Feature ArticleIce-Steel Impact In Arctic Operations
CTD measurements during descent and ascent of the AUV in December 2011.
Sustainable Technology for Polar Ships and Structures (STePS2) at the Faculty of Engineering and Applied Science at Memorial University in St. John’s, Newfoundland, Canada, is a five-year project focused on these objectives. Currently in its fourth year, the STePS2 team has conducted hydrodynamic ice-ship hull interaction tests and a range of static and dynamic experiments that collide 1-meter-diameter ice cones with steel structures at speeds up to 6 meters per second.
Typical ships in the Arctic sail at 4 to 6 knots (2 to 3 meters per second), whereas a ship accidentally striking an iceberg off the east coast typically sails at 12 to 16 knots (6 to 8 meters per second). To date, experiments have involved the use of two stationary steel frames and a small (1/4-scale) double-pendulum apparatus (1-meter-cube structure).
A large double-pendulum apparatus (4 by 4 meters) is in the final stages of construction and will provide near full-scale ice-impact scenarios. Experiments scheduled to begin in April will be the first laboratory tests of ice and steel-structure collisions at full-scale Arctic speeds.
In offshore areas where historical ice data are available, in order for oil and gas operators to predict outcomes, a better understanding of the conditions and elements that influence ice loads is required. Variables include the type of ice, its shape, temperature, grain size, the speed of interaction, and the shape and surface condition of the object with which it collides. For the past 25 years, the consensus among researchers has been that ice pressure follows a standard pressure-area curve: If you know the area, you will know the pressure and will be able to calculate a load.
The STePS2 experiments have shown that this is often not the case because the load changes with subtle changes in shape, structural stiffness and impact velocity. One finding, for example, is that pressures are very high at very slow speeds, contrary to the notion that an impact occurring at a high speed creates a larger load.
Static Tests. In November, an experiment was performed to test the overload capacity of the side shell of a ship’s hull (design load: 220 kilonewtons), which was held in place by a steel support frame. An ice cone was slowly pushed against the ice-class ship structural panel, at the rate of 1 millimeter per second, to a maximum load of 2.7 meganewtons—more than 10 times the design load&mdand nearly the limit of the hydraulic ram.
The panel was deformed into the shape of the cone pushing against it, which had slowly flattened from its original pointed tip, but there were no tears or through-thickness cracks in the steel, contrary to expectations. During the early stage of the loading, a crack appeared in one of the welds that joined one section of the frame to another, but as the deformation increased with additional pressure, the crack closed.
This experiment demonstrated that at slow loading rates, ice exhibits creep plasticity—increased ductility at low strain rates—and can, in effect, heal itself while being deformed. Due to the stroke limitations of the hydraulic ram, what is still unknown is the maximum load the hull could have withstood before tearing.
This test indicates that marine steel structures can sustain loads far in excess of their design point when ice is pushed against them at extremely low speeds. This would occur if, for example, a vessel is trapped under pressure in a moving ice field, or ice pushes against a fixed or moored stationary offshore structure at a very low speed. If a real ship were to sustain similar damage, it would be able to reach its home port, and repair costs would be modest. If, instead, a tear occurred, the loss would be orders of magnitude greater—as both lives and the environment would be at risk.
This experiment, and other static tests conducted for STePS2, have contributed to an improved understanding of the overload capacity of steel structures under ice loads. Before these tests, it was believed that there was a pressing need to devise new ways to lay out a ship’s structure in order to prevent fracture. Based on the test results, it appears that the current methods are adequate, recognizing that there is always room for improvement.
This experiment also suggests that a collision-resistant steel structure could be made from lighter material. Dents, such as the one that was produced in the lab in which the vessel remained watertight, may be deemed to be acceptable in extreme circumstances. This could result in considerable savings in up-front capital costs. To continue this article please click here.
Dr. Claude Daley is the STePS2 principal investigator and a professor and chair of the Ocean and Naval Architectural Engineering program at Memorial University in St. John’s, Newfoundland, Canada. He has led research and development projects on ships in ice and developed mathematical models for ship-ice interaction. He is a graduate of University of Western Ontario and Princeton University, and holds a doctorate in ice mechanics and Arctic naval architecture from Helsinki University of Technology.
Dr. Bruce Colbourne is a STePS2 co-investigator and project manager. He is a professor and researcher specializing in predicting loads from ice and waves on ships and offshore structures. He also works with the Canadian Standards Association’s committee for offshore structures standards. Colbourne is a graduate of Memorial University of Newfoundland (MUN) and the Massachusetts Institute of Technology, and holds a Ph.D. in Ocean Engineering from MUN.
Andrew Safer is a writer based in St. John’s, Newfoundland, Canada, who specializes in articles for international magazines about ocean technology innovations and their business applications. His previous Sea Technology articles include “An Information Hub for Vessel Traffic Operations Aids Users in Newfoundland,” “Spatial Visualization of the Marine Environment” and “Canada’s Multibeam Platform: Advantages and Applications.”