Numerical and analytical study of a spar-like floating offshore wind turbine impacted by a ship

It is estimated that Europe will develop 85% of the offshore wind needs in the North Seas. Moreover, floating offshore structures have been developed and installed in Scotland and other countries, this technology is still under development, with turbines that reach up to 12MW and more than 200 m diameter.
Recently we have seen how a ship grounding can cause several problems to the economy world wide (with the Evergreen accident in the Suez canal 2021). This is the type of situations that we want to avoid when close to a wind farm, as the consequences of such collisions could include human lives loss or high environmental and economic issues. Design offces should always do a risk analysis before the installation of offshore structures. Normally Finite Element software are used to compute the resistance of an offshore wind turbine upon a collision with a ship. However, ship collisions involve several parameters (such as ship velocity, trajectory, mass and properties of the collided structure), which would mean the study of thousands of collision cases in order to find the worst scenario. Such studies would be time demanding and high computational resources are needed, which is not ideal in a pre-design stage.
The purpose of the present thesis is to study, by means of Finite Element Methods, the energy dissipation modes of an spar-type Floating Offshore Wind Turbine and propose a simpli ed method to assess quickly the resistance of such structure after a collision with a ship. This proposal uses a combination of tools to calculate the different phases of the collision, including internal and external dynamics of the response. This can be an interesting way to identify the most critical collision scenarios for the impacted Floating Offshore Wind Turbine, and then use the powerful Finite Element Methods to analyse in detail the collision scenario chosen. The energy dissipation modes identi ed are namely: local crushing of the tubular tower, global bending of the whole structure and external dynamics, including hydrodynamic effects and mooring lines reaction.
For the first two, the structure is considered as clamped at one end and closed-form expressions describing the evolution of the resistant force with respect to the ship penetration are presented, based on previous studies on ship-jacket structures collisions (for the modes involving cross-section crushing) and classical beam theory (based on the upper-bound theorem associated with a plastic limit analysis). For the mooring analysis, as the collision time is large in comparison to the mooring vibration modes, and the surge motion of the FOWT is small in comparison to its diameter, the simple quasi-static catenary equation is proposed, taking into account contact with the seabed. For the dynamic external response, the external dynamics calculator MCOL is proposed, as it solves the equation of motion taking into account the forces and moments calculated analytically during the contact. It has in consideration the hydrodynamic properties and wave response of the
floating structure.