C3/1998M Ms OIHONNA, Breaking Loose of a Hydrogen Peroxide Tank-container During Storm on 31.1.1998
Ms Oihonna departed Lübeck, Germany, bound for Rauma, Finland, on January 29th 1998 at 15:05. The ship carried a cargo of 268 tons and 3657 tons water in ballast. The cargo consisted of, among other things, four tank-containers of hydrogen peroxide placed on the IMO deck at the stern of the ship. Each tank-container was loaded with 25 tons of 50 per cent hydrogen peroxide.
At the beginning of the voyage the weather was so rough that the sheltered route between Gotland and the Swedish coast was selected. After the ship had arrived in the Aland Sea at midnight on January 30th, the speed was slowed down to about five knots because of the stormy wind and the heavy seas. The ship was steered so that the wave direction was ten to fifteen degrees on the port bow, according to the officer of the watch (OOW).
According to the Finnish Institute of Marine Research the estimated maximum significant wave height was 6.1 metres at that sea area at about 23 o´clock or three hours before the accident. The wind speed observed at the nearby Märket meteorological station was then 25 metres per second from the direction of 10 degrees. At the time the accident happened, the wave height estimate was 5.9 metres.
At 2:10 am on January 31st the ship steward observed that one tank-container had come loose on the open third deck at the stern. The steward went directly to the bridge to inform of the swinging tank-container. The crew was alarmed. A group of seven crew members under the command of the first mate commenced to re-secure the tank-container. At that time the tank-container was fully detached from its fastenings, and it was sliding uncontrollably on the partly icy deck. The master contacted the MRCC Turku by mobile phone to give a notice of the potentially dangerous situation on the ship. The crew made several attempts to re-fasten the tank-container, which was still on its roll-trailer. These attempts were successful only partly, since the chains broke off at least twice.
At about three o´clock the first mate asked the master to heel the vessel to starboard by ballasting. The ship already had a minor list to starboard, and in addition, there was more free space on the deck on that side. The ballast transfer took 30 to 45 minutes. With the help of the list (abt. 8 degrees) the tank-container held its position better. Before the heeling was completed, the tank-container hit bulkhead and began to leak. The crew immediately went inside the ship. The flushing of the deck was started without delay. Three crew members used chemical protective suits with breathing apparatus. Working with this gear, however, was difficult in the prevailing conditions. The tank-container and the roll-trailer were finally secured at 06:30. The other cargo units, which had stayed in place, were checked and fastened more tightly after dawn.
The breaking of one of the rear fastening hooks can be regarded as the primary reason for breaking loose of the tank-container and its roll-trailer. The inertia forces exerted to the cargo unit due to high vessel accelerations exceeded the maximum tensile strength of the hook. After one hook was broken the rear part of the tank-container began to move transversally, which also broke the other rear hook and eventually the remaining two fastenings.
The exceptionally heavy sea combined with the light loading condition excited violent ship motions and accelerations. The bottom slamming at the stern increased the accelerations even more.
The following factors contributed to the occurrence of the incident:
1) Icy conditions generated a layer of ice and slush on the open deck decreasing the friction between the deck and roll-trailer substantially. Reduced friction increases the loads on the fastenings considerably.
2) One or more of the fastening hooks could have been fastened to the roll-trailer in such a way that they were not free to align with the chains. The angle between the hook and the chain could have been so large, that the tensile strength of the hook in the direction of the chain force was below the breaking strength of the chain. This lowers the total maximum strength of the fastening.
3) The angle between the fastening chain and vertical direction may have been larger than the optimum value. The chain load increases significantly with the chain angle.
4) The tightening forces in one or more fastening chains were conceivably too low, which can reduce the friction further. Possible reasons for low forces were defective or even broken rigging screws, ineffective pneumatic torque wrench used in tightening the rigging screws and fall of pressure in the compressed-air supply of the vessel due to simultaneous users or malfunction of the compressor.
More than 20 tons of 50 per cent hydrogen peroxide leaked on the deck and into other ship compartments. The fast and professional action of the crew prevented the infliction of major damages to the vessel and cargo. In spite of the existing health hazards the crew performed the operation without injuries.