Summary
On 19 January 1994, the Australian flag tanker Osco Star was loading a cargo of petroleum products at No.2 Jetty of the BP Refinery at Kwinana, near Fremantle, W.A.
At about 1420, aviation jet fuel (Avtur) was being loaded into No.5 port and starboard wing tanks and nearing, the required finishing, ullage in both tanks when the duty mate in the cargo control room shut No.5 port wing tank filling valve. This action put the full loading rate, of about 1200 m /hour, into No.5 starboard tank and, shortly afterwards, cargo overflowed onto the deck from the pressure/vacuum release valve on No.5 starboard cargo tank.
Immediately, shore pumping was stopped, no.1 centre was opened as a "crash tank" and the manifold valves and all cargo tank filling valves were closed.
When the clean up of the spilt oil was completed, soundings were taken of all cargo tanks. From these, it was established that the level in No.5 starboard tank was falling, while that in No.4 starboard was increasing - an indication that these two tanks were now common in some way.
The cargo in Nos.4 and 5 starboard tanks was pumped to other tanks in the ship and the two tanks were then cleaned and gas-freed prior to being inspected.
Inspection revealed that there was extensive damage to the structure between the two tanks caused by No.5 starboard tank having been hydraulically over-pressurised. The bulkhead had ruptured, leaving a hole of approximately 1 X 1.5 metres. Various other sections of the corrugated bulkhead were bulging, and a weld fracture was found in the aft bulkhead of No.5 starboard tank.
The vessel was eventually allowed to load its original cargo and to proceed to Sydney and thence Geelong, for temporary repairs.
The incident was investigated by the Marine Incident Investigation Unit under the provisions of the Navigation (Marine Casualty) Regulations.
Conclusions
These conclusions identify the different factors contributing to the accident and should not be read as apportioning blame or liability to any particular organisation or individual.
1. The structural damage to the vessel was caused by hydraulic over-pressurisation of No.5 starboard wing cargo tank during the loading of a cargo of jet fuel. The facility for relief of pressure, i.e. a single "Press-Vac" pressure/ vacuum relief valve is not designed to be able to relieve excess pressure under these circumstances.
2. The over-pressurisation of No.5 starboard wing tank was caused by operational errors on the part of the duty deck officer, the Second Mate, in the Cargo Control Room. These were:
i) Loading at an excessive rate into a single wing, tank.
ii) Not opening, the filling valve to No. 1 centre cargo tank before closing off the filling, valve to No.5 port.
iii) Not switching the digital display readout to no.5 starboard tank immediately after shutting off No.5 port.
iv) A mathematical error in his calculations for "time to go" before topping-off No.5 wing tanks.
v) Not accepting an alarm displayed on the Autronica VDU.
3. The Second Mate was not fully aware of all aspects of the operation of the Autronica ullage monitoring system, in particular, the fact that "unaccented" alarms on the computer screen will inhibit the sounding of any further alarms which may be activated.
4. It is probable that the Second Mate's thought processes and concentration were affected to some degree by both fatigue and personal problems. This may account for his forgetting to open the filling valve to No.1 centre, forgetting to switch over the selector switch on the digital display to No.5 starboard tank after closing off the cargo filling to No.5 port, and his mathematical error when calculating the "time to go" before topping-off the wing tanks.
5. Both the training, and experience of the Second Mate appear to the Inspector to be insufficient for the operation in which he was employed and the responsibility which he held at the time of the incident. This was due, in part, to the lack of opportunity for an effective induction into the ship's routines and systems, and in part to the lack of information, concerning the level of his experience, being, passed to the Master by ASP Ship Management.
6. The definition and literal meaning of the responsibilities of a "responsible officer" as detailed in Marine Orders Put 3, Seagoing Qualifications, indicate that it was not appropriate for the Second Mate to take responsibility for critical cargo-handling duties, such as the topping-off of cargo tanks.
7. The position of the cargo valve control console, the Autronica computer VDU screen and the tank digital display unit a-re such as to constitute a poor ergonomic layout for single person operation of the control room.