Session: 09-01-02 Wind Energy: Aero-hydrodynamics 2

Paper Number: 123630

123630 - Validation of a Mooring System With Clump Weights for a Spar Floating Wind Turbine Model Using Openfast 

Floating wind power is considered one of the most promising sources of sustainable energy today and is considered a key technology to meet decarbonization goals. Stronger and more consistent wind conditions found in deep water places are more profitable in comparison with coastal areas; as a consequence, in the last decade, there has been a clear tendency to place Floating Wind Turbines (FWT) in deeper waters. New challenges arise with respect to their ability to remain in position within certain spatial limits, demanding innovative, highly reliable and cost-effective mooring solutions.

This research focuses on analyzing different mooring systems for a FWT mounted on a spar-buoy platform. For this purpose, we selected the widely studied in the literature 5MW OC3-Hywind prototype (Jonkman et al. 2008, Jonkman et al. 2009). We propose an initial realistic catenary mooring system designed for a deep-water location. This mooring design objective of minimizing material cost in production and manufacturing is achieved by means of two segment lines: an upper steel wire segment and a lower studless chain segment. In the context of offshore wind energy, the reduction of turbine's motions allows to produce power in a wider range of sea states. In addition, lower motions and accelerations reduce the tension of the lines and minimize the associated building and maintenance costs. Several recent research studies (Bruschi et al. 2020, Xu et al. 2021) underlie the advantages of adding clumped weights at discrete positions of the mooring line, in terms of both mooring tensions and motions reductions. Therefore, building on this mooring system, we introduce two innovative mooring configurations that incorporate the use of one clumped weight at two different locations: one close to the seabed and the other one close to the fairlead.

These three mooring configurations are evaluated under different wave conditions, which include regular and irregular wave scenarios representing typical operational and extreme conditions in the North Sea. To assess these different mooring systems, we conducted 1:47 scale model experiments at the CEHIPAR ocean basin, where we measured mooring tensions and platform motions. These experiments are compared with a set of numerical simulations carried out using OpenFAST, where the module MoorDyn is implemented to model the mooring dynamics. The numerical model is calibrated with additional platform damping and stiffness coefficients to match the experimental decay tests in the 6 DOF.

The additional damping coefficients which model the viscous effects together with the OpenFAST's BEM numerical tool show good agreement for the dynamic performance of the FWT. The results demonstrate the remarkable accuracy of OpenFAST with MoorDyn when compared to the experimental data, specifically in terms of platform motions. Since considerable differences are found when comparing the mooring tensions obtained with MoorDyn and the experimental results, a discussion on the difficulty of scaling the material axial stiffness of the line in the model tests is included.

Since the maximum tension in the lines under extreme conditions is decreased around 5%, the goal to obtain a load reduction of the mooring lines by the inclusion of clumped weights is achieved in this study.  Consequently, this research contributes significantly to estimate the change of the seakeeping performance of the FWT when clumped weights are attached to the lines.

Presenting Author: Tomas Lopez-Olocco Universidad Politécnica de Madrid

Presenting Author Biography: I am currently completing my PhD in Naval Architecture and Ocean Engineering at the Universidad Politécnica de Madrid (UPM). My PhD is focused on the modeling and analysis of novel mooring systems and dynamics of Floating Wind Turbines. The research that I conduct focuses on modeling, analyzing, and testing Floating Wind Turbines. Also, part of my work is the validation of servo-elastic-hydro-aero numerical models with the experimental data obtained from ocean basin tests that I analyze.

Authors:

Tomas Lopez-Olocco Universidad Politécnica de Madrid
Leo M. González Gutiérrez Universidad Politécnica de Madrid
Krish Thiagarajan University of Massachusetts Amherst