COVID-19 has had an unprecedented impact on the shipping industry, particularly the cruise shipping industry. World over many ships – not just cruise vessels, but cargo carriers, bulk carriers and even off-shore supply vessels are being laid up with no real clarity on when their businesses will return to normalcy. According to some reports some sectors have between 15-100% of vessels lying at anchor. Of these sectors, bulkers were the first to be hit as raw material cargoes to China softened and the economy contracted earlier this year. Meantime, in the container sector around 14% of global TEU capacity is said to be lying idle and around 100-200 of the world fleet of 770 VLCCs may be needed to satisfy the demand for floating oil storage. It is, of course, the cruise industry that is suffering the most, with just about every ship in the fleet idled, other than some being used by cruise lines to repatriate crew.
Corrosion protection of laid up ships
There are, as is well known, two types of vessel layups – cold layup and hot layup. In hot layup some of the critical powered functions are kept on while in a cold layup almost all will be shut down. One of the issues during layup is that corrosion continues unabated and needs to be controlled to prevent a degradation of the hull plate and vessel life and expensive repairs on starting up.
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For a vessel protected by sacrificial anodes, these anodes will continue to protect the vessel even when laid up but will get consumed needing replacement and an early dry-docking even if other aspects of the vessel do not require docking. For vessels protected by impressed current systems the systems will need to be on, readings recorded and as with any electrical or electronic system periodic maintenance would be required.
A simpler option could be to use a system of suspended sacrificial anodes for as long as the vessel is laid up. Such a system comprises several anodes suspended on cables around the ship’s side to about mid keel depth. Aluminium is the most efficient anode material for this unless the vessel is laid up in fresh water when Zinc or Magnesium could be preferred. The cables should be connected to the hull making good electrical contact. In addition the protection can be easily monitored periodically by carrying out a simple hull potential survey using a multi-meter and a portable silver/ silver chloride reference electrode and measuring the potential of the hull at 3 to 5 locations (depending on the size of the vessel) on each of the Port and Starboard sides. The potential should ideally be -800 ± 50mV. If readings are higher than -800 (-700, -600 etc.) more anodes should be lowered into the water and if lower (-900 -950 etc.) some anodes should be lifted out of the water.
If the system is properly designed and sufficient quantity of anodes deployed, then, with so many issues to worry about, corrosion and consequent issues on the hull, early docking for sacrificial anode replacement or the issue of keeping an ICCP system on and maintaining it with limited crew are problems vessel owners can put out of their minds.
When the vessels are being re-commissioned, the suspended anodes just need to be pulled up, cleaned and stowed away for future use and it can be ‘anchors aweigh’ for the vessel.
SARGAM has extensive experience in designing and implementing Cathodic protection systems for a wide range of vessels ranging from tugboats and OSVS to FPSOs and aircraft carriers and will be happy to assist in designing and supplying a low-cost temporary cathodic protection system for any laid-up vessel.