Within the NEST project and clustered with research work conducted under the STRICE project Chinese scientists from the Dalian University of Technology, VTT and HUT visited the European ARCTECLAB at HSVA.

The Chinese-German-Finnish team conducted a series of ice force and ice-structure interaction measurements in the large ice tank of HSVA. German and Finnish test and analyses equipment and tools were used to gain deeper information on ice-structure interaction by a series of specifically defined tests under controlled conditions. Such tests are only achievable in the laboratory as usually in nature process variability and non-linear interactions prevail, making it difficult or even impossible to obtain dependencies on specific ice conditions and resulting reactions from and interactions with structures.

Assembly for ice-structure interaction tests
in the large ice tank of HSVA.

The joint STRICE-NEST model tests focussed on improved knowledge on the structural response at different ice thickness and drift speeds taking into account variations in structural stiffness and damping behaviour.

For these investigations a special set-up of mechanical devices and sensors were fixed under the movable bridge of the ice tank.

Stiffness and damping of the test model structure could be adjusted within controlled margins.

The speed of the model bridge with the attached structure moving through the model ice could be variably adjusted. The properties of the ice covering the ice tank is also possible to vary in order to simulate different ice thickness and conditions, for instance level and rafted ice or even ridges.

Together with a series of mounted sensors this model arrangement simulates ice movements and interaction with a fixed structure at different speeds and for different ice conditions as ocurring, with much more heterogeneity, in nature. Certainly applicable scaling effects and related coefficients between model and full scale as well as between model and natural ice need to be adequately considered.

A conical indentor was applied to investigate the effect of the drift speed of the ice.

Generally one can observe different response behaviour of the structure, for instance

• Response to ductile ice failure at very low ice velocities,
• Quasi-static response and transient vibration at low ice velocities,
• Steady-state vibration at medium ice velocities,
• or, random response at high ice velocities.

The photo at the right displays nicely piling of crushed ice at the contact face of the structure as well as buckling of the ice surface and conical cracks in upstream direction. Model and nature behave very simialarly as can be seen for instance in the STRICE gallery of videos and photos from winter 2003 where such processes are illustrated in full scale.

Ice-structure interaction in the model tank
with an applied conical indentor.

Ice-structure interaction in the model tank
with an applied vertical indentor.

Application of a more simply shaped vertical indentor allows, in addition to ice velocity impacts, the investigation of single and combined effects, such as stiffness and damping behaviour of the structure.

In real offshore constructions these parameters can be controlled by a variety of design and construction criteria like selection or different building materials with variation of their parameters (for instance sorts of concrete or steel), variation in the wall thickness or foundation of a structure, adequate adjustments and junctions of segments just to mention a few.

These parameters determine the oscillation and damping behaviour as well as the response of an offshore structure to external forces (ice, wind, waves, currents).

In the model stiffness and damping can be precisely controlled by a specific set of adjustable mechanical devices at the indentor and the connector to the moving bridge.

The results obtained from these investigations are used to generally better understand and numerically simulate ice-structure interaction processes.

Besides the structural stability of a building, whether off- or onshore) its vibration and resonance behaviour in response to a wide spectrum of external forces is of utmost importance. If, for instance, resonance vibrations can be caused by dynamic ice forces a structure is under high risk of damage or even destruction. To get an overview what kind of damage may occur to different offshore structures we invite to view this presentation from the NEST workshop.

More details on these model tests are briefly described in the joint presentation of the investigation team given on the NEST Workshop in Dalian. The comprehensive report on the joint STRICE-NEST model tests remains confidential property of the project participants.

Please refer also to the other presentation of the NEST Workshop which consider complementary and affiliated topics.