SAMCoT - Annual Report 2015

SAMC

• ANNUAL REPORT 2015

While moving in managed ice, the energy of the vibrations

caused by the ice were hardly measurable. Upon entering

the unbroken ice field, the ice around the vessel failed by

crushing and bending. Due to the bending of the ice, the

ice failed mostly through circumferential cracking. The

transition into unbroken ice is clearly visible in the time-

frequency distribution due to a significant increase in the

energy of the signal over all frequencies. In particular,

vibrations with frequencies between 20 Hz and 30 Hz show

a significant increase in energy. After a few seconds, the

excitation decreases but the energies of the heave-vibra-

tion signals are still significantly higher than in the broken

ice field. The significant excitations upon exiting the ice

channel can be explained by a sudden decrease in veloc-

ity of the vessel. The kinetic energy upon contact with the

unbroken ice might have caused a significant amount of

crushing prior to bending of the ice. As soon as the ship’s

speed decreased, bending failure might have been the

dominant failure mode, which created significantly fewer

vibrations than crushing but still more than the ice interac-

tion with already broken ice inside the ice channel.

The distribution of the frequencies allows for the identi-

fication of the dominant ice failure mode. Figure WP5_7

and Figure WP5_8 show the power spectral densities

of two situations with different dominant failure modes.

In the case of ice crushing, as shown in Figure WP5_7,

high frequency components become dominant. Crushing

against the hull can cause high frequency, random vibra-

tions, which can explain the significant increase in power at

the higher frequencies between 25 Hz and 40 Hz. When the

ice fails by bending, a repeating force pattern is measured,

which is the reason for the excitation in the low frequen-

cies, as shown in Figure WP5_8.

The technique presented allows for a significant improve-

ment in ice observing systems on ice-going vessels or on

structures in ice. Further research is ongoing to identify

the direction of the ice force, so that the system can be used

as feedback in ice-capable control and monitoring systems

for Arctic offshore operations.

WP5_Fig. 9: Time-Frequency distribution of the transition from broken ice to unbroken level-ice

WP5_Fig. 10: Ice condition while moving from broken to unbroken level-ice

The distribution of the frequencies allows for the identification of the dominating ice failure

WP5_Fig. 11 and WP5_Fig. 12 show the power spectral densities of two situations with di

dominating f ilure mode. I case of ice crushing, as shown in WP5_Fig. 11, high fre

components become dominant. Crushing against the hull can cause high frequency, r

vibrations, which can explain the significant increase in power for the higher frequencies betw

Hz and 40 Hz. When the c fails by bending, a repeating force pattern is measured, which

reason for the excitation in the low frequency, as shown in WP5_Fig. 12.

WP5_Fig. 11: Power spectral density of 20 seconds of measurements while crushing against the hull occured

Frequency [Hz]

Power Spectral Density

2000

4000

6000

8000

10000

IMU 1

IMU 3

IMU 4

WP5_Fig. 12: Power spectral density of 20 seconds of measurements while failure by bending of the ice occurred

The presented technique allows for a significant improvement of ice observing systems on ice

vessels or on structures in ice. Further research is conducted to identify the direction of the ice

such that the system can be used as feedback in ice-capable control and monitoring syste

rctic offshore operations.