While loading at a terminal, the ship listed heavily to port during shifting of ballast. She finally became stable at about 16 degrees list whilst touching the bottom. At the time of the accident, the ship was approximately at even keel with a draft of about 9m. The owner’s office and DNV ERS (Emergency Response Service) were immediately informed of the situation by the Master. They communicated by telephone whilst stability and strength calculations were carried out. Ballast was carefully shifted to starboard and after about five hours, the ship was reported to be back in an upright position.
Extent of damage
There was no damage to the hull but cargo operations had to be stopped for a few hours. Fortunately, no cargo hoses were connected to the terminal at the time of the accident, otherwise they would have been damaged and might have resulted in pollution.
The ship is a double hull tanker. The cargo tanks are built without longitudinal centre line bulkhead and the wing ballast tanks are “L” shaped. Partially filled cargo tanks with no longitudinal bulkheads may cause large free surface effects to the extent that the initial metacentric height (GM) becomes negative. This creates initial instability and results in heeling to an angle where the ship becomes stable again (lolling). For a tanker of this size, the stable condition after listing would normally be less than 5 degrees if lolling was the only contributory factor.
However, it was afterwards confirmed by sounding that there had been considerable asymetrical distribution of ballast before the ship listed. Consequently, it is believed that the initial lolling to port caused a flow of ballast water through the cross over valves, which might have been left open due to human error or mechanical/electrical failures. Intact stability calculations, based on the reported asymetrical distribution of ballast, shows that the cargo loading conditions have a negative righting lever arm (GZ) from 0 degrees to 17.3 degrees but a satisfactory GZ above that angle. This explained the soft touch of the bottom and the absence of hull damage.
Lessons to be learned
- The above effect of free surface is well known among deck officers who have operational experience on Ore/Bulk/Oil carriers. Other deck officers may not be familiar with the special stability characteristic of double hull tankers not fitted with longitudinal centre line bulkheads. Thus,
- Deck officers to be aware of the stability characteristics of this ship type.
- For existing ships, IACS (International Association of Classification Societies) Unified Requirement L3 stipulates that positive GM must be maintained throughout the loading/ballasting operation, in port or at sea. Special approved procedures have to be followed in this case this requirement has not already been met through design measures (longitudinal centre line bulkhead).
- For new ships, MARPOL 73/78 Annex 1 (1997 Amendment) Reg 25A, which came into force on 01.02.1999, includes similar requirements. This regulation practically requires longitudinal centre line bulkheads in the cargo tanks. (The exception is for OBO’s for which simple supplementary operational procedures may be allowed).
- Regular checking of critical components in the ballast system, e.g. cross over valves, is essential.
Source: Mars/Nautical Institute