Bulletin No.: 09/1996

Date (Y-M-D): 1996-07-10
 

Subject: Ships Operating in Winter Conditions

This bulletin has been prompted by a number of reports, incidents and casualty investigations respecting ships operating in winter conditions. Its purpose is to serve as a reminder of some of the problems which may be encountered due to spray icing, low ambient air temperatures and slush ice.

Air vents may become blocked due to external spray icing, also, where low air temperatures are encountered, there is the possibility of ice build-up inside air vents and in tanks situated above the waterline, particularly those exposed to the wind. As a consequence of air vent blockage, subsequent pumping may cause damage to the tank or ship structure due to changes in tank internal pressure.

Attention should therefore be paid to all air vents and, in particular, to those connected to sea water cooling systems, tanks being used for cooling water recirculation and tanks undergoing ballasting or de ballasting. It is suggested that in the case of ships operating in winter conditions it would be advisable to provide protection to air vents by suitable siting in unexposed locations, screening or heat tracing.

Ships operating in slush ice conditions are prone to blockage of sea water cooling intakes unless special arrangements are made. It is suggested that, in addition to steam or compressed air clearing connections at the intakes, a method of directing warm cooling water discharges into the intakes should be provided. The basis of one such arrangement is described in the attached annex. Many ships have, as a back-up, a system of cooling water recirculation using designated tanks. In such installations, experience has shown that double-bottom ballast tanks are to be preferred.

Annex
Design and Construction of Sea Inlets

Operating Under Slush Ice Conditions

1. On a number of occasions concern has been expressed by pilots in Canada with regard to ships operating in slush ice conditions. Casualty records also indicate that a substantial number of incidents have occurred during the winter navigation season in Canada. The primary cause of many of these incidents has been due to the loss of cooling water to the ship's main engine and/or generators due to the blockage of sea inlets by slush ice. Incidents have ranged from minor vessel delays to complete power failures which in some cases have resulted in groundings.
2. In reviewing the requirements of Regulation 26 of part C, chapter 11-1 of SOLAS 1974, it is noted that only a general reference is made to the design and equipment with regard to environmental conditions and that detailed requirements are therefore left to the discretion of the flag administration.
3. Although it is common practice to provide a supply of low pressure steam or compressed air to cooling water inlets, experience in Canada has shown that when ships are operating for extended periods in anything but the lightest ice conditions, this provision will, in all probability, be inadequate. It is our opinion that it would be prudent from a safety point of view to address in some detail, a suitable system for maintaining a supply of cooling water to the essential machinery of ships operating under these conditions.
4. The following system of circulation has been used on a number of Canadian ships and provides one possible solution to this problem
    
   1. The ship should be provided with at least one sea bay from which pumps supplying cooling water to essential machinery can draw.
   2. The sea bay should:
       
      1. be supplied with water from at least two sea inlet boxes; and
      2. be connected to the sea inlet boxes by pipes, valves and strainers with a cross sectional area equal to the total area of the suctions served by the sea bay;
   3. The sea inlet boxes should:
       
      1. be fitted on each side of the ship;
      2. be as deeply submerged as possible;
      3. have an area open to the sea of five to six times the total area of the pump suctions served by the sea bay;
      4. be fitted with a strainer plate at the ship's side having perforations approximately 20 mm dia. to prevent ingestion of large ice particles;
      5. be fitted with a low pressure steam connection for clearing the strainer; and
      6. be vented to atmosphere by a valved pipe with a cross sectional area at least equal to that of the cooling suctions.
   4. Diversion valves and piping should be provided at overboard cooling water discharges to permit warm water to be returned to the sea inlet boxes to prevent blockage.
5. Eastern Canadian waters are noted for heavy spray ice accumulation during winter and precautions should be taken to avoid structural damage due to blockage of air pipes by spray ice. Air pipes serving cooling systems should be positioned in protected areas or heat traced as a preventive measure.
   
   The attached sketch illustrates the recommended layout of a successful system incorporated in Canadian vessels operating under slush ice conditions in Canadian waters.

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