Experiments in a state-of-the-art wave tank suggest we have underestimated the potential size and power of rogue waves, and the risk they pose to offshore infrastructure.
Ships, wind farms, oil rigs and other offshore structures may be at risk of damage from extreme ocean waves due to underestimations in potential wave sizes and forces.
 |
| Multidirectional waves created in a circular tank grew higher and larger than those in the one-way tanks Thomas Davey and Ross Calvert |
Using a circular tank that generates waves from multiple directions, scientists have produced “3D” waves that accurately mimic real ones in the ocean – in particular rogue waves, which can grow exceptionally steep and large. Compared to those created by standard one-way tanks, the waves in the multidirectional, circular tank grew up to four times as steep – potentially pointing to oversights in previous force calculations, says Mark McAllister at the University of Oxford.
“I spend a lot of my research time telling people that the ocean is 3D and not 2D, and we should be mindful of that,” he says.
Waves break when they become so steep that they crest over, causing a major shift in the wave’s mass and energy, as well as in heat and gas exchanges between air and sea. What exactly affects that slope and the breakover point, however, remains somewhat mysterious, despite abundant research, McAllister says.
The physics of wave-breaking is so complex, in fact, even computers can take several months to model a single wave, he says. To combat that problem, scientists usually study seawater in rectangular wave tanks, triggering waves at one end which evolve in slope, height, length and other measures as they travel along the channel. Some teams trigger waves from a second side of a square tank in efforts to add a third dimension, but that shape limits the number of directions waves can come from.
Instead, McAllister and his colleagues teamed up with engineers at the University of Edinburgh, who developed a 25-meter-wide circular wave tank surrounded by 168 wavemaker devices.
In an earlier experiment, McAllister’s team used the tank to find just the right multidirectional wave forces and angles to reproduce a scaled version of the 25.6-meter-high Draupner wave, which occurred near Norway in 1995.
Inspired by that finding, the researchers then ran an additional series of tests with different directional forces to imitate real-world conditions – including crosswinds. They found the waves grew higher and larger than those in the one-way tanks, probably because the water moved mostly up and down rather than side to side. “Essentially, the waves can become much larger before they break,” McAllister says. In addition, multidirectional waves continued to grow up to 80 per cent higher even after breaking because of water shooting upwards.
The researchers also observed three categories of breaking behavior – a travelling wave break similar to those used for surfing, a standing wave break with up-shooting jets of water and a mix of the two that involves a fast-moving ridge of up-shooting water. These different kinds of breaks could significantly change the way the waves dissipate energy as they crash – as well as the forces they might exert on a ship or structure, says Wouter Mostert at the University of Oxford, who was not involved in the study.
“This is indeed an important and fascinating piece of work,” Mostert says.
Nick Pizzo at the University of Rhode Island says the study team has finally succeeded in exposing important details about how and when multidirectional waves break – which should have been obvious all along. “The paper uncovers a simple truth that has been hiding in plain sight,” says Pizzo.
While the findings should help engineers design structures more suitable to such forces in the future, it’s important to remember current designs were made to withstand much stronger forces than previous models predicted they would face, says McAllister.
Even so, extremely large and steep waves could cause “great damage”, Mostert says, despite the fact that we still don’t know how often they actually occur.
“I think it’s crucial that existing models be extended to include this important phenomenon,” he says.
Journal reference:
0 Comments