Double-bottomed ships
Åse Dragland
New regulations for double bottoms on tankers arrived in the wake of the "Exxon Valdez," "Arisan" and "Braer" disasters. Researchers at NTH`s Dept. of Marine Technology have been using modern computer simulation methods to find optimal solutions.
Jørgen Amdahl with the steel stucture that simulates the double bottom of a tanker. Amdahl is "inside" the vessel, whose outer hull has been destroyed by the experiment, while the tank itself is intact.
Large scale computer simulations have been used as a tool in the hunt for these criteria. By feeding in data on current hull designs and seabed conditions, the researchers have been able to simulate groundings which allow them to predict the consequences of shipping accidents.
The specific experiments have been done in the Strength Laboratory at the Dept. of Marine Structures, where steel structures are bolted firmly to the floor while a powerful jack is lowered, pushing the bottom of the structures in with great force. The jack simulates the seabed on which the vessel is resting, while the 3 x 3 m steel structure represents a section of a double bottom. The deformed "hull" enables the researchers to measure how the "ship" will behave when it is at rest on the obstruction, how a falling tide will affect the forces involved, and the process of being pressed in. The whole process is subsequently simulated on the computer in order to verify the theoretical model.
The first questions posed by researchers are "How deep will the damage be? When will the inner bottom be penetrated? How much can the vessel withstand before it breaks up?"
Inside the inner bottom is the thick-flowing, dangerous oil. "Exxon Valdez" went stranded off Alaska in l989. "Braaer" lost its engine power and grounded on the coast of Shetland while its oil flowed out. The wrecked "Arisan" threatened Runde with its serious pollution. The accidents have taken different forms, but each of them has provided the scientists with valuable information.