Collision Study of Rigid Ships with a Deformable Offshore Wind Turbine Jacket Structure

Abstract

The higher energetic demand of today’s society has required the endeavor of advancing into harsher environments to convert and obtain energy to ensure its productivity and functioning. For this reason, wind energy has become an attractive source of renewable energy, as in the long term it is cleaner and safer for the environment than burning fossil fuels. Recently, wind farms are being developed further offshore. These require more advanced technologies and more robust structures for support. The jacket support system originated in the oil and gas field, where it is used at water depths of up to 500 m. It has been successfully employed in the renewable energy field as the support for offshore wind turbines in depths of up to 45 m1. Due to the larger area density of an offshore wind farm, there are concerns on the possible collisions of either passing or service ships with the wind turbine support structures. Therefore a risk analysis becomes necessary during the planning stages of the offshore wind farm to ensure a safe operation throughout its service life and to identify the most likely collision incident. This thesis deals with the in depth analysis of the collision process of ships with offshore wind turbine jackets. The results of the collision simulations will be used to validate a simplified collision analysis software based on the super element method, which will reduce overall design costs serving as a replacement to Finite Element Analysis during the predesign stage of the wind turbine jackets. Concretely, CAD models of an Offshore Supply Vessel and a Crude Oil Carrier were implemented into the collision simulations to study the collapse process of a jacket. The nonlinear finite element code LS DYNA was used for both scenarios, where different velocities and collision angles were assessed, maintaining the ship models as rigid and the jacket model deformable. Initial simulations did not account for the gravity or the effects of the added weight of the wind turbine on the jacket structure. Additional two part simulations were carried out to study and compare the collapse behavior of the jacket considering the weight effects of the wind turbine. A “preloading” implicit simulation loaded the stresses imposed by the wind turbine and the gravity on the jacket structure and a secondary explicit simulation accounted for the collision of the ship. With the completed simulations a detailed analysis of the internal energy, crushing force and stress distribution throughout the jacket with varying kinetic energies and collision angles allowed to characterize, for the geometry in question, its sensitivity to the added weight of the wind turbine and whether or not this parameter has to be considered in the simplified super element software. Also, for all the different scenarios the importance of local (tube crushing) and global (structure bending) deformation was established. From the results it was observed that high kinetic energy simulations are less sensitive to the added weight of the wind turbine and the jacket, while low kinetic energy simulations present a higher sensitivity to the weight of both the wind turbine and the jacket, which occurs because of the different stress distribution throughout the structure. However, the overall tendency observed was that the added weight of the wind turbine and the jacket do not affect the collision simulation in a considerable enough manner as to account for it in the development of the simplified software for scenarios similar to the ones modeled.