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Increased safety risk due to severe corrosion related to box cooler arrangements

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DNV GL has experienced a number of vessels with severe corrosion of the steel structure in way of the box cooler. In the most severe cases, the bolted flange of the top plate of the cooling box was involved. This leads to increased risk of collapse of the top plate. This article provides insights and recommendations on how to avoid this.
Heavy local corrosion of the flange of a box cooler
Heavy local corrosion of the flange of a box cooler.
Coating failure of the cooling pipes on a box cooler
Coating failure of the cooling pipes on a box cooler.

Relevant for ship owners and managers as well as design offices, shipyards and suppliers.

Extent of damage 

A box cooler eliminates the secondary cooling water circuit of sea water inside the engine room (pumps, filters, valves, pipes, etc.). The cooling effect is achieved by natural circulation of the surrounding sea water and is considered to be a cost-effective alternative.

There are two main challenges related to box coolers:

  1. Aggressive galvanic corrosion due to more noble material in the box cooler compared with the ship steel 
  2. Marine growth on the cooler tubes, which can reduce the cooling effect

The most common systems of box coolers in the existing fleet are:

  1. U-tube bundle made of aluminium brass (CuZn20Al2), coated to prevent harmful galvanic effects on the carbon-steel sea chest; additional marine growth prevention system may be installed to prevent fouling
  2. U-tube bundle made of copper nickel (CuNi10), uncoated to prevent marine growth on the tubes

In a number of cases, severe corrosion damage has been observed in the carbon-steel mounting flange on top of the sea chest, including the bolts and shell plating of a sea chest in way of the box cooler. Considerable sea water leakage in a corroded sea chest may cause a large ingress of water in the engine room if not noticed in time.

Probable causes

The main drivers of corrosion damage are:

  • The natural circulation around the cooling elements causes warm sea water to rise towards the top of the sea chest (top plating), creating environmental conditions which increase both corrosion rates and marine growth (barnacles, shells, etc.).
  • Exposed noble materials (corrosion resistant) in the box cooler tubes impose galvanic currents between the cooler tubes and the sea chest, causing galvanic corrosion of the adjacent steel structure, if proper electrical insulation is not maintained.
  • The corrosion protection systems applied (coating of U-tube bundle, if applicable; coating of the surrounding steel structure and/or sacrificial anodes) are not able to suppress the corrosion rate of the sea chest. This can occur due to poor design, damage during installation or in service, or lack of necessary inspection and maintenance.
  • Insufficiently cathodic protection in the upper part of the box cooler compartments

Recommendations

Box coolers are sometimes assumed to be maintenance free. However, DNV GL’s experience has shown that the following items should be regularly verified by the crew:

  • Inspection and maintenance of the marine growth prevention system. Check the wear of the anodes and correct settings according to the maker’s specifications (if installed).
  • In connection with cleaning of the U-tube bundle do not damage the coating or remove the copper-oxide layer.
  • Check the condition of the corrosion protection system (coating and sacrificial anodes as applicable) of the specific box cooler and sea chest. A small area of damaged coating in the sea chest will concentrate galvanic corrosion to the area of exposed steel. If new coating is applied, do not coat the U-tube bundle, antifouling loading plates or anodes, as applicable.
    Note: The smaller area, the more aggressive the corrosion.
  • Look for signs of corrosion and leakage in the mounting frame and the corresponding sea chest.
  • If sacrificial anodes installed in the sea chest are consumed faster than expected, this may be a sign of damage to the coating system applied or defective electrical insulation of the bundles. Consult the relevant box cooler cathodic protection supplier for guidance on the positioning of sacrificial anodes.
  • Ensure electrical isolation between the box cooler bundles and hull.
  • Intact condition of the mounting flange bolts and gasket.
  • Sacrificial anodes are recommended close to the flange as an extra safety measure against galvanic corrosion; in order for these to work, they have to be immersed in seawater (air pocket close to the flange to be avoided); contact the manufacturer for advice.

Due to the severe corrosion issues seen with box cooler arrangements, DNV GL will follow up these details carefully in connection with dry-docking every five years. This may include withdrawal of the box cooler.

  • Class inspection will focus on: Inspection of the mounting flange, bolts and gasket(s)
  • Condition of coating and sacrificial anodes in the upper box cooler compartments if applicable
  • Inspection of the electrical isolation between box cooler bundles and hull if applicable
  • Capacity and arrangement of the antifouling system applied.
  • Inspection of the sea chest inlet and outlet grids (to ensure sufficient free flow area)
  • Marine growth (barnacles, shells, etc.) may cover severe local corrosion, especially in the upper part of the sea chest. Thorough cleaning is therefore necessary to check surface condition

Reference

DNV GL Rules Pt. 7. Ch.1 Sec. 5. 1

Contact

If you have any question concerning this news, please email to
Fleet in Operation – Hull & Materials.