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Summary of Working Paper No. 7-1995

I.1.7: Ice Flaking Tests Conducted with a Gas Actuator System

By Stephen J. Jones, Institute for Marine Dynamics, National Research Council of Canada.

Since 1969, there have been efforts to devise methods of deriving global ice loads on structures by means of small or medium scale tests. Since the time of the early tests, gravity base structures have been placed in the Beaufort Sea and full scale load measurements have been made. Large scale data have also been obtained from ship trials, and forces were measured at Hans Island when southward drifting, multi-year ice impacted the island. However, there is still controversy over the magnitude of loads experienced by the Molikpaq structure, and the loads measured at Hans Island are considered to be a lower bound.

The use of the flatjack, a thin walled, flat envelope of steel which is fitted into an ice slot and into which fluid is pumped to achieve the failure load of the ice, made it possible to load large areas of ice at relatively low cost. Tests conducted with these flat jacks to areas of 4.5 m2 showed that the strength of competent, intact ice did not change significantly with the size of loaded area. The compliant flat jacks themselves provided a perfect loading with no stress concentrations over the loaded area. It was reasoned that if a stiff plate were placed in the ice slot alongside the flatjack, then the resulting stress concentrations from the stiff plate would result in a lower failure pressure more reasonably approximating that measured on large structures and ships.

The object of Phase 1 of Project I.1.7 was, therefore, to investigate (a) the effect of such a stiff plate and (b) the feasibility of using cheaper, gas-filled flatjacks instead of the more expensive, liquid-filled.

The program was conducted on lake ice near Calgary in February, 1994 and used a 76 mm thick aluminum plate. The results were very encouraging. The gas-filled flat jacks worked very well provided certain safety precautions were taken. However, so perfect were the saw cuts made using a specially built guide, that the pressures, while reduced somewhat, still resembled those measured in the laboratory. An aluminum plate 1.5 m by 0.2 m by 76 mm thick was able to produce pressures of around 5.5 MPa. Meanwhile, average pressures on large structures due to global loading of 1 to 2 Mpa have been measured.

Two tests with the rigid indentor were run in pre-damaged ice and the pressures obtained averaged 1.5 MPa, a value in line with global pressure measured on large structures. This constitutes a very important observation and provides concrete direction for further tests in Phase II.