Optimization of grillage design for the transportation oof suction caisson jackets

Abstract

Suction caisson jacket (SCJ) is one of the most promising substructures, under certain conditions, for offshore wind turbines. Their unique characteristic of large diameter (D) compared to their small wall thickness (t) conducts them an object of study, concerning their transportation to the offshore location, as their structural stability should be ensured. Their large D/t ratio leads the caisson into being a flexible structure, which should be stabilized in an already moving barge (due to wave motions and accelerations). The caisson is partially supported in the base perimeter, in order to leave enough space for both loadout equipment and the barge to coexist. The main objective of this thesis is to apply the most optimal configuration to sufficiently support the caisson on barge. Prerequisite of that is to examine the behavior of the caisson, when interacting with the barge during transportation. A parametric study is performed where the stresses and deformations of the caisson are retrieved and the buckling behavior is examined. The boundary conditions are applied in the bottom perimeter of the caisson and they are designed, so as the stiffness of the barge is taken into consideration. The parametric study considers changing the boundary conditions (supports) in the perimeter of the caisson. In total 6 cases were examined, in which the supports vary with respect to their length, their location in the base of the caisson and finally the value of their stiffness.
A finite element analysis is performed and the results were derived for every case of the parametric study. The equivalent von Mises stress is examined in the full length of the caisson and in the supported perimeter, as well as its most important components. For the supported perimeter, directional deformations are derived in radial, tangential and vertical direction. The results showed that the stresses developed very large peaks above yield limit at the edges of the supports, for each case, and the most influential component causing these peaks turned out to be bending stress in tangential direction. The bending stresses are caused by radial deformations, thus in order to reduce this stress component, radial deformations should be reduced. The comparison of the cases showed that the trend of the stresses remain the same for every case. It is also realized that the peaks in stresses and deformations in radial and tangential direction show the most reduction when the distance between the supported segments is reduced. A non-linear analysis performed indicates that the high stresses in every case (above yielding) are acceptable and this will lead to a redistribution of stresses in the caisson. The buckling behavior of the caisson is examined and in every case buckling failure does not have a high possibility to occur for the applied load combination. As a result of this study it is concluded that the most optimal grillage configuration has to do with reducing the unsupported area between the supported segments.