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• ANNUAL REPORT 2013
Figure 4. The numerical model of a tanker and broken ice
floes.
Figures 5 and 6 show the trajectories and ‘distribution
density’ of drifting ice floes close to the tanker with a
heading of 45° with respect to the current. This example
is a good demonstration of the hydrodynamic effect of a
floating structure on broken ice.
Frequency lock-in for floating offshore
structures.
Every structure operating in ice-covered offshore
conditions experiences ice induced vibration. PhD
candidate Chris Keijdener is studying this phenom-
enon. Generally ice induced vibrations do not cause
more problems than vibrations from other sources. A
special phenomenon called frequency lock-in, how-
ever, has been observed when level ice interacts with
fixed offshore structures over a certain range of ice
velocities. Over this range, the ice seems to be ca-
pable of tuning its crushing frequency to that of the
structure causing amplification of the response. Al-
though many researchers have studied this problem
during the last few decades, the exact cause of fre-
quency lock-in remains highly disputable.
Frequency lock-in can cause strong responses by
fixed offshore structures. This phenomenon has
caused trouble on several occasions such as with the
gravity-based structure Molikpaq, where it caused
an unpleasant working environment for platform
personnel. In the Bohai Sea, frequency lock-in re-
sulted in damage to a jacket and in Finnish waters,
it caused strong vibrations on the Norströmsgrund
lighthouse. Researchers and engineers agree that
frequency lock-in is a dangerous phenomenon for
fixed structures and much research has and is being
done on this subject so that it can be fully understood
and designed for accordingly.
Although frequency lock-in has been observed on
fixed structure, it has yet to be observed on floating
structures. The main issue here is that the experi-
ence with moored vessels interacting with level ice
is very limited. To date the Kulluk is the only vessel
that can boast of this achievement. In the 90s the
vessel operated in the Canadian Arctic. During this
period, much data was gathered and studied. The
vessel nearly always operated with icebreaker assis-
tance, thus limiting its exposure to level ice, possibly
explaining why frequency lock-in was not observed.
The dangerous consequences for fixed structures
provide a strong incentive to start research into this
topic also for floaters.
Figure 5. The ice floes’ trajectories.
Figure 6. The ice floes’ concentration.