Experimental Investigations of Tension Piles in Sand Subjected to Static and Cyclic Loading

The present thesis regards the behavior of the piles in jacket pile foundations used for offshore wind turbines. The piles are often loaded in tension because of the combination of wind and wave conditions and the low self-weight of the wind turbine. The repeated cyclic loading can lead to accumulated upwards displacement of the piles and, thus, undesired deflection of the wind turbine structure.

This study concerns the effect of cyclic loading on a pile installed in dense sand and loaded in tension. A new laboratory test setup was constructed to make these pile load tests. The thesis discusses the advantages and disadvantages of the test setup. The results of cyclic loading tests showed that the loading conditions are very important for the behavior of piles. Some wind and wave conditions can be beneficial and increase the pile capacity while other conditions can be damaging and reduce the pile capacity and result in large permanent displacement of the piles. Hence, it is vital to account for the cyclic load conditions at the site of an offshore wind farm when designing the wind turbines.