Structural damage on board Osco Star

Summary

On 10 December 1993, the Australian flag tanker Osco Star was loading a cargo of petroleum products at the Shell refinery Geelong (Corio). A little after midnight, as the officer of the off-going watch was checking the deck, he opened the sighting port to no.4 port cargo tank and heard the rush of liquid falling into what should have been an empty tank. On checking he found that the gas oil cargo in no.3 centre tank was flowing into no.4 port.

Cargo operations were suspended and an inspection made of the area. It was subsequently discovered that gas oil had also entered the trunk-way to no.4 double bottom ballast tank.

Initially, it was considered that the ship had been over-stressed during cargo operations. However, by late morning on the 11 th, it was accepted that the tank structure had suffered damage due to over-pressurisation, caused by the failure of the pressure/ vacuum relief valve on no.3 centre cargo tank, a valve which is designed to vent the displaced gas in the tank when loading, or to relieve vacuum when discharging.

Substantial damage was sustained by the vessel's structure, with splits occurring between no.3 centre cargo tank and no.4 port wing tank and the trunk-way to no.4 double-bottom ballast tank. The top of the bulkhead between nos.3 and 4 centre cargo tanks was set back approximately 750mm and the main deck above the tank was set up by 116mm.

The Marine Incident Investigation Unit investigated the incident under the provisions of the Navigation (Marine Casualty) Regulations.

Conclusions

1. The structural damage experienced by the vessel, in the vicinity of no.3 centre and no.4 port wing cargo tanks, was caused by an excessive buildup of gas pressure in no.3 centre tank as the tank was loaded. This was, in turn, caused by the failure of the single PIV valve on no.3 centre tank to open and vent the tank.
2. The PIV valve met the International Maritime Organisation standard and was of proven design. However, the cast iron used in the PIV valves on Osco Star resulted in a build-up of rust and cargo residues in the critical area between the valve housing and the edge of the valve disk and also between the valve housing and the top cone.
3. Due to the fact that the ship made relatively few voyages in ballast, the opportunity to overhaul the P/V valves at sea was limited.
4. The Chief Mate knew of a deficiency in no.3 centre tank PN valve but did not ensure that the valve was repaired or that alternative open loading procedures were followed before loading cargo to that tank. It is probable that the need to ensure that the valve was repaired slipped his mind.
5. Other members of the ship's staff were aware of the defect in no.3 centre PIV valve, but no action was taken to rectify the situation, and only a limited and low-key reminder was given to the officer of the watch, and no alternative open loading procedure was suggested.
6. The Third Mate, when handing over his watch, indicated that the PN valve on no.3 centre tank was functioning correctly, without having previously established that this was the case.
7. There was a break-down in procedures in addressing the repair of no.3 centre PN valve and a failure of communications concerning the importance of ensuring the valve worked correctly.
8. The information obtained during the course of this investigation, with particular reference to the report of the Salvage Association, UK, would demonstrate that incidents of over-pressurisation of cargo and ballast tanks, with the associated danger to life and the risk of pollution, occur very much more frequently than is commonly appreciated throughout the maritime industry.