Lack of knowledge about the effect of strong waves on offshore structures

Equinor’s Sleipnir A Platform in a Storm (Equinor)

Posted on August 22, 2021 at 3:42 PM by

The maritime executive

The force of waves hitting offshore platforms, windmill pillars, ships or other offshore structures can cause enormous damage.

One of the fundamental – and unresolved – problems in designing this type of large structure is being able to predict exactly how they will react to extreme stresses. What exactly is the force load of powerful waves crashing into structures?

Solving these challenges will be a major step towards safer and more profitable maritime operations. “It is crucial to understand the mutual interaction between the impact wave and the response of the structure,” explains René Kaufmann.

Kaufmann is a postdoctoral researcher at NTNU SIMLab (Structural Impact Laboratory) and one of the researchers of the SLADE KPN project. This is a Knowledge Building for Industry (KPN) project funded by the Research Council of Norway, in which researchers from SINTEF Ocean and NTNU collaborate on basic research. The overall objective is to increase safety at sea.

It is important to expand what is known about these challenges, but this will require systematic experimental studies of wave impact scenarios. The project will do just that, which should allow researchers to understand how the behavior of a structure interacts with the loads applied to it.

Researchers are developing experimental methods to measure this interaction. Better calculation methods can help the industry when designing new offshore structures.

Kaufmann’s goal is to measure the impact of local surface deformations caused by massive loads. An important aspect of his research is to ensure that the measuring equipment itself does not affect the properties of the structure. SIMLab researchers used their experience with camera-based techniques to measure the structural response to loads resulting from impacts and explosions.

But more on that later. We first set off for a trip to the Norwegian Sea.

“Huge wave on its way”

The monstrous horizontal waves that can violently hit ships and other structures at sea originate from so-called century-old storms.

In 1995, the “Draugen” offshore platform was put to the test in the Halten Bank region on the Norwegian continental shelf. On March 12, a hurricane swept through the Norwegian Sea and rig manager Magne Gundersen received an unexpected phone call from the Aberdeen Meteorological Center.

The Center warned of a massive wave heading towards the platform. The crew only had 30 minutes to prepare. Production was immediately halted. Gundersen rounded up the crew of 134 in the gymnasium inside the platform. There, he reassures everyone by expressing his unconditional confidence in the engineers who designed the Draugen platform.

The Draugen platform on Halten Bank, in the Norwegian Sea. Photo: Svema / Creative Commons Attribution-Share Alike 4.0

“Right after I said these words, the loudest, shakiest and most violent ‘BANG’ I have ever heard sounded,” Gundersen said in an interview after the incident. “We started to feel more and more movement under our feet. (…) [T]The room kept rocking. I couldn’t say exactly how long this lasted, but I guess it would be over a minute. “

First, the huge wave had hit the shaft, before lifting itself under the bridge with tremendous force. The distance between the standing water level and the platform cellar apron is 30 meters.

In the physics of crashing waves

A key question for SLADE is: what is the effective stress of these types of loads? “We need to understand the load before we can study the details of a structure’s behavior,” says Vegard Aune, associate professor at SIMLab.

Another incident that helped motivate SLADE occurred in the North Sea in December 2015, when a large, steep wave thundered in the COSL innovator drilling rig. The platform was designed in accordance with the regulations, but still did not withstand the load. The incident claimed a human life and four people were injured. The platform was also extensively damaged.

“Accidents like COSL innovator event raise the question of whether we fully understand the underlying physics of loading during violent crashing waves. Providing construction engineers with detailed knowledge of loads, underlying physics and materials is crucial. All of this is essential in order to understand and predict how structures react in the event of extreme stress, ”explains Aune.

Kaufmann, together with fellow researchers Bjørn Christian Abrahamsen from SINTEF Ocean Transport & Energy and project engineers Trond Auestad (SIMLab) and Jens Åge Havmo (SINTEF Ocean), recently spent several days in Tyholt’s Ocean Basin Laboratory in Trondheim, where they have slamming tests on scale models. The researchers tested the measuring technique they had devised, which involves measuring the charge when the wave hits the structure.

Today, this is measured with point counters that rely on wires. The SLADE team explored the use of lasers and camera techniques that not only provide measurements at a given point, but can potentially determine over time and space how the entire incoming wave field is changing. as it gets closer to the structure.

This development leads to another goal for SLADE, that of bridging the gap between physical testing and reliable computer simulations.

The researchers recreated crashing waves in the basin, slamming them into a flat steel plate. The plate was integrated into a steel pillar representing a reduced steel structure at sea.

This article is courtesy of Gemini News and is reproduced here in an abridged form. The original can be found here.

The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.

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