Session: 02-10-01 Collision and Crashworthiness 1

Paper Number: 79672

79672 - A Semi-Analytical Methodology to Assess the Dynamic Elastic Response of a Monopile Offshore Wind Turbine Subjected to Low-Energy Ship Impacts 

With the increased presence of offshore wind turbines (OWT), not only in shallow coastal waters, but also in deepwater zones, the risk of ship collision accidents also increases. The outcome of a ship-OWT collision can range from minor structural damage to full collapse of the turbine depending on the kinetic energy of the striking ship. Low-energy impacts, for instance, can occur during maintenance operations when service vessels need to approach the turbine, sometimes in harsh sea states. While this type of accident might not result in significant permanent damage of the tower or the foundation, the dynamic elastic response of the overall structure can induce high accelerations at the rotor-nacelle assembly (RNA), where sensitive subcomponents such as the generator, bearings and gearbox are contained.

This study presents a two-step methodology to quickly assess the dynamic elastic response of a monopile OWT subjected to a low-energy ship impact. The first step consists in modelling the monopile as a reduced set of Euler-Bernoulli beams with different cross-sections and a tip lumped mass representing the RNA. A semi-analytical framework is used to derive closed-form expressions for the natural frequencies and mode shapes of a Multiple Discontinuity Beam. This method allows for an accurate description of the structure’s geometry and a rapid calculation of its modal characteristics. 

In the second step, the modal superposition method is applied to calculate the monopile’s response to a pre-defined collision force. Validation of the proposed semi-analytical model is performed using the explicit non-linear FEM code LS-DYNA. Special emphasis is attributed to the water added mass effect by comparing a simple approach, where hydrodynamic masses are added to the monopile’s immersed section, and a fully coupled approach where the surrounding water is explicitly modelled.

The proposed methodology is intended for pre-design stages, where risk assessment is performed and a large number of collision simulations is required. In this case, fast calculation tools are generally more suitable than FEM, allowing for a good compromise between computational cost and accuracy.

Presenting Author: Icaro Ladeira Institut Catholique d'Arts et Metiers

Authors:

Icaro Ladeira Institut Catholique d'Arts et Metiers
Hervé Le Sourne Institut Catholique d'Arts et Metiers
Jonathan Alexander Morán Arellano University of Liège - ANAST
Lissette Priscilla Salazar Loor University of Liège - Anast