Friday, 24 October 2003
Anna Salleh
ABC
An experiment showing a rising bubble about to sink a ship
Methane bubbles from the sea floor could be responsible for the mysterious sinking of ships in areas like the Bermuda Triangle and the North Sea, new Australian research confirms.
Computational mathematics honours student David May and supervisor, Professor Joseph Monaghan of Monash University in Melbourne report their research in the American Journal of Physics.
Their modelling suggests that giant bubbles are much more likely to sink ships than previously thought, adding new weight to warnings about ships travelling in areas where bubbles are likely to be.
Huge bubbles can erupt from undersea deposits of solid methane, known as gas hydrates. The methane - found as an odourless gas in swamps and mines - becomes solid under the enormous pressures at the deep sea floor. Under the sea, however, the ice-like methane deposits can break off and become gaseous as they rise, creating bubbles at the surface.
"Sonar surveys of the ocean floor in the North Sea (between Britain and continental Europe) have revealed large quantities of methane hydrates and eruption sites," say May and Monaghan.
These bubbles aren't any old round sphere, according to May. In fact, they are lens-shaped, with a flat bottom and a domed upper surface.
While previous experimental research - using, for example, bubbles in large glass beakers filled with water - have supported the theory that plumes of bubbles can sink ships, May and Monaghan took these ideas further by simulating the event with a computer model.
But first, they trapped water between vertical glass plates and launched gas bubbles from the bottom to see what would happen to a toy ship floating on the surface. They found that a single giant bubble, the same width as the length of a ship, could swamp a ship under certain circumstances.
The researchers also developed a numerical computer model that was able to predict whether the toy ship would sink under different conditions. The computer model, based on the principles of fluid dynamics, related velocity, pressure, and density measurements of both water and gas, in two dimensions. A display showed the movement of the water resulting from a giant bubble and its impact on a computerised "ship".
May and Monaghan checked the accuracy of the computer model by feeding different sized real bubbles into the glass tank and seeing whether the ship sank as it was placed in different positions in relation to the bubble.
Whether or not the ship will sink depends on its position relative to the bubble. If it is far enough from the bubble, it is safe, they say. If it is exactly above the bubble, it also is safe - the danger position is between the bubble's middle point and the edge of the mound where the trough formed.
"When we started playing around with the model, we saw lots of interesting features at the surface that hadn't been discussed in the literature," May told ABC Science Online.
"I thought the bubble would rise up, burst and create a cavity that the ship would fall into and it wouldn't sink. But instead, you got an elevation of water - a sphere of water that the boat would slide off. But when the bubble burst, you got this high velocity jet of fluid spurting down into the water, pushing the boat under with it."
The researchers say a recent survey has revealed the presence of a sunken vessel within the centre of one particularly large eruption site, now known as the Witches Hole, suspected to be the victim of a bubble.
No one has seen such an eruption in real life, and no one knows how large the bubbles coming off a methane deposit would be or what configuration they would be in. However as soon as bubbles are characterised, measurements can be collected and plugged into the computer model to assess the potential risk to ships passing by, May says.
http://www.abc.net.au/science/articles/2003/10/24/973492.htm
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