Session: Mooring & Cable

Paper Number: 165418

165418 - Floating Offshore Wind Farm Design Optimization, Including Mooring Line Orientations and Fatigue Design 

Abstract: 

Floating offshore wind turbines (FOWTs) offer a promising way to produce renewable electricity in offshore deep-water environments, where conventional fixed-bottom foundations are impractical. But this emerging technology poses new challenges, especially in setting up the mooring lines.

Offshore wind farm layout is generally optimized to maximize annual energy production by minimizing the wake effects due to upstream turbines, while taking into account the limits of the geographic area allocated by the public authorities for this farm and alignment constraints designed to facilitate installation and maintenance.

In the case of floating wind farms, additional constraints such as minimal spacing between mooring lines and preventing mooring lines crossing in the x-y plane must be considered. Designers may also aim to arrange these mooring lines similarly across each row of turbines to simplify the design and installation process. These constraints can be incorporated directly into the optimization or addressed later. In all cases, the final design of the mooring lines must take into account the fatigue constraints which are emphasized by the turbulence added by the presence of upstream turbines. Estimating accumulated fatigue damage requires a huge number of simulations to compute its expectation over the joint probability of wind, wave and current parameters. Reproducing a regular pattern in the farm layout and mooring lines orientation then facilitates this design, by limiting the number of cases to study.

In this work, we present several results after the dedicated study of IEA Wind task 49 collaborative project IDEA. We consider a farm composed of 67 IEA 15 MW wind turbines to be placed within a rectangular area of 256 km². Each turbine is supported by a UMaine semi-submersible floater equipped with 3 semi-taut lines adapted for the Humbolt deep water site.

We present a method to place the turbines and their mooring lines in the allocated area. It is based on the construction of a regular pattern for the turbine’s positions and mooring lines orientation and have two steps. It first places the turbines within the allocated area in order to maximize the annual energy production (AEP). And in a second step, it optimizes the mooring lines positions to maximize the distance between these lines. For the first step, different algorithms can be used, leading to different farm layouts. We propose to compare these layouts according to several criteria (AEP and impact on fatigue). Finally, we provide a conservative fatigue estimate on the mooring lines taking into account the impact of stationary wake deficit and added turbulence and the impact of the mooring lines orientation.

Presenting Author: Alice Nassor IFP Energies nouvelles

Presenting Author Biography: Alice Nassor is a research engineer at IFP Energies Nouvelles, specializing in mechanics applied to renewable energies, particularly floating offshore wind energy. She holds a PhD from the Institut Polytechnique de Paris, obtained in 2023. Her current work focuses on mooring lines design and farm layout optimization for floating offshore wind farms.

Authors:

Alice Nassor IFP Energies nouvelles
Yuksel Alkarem National Renewable Energy Laboratory
Michel Castagne IFP Energies Nouvelles
Matthew Hall National Renewable Energy Laboratory
Junho Lee Deep Anchor Solutions Inc
Ericka Lozon National Renewable Energy Laboratory
Paul Malisani IFP Energies Nouvelles
Yann Poirette IFP Energies nouvelles
Vishnu Ramachandran Norwegian University of Science and Technology