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17 December 2008
The phenomenon of pitting corrosion in cargo, ballast and slop tanks of tanker vessels carrying crude oil products has been recognized for decades. However, the shift from single hull to double hull tankers has brought about new challenges because double hull tankers are more exposed to pitting. Pitting can require repairs that disrupt the commercial operation of a vessel and can potentially create a risk of pollution and jeopardize the safety of the vessel and crew. Pitting thus needs to be dealt with both commercially and contractually by the affected parties.
Pitting corrosion, or pitting, is a form of localized corrosion that produces cavities or pits in metal. It is considered more dangerous than uniform corrosion because it is more difficult to design against, predict and detect. The driving force behind pitting is a lack of oxygen, creating breeding grounds for sulphur-reducing microbes which produce acids that locally corrode steel. These microbes are found in crude oil, placing oil tankers at particular risk. However, they are also common in seawater and mud.
Pitting is common in single hull vessels but normally does not require attention until the vessel is 15 to 20 years old. However, in double hull vessels it can occur after only a few years. This is because the double hull acts as insulation and causes the crude oil to stay at elevated temperatures for a longer period of time than in single hull tankers. Thus, the microbes thrive and the effect of microbe-induced pitting is intensified. Following the adoption of the 1992 amendments to Annex I of the International Convention for the Prevention of Pollution from Ships, all crude oil tankers have been built with double hulls.
Reports of aggressive pitting in double hull oil tankers started to break in the mid-1990s, when double hulls had been used for some years. Most double hull tankers of the 1990s were built without any protection for the steel. In severe cases, pitting as deep as seven to nine millimetres (mm) was found in 18 to 20 mm thick tank bottom plates on vessels less than five years old. If the pitting process goes unnoticed, it can eventually lead to crude oil leaking into the ballast tanks, contaminating the ballast water and releasing combustible gases, causing a risk of explosion (although leaks are usually detected by hydrocarbon alarms), which in turn poses a risk to the safety of the vessel and to those onboard, and creates a risk of oil spills. It has been suggested that the mandatory requirement for double hulls, introduced in the aftermath of the Exxon Valdez disaster in 1989, has actually created an unforeseen potential for oil pollution.
The most common method for prevention of microbe-induced pitting is to utilize a strong and hard, yet flexible, multi-layer and multi-colour paint coating to a height of one or two metres from the tank bottom, along with the creation of more inspection-friendly newbuilding designs. Japanese engineers have also designed a new type of steel which is supposed to be more resistant to microbe-induced pitting.
The costs related to dry-docking and repair due to pitting damage may be extensive and may not be covered by insurance. Prospective buyers of a double hull oil tanker yet to be built should therefore ascertain that the vessel is being built to withstand pitting corrosion. Such preventative measures may be provided for in the shipbuilding contract and the relevant specifications, with the costs forming part of the total price for the building of the vessel. If the necessary measures have not been provided for in the shipbuilding contract, buyers would be well advised to consider taking the necessary measures by way of a variation order during the building process rather than running the risk of pitting damage and consequential losses at a later stage.
Most paint coating manufacturers now offer long guarantees of several years' duration which may be assigned to the owner under the newbuilding contract. However, the owner should consider consulting the paint manufacturer before signing the contract, working out adequate paint specifications and employing on-site teams to monitor the coating process.
Sale and purchase
Pitting is not always easily detectible through inspection. Buyers would therefore be well advised to insist on cleaning and inspection of areas where there is risk of pitting, or at least to ascertain that the seller’s classification society has conducted sufficient inspection during recent surveys. However, buyers should be aware that the various classification societies have different steel repair requirements. Taking delivery of a vessel with latent pitting damage may prove costly, especially under a standard outright sale such as under the Norwegian Sale Form, where the risk is placed on the buyer as a main rule.
Given the risk of pitting inherently involved with the carriage of crude oil products, owners should ensure that this risk is adequately allocated in the charterparty. Although the risk of pitting becomes more significant the longer the duration of the charterparty, the risk might also become significant for owners that, over some time, trade their vessels in the spot market.
Under Norwegian law, the owner must bear the costs of repairing damage to the vessel related to pitting, unless the risk is shifted to the charterers by express wording. If the owner is uncomfortable with this it could, for example, negotiate a cost-sharing scheme for preventive measures or possible repair costs with the charterer before the vessel commences service. It is important that owners are conscious of the pitting issue, especially when concluding long-term contracts. Otherwise, pitting may quickly ‘eat’ their margins.
Preventive measures reduce the risk of pitting, which in turn reduces the risk of oil spills. The need for such a preventive approach has been acknowledged by most major players in the oil trade community, and it thus seems wise for owners to start to prepare for new regulations and possible expanded vetting requirements sooner rather than later. Apart from being environmentally responsible, owners could very well find such measures reflected in a positive way on their balance sheets.
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