Huge offshore structures like wind turbines and oil rigs are designed to endure the innumerable challenges the ocean metes out. However, over time, just the saltwater itself can significantly reduce the durability of a structure’s welds.
Professors Zhenzhen Yu (left) and Michael Joachim Andreassen use neutrons at ORNL's High Flux Isotope Reactor to investigate residual stresses expected to occur in the welds of offshore underwater wind turbine foundations. Credit: Genevieve Martin/Oak Ridge National Laboratory, U.S. Dept. of Energy
That is why professors Michael Joachim Andreassen from the Technical University of Denmark and Zhenzhen Yu from the Colorado School of Mines are using neutron analysis at the Department of Energy’s
Oak Ridge National Laboratory to corroborate a more advanced technique of welding involving high-power lasers. Neutrons have extremely penetrating properties—more so than X-rays—and can probe nearly any material in a nondestructive manner.
The Neutron Residual Stress Mapping Facility at ORNL’s High Flux Isotope Reactor—a DOE Office of Science User Facility—enables researchers to investigate the quality of their welds at the atomic scale. The team’s findings could result in faster, more cost-effective production techniques, as well as considerably stronger, longer lasting welds.
We’re studying residual stresses in really huge structures, especially supersized monopiles—enormous steel cylinders that form the underwater foundations for wind turbines. We want to look at the relationship between residual stress and varying thicknesses in the steel plates used in construction, by comparing two different welding methods.
Michael Joachim Andreassen from the Technical University of Denmark
Generally, residual stresses are stresses that remain in the weld’s structure after applied loads or pressures have been removed. In certain cases, residual stresses can cause premature failures like leaks or cracks. They can be caused by several elements, such as exposure to harmful chemicals, fluctuations in temperature, or metal fatigue, caused by frequently applied loads.
The steel plates used to construct monopiles can be up to 130 mm in thickness, said Andreassen. They are typically welded together using a traditional technique called submerged arc welding, where electric arcs are used to melt the joining materials. Therein, the weld’s molten seam, or weld pool, is constantly “submerged,” or covered, in a granular flux of several compounds used to support the weld and protect it from contaminants in the atmosphere.
There are a range of advantages to submerged arc welding. Amongst other things, the method yields fewer sparks, impurities, and toxic fumes than similar approaches. However, says Andreassen, there are substantial burdens, too.
“You have to remove a lot of material to do the weld, and then add filler material after. It costs a lot to remove and add the materials, and in the end, you have a really huge groove with lots of introduced residual stresses,” he explained.
Naturally, the more tensile residual stresses there are, the more vulnerable a weld will be to failure.
The hybrid laser-arc welding technique introduces a more focused heat source that allows us to mitigate residual stress, in the ocean, saltwater eventually creates corrosion, and if you have high degrees of tensile residual stress, the faster corrosion occurs and the greater the likelihood of fractures or cracks propagating through welded regions.
Zhenzhen Yu from the Colorado School of Mines
Neutrons offer an extremely comprehensive picture of how the atoms are acting deep inside the welds, comparing residual stresses from both the submerged arc and hybrid laser-arc samples. The neutron measurements reveal any changes in residual stress as Andreassen and Yu raise the steel plate sample sizes from 10 to 20, 40 and 60 mm thick.
“The reason we like neutrons for this research is because it’s the only technique that can penetrate through the steel plates to give us a complete profile of the residual stress,” said Yu. “We will use the neutron data and compare it with simulation work from Michael’s group that we can apply directly to the actual structure.”
HFIR is a DOE Office of Science User Facility. UT-Battelle manages ORNL for the DOE’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to look into some of the most pressing challenges of today.