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.