6.1 Process considerations when coating repairs may be performed
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC.1/Circular.1399 – Guildelines on Procedures for In-Service Maintenance and Repair of Coating Systems for Cargo Oil Tanks of Crude Oil Tankers – (10 June 2011) - Annex – Guidelines for Maintenance and Repair of Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers - 6 Coating Repairs - 6.1 Process considerations when coating repairs may be performed

6.1 Process considerations when coating repairs may be performed

  6.1.1 The following considerations should be taken into account when coating repairs are undertaken:

  • .1 safety, including tank entry requirements;

  • .2 tank cleaning;

  • .3 staging;

  • .4 salt contamination;

  • .5 rust scale;

  • .6 pitting corrosion;

  • .7 temperature;

  • .8 condensation;

  • .9 ventilation;

  • .10 dehumidification;

  • .11 compatibility of coating systems; and

  • .12 stripe coating/design/surface area.

  6.1.2 Safety. Refer to the Recommendations for entering enclosed spaces aboard ships (resolution A.864(20)), as amended, and ISGOTTfootnote. It is an absolute requirement that all of the ship's safety and tank entry procedures and policies are adhered to. When a ship is out of service, in a ship yard repair, local regulations apply regarding safety. The ship yard is responsible for their implementation.

  6.1.3 Tank cleaning. Successful tank cleaning requires longer termed planning ahead, even for previous voyages to ensure concentrated Crude Oil Washing (COW) is carried out at the port(s) of discharge for the relevant cargo tanks. Especial attention should be given to tanks and areas to be cleaned and treated.

  6.1.4 Subsequent to COW of the relevant tanks, water washing, that may include the use of suitable tank cleaning detergent, and the use of fresh water, will be required. If deadweight and draft limitations of preceding voyage allow collecting substantial quantities of fresh water from rivers or other sources, this will make for a much more successful water washing as it will limit the salt contamination of tank surfaces and facilitate hand washing during surface preparations. The aim of the tank cleaning is to provide surfaces without oil residues on areas to be repaired.

  6.1.5 The shipowner's office must be contacted to confirm availability and reserve capacity for oily tank washings disposal ashore at subsequent ports. Similar good communication and co-operation will also be required even for programmed coating repairs.

  6.1.6 Special care must be taken during the use of solvents and detergents which are essential to ensure oil free surfaces for good adhesion of future coats. Due consideration should be paid to the disposal of these solvents and detergents from the view points of protection and environment. The gases released to the tank atmosphere by these solvents are explosive and toxic or poisonous and should be removed as fast as possible from the tank atmosphere. Thus venting and gas freeing equipment and procedures as recommended in ISGOTT should be established.

  6.1.7 When possible, control of the relative humidity during actual application of coating would increase the longevity of the coating and its adherence to the structure. Dehumidification is usually only an option during repairs alongside at an organized repair facility.

  6.1.8 Staging must be arranged to allow good access to all surfaces. Staging must be arranged according to prevailing safety regulations. Staging poles and working platforms should be placed in a distance from the surface to provide suitable work space for all subsequent operations, special care should be taken secure access to corrugated bulkheads.

  6.1.9 Salt contamination will cause accelerated deterioration of the coating if not removed prior to coating application. A recommended procedure to reduce salt contamination is to remove loose rust scale followed by thorough fresh water rinsing, preferably at elevated temperatures and high pressure. Test the salt content after washing and before coating using standard ISO 8502-9 or other equivalent methodfootnote and rewash if necessary. Observe, that salt contamination is concentrated in pits on pitted surfaces and the use of ISO 8502-6 and ISO 8502-9 may result in misleading results. This should be the starting point in any surface preparation process in cargo oil tanks on board ships after having thoroughly removed any oil contamination. In case of major repair or full recoating, any deviation should be agreed between the parties concerned and noted in the CTF.

  6.1.10 Rust scale that is not removed prior to coating application will cause early failure. Loose top-scale is easy to remove, however the inner (black) hard scale is much more adherent. When over-coated it will soon detach between the steel and the scale and come off, typically with the coating adhering very well to the outside of it. If the hard scale cannot be removed, the service life expectancy of the treatment is 1 to 2 years regardless of the coating used.

  6.1.11 Pitting corrosion is a major problem on board ships on area that have been exposed to seawater for some time. If it has been accepted that the pits need not be welded up in order to prevent further accelerated damage, a coating should be applied. Soluble salts will be present within the pits and it is essential that these are removed otherwise corrosion will soon start inside over-coated pits, affecting the service life. Various methods of salt removal from pits have been proposed. For example, water jetting followed by blast cleaning or possibly exposure to high humidity and repeated water jetting. Whichever method is chosen, any residues from the washing processes should be removed otherwise the soluble salt will precipitate out of the water on drying.

  6.1.12 Temperature is a critical parameter to consider. When repairs are carried out in a shipyard, proper surface temperature control can more readily be achieved in the areas requiring coating.

  6.1.13 Condensation is always a risk. It is an absolute necessity that the contractors have a good understanding about relative humidity and its relation to substrate temperature and dew point.

  6.1.14 Applying coating on the surface that is at or below the dew point, or that will be at or below the dew point while the coating is wet, will not perform. Ideally the temperature should be at least 3°C above the dew point.

  6.1.15 Ventilation is a vital factor for safety and quality of the coating application and must be carried out continuously during surface preparation, paint application, drying and curing. Ventilation arrangements must provide maximum efficiency, e.g., by arranging the ventilation so it extracts from the lowest and furthest corners to ensure the fast and efficient removal of solvents. The use of solvent free coating systems eliminates solvent release from the paint, but ventilation is still required during surface preparation and curing.

  6.1.16 Dehumidification of the tank or space to be coated effectively prevents rerusting of the steel after surface preparation and allows paint application on a dry steel substrate. This will not only ensure that the paint is applied under proper conditions, but it will also reduce delays and thus improves productivity. There are two different types of dehumidification, i.e. desiccant and refrigeration. Both work well, the desiccant type being ideal in moderate and cold climates, and the refrigeration type in warmer climates. Dehumidification to 40% to 50% relative humidity is recommended.

  6.1.17 Compatibility of coating systems is of utmost importance for a good end result. Unless the original coating system is totally removed, a coating system compatible to the original system should be used in accordance with the paint manufacturer recommendations. The coating system requires a Statement of Compliance or Type Approval Certificate according to the Performance standard for protective coatings for cargo oil tanks of crude oil tankers (resolution MSC.288(87)).

  6.1.18 Stripe coating/design/surface areas should be differentiated with respect to coating application as degree of access varies. Edges, corners, weld seams and other areas that are difficult to coat need special treatment. "Stripe coating" is used to produce a satisfactory coating and to obtain specified Dry Film Thickness (DFT) on such areas. Stripe coats should be applied as a coherent film showing good film formation and no visible defects, such as pores or de-wetted areas. The application method employed should ensure that all areas which cannot be adequately coated by spray application are properly stripe coated.

  6.1.19 It is recommended to apply a stripe coat before or after each main coat. This should be done using a colour that contrasts with each main coat, as this makes it easier to see that the stripe coat is satisfactory.

Parent topic: 6 Coating Repairs
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