Lessons Learnt: Defective lube oil cooler causes pollution

Oil tanker vessel at sunset - cropped

Vessel Type: Tanker

Incident description

While the vessel was alongside, oil sheens were observed floating on the dock water around the stern. The source was suspected to be lubricating oil leaking from the stern tube seal. After mitigation and clean up measures were carried out, arrangements were made to dry dock the vessel and effect necessary repairs. Upon inspection, pieces of fishing net were fouling the propeller shaft in way of the stern tube seal. After renewal of the seal, the vessel was re-floated and returned to service. About one week later when at anchor, oil sheens were again seen around the vessel's stern upon which the master informed the local authorities and arrangements made for cleaning up the pollution. As it was assumed the oil was again leaking from the stern tube, the vessel proceeded to a nearby shipyard to investigate the problem. Further inspection of the stern tube seals did not reveal any deficiency or oil leaks but after re-floating and shifting to a lay-by berth, oil sheens were again observed around the vessel's stern above the main engine lube oil cooler overboard discharge. A diving inspection confirmed the presence of oil in the discharge water when the lube oil cooling water pump was running.


The main engine lube oil cooler was found to be leaking at the sea water outlet flange and collected water showed signs of oil contamination. The Owners arranged for the cooler to be overhauled which involved the renewal of sealing gaskets and a number of heat exchanger plates and after reassembly, the unit was successfully hydraulically tested. It is understandable the management assumed the second oil leak originated from the stern tube, but with hindsight, all other possibilities for the leakage of oil should have been investigated (it was reported that the main engine and stern tube lube oils were of the same specification). The deficient condition of the lube oil cooler resulted not only in a pollution incident but also significant costs being incurred in respect clean up, inspections, repairs and off-hire.

Lessons Learnt

  • Machinery relating to oil systems must be properly maintained in accordance with the manufacturer's instructions
  • The condition of overboard discharges should be closely monitored as is practicable
  • Closely monitor LO system storage tank levels for any unusual consumption
  • When oil release from a vessel is observed or suspected, respond in accordance with the vessel SOPEP
  • Report any pollution to the proper authorities, inform the Club, take samples and ensure all possible reasons for leakage are considered and eliminated
  • Poor machinery maintenance can risk the environment and be very expensive

Captain David Nichol

Senior Loss Prevention Executive (Greece)