4 Design for Reducing Accumulation of Sediment
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Statutory Documents - IMO Publications and Documents - Resolutions - Marine Environment Protection Committee - Resolution MEPC.209(63) – 2012 Guidelines on Design and Construction to Facilitate Sediment Control on Ships (G12) – (Adopted on 2 March 2012) - Annex – 2012 Guidelines on Design and Construction to Facilitate Sediment Control on Ships (G12) - 4 Design for Reducing Accumulation of Sediment

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:

  • .1 horizontal surfaces to be avoided wherever possible;

  • .2 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;

  • .3 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;

  • .4 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;

  • .5 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;

  • .6 where inner members butt against bulkheads, their installation should be such as to prevent the formation of stagnant pools or sediment traps;

  • .7 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;

  • .8 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

  • .9 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.

Parent topic: Annex – 2012 Guidelines on Design and Construction to Facilitate Sediment Control on Ships (G12)
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