Environmental Engineering Reference
In-Depth Information
BWE depends very much on the BWE method used, as well as on the quantity of
ballast water to be exchanged.
To calculate the possibility of vessels to conduct BWE in the BWEA, the length
of the route through the area, the quantity of ballast water to be exchanged, the ves-
sel's speed and BWE PRC are factors to be considered. Different approaches have
been used in previous studies to prepare assessments of the time needed to conduct
BWE (e.g., AQIS
1993
; Royal Haskoning
2001
; Dragsund et al.
2005
). However the
results are not helpful for the Port of Koper situation because the vessels considered
are mostly of different profi le than those discharging ballast in or when approaching
this port.
One of the options to assess BWE PRC is to calculate it as the minimum ballast
water pump capacity of the vessels' ballast water system (i.e., theoretical capacity)
according to the shipbuilding rules (e.g., ABS
2006
), or by the ballast water pump
capacity reported with BWRF (i.e., the one based on vessels documentation). It was
recognised that the theoretical BWE capacity was always lower than the practical
capacity. In reality, BWE PRC is expected to be lower then the ballast water pump
capacity based on ships documentation, because:
when using the sequential method, more than 95 % of the water needs to be
exchanged to comply with the D-1 standard and this may require partially
closing of valves to avoid suction of air on pumps, or even stripping may need to
be conducted;
when using the pump-through method the vents through which water is pumped
out may slow down the pumping rate, and
additional time is needed to switch between tanks when more tanks need to be
exchanged, i.e., usually tanks would be exchanged in pairs in diagonal to lower
the stresses on vessels hull.
To illustrate this, a vessel with a ballast water pump capacity of 500 cubic metres
per hour (two pumps each with 250 m
3
/h) and 5,000 m
3
of ballast water on board to
be discharged would need approximately 21-22 h to complete the BWE when
utilizing the sequential BWE method. With an average speed of 15 knots, the vessel
will sail for 330 NM in 22 h. The same vessel, when using the pump-through
method, would need approximately 50 % time in addition than needed for the
sequential method (i.e., 33 h), and consequently a larger BWEA distance to complete
the exchange (i.e., 495 NM). Some mainly very large dry bulk carriers and tankers
may need to conduct BWE according to their BWMP using a combination of the
sequential and pump-through methods, i.e., for some tanks the sequential method is
selected, for others the pump-through method is used.
Because of this BWE PRC was based on the minimum calculated ballast water
pump capacity, i.e., the theoretical capacity. For the sequential method ballast water
needs to be pumped out of the tank and thereafter that tank needs to be fi lled again,
what would approximate to two times the water volume to be intended for dis-
charge. For the pump-through method three times the volume of the ballast water to
be discharged needs to be pumped through.
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