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• ANNUAL REPORT 2013
For fixed structures, the frequency of crushing fail-
ure is often believed to be in the same range as the
natural frequency of the structure. The stiffness of
the floaters comes from the mooring system and not
directly from seabed reaction as is the case for fixed
structures. This means that the natural frequency of
the floaters is much lower. As a result, the ice fail-
ure mode for fixed structures, namely crushing, can-
not cause frequency lock-in for floaters. However,
when interacting with floaters the ice generally fails
by bending. The bending frequency of the ice falls in
the same frequency range as that of a moored ves-
sel, thus suggesting that resonance and, probably,
frequency lock-in may be expected.
Frequency lock-in is differentiated from normal
resonance in that frequency lock-in can occur over a
range of ice velocities. As the name suggests, the ice
failure can lock itself into the frequency of the struc-
ture. This implies that the ice has the ability to adapt
its failure frequency.
In order to investigate the possibility of frequency
lock-in, Keijdener has recently been working on a
model that will be capable of determining whether
the bending failure frequency of level ice has the abil-
ity to adapt itself to the motion of the floater. While
working on this model it was found that the contact
model, which predicts the normal forces caused by
the hull as the ice pushes against it, is critical when
it comes to studying the breaking length. Keijdener is
now studying the contact model and its effects on the
ice-structure interaction process. The final model
will hopefully provide more insight into the energy
exchange during the process and how the interaction
itself influences the breaking length and how this in
turn will impact the possibility of frequency lock-in.
Accidental collisions with ice masses
In 2013, PhD candidate Ekaterina Kim submitted her
thesis entitled “Experimental and numerical studies
related to the coupled behaviour of ice mass and steel
structures during accidental collisions”, the defence is
scheduled for 9 May 2014. Her work has been carried
out in three fields of interest: first, the avoidance of
accidents due to rare, extreme ice actions, second,
experimental investigations of the energy absorption
capacity of ice during crashing, and third, the design of
ice models.
More and more people are attracted to move opera-
tions north. Unfortunately one must rely on limited
knowledge, data and experience when designing a
floating structure for Arctic conditions, setting opera-
tional limits or specifying certain safety requirements.
Several marine incidents have shown that absolute
safety does not exist. Accidents include the holing and
sinking of the cruise ship Explorer in 2007 and the holing
of the tanker Nordvik in 2013. Despite the development
of global positioning systems, on-board radars, satellite
Figure 7. The tension stresses at the top of the beam caused by bending are shown over the length of the ice sheet at the
moment.