of the ship during loading and unloading operations. Water is transferred from the port heeling tank to the starboard heeling tank, or vice versa, in order to keep the ship in the upright position using a dedicated anti-heeling pump system.
The heeling tanks are filled and emptied by way of the ballast pumping system. In the event of a failure of the anti-heeling system, the ballast pump is used to transfer ballast as required between the port and starboard heeling tanks.
After an incident, the heeling system must be able to move huge amounts of water in a short period of time and give a heeling compensation
Both a tie line and a collecting pipe system are options for ballast system pipework. Simple collecting pipe systems have been most common, with the main line running through a ship’s pipe tunnel.
For ballasting and eductor drive operations, the ballast pumps can draw suction from the main sea water main crossing pipe in the engine room. As part of de-ballasting procedures, the ballast pump discharges directly overboard. Before beginning ballasting operations, care must be taken to ensure that the ballast main is properly flooded.
The ballast system is controlled from the ship’s control centre panel which enables the pumps to be started and stopped and ballast system valves to be opened and closed remotely. Usually the valves are hydraulically operated and all of them are normally located outside of the respective ballast tanks which allows maintenance and inspections to be carried out on all of the valves without having to enter a tank.
All ballast and de-ballasting operations for each tank must be entered into the Ballast Log Record Book, stating date, ship’s position, temperature, specific gravity, pumped quantity, tank quantity and any further remarks. Refer to IMO Resolution A868(20). The International Convention for Control and Management of Ship’s Ballast Water and Sediments has been accepted in 2004 and it applies to all vessels that carry ballast water. There are some basic procedures available for ballast water management:
Ballast water exchange – either all of the water is pumped out and then filled back up (sequential method) or fresh sea water is continuously added (flow-through method). With the sequential method, the tank is completely emptied and then filled again, but this changes the vessel’s load condition, which is not good. In the flow through or overflow method, the water is pumped through the tank until it overflows. To replace 98 percent of the total volume of the old ballast water with new water, the whole volume needs to be replaced about three times. This method doesn’t affect the vessel’s load condition, but it does increase the handling capacity of the ballast pump and the amount of time it is running.
Ballast water re-circulation – ballast water is recirculated within the ship instead of being released or taken in using this approach, which is dependent on the ship’s load. Ships carrying containers are particularly interested in this strategy, which keeps the ship’s overall mass constant while optimizing load distribution.
Ballast water treatment – when the tanks are filled and when they are emptied, the ballast water is subjected to mechanical, physical and chemical treatment. There are a variety of ways in which water can be heated or subjected to UV radiation or ultrasound depending on the manufacturer, but the sediments are separated by means of filters.
When using ballast water treatment system, prior to the commencement of ballasting operations, the BWT units lamps commence a start-up sequence at which time the units are cooled by sea water. The incoming ballast passes through the filter which performs the function of removing larger organisms and particles, after the filter, the ballast water passes through the UV or chemical injection units which treat the water to the required IMO standard before entering the ballast tanks. On completion of ballasting, an automated sequence flushes the treatment unit with fresh water, following this, a CIP (Cleaning-in Place) cycle commences to clean the unit, which takes approximately 15 minutes. The cleaning cycle may also be operated manually from the local control panel.
During de-ballasting, the main difference to the ballasting procedure is that the filter is bypassed because the water would already have been filtered during ballasting. During the discharge process, water from the tanks is again passed through the UV or chemical injection units to destroy any organisms that may have formed during the time the ballast water has remained in the tanks.
Ballast system is normally operated by vessel Chief officer or one of the deck officers during their watchkeeping under Chief Officer supervision and the engine room should be informed by the imminent use of the system 9especially operating the ballast pumps).
Under normal circumstances no more than one pair of ballast tanks (port and starboard) should be partly filled at any one time in order to prevent stability problems due to the effect of slack tanks. Tanks not currently being filled or emptied should be completely filled or empty. Check the quantity of ballast water to be removed from the particular pair of tanks. Water should normally be removed from the port and starboard ballast tanks at the same time with heeling being controlled by the anti-heeling system.
It is important to note that hydraulic hammer in ballast lines can cause serious damage and must
be prevented at all times. Valves must only be opened in a manner that will prevent damage to pipes, pumps and other valves in the system. In the planning and execution stages of ballast operations, consideration must be given to the following:
– The opening of valve(s) from an empty tank into a line that may or may not be empty or in partial vacuum. This will allow the pressure or vacuum that may be present to decay slowly.
– Back filling of the lines from the sea chest should be done in a controlled manner by only opening the appropriate valves to the pumps and the ballast lines. This will again allow the pressure or vacuum that may be present to decay slowly. It may also be possible to vent any displaced air in the lines through the ballast overboard discharges.
It is the responsibility of all those either directly involved in or assisting in supervising cargo/ballast operations to ensure that the system valves are operated in a safe and proper manner and that the systems, including pump casings are vented before operations commence.
If the operator is to leave the Ship Control Center then the discharge, in case of de-ballasting, should be set to eductor discharge only, in order that the ballast pump does not run dry with the subsequent failure of the pump elements and shaft seal.
In order to strip out the tanks, different systems are used like water eductors or self-priming stripping pumps (usually lobe type) which are designed to run with low suction and without any additional water which is an advantage as the vessels are fitted with a Ballast Water Treatment System. An adequate trim should be maintained throughout the stripping operation to achieve good suction to the pump.
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