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31
SAMC
o
T • Annual report 2012
Ice-Induced Vibrations
Bottom founded offshore structures occasionally experience sustained ice-
induced vibrations. This vibration causes operational problems and may be a
risk for structural safety.
These ice-induced vibrations are often characterized
into three regimes, depending on the incoming ice veloc-
ity (for more information, see the recent ISO standard
Arctic Offshore Structures): a) Intermittent crushing
taking place at the lowest ice velocities; b) Frequency
lock-in for intermediate ice velocities and; c) Continuous
brittle crushing occurring when the ice velocity is high.
Insufficient experimental data exists, both full-scale
and laboratory-scale, to explain the physical reasons
for these vibrations. As the phenomena are not well
understood, none of the present numerical models
capture the measured forces and accelerations from
experiments.
In order to make reliable predictions of ice-induced
vibrations, a numerical model based on experimental
data needs to be developed. In August 2011, a series of
experiments were carried out in the large ice basin at
the Hamburg Ship Model Basin (HSVA) through the EU
HYDRLAB-IV facilities. In 2012, the SAMCoT special-
ized research team analysed this data and has contin-
ued the development of a numerical model.
An international ice load survey carried out by Timco
and Croasdale (2006) shows that there is considerable
uncertainty in prediction of ice loads on structures.
Laboratory experiments are one of the ways that we
can control to some extent both structure and ice
conditions. Different laboratory studies on compliant
structures use different techniques and measurement
setups to derive the ice forces.
Many of the test setups use direct measurement of
the global ice force with load cells and dynamom-
eters, while others use indirect calculations of force
from structural response. The dynamometers give
easy access to forces, still the added mass from water
and model mass have to be dealt with. Load cells built
into the indentor, together with an accelerometer, are
possible in the laboratory, but scarcely applicable on
full-scale offshore structures.
Inverse force identification is a research topic adopted
into structural dynamics and applied on systems
where the forces are difficult to measure directly. For
civil engineering purposes, bridges, piers, lighthouses
and offshore structures interacting with ice are all
examples where the ice force is difficult to quantify.
In ice, available works on inverse force and system
identification is limited and restricted to deterministic
frequency domain methods.
Fig. 27. Sketch of the numerical model. In
green, a four-legged offshore structure is
shown. In blue, the ice sheet is divided into a
lattice near the structure and into a continuum
at a distance from the structure. Important
aspects of the interaction are characterised
as: confinement, driving force, inhomogene-
ous ice property, friction at the ice-structure
interface and interaction zone mechanics.