Vessels and Ballast Water - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 22

however, generally does not apply to fore-peak and after-peak tanks since these are
frequently fi lled partially because of trimming the vessel.
Deballasting is conducted in the opposite sequence by:

- gravity through opening the valves that enables a vessel to discharge ballast

water into the surrounding environment from ballast tanks (or cargo holds used
for ballast) above the water line;

- pumping out the ballast water from ballast tanks (or cargo holds used for ballast)

below the water line.

Nevertheless, all ballast water may be discharged into the surrounding environment
by pumping, instead of using the gravity method (David 2007 ).
When tanks are getting close to empty, ballast pumps start loosing suction as
they start getting air in the system. The remaining water in tanks after pumping
with ballast pumps is in general between 5 and 10 % of ballast water tank volume,
what is mainly depending on the vessels trim. The ballast pipes suction heads are
usually installed on the back side of the ballast tanks, hence for pumping out most
of the remaining ballast water the vessel needs to be trimmed astern, what is also
a very general practice. This astern trimming is to compensate the change of trim
during the voyage because of fuel consumption from tanks, which are more in the
stern part of the vessel, to arrive in the next port of call approximately on even
keel. However, when Gollasch and David conducted shipboard tests of different
BWM methods we noticed that on vessels which were trimmed ahead about 15 %
and more of unpumpable water remained in the tanks during the empty-refi ll
(sequential) BWE. Actually, practice on some newer container vessels has shown
that when trimmed ahead the vessel consumes less fuel during navigation proba-
bly due to better hydrodynamics, hence these would nowadays usually start the
voyage on even keel or even be trimmed ahead (Captain Alok Kumar personal
communication, Chief Offi cer Guy Mali personal communication). When at the
start of the voyage a vessel could not be trimmed ahead because of some limita-
tions (e.g., limited maximum draft, required even keel), ballast operations would
be conducted at see what is done by internal transfer of ballast water or pumping
in some additional ballast water. For almost total deballasting of tanks, i.e., 1-2 %
of the ballast water tank volume remaining as unpumpable ballast, a ballast ejector
pump is used. This is also so called “stripping” and is done by using the fi repump
together with the ballast stripping eductor (Chief Offi cer Guy Mali personal
communication).
All ballasting and deballasting activities are usually led by the fi rst (chief)
offi cer, who is responsible for the vessel's stability. Following his instructions, the
pumps and valves are operated automatically from a ballast control console or from
a computer by an offi cer ( Fig. 8 ).
Some older vessels do not have an automated control over ballast pumps and
valves, then this may be done manually by an engineer, while the bosun (senior deck
crewman, ranked below the deck offi cers) has to monitor the conditions of ballast in
the ballast tanks by measuring the water level via sounding pipes at adequate