A recurring pollution risk involving overboard piping routed through fuel oil tanks, following two further incidents involving a similar arrangement.
In both recent cases, Oil Discharge Monitoring Equipment (ODME) overboard discharge pipes passed through bunker fuel tanks before terminating at the ship’s side, allowing fuel to escape overboard after the pipe sections failed.
The incidents follow an earlier case in which a deck scupper drain pipe routed through a fuel oil tank developed a defect, allowing bunkering fuel to enter the damaged pipe and discharge into the sea.
Although all three incidents involved tankers, the recurrence of this failure mode across unrelated vessels highlights a broader risk. These incidents demonstrate how seemingly isolated overboard piping can create a direct pathway between an oil-containing tank and the sea if the pipe loses its integrity.
ODME overboard pipe failures
In both recent cases, the section of ODME overboard discharge piping located within a bunker tank failed. Bunker fuel entered the damaged pipe and escaped directly into the sea.
Importantly, the discharge occurred despite the ODME overboard valve being closed and the removable spool piece having been removed inside the vessel.
These measures effectively isolated the inboard ODME system, but did not isolate the section of pipe extending through the bunker tank to the vessel’s side. The apparent causes of the failures differed. In one case, corrosion and wastage were believed to have caused the deterioration.
In the other, a crack had developed along a welded seam dating from the vessel’s original construction.
While the causes were different, the outcome was the same. Once the pipe wall lost its integrity inside the bunker tank, the pipe became an uncontrolled pollution pathway, allowing fuel oil to escape directly overboard.
Why these arrangements warrant attention
An overboard pipe routed through a bunker or cargo oil tank can create a single-point failure with potentially significant pollution consequences.
Depending on its location and service, the pipe may be exposed internally to water and externally to heated fuel oil. Corrosion, wastage, vibration, thermal cycling and local stress concentrations may all contribute to deterioration over time.
Inspection can also be challenging. Pipe sections located within fuel or cargo tanks may be difficult to access and examine effectively during routine inspections. There is also no routine practice of pressure testing such pipe sections.
As a result, corrosion, cracking or localised wastage may remain undetected until failure occurs.
The incidents highlight an important limitation of this type of piping arrangement. If a defect develops in a section of pipe located between the isolation overboard valve and the vessel’s shell, fuel oil may enter the pipe downstream of the closed valve and continue flowing directly overboard.
Such an arrangement should therefore not be assumed to be safe simply because the associated system has been isolated or is no longer in service. Closing the overboard valve does not necessarily prevent pollution if the pipe section downstream of the valve remains exposed to oil within the tank.
Recommended actions
Shipowners and managers should review overboard piping arrangements across their fleets, giving particular attention to any overboard pipe that passes through a bunker or cargo oil tank and could create a direct route between a tank containing oil and the sea following a loss of pipe integrity.
While the known incidents involved tankers, operators of other vessel types should also verify whether similar arrangements exist on board their vessels.
- Identify vulnerable arrangements: Review piping diagrams, tank plans, construction drawings and onboard arrangements to identify any overboard pipe routed through a bunker or cargo oil tank.
- Verify pipe condition: Inspection should cover the complete pipe length within the tank, including welded seams, joints, supports and areas susceptible to vibration, stress concentration or localised corrosion.
- Assess inspection limitations: Consider whether the pipe can be effectively examined using routine visual inspection and thickness measurement techniques. Where access is restricted, alternative examination methods may be appropriate.
- Review isolation arrangements: Confirm whether existing isolation measures effectively isolate the entire overboard pipe, including any section located downstream of the isolation point and upstream of the vessel’s shell plating.
- Review maintenance and inspection programmes: Planned maintenance systems should identify and specifically address concealed or difficult-to-access overboard pipework routed through oil tanks. Inspection frequency should take into account the pipe’s age, operating environment, material and accessibility.
- Consider alternative pipe routing: Where practical, overboard piping should be routed through void spaces or other non-oil compartments rather than through fuel or cargo oil tanks. For existing arrangements, shipowners and managers may wish to consult their Classification Society regarding possible retrofit modifications.
- Similar considerations should be addressed during the design and construction stages of newbuilding’s.
- Report defects to class: Any identified cracking, defective welds, corrosion, wastage or other deterioration should be reported promptly to the vessel’s Classification Society. Repairs and modifications should be carried out under the approval and supervision of Class.
- Consider interim risk controls: Where vulnerable arrangements remain in service, operators may wish to consider limiting maximum tank filling levels as an interim measure to reduce the pollution risk.
- Prepare emergency isolation measures: Ship-specific contingency plans should address how leakage from an overboard pipe routed through an oil-containing tank can be stopped or controlled. Measures may include reducing tank levels, transferring oil to alternative tanks, sealing the external outlet where practicable and arranging temporary repairs under Classification Society oversight.
Emergency response considerations
Where leakage is suspected, prompt action is essential. The vessel should activate its shipboard pollution response procedures, notify the relevant parties and take immediate steps to reduce or eliminate the pressure acting on the damaged pipe.
During an active pollution incident, priority should be given to stopping or reducing the discharge. Operational concerns associated with fuel transfer, including compatibility considerations, should be balanced against the environmental consequences of allowing the leak to continue.
Any temporary plugging or sealing arrangement should be regarded as an emergency control measure only. Permanent repairs should be undertaken following safe tank preparation and in accordance with applicable Classification Society requirements.
Key lessons
As highlighted by West, these incidents demonstrate how the failure of a single pipe section can bypass conventional pollution-prevention barriers when overboard piping passes through an oil-containing tank. A closed valve or disconnected spool piece may not prevent pollution if the defect develops downstream of the isolation point.
Where practical, eliminating the direct interface between oil-containing tanks and overboard piping remains the most effective long-term risk-control measure. Effective inspection, maintenance, emergency preparedness and appropriate piping arrangements all play an important role in preventing similar incidents.
The recurrence of these incidents also underlines the importance of identifying concealed pipework that may not receive the same level of attention as more accessible systems. A piping arrangement that appears isolated during normal operations may still present a direct pollution pathway if its integrity is compromised inside an oil tank.