On the influence of jack-up footprints on the dynamic behaviour of offshore foundations

Jack-up vessels are frequently used to install offshore structures. These vessels are shipped to the construction site and then lifted out of the water using their legs to create a stable working platform. Shallow foundations, so-called spudcans, are attached to the legs and pressed into the ground until sufficient bearing capacity is achieved. When the construction work is finished, the spudcan is pulled out of the ground again and usually a footprint remains representing a hole in the seabed. As the installation of offshore structures usually consists of several construction phases, several jack-up vessels are placed around the structure at different times and each vessel leaves several footprints. The influence of footprints on the dynamic behaviour of offshore structures is exemplified by the foundation of offshore wind turbines. The investigations were carried out with numerical simulations using the FEM. The influence of the footprint location as well as its distance to the structure is investigated. General recommendations are given and how this influence can be addressed in the design of the structure.