Two recent pollution incidents on oil tankers occurred due to improper design of oil discharge pipes. In both cases, the discharge pipes of the Oil Discharge Monitoring Equipment (ODME) were routed through the bunker fuel tanks towards the ship's hull. These incidents demonstrate the serious hazards of such connections and the potential for environmental pollution.
Details of the Incidents
In these two incidents, the section of the ODME discharge pipes located within the fuel tank experienced a failure, resulting in fuel being discharged into the sea. This discharge occurred while the ODME discharge valve was closed and the removable component inside the ship had been removed. These actions effectively isolated the ODME system within the ship, but the pipe section leading from the fuel tank to the ship's hull was not isolated.
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The cause of the failures in these two incidents was different. In one case, corrosion and wear were identified as the main factors, while in the other case, a crack had developed at the ship's welding point since its construction. However, the outcome of both incidents was the same. When the pipe wall within the fuel tank failed, the pipe became an uncontrollable pathway for pollution, and fuel was discharged directly into the sea.
The Importance of Attention to Pipe Design
A discharge pipe routed through the bunker or cargo tank of a ship can act as a single point of failure that carries significant pollution consequences. Depending on its location and service, the pipe may be exposed to internal water and heated fuel outside of it. Corrosion, wear, vibration, thermal cycling, and local stress concentrations may contribute to its deterioration over time.
Inspecting these pipes can also be challenging. Pipe sections located within fuel or cargo tanks typically present difficulties for effective access and inspection during routine inspections. Additionally, there is no procedure for pressure testing these sections, and as a result, corrosion, cracks, or local wear may remain unnoticed until a failure occurs.
These incidents reveal significant limitations in the design of such connections. If a failure occurs in any part of the pipe located between the isolation discharge valve and the ship's hull, fuel may uncontrollably pass through the closed valve and discharge into the sea. This type of design should not be assumed to be safe, even if the associated system is isolated or no longer in use.
Recommended Actions
The West of England P&I Club advises its members and ship managers to review the design of their discharge pipes across their entire fleet, paying particular attention to any pipe that passes through a bunker or cargo tank and could create a direct pathway between oil tanks and the sea.
Members should also focus on identifying vulnerable connections, reviewing inspection limitations, confirming isolation procedures, and maintenance and inspection programs. In cases where vulnerable connections are still in use, limiting the maximum fill level of tanks is suggested as a temporary measure to reduce the risk of pollution.
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