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Experiments by Invited Researchers

 

Ice Actions on Conical Structures

Project acronym: HyIII-HSVA-04
Name of Group Leader: Dr. Tuomo Kärnä, Kärnä Research and Consulting, He
User-Project Title: Ice Actions on Conical Structures
Facility: Large Ice Model Basin (LIMB)
Proceedings TA Project: Ice failure process on fixed and compliant cones
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

This project addresses offshore structures that have a cone at the waterline to reduce static and dynamic ice actions. The general objective was to collect new data for the design of such structures. Four specific objectives are described as follows.

Dynamic ice actions on conical offshore structures are not fully understood at present. Structures with narrow cones experience vibrations in the Bohai Sea while other structures with wider cones have behaved much better in the Baltic Sea and in the Canadian arctic waters. Therefore, the first specific objective was to study how the width of the cone influences the dynamic ice actions. Both up- and downward breaking cones were considered.

The peak values of time-varying ice actions are believed to depend on the ice speed. It has been proposed that speed effects may arise due to three-dimensional hydrodynamic effects in shallow waters. A theoretical model is being developed for this phenomenon. Accordingly, the second specific objective was to obtain data for the development of such a model.

The third specific objective was to study the possibility of mitigating ice actions by using a compliant cone. The idea here is to create mechanical compliance between the structure and the cone, such that the motion of the cone enhances the failure of ice sheet acting on the structure.

Finally, the research team also needed data about ice actions on floating structures that have a downward breaking shape at the waterline. The test set-up with a compliant cone was used for this purpose.

User-Project achievements and difficulties encountered:

The test laboratory had recently acquired a new data acquisition system that can be used to perform wireless measurements on moving objects. This project made extensive use of this system by measuring the deflections of the ice sheet. Practical difficulties were first encountered as the data acquisition facility was brand new. However, very useful results on the hydrodynamic ice-water interaction were obtained due to sequential improvements in the test set-up.

Tests were done at several indentation speeds. The raw data on ice loads shows that the peaks of the ice action increase strongly with the speed. Calibration data of the test set-up were acquired so that the contributions due to water drag and the dynamic magnification can subsequently be considered.

The time signals on ice load and the cone response were supplemented with video records and photos, which show all details of the ice failure process as well as the ice ride-up and clearing.

Tests with the wide downward breaking cone showed, surprisingly, that the ice loads were significantly lower than predicted by the well-known model by Ralston. This result will probably call for modifications in the model concerned. During the tests the low level of ice actions posed problems for the tests with the compliant cone. The project solved this problem by increasing the buoyancy forces on the test structure. Accordingly, successful results were obtained also for the compliant cone.

Highlights important research results:

This project helped the test laboratory to demonstrate the feasibility of a new wireless data acquisition system that can be used to measure the dynamic deflections of the surface of an ice sheet. These deflections appear as wave propagation that emanates from the point where a model of a ship or a structure acts on the ice edge. The data that this project received from these measurements are of uttermost importance for the present project team. In addition, it seems clear that this test system will be very useful for the test laboratory in its efforts to enhance the quality of ice research.

This project successfully compiled a significant amount of new data that four PhD students will use in modeling

  • hydrodynamic effects in dynamic ice actions,
  • the ice failure-and clearing processes that arise while an ice sheet acts on a conical structure,
  • mitigation of ice actions by using the concept of a compliant cone and
  • ice actions on floating platforms.

In addition, the project team will introduce the new data as background information for an international expert group, which is finalizing the forthcoming standard ISO 19906 for arctic offshore structures.

Fig 1. (Top) Underwater pressure transducers upstream the cone

Fig 2. Ice failure on a narrow model cone Fig 3. Compliant cone acting on an ice sheet.
Waterline Diameter 1.36 m and the cone angle
60o, downward breaking

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