^ Applying Surface Treated Composite (STC) Coatings – Image Courtesy of Hydex Solutions at http://shiphullperformance.org/getJournalPDF/15.pdf
This is the second in a series of two articles and is largely based on the whitepaper Extending the Interval Between Drydocking to Ten Years by Hydex Solutions. The first article viz. Is Dry Docking a Necessary Evil? deals with the numerous aspects of drydocking.
The Path Between Two Extremes
Drydocking ships is necessary to counter fouling and corrosion, the two most potent destroyers of ship hulls. And because painting hulls after cleaning and shot blasting them deals with both these insidious raiders, it is the most important operation in drydocking.
Lest you start regarding drydocking as a pleasant exercise, think again. It is a complex, expensive, lengthy, and stressful task that requires you to stop lucrative ship operations and dispatch the vessel to a shipyard. Chances are, this yard will be miles away from the ship’s regular route.
But if you do not drydock the vessel, your fuel expenses will soar to astronomical levels. This is because fouling and corrosion roughen the hull and escalate the drag that, in turn, hikes fuel consumption.
That is not all. Your ship will emit more than the permitted amount of carbon dioxide (CO2). It will also transfer more invasive species from one area of the ocean to another. And when you finally drydock the vessel, it will cause more pollution in the water around the yard.
All this can attract heavy financial penalties even as you have to pay larger sums to get your ship insured. Seems both the ends – frequent drydocking and seldom drydocking – are both financially unviable. What then is the solution to this pressing concern?
Fortunately, there is an answer to this vexatious issue – the application of Surface Treated Composite (STC) coatings. Moreover, STC paints achieve the primary objective of hull painting viz. preventing fouling and corrosion without adverse impact on health and environment.
Hull Killers Named Fouling & Corrosion
Fouling is the accumulation of colonies of algae, seaweed, tubeworms, polyzoans, barnacles, and mussels on a ship’s hull. These increase the drag on the vessel, lower its speed, escalate fuel use by a whopping 40%, reduce a hull’s useful life, and hike its vulnerability to corrosion.
With fouling, such organisms also get a free ride from one part of the ocean to another. At their destination, these species can grow rapidly in the absence of natural checks and destroy the local ecosystem.
A rise in salinity and temperature of seawater increases fouling. Stationary and slow moving ships are more prone because the larvae of the aforementioned organisms cannot get a foothold on ships moving at over 1 knot.
Corrosion is the destructive attack of the environment on the hull. There are various forms of corrosion, all of which remove material from the hull. Dissolved oxygen, air and water temperatures, humidity, pollutants, and wind affect the rate of marine corrosion. It can be:
- Rusting: the combination of metals with oxygen in the presence of moisture / water
- Galvanic Corrosion: results when two different metals are connected via an electrolyte. Seawater acts as electrolyte in this case. The less noble metal becomes the anode (positive terminal in an electrochemical cell) and corrodes faster
- Pitting: is the creation of small holes on the hull surface. Changes in oxygen content, temperature, and flow rate of water make an anode out of the exposed area. Anodes lose material faster
- Electrochemical Corrosion: is very much like galvanic corrosion except that it needs an external stray current
- Cavitation Corrosion: follows the formation and collapse of low-pressure vapor bubbles near the hull. Extreme pressure differences during turbulent water flow create these bubbles. Their collapse causes surrounding water to gush in at very high pressure
Coatings prevent rusting because they preclude contact between metals on one hand and water and oxygen on the other. Harder metals are more resistant to cavitation. Streamlining flow patters around the hull minimizes the formation of vapor bubbles.
Cathodic protection inhibits galvanic, electrochemical, and pitting corrosion. You install a less noble material near the metal surface you want to protect. Seawater completes the electrochemical cell. The less noble metal becomes the sacrificial anode and protects the cathode.
Limitations of Conventional Anti-Fouling Paints
A rough hull indicates that you need to get the vessel drydocked. When you discover a rough hull, you can:
- Continue with Business as Usual
- Opt for Underwater Cleaning
- Undertake Drydocking
As mentioned earlier, a business as usual attitude will make you spend more on fuel. It can also make you liable for fines as you exceed the regulatory caps for emissions of polluting gases and transfer of invasive aquatic species.
For example, it you do not re-apply a conventional coating system on a hull for around 10 years, the fuel penalty i.e. the rise in fuel consumption can be as high as 25-40%. Please note, fuel costs account for 47% of a ship’s voyage costs.
Next, you can clean the hull in-water. This presents its own set of problems related chiefly to conventional anti-fouling coatings. These coatings contain toxins or biotins that slowly leech into water and prevent fouling on hulls.
Two specific issues are particularly noteworthy:
- Rapid Wear
- Environmental Degradation
Traditional coatings are soft and erode quickly. You have to repair them frequently. Shipyard personnel shot blast the hull and repaint it for the first 2-3 drydockings. And when the hull becomes intolerably rugged every 10-12 years, they blast it to bare steel and re-apply the paint.
In the meantime, you incur a sizable fuel penalty. Cleaning also releases these paints and promotes marine pollution. Loaded as they are with heavy metals and other toxic materials, these paints pollute water around shipyards.
Rough tools used for in-water or underwater hull cleaning further accelerate the removal of these coatings. Such tools also roughen the hull. Rough hulls inflate fuel use and provide better grip to aquatic organisms. This actually promotes fouling.
The issues do not end here. Most conventional paints used the two organotin compounds viz. tributylin (TBT) and triphenyltin (TPT). IMO’s Convention on the Control of Harmful Antifouling systems for Ships 2008 has banned the use of organotins in anti-fouling paints.
Now, there are multiple manufacturers who will tell you their post-TBT antifouling paints protect hulls for five years. Practical experience tells a different story. This duration is only a few months, a year at the most.
Drydocking therefore is the best option but, as mentioned, is expensive, intricate, time consuming, and stressful. We can however initiate measures that extend the interval between drydocking to 10-12 years.
Regulations mandate one drydocking every 2.5 to 5 years depending on the type and age of a ship. Classification societies have started allowing ships less than 15 years of age to drydock once every 7.5 years provided they meet certain conditions.
Surface Treated Composite (STC) Coatings
Glassflake-based STC coatings are the best available underwater hull paints of the day. These are more durable, tougher, and more resilient. They have successfully shielded hulls in the harshest of conditions – 2.5 meter ice mixed with volcanic lava in Antarctica.
Apply STC coatings to the entire wetted area of the hull of your newbuild after grit blasting the hull to Sa 2.5 roughness. It helps them gain traction on the surface. These are compatible with steel, aluminum, and glass-reinforced plastic hulls.
Coming with a 10-year guarantee, STC coatings last for our target 10-year interval between successive drydockings. In reality, they last for the entire 25-30 year lifespan of the hull. STC paints also slash cavitation damage to steel rudders. Do not apply them on propellers though.
With STC coated hulls, all you have to do in the name of maintenance and repair is conduct routine in-water hull cleaning – without drydocking the vessel. This removes all macro-fouling and most micro-fouling.
And when you drydock the vessel, you only have to undertake a few trivial touch-ups that involves painting of less than 1% of the hull’s wetted area. This does not take more than a few cans of paint.
In-water cleaning of hulls is a customized process. Ships plying in warmer waters and spending more times at port require greater cleaning as compared to vessels voyaging in cooler waters and spending less time anchored in ports.
Using STC paints and following the aforementioned procedure, you slash the cleaning time to a meager 6-12 hours including the time taken to clean the niche areas of your vessel. This way you can clean the hull every 1-2 months.
Such a system has multiple benefits. It cuts down the expenses for cleaning, drydocking, and fuel. Your ship spends less time at the dry dock and more time in the ocean earning revenue for you.
In marked contrast to traditional coatings, these paints actually soften with every in-water hull cleaning. Being harder, STC paints do not leech into water when you clean the hull. This minimizes water pollution around the shipyard.
STCs are eco-friendly for two more reasons. One, your vessel gobbles up less fuel and therefore emits lesser pollutants. And it transfers lesser invasive aquatic species because it makes it near-impossible for these organisms to cling to the hull.
Many ports restrict in-water cleaning because cleaning hulls coated with conventional paints pollutes port waters. This does not apply to STC paints. Such restrictions will soon be obsolete as STC paints acquire greater popularity.
Despite all this, you will still have to drydock your ships at less-than-10-year intervals to inspect, repair, maintain, or repair tail shafts. You cannot conduct these operations in-water. The duration, complexity, and expense for such drydockings will however be lesser than before.
Advances in material technology have always expanded the frontier of human civilization. STCs might just be one such technology that transform the way we maintain hulls.
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