The vessel was discharging a cargo of crude oil when she was arrested for leaking oil. When divers were called in, it was verified that the source of leakage was the oil/ballast (O/B) overboard line from the oil discharging monitoring (ODM) system.
The overboard oil/ballast butterfly valve was found to be in a partly open position while the remote valve-indicating-system in the control room showed the valve to be in a fully closed position.
The O/B overboard valve was located 0.5 meter above the deep ballast water line (DWBL) and fitted to the ship side. During discharging of cargo the vessel came higher in the water and as a result the oil/water mixture trapped in the piping between the overboard valve and the ODM regulating valve was seeping out.
Principle sketch of oily water overboard arrangement
Oil leaks, even very small amounts, are a cause of serious concern and may have major consequences for the owner/operator especially when occurring in ports or other sensitive areas.
The overboard butterfly valve was after dismantling found to be non-operable as the packing of the hydraulic piston operating the valve was found disintegrated and age-hardened.
The overboard butterfly valve in question is located on the ship side inside a heated fuel oil tank and remotely hydraulic operated from the control room. Access to the valve can only be gained from inside the fuel oil tank.
The packing of the hydraulic piston of the valve was of a material that did not withstand the heat from the heated fuel oil (approximately 55°C). Over time this resulted in the packing being severely damaged. During the remote operation of the valve the hydraulic oil was leaking and passing the piston without the piston itself being moved, thus preventing the ship side valve to follow the control room commands.
The above incident provides the following lessons to be learned:
- All outlet and sea inlet valves should be easy accessible.
- Locating overboard valves remotely operated inside fuel oil tanks, should be avoided.
- Remotely operated sea valves should in addition have possibilities for manual operation.
- The valve position should be verifiable directly on the valve.
- Sea inlets and outlets piping should not pass through fuel oil tanks or similar inaccessible spaces. In particular passage through heated fuel oil tanks should be avoided because the combination of heat and seawater results in accelerated corrosion with significant risk of oil pollution over time as the pipe wall is corroded away.
Source: DNV
Officer of the Watch 
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