The influence of ice jams on ice-induced vibrations of multi-legged sub-structures

Abstract

The world is transforming
its energy production towards more sustainable sources of energy. In Europe,
there is currently 25 GW of installed offshore wind power capacity. This is
expected to grow with 29 GW over the next five years. Offshore wind farms can
be expensive and challenging to build, design and maintain. Understanding the
offshore environment will ensure that the to be produced offshore wind turbines
are of sufficient quality while reducing costs. Monopiles are currently
the most common sub-structure, but jacket sub-structures are becoming more
relevant due to increasing water depth or changing soil conditions. Structures
in icy waters, such as the Baltic Sea, may be subjected to ice induced
vibrations while they encounter sea ice. These vibrations have to be considered
in vertically-sided offshore structures' design and are the most critical load
case when ice is concerned.

Multi-legged
sub-structures, such as jackets, can have a problem that does not exist for
monopiles, namely ice jamming, where ice fills the space between the legs of a
multi-legged sub-structure. The legs and the jammed ice may then act as a
single structural unit. Which leads to the main research question: how does an
ice jam influence ice-induced vibrations of a multi-legged sub-structure?

First, a literature study
of ice jams and multi-legged sub-structures was performed. This study concluded
that different ice jamming situations are possible and have occurred with
multi-legged structures, which not all have survived. The ratio between leg
spacing and diameter plays a vital role in the ice action on multi-legged structures.
Furthermore, the combination of ice-induced vibrations and ice jamming had not
been studied yet.

Secondly, a model is made
based on a phenomenological ice crushing model using COMSOL Multiphysics and
MATLAB to simulate the structural response. The sub-structure is based on the
jacket design for the NREL 5-MW reference turbine. Different situations from
the literature study are used to make several design scenarios for which the
structural response is calculated. In total, there are five situations: a base
case, an angled base case, an internal jam, a frontal jam and an angled frontal
ice jam. The base case does not have an ice jam, and the angled frontal jam has
an increased thickness of the jam to twice the incoming ice. For the other jams,
the thickness is equal to the incoming ice.

The different scenarios are
simulated for a range of ice drift velocities to capture the different
ice-induced vibration regimes and see how the structural response changes due
to the presence of an ice jam. First, a baseline was established of the jacket's
structural response for the base case. Afterwards, the three different ice jams
were simulated. Results show that the base case is excited in all three
ice-induced vibration regimes. At lower ice drift speeds, intermittent crushing
is observed. Then at around 0.05 ms^-1, it transitions into the frequency
lock-in regime. Here the structure is excited at its second natural frequency.
For higher ice drift velocities (>0.2 ms^-1), continuous brittle
crushing is seen. For the angled base case, the transition between intermittent
crushing and frequency lock-in happens at around 0.1 ms^-1, and it stays
longer in the frequency lock-in regime. The internal stresses around the
contact area between ice and leg for the internal and frontal jam did significantly
exceed the ice strength. Thus these jams would have failed on crushing at the
contact area. The stresses inside the angled frontal jam exceed the ice
strength but by a small margin. With all the assumptions made taken into
account, the jam might hold. The structural response shows an increase in
period for intermittent crushing and a lower amplitude in structural
displacement than the base case.

The main conclusion is that an ice jam that
would significantly impact the ice-induced vibrations cannot be sustained. The
internal stresses exceed the ice strength which would cause the jam to fail.
The ice jam that can be sustained acts as additional stiffness for the system
and decreases the structure's displacement amplitude for the intermittent crushing
regime. In the frequency lock-in regime the structure's displacement frequency
increases a bit. But the amplitude is similar in all scenarios because the
maximum velocity of the structure will roughly be the same as the incoming ice
floe because that is what excites the structure, and this doesn't change.