Ship Corrosion Case Study: Factors for preserving & coating ships hulls

This case study illustrates what can drive the selection of the right biofouling solution

The damages and losses of speed and efficiency that can be caused by the presence of fouling on a ship’s hull—more specifically on the bottom below the waterline—are well known. The attachment of fouling is aggravated when the ship is docked for a long time without the opportunity to navigate and to allow the antifouling coatings to achieve their purpose, which is losing the materials with the assistance of moving water.

Ship hull preservation and coating requires ship owners to consider:

• Cleaning processes used to remove biofouling and other contaminants;

• Surface preparation: methodology choice, cost vs. efficiency; and 

• Coatings: total removal and recoating or repainting.

The case study that follows will illustrate these considerations.

On this particular ship, the hull above the waterline was very damaged and eroded due to years of dock or fender impacts. Corrosion was present in almost 30% of the overall area. Section reduction of the steel plates was verified and could be seen with the naked eye. Below the waterline, there was a massive concentration of biofouling, mainly calcareous biofouling. In addition, signs of corrosion were also visible. No anodes were present, indicating the presence of impressed current
cathodic protection; the anodes were either turned off or inefficient.

Methodology

Above the waterline, methods included the following:

• Pressure washing up to 500 bar (7,252 psi) of all hull areas;

• Spot blasting per Sa 2½, according to International Organization for Standardization (ISO) 8501 of corrosion spots, two layers of spot coating to approximately 30% of the overall area, and one general layer of finishing coating to all areas;

• As additional information, about 90 m2 (969 ft2) of steel plates in the hull were replaced due to the lack of thickness and deformation caused by impacts. 

Below the waterline, methods included:

• Pressure washing up to 500 bar of all hull areas;

• Scrapping biofouling in spot areas with tighter biofouling;

• Power tool cleaning up to degree per St 3, according to ISO 8501, two layers of spot coating up to 5% of the overall area, and two general layers: one sealant and one finishing coating antifouling silicone based.

Attention is focused on coatings below the waterline as this is an area important for ship performance — specifically, speed and fuel efficiency, both of which are key factors in the overall efficiency of vessel performance and its cost effectiveness.

Pressure washing was not effective in this area. Years of docking were harmful for the antifouling; hard biofouling was attached to the vessel hull and was very difficult to remove. More incisive actions (e.g., persistent pressure washing) damaged the coating, making scratches as the rotational water jet passed. Having noticing this, the ship owner, with input from the coating manufacturer, decided to scrape. However, calcareous residues remained in the coating.

While other options were available, such as power tool sanding, grit sweeping, or blasting to bare steel, they were not used possibly due to cost or time constraints. Regardless, the owner decided on pressure wash cleaning again, spot blasting, and repainting per the specification.

Conclusions

These solutions, techniques, and their execution did not follow best practices:

• Damage or sensitive paint was not removed;

• Mud cracked coatings remained and were overcoated;

• Edges were not feathered and in fact, poor coating conditions would not allow for it; 

• Coating delamination due to solvent sensitivity of subsequent coating layer was not removed; and

• Final silicone coating roughness, caused by underneath old coating
poor conditions, was not considered as a relevant factor.

What then, drove the owner to the biofouling solution chosen?  (See Table).  On the one hand, a rise of 10 to 20 μm (0.4 to 0.8 mils) in the antifouling finishing layer roughness (Rz) can increase up to 1% total resistance experienced by a ship in navigation. Moderate concentration of fouling in a ship hull can reduce cruising speed by more than 10% and increase fuel consumption and manmade gas emissions to the atmosphere.

On the other hand, a total coating removal, including total blasting and the application of a more effective coating, could have increased maintenance costs, including unavailability periods and higher
drydock costs.

Techniques, technicians, equipment, and materials are always available despite time constraints. An effective intervention allows for the best result in a shorter period, reducing the risk of loss and damages for the intended period between maintenance procedures.

hull damage visible after blasting

IMAGE 1: Hull damage visible after blasting 

damage caused by pressure water jetting

IMAGE 2: Damage caused by pressure water jetting 

final layer of coating on hull bottom

IMAGE 3: Final layer of coating on hull bottom 

The real concern should be to ensure that proper corrosion protection
is performed. Standards such as those from ISO and NACE International, can and should be used to ensure proper performance. There should be a balance between maintenance and unavailability
costs and production losses or, in a worst case scenario, total material (i.e., vessel) loss. 

             Solution                 Consequence                      Decision Driver 

Pressure water
cleaning & scraping,
spot blasting, &
general coating
  Defective coating remains, introducing
delamination in new coating & general coarse roughness in final coating
  Less dry dock time, less cost
    The corrosion protection of the vessel hull under waterline will be more dependable in CP   1) Use less expensive - although less efficient - cleaning & surface prep system; 2) Availability for new shipping's earlier 3) Maintenance schedule, new dry docking

 

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Source: Originally appeared in Maritime News, Spring 2020; authored by Ricardo Jorge Carvalho Soares, Civil Engineer and COO at MONTACO, S.A.

 

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