The following is an excerpt.

Heavy storms can cause steep and violent waves slamming into platforms and offshore wind turbines.
SIMLab’s Rene Kaufmann measures the pressure and deformations from such massive loads – to improve safety at sea.

In the SLADE KPN project, It is crucial to understand the mutual interaction between the impacting waves and the structure’s response. (Photo of the Sleipner A platform: Equinor)

 

Wave slamming has the potential of causing massive damage. At risk are offshore rigs, wind turbine pillars, ships, or any other structures at Sea. The accurate prediction of structural response due to wave slamming is a challenge. In fact, it is one of the fundamental – and unresolved – problems in designing such large ocean structures. Solving these challenges will be a big step towards safer and more cost-efficient marine operations.

UNDERSTANDING THE MUTUAL INTERACTION
«It is crucial to understand the mutual interaction between the impacting wave and the structure’s response», says Rene Kaufmann (photo right). The post-doctoral fellow at SIMLab is involved in the SLADE KPN project.

Here, scientists from SINTEF Ocean and NTNU join forces on fundamental investigations. SLADE is a knowledge building project (KPN) financed by the Norwegian Research Council (NRC) and prominent maritime players. Among them, the leading international offshore operator Equinor. Plus Aker Solutions, a global leader in the delivery of offshore energy production facilities. Also, consulting engineers Multiconsult, the Norwegian Shipowner’s Association, the Norwegian Maritime Authority and the Norwegian Petroleum Safety Authority support the project. And The United States Navy is an international partner. 

 

BRIDGE THE GAP AND BUILD BETTER DESIGNS 
The path to new knowledge goes through systematic experimental studies of relevant wave-impact scenarios. These must be combined with the development of experimental, numerical, and analytical methods. Thus, SLADE aims to bridge the gap between physical tests and reliable computer simulations. 
SIMLab’s task is to measure the full-field surface deformations. To do so, Rene Kaufmann uses powerful, non-intrusive, and optical techniques. The measurements provide new insight into the actual loading during violent wave impacts. He records the experiments with high-speed cameras able to film several thousand frames per second. Also, for the first time, he extracts loading information from the deformation measurements during wave impact. More details on SLADE and Kaufmann’s measuring techniques comes later.  First, we take a trip into the Norwegian Sea.  

 

The critical waves causing the most significant
horizontal slamming loads on ocean structures
originate from typical 100-year storms.

 

 

 

«A VERY LARGE WAVE COMING»
In 1995, the offshore oil and gas platform «Draugen» was put to a severe test in the Haltenbanken area. 12 March, a hurricane raged in the Norwegian Sea. Suddenly, the Platform Manager, Mr Gundersen, received an unexpected call from the Aberdeen Weather Center. They warned him of a massive wave coming in their direction. 30 minutes was all they had to prepare. The production was stopped. Mr Gundersen summoned the crew of 134 people to the gymnasium in front of the platform’s inner walls. There, he reassured everyone present on his unconditional faith in the platform designers’ work. 

 

«INCREASINGLY LARGE MOTIONS UNDER OUR FEET»
«After I had finished my speech, the loudest, most shivery and violent “BANG” I have ever heard rang out in the gymnasium», he said in an interview later.  «We started to feel increasingly large motions under our feet. (…) The room kept on pitching. I could not tell exactly how long it lasted, but my guess would be more than a minute».
First, the monstrous wave had hit the shaft and then lifted itself up under the deck with a violent force. The air gap on Draugen, from still water level to cellar deck, is 30 meters. 

 

Young man wearing face mask in marine lab
Vegard Aune, Associate Professor at SIMLab.

 

INTO THE PHYSICS OF VIOLENT WAVE SLAMMING
A key question for SLADE is: What is the effective action of such loading conditions? 
«We must understand this before we can study the details in the structures behaviour», says Vegard Aune, Associate professor at SIMLab. Another incident that forms some of the project’s motivation happened in the North Sea in December 2015. A big, steep wave struck into the deep-water drilling rig COSL Innovator. The platform was designed according to the regulations. Still, it failed to withstand the load. The incident caused one fatality, four people injured and extensive damage.

UNDERSTANDING AND PREDICTING RESPONSE
«Accidents like COSL Innovator raise the question if we fully understand the underlying physics of the loading during violent wave slamming. It is of utmost importance to provide structural engineers with detailed knowledge. About loads, underlying physics, and materials. All to understand and predict how structures respond during extreme loading events», says Aune.

 

 

CONTROLLED SMALL-SCALE MODEL TESTS 
Recently, Rene Kaufmann and fellow SLADE researchers spent several days performing slamming tests on small-scale models in the SINTEF Ocean Basin Laboratory.
In the basin, they generated steep waves slamming into a flat steel plate. The plate was integrated into a pillar, representing a downscaled offshore steel structure.  It has been assumed that the loading could be obtained from force panel measurements on massive, non-deformable structures. Techniques to determine the full-field surface pressures and the mutual interaction between the load and the structure’s response have been lacking.

 

 

 

 

Wave Slamming Test

 

FULL-FIELD SURFACE SLOPE MEASUREMENTS 
Kaufmann would like to change this. He uses the full-field deformation measurements of a flexible, plated structure to access the surface pressure distributions acting on the plate.
SIMLab’s post-doc has a background in fluid mechanics and is an expert in deflectometry. He describes it as «a full-field surface slope measurement technique». It is highly sensitive and allows measuring deformations down to the micrometre level. Kaufmann earned his PhD in 2019 at the University of Southampton, UK. The technique he uses in SLADE relates to his thesis titled «Full-Field Surface Pressure Reconstruction Using Deflectometry and the Virtual Fields Method» (VFM).

 

SLADE WAVE SLAMMING:
The path to new knowledge goes through systematic experimental studies of relevant wave-impact scenarios. These must be combined with the development of experimental, numerical, and analytical methods. Thus, SLADE aims to bridge the gap between physical tests and reliable computer simulations. 

 

 

 

BLASTS, DROP-TESTS AND SLAMMING TESTS
At SIMLab, Kaufmann works closely with Associate Professor Vegard Aune and Senior Researcher Egil Fagerholt. He entered SLADE in autumn 2019 when the project was well underway. In addition to the recent wave slamming tests, he has also performed some initial experiments in SIMLab’s shock tube. And late last year, they did a series of drop tests in one of the basins at SINTEF Ocean (photos below). Then, they dropped a 3D-printed stiffened panel from different heights into the water. They used 3D Digital Image Correlation, Egil Fagerholt’s field of expertise, to measure the dynamic deformations.

 

 

 

 

 

 

3D-DIC measurement of impact on a 3D printed panel

 


SCIENTISTS COMPLEMENTING EACH OTHER 
The project manager of SLADE KPN, Dr Øyvind Hellan, describes the project as an «inspiring collaboration between research environments that complement each other».  Hellan is Vice President for Research in SINTEF Ocean. He says that, when they launched SLADE they only wanted to assemble a top Norwegian national team. SINTEF Ocean, a world leader in marine technology and marine bioresearch, teamed up with NTNUs Department of marine technology (IMT) and SIMLab at the Department of Structural Engineering (KT). IMT is a world leader in education, research, and innovation for engineering systems in the marine environment. SIMLab holds the same status within the field of load-bearing structures subjected to extreme loads. 

 

Portrait of a man

 

 

We obviously have a common curiosity for the possibilities
that new technology offers. Frequent and fruitful professional
discussions unite the people of SLADE KPN.

Dr Øyvind Hellan, project manager

 

A LONG HISTORY OF COLLABORATION
«These are the two academic environments at NTNU that rank highest in the NRC’s international evaluations within their respective fields», Hellan says.
Although SLADE KPN was launched in 2019, the involved partners have a history of collaboration dating long back. «I think the people involved in this project share the same attitude, concerning both basic research and the value creation and innovation that must come out of the research. Also, we obviously have a common curiosity for the possibilities that new technology offers. Frequent and fruitful professional discussions unite the people of SLADE KPN», says Dr Øyvind Hellan.