Measuring the Impact of Extreme Waves on Offshore Structures
Strong storms can trigger steep, breaking waves that slam into platform and wind turbines with tremendous force. Scientists at the Norwegian University of Science and Technology and SINTEF are studying the behavior of offshore structures subjected to these kinds of waves. Their goal is to increase safety at sea.
Strong storms can trigger steep, breaking waves that slam into platform and wind turbines with tremendous force. Scientists at the Norwegian University of Science and Technology (NTNU) and SINTEF are studying the behavior of offshore structures subjected to these kinds of waves. Their goal is to increase safety at sea.
The force of waves slamming into offshore rigs, wind turbine pillars, ships or other offshore structures can do an enormous amount of damage.
One of the fundamental—and unresolved—problems with designing these kinds of large structures is being able to predict exactly how they will react to extreme stresses. What exactly is the load from the force of powerful waves slamming into structures?
Solving these challenges will be a major step toward safer and more cost-effective marine operations.
Interaction Between Load and Load Effect
“It’s crucial to understand the mutual interaction between the impacting wave and the response of the structure,” said Rene Kaufmann.
Kaufmann is a postdoctoral fellow at the NTNU Structural Impact Laboratory (SIMLab ) and one of the researchers in the SLADE KPN project, a knowledge-building project for industry (KPN) funded by the Research Council of Norway, in which researchers from SINTEF Ocean and NTNU are collaborating on basic research.
The overall goal is to increase the safety at sea.
Building Bridges and Better Design
It is important to expand what is known about these challenges, but that will require systematic experimental studies of wave-impact scenarios. The project will do exactly that, which should allow researchers to figure out how a structure’s behavior interacts with the loads that are applied to it.
The researchers are developing experimental methods to measure this interaction. Better calculation methods can help the industry when new offshore structures are designed.
One important aspect of Kaufmann’s research is to make sure the measuring equipment itself doesn’t affect the structure’s properties. Researchers at SIMLab have used their experience with camera-based techniques to measure the structural response to loads from impacts and explosions.
“Huge Wave on Its Way”
The monster horizontal waves that can slam violently against ships and other structures at sea originate from what are called 100-year storms.
In 1995, the offshore platform Draugen was put to a serious test at the Halten Bank area on the Norwegian Continental Shelf. On 12 March, a hurricane swept through the Norwegian Sea and platform manager Magne Gundersen received an unexpected phone call from the Aberdeen Weather Center.
The center warned of a massive wave on its way to the platform. The crew had only 30 minutes to prepare. Production was immediately stopped. Gundersen gathered the crew of 134 people into the gymnasium in the interior of the platform. There he reassured everyone by expressing his unconditional trust in the engineers who had designed the Draugen platform.
Platform Shook Under Crew
“Just after I said those words, the loudest, most shivery and violent 'bang!' I have ever heard rang out,” Gundersen said in an interview after the incident.
“We started to feel an increasingly large amount of movement under our feet. … The room kept pitching. I couldn’t tell exactly how long it lasted, but my guess would be more than a minute,” he said.
First, the huge wave hit the shaft before it lifted itself up under the deck with tremendous force. The distance between the still water level to cellar deck of the platform is 30 m.
Into the Physics of Wave Slamming
A key question for SLADE is: What is the effective stress of these kinds of loads?
“We have to understand the load before we can study the details of a structure’s behavior,” said Vegard Aune, an associate professor at SIMLab.
Another incident that contributed to the motivation for SLADE occurred in the North Sea in December 2015, when a large, steep wave thundered into the COSL Innovator drilling rig. The platform was designed in accordance with regulations but still failed to withstand the load.