Ingangsdatum: 02-03-2012
4 - Design for reducing accumulation of sediment
4.1 Ballast water tanks and their internal structure
should be designed to avoid the accumulation of sediment in a
ballast tank. The following should, as far as is practicable, be
taken into account when designing ballast tanks:
- horizontal surfaces to be avoided wherever possible;
- where longitudinals are fitted with face bar stiffeners,
consideration should be given to fit the face bar stiffeners
below the horizontal surfaces to aid drain off from the
stiffeners;
- arrange for induced flows of water, either by pump forces or
gravitational forces, to wash along horizontal or near
horizontal surfaces so that it re-suspends already settled
sediment;
- where horizontal stringers or webs are required, drainage holes
to be as large as possible, especially if edge toe-stops are
fitted where horizontal stringers are used as walkways, to
encourage rapid flow of water off them as the water level in the
tank falls;
- internal girders, longitudinals, stiffeners, intercostals and
floors, where fitted, should incorporate extra drain holes which
allow water to flow with minimal restriction during discharge
and stripping operations;
- where inner members butt against bulkheads, their installation
should be such as to prevent the formation of stagnant pools or
sediment traps;
- scallops should be located at the joints of the inner bottom
(tank top) longitudinals or intercostals and floors to allow for
good airflow, and thus drying out of an empty tank. This will
also allow air to escape to the air pipe during filling so that
minimum air is trapped within the tank;
- pipeline systems should be designed such that, when
deballasting, disturbance of the water in the tank is as
powerful as possible, so that the turbulence re-suspends
sediment; and
- flow patterns in ballast water tanks should be studied (for
example by the use of Computational Fluid Dynamics (CFD)) and
considered, so that internal structure can be designed to
provide effective flushing. The amount of internal structure in
double bottom tanks will reduce the scope for improving flow
patterns. The hydrodynamic performance of the ballast tank is
crucial to ensure sediment scouring.
4.2 Any designs depending upon water flow to
re-suspend sediment should, as far as possible, be independent of
human intervention, in order that the workload of ships' crews is
minimal when operating the system.
4.3 The benefits of design concepts for reducing
sediment accumulation are that there is likely to be good sediment
removal while deballasting, with minimum retention of sediment in
the tanks, and therefore a reduction or no need for removal by other
means.
4.4 The design of all ships should provide safe
access to allow for sediment removal and sampling.
4.5 The design of ballast water systems should, as
far as practicable, facilitate installation of high sea suction
points on each side of the ship.
4.6 When practical, equipment to remove suspended
matter at the point of uptake should be installed.