Project Summary Sheet

Project Summary Sheet
Project Acronym:
Name of Group Leader:
HYIII-HSVA-G4
Dr. Tuomo Kärnä, Kärnä Research and Consulting, Helsinki,
Finland
User -Project Title:
Facility:
Ice Actions on Conical Structures
HSVA ARCTECLAB – Large Ice Model Basin (LIMB)
User Project Objectives:
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.
Fig 1. Underwater pressure transducers upstream
the cone
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
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Project Summary Sheet
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 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
Publications:
Kärnä, T. et al. (2010). Ice Failure Process on Fixed and Compliant Cones. Proceedings of the
HYDRALAB III Joint Transnational Access User Meeting, Hannover, February 2010
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