Monday, October 20, 2008
If a ship floods, the loss of stability is due to the free surface effect, as the water accumulating in the hull will be in the bilges, lowering the centre of gravity and actually increasing the metacentric height. This additional mass will however reduce freeboard (distance from water to the deck) and the ship's angle of down flooding (minimum angle of heel at which water will be able to flow into the hull). The range of positive stability will be reduced to the angle of down flooding resulting in a reduced righting lever. When the vessel is inclined, the fluid in the bilge will move to the lower side, shifting its center of gravity toward the list, further extending the heeling force. This is known as the free surface effect
Flooding, liquid cargo leakage, or unintended water (from precipitation or waves) in any compartments or on any decks of watercraft, and the resulting free surface effect are often a contributing cause of accidents, capsizes and casualties e.g. the loss of TEV Wahine (Wellington, New Zealand, April 1968), the MS Herald of Free Enterprise (Zeebrugge, Belgium, March 1987), and MS Estonia (Baltic Sea, September 1994).
The free surface effect is one of several mechanisms where a craft can become unstable and roll-over (capsize). It refers to the tendency of liquids — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves & wind acting upon the craft).
The free surface effect can become a problem in a craft with large partially-full bulk cargo compartments, fuel or water tanks, especially if they are located spanning its fore to aft centerline. If a compartment or tank is either empty or full, there is no change in the loading of the mass, or the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). If the tank is only half-full, however, the liquid in the tank will respond to the craft's heave, pitch, roll, surge, sway or yaw. For example, as the craft rolls to the left (port), a liquid will move so that much of it is now on the left (port) side of a tank, and this will move the craft's center of mass and center of moment towards the left (port). This has the effect of slowing the craft's return to vertical.
The free surface effect becomes worse if the craft then rolls through the vertical towards the right (starboard). It takes time for the liquid in the tank to respond and move towards the right (starboard) side of the tank. After the craft rolls through the vertical towards right (starboard), most of the liquid moving in the craft's tank then slams into the right (starboard) side of the tank, often with the effect of causing the craft to heel further over, as the liquid mass hits the bulkheads of the tank. In turbulent winds, heavy sea states, or on rough roads, this can become a positive feedback loop, causing each roll to become more and more extreme, until the craft rolls-over (capsizes).
The higher up these fluid motions occur, especially above either the craft's center of moment (buoyancy) or center of mass, the more pronounced the instabilities.
To mitigate this hazard, cargo vessels use multiple smaller bulk compartments or liquid tanks, instead of fewer larger ones, and possibly baffling within bulk compartments or liquid tanks to minimize the free surface effects on the craft as a whole. Keeping individual bulk compartments or liquid tanks either relatively empty or full is another way to minimize the effect and its attendant problems.
Free surface effect can affect any kind of craft: ranging from watercraft (where it is most common) to bulk cargo or liquid tanker semi-trailers and trucks (causing either jackknifing or roll-overs), or aircraft (especially fire-fighting water-drop and refueling tankers where baffles mitigate but do not eliminate the effects).
Posted by Capt DV at 6:50 PM
Sunday, October 12, 2008
The Toya Maru (洞爺丸, Tōyamaru?) was a Japanese freightliner constructed by the Japanese National Railways which sank during a typhoon in the Tsugaru Strait between the Japanese islands of Hokkaidō and Honshū on September 26, 1954. It is said that 1153 people (Japanese National Railways announcement in September 1955) aboard were killed in the accident, however the exact number of fatalities remains unknown because there were victims who managed to ride the ship and those who forcedly cancelled their rides just before the incident.
The Toya Maru was launched on November 21, 1947. It was 118.7 meters (approximately 130 yards) long and 15.85 meters (60 ft) at its beam and it had a Gross Register Tonnage of 3,898 tons. It could accommodate 1,128 passengers and was operated by 120 crew. She covered the distance from Aomori to Hakodate in 4 hours and 30 minutes.
As early as 1950, it was fitted with Radar equipment, becoming one of the first Japanese sea liners to do so. It was used by the Emperor the month before her demise. It was also famous as the flag ship of the Tsugaru Strait.
The following refers to what happened on September 26, 1954 to the Toya Maru.
Typhoon No.15, Marie, which had blown through Honshū, was in the Sea of Japan at 12:00 hours on September 26, 1954, proceeding Northeast at a speed of more than 100 kilometers an hour. It was predicted to reach the Tsugaru Strait at around 17:00 hours.
At 11:00 hours, the Toya Maru arrived at Hakodate after its first journey that day from Aomori. She was originally scheduled to return at 14:40 hours, to arrive at Aomori just before Typhoon Mary. However, due to the expected storm, another ferry the Dai 11 Seikan Maru, a somewhat poorer quality vessel, could not depart on its scheduled journey to Hakodate. Therefore, passengers and vehicles were transferred to the Toya Maru delaying departure.
The captain of the Toya Maru decided to cancel its journey at 15:10 hours.
At 17:00 hours, following heavy rainfall in Hakodate, the weather cleared up and the outlook improved. The captain presuming that the Typhoon had now passed as predicted decided to proceed with the journey for Aomori. However, by this time the Typhoon had slowed and was predicted to stay over the strait for an entire day.
Strangely, the typhoon was gaining strength on Japan Sea. The Typhoon is considered to have already become an extratropical cyclone when it reached Japan, in recent years.
At 18:39 the Toya Maru departed from Hakodate with approximately 1,300 passengers aboard. Shortly thereafter the wind picked up coming from a SSE direction.
At 19:01, the Toya Maru lowered its anchor at the very side of Hakodate Port to wait for the weather to clear up again. However, due to the high winds, the anchor did not hold and the Toya Maru was cast adrift. Water entered to the engine room due to poor design of the vehicle decks, causing its steam engine to stop and the Toya Maru to become uncontrollable. The captain decided to beach the sea liner onto Nanae Beach, in the outskirts of Hakodate City.
At 22:26 hours, the Toya Maru beached and an SOS call was made. However the waves were so strong that the sea liner could no longer remain upright and at around 22:43 hours, the Toya Maru capsized and sank at sea several hundred meters off the shore of Hakodate. Of the 1,309 on board only 150 people survived, while 1,159 (1,041 passengers, 73 crew and 41 others) died.
Four other ferries sank in the same typhoon with a total loss of life of 1,430.
Posted by Capt DV at 9:44 PM