Understanding the Risks of Corrosion in Bulk Carriers
Introduction to Bulk Carriers and Their Cargo
Bulk carriers, specialized vessels designed for transporting unpackaged bulk cargo, are vital to global trade. They typically carry large quantities of goods such as coal, grain, and sulphur. However, the nature of their cargo, particularly elemental sulphur, poses unique challenges concerning vessel integrity.
The Threat of Accelerated Corrosion
Recently, marine consultancy Brookes Bell has issued a dire warning for bulk carriers transporting sulphur cargoes in the Strait of Hormuz. With some vessels remaining anchored for more than 60 days, the risk of structural corrosion has significantly increased. This situation arises because the protective treatment applied to cargo holds has a limited lifespan, normally around 20 days. Extended anchorage beyond this period leads to detrimental effects on the vessel’s steel structure.
The Process of Corrosion
Elemental sulphur is loaded into holds that are typically treated with a limewash coating, intended to create a barrier between the cargo and the vessel’s steel. Once the cargo is on board, it is dampened with water to maintain protection for approximately 20 days. Post this period, unprotected steel starts to corrode when it comes into contact with moisture and sulphur.
Proliferation of Corrosion
As Arron Jackaman, Director of Non-Destructive Testing at Brookes Bell, notes, when steel is exposed for prolonged periods, corrosion forms at alarming rates. The acidic environment includes iron sulphide, which leads to rapid and localized pitting corrosion — a rate of decay far higher than what would occur in regular seawater.
Evidence shows that unprotected steel in affected vessels has showcased pitting depths of up to 5 mm, with some extreme cases reporting wastage of up to 7 mm over just 50 days. For comparison, the standard corrosion rate for steel in seawater is about 0.2 mm per year.
Misinterpretations of Visual Damage
Interestingly, sulphur corrosion often appears more severe than it is. Damage frequently concentrates where equipment for cargo discharge has compromised the protective coating, and on cargo hold tank tops where coating is absent by design. Without precise measurements, visual assessments can mislead stakeholders into overestimating the extent of damage and underestimating the necessary repair work.
Implications for Repairs and Costs
Under Common Structural Rules, bulk carrier structures are allowed for a sacrificial corrosion margin of 5.5 mm. Visual assessments alone can overlook this margin, leading to excessive repair claims. Jackaman emphasizes that basing repair decisions on appearances rather than measuring the actual structural integrity can inflate costs significantly; wholesale steel replacement can cost up to ten times that of targeted localized repairs.
Safety Considerations for Crew
Additionally, Brookes Bell has stressed the importance of safety measures to protect crew members working in potentially hazardous environments due to corrosion. Proper atmospheric testing, risk assessments, and protocols for confined space entry need to be established before any personnel can safely enter affected cargo holds or adjoining tank spaces.
The Importance of Early Engagement
To effectively tackle the challenges posed by corrosion in bulk carriers, early engagement with marine consultancy is critical. The most reliable data regarding corrosion is typically obtained before cargo residues are cleared, and surfaces are cleaned. Brookes Bell advocates for involvement immediately after corrosion is identified, allowing for accurate assessments and preventing unnecessary steel replacement.
Jackaman reiterates that early assessment provides shipowners and insurers with the strongest position to challenge inflated repair claims, ensuring that repair costs are fair, proportionate, and justifiable.
The state of bulk carriers, particularly those transporting elemental sulphur, calls for urgent attention and proactive measures to mitigate financial and structural losses.