Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Sensitivity Analysis of Mooring Tension Fatigue For a 15 MW Spar-type Floating Offshore Wind Turbine
Qi Pan, Po Wen Cheng
Stuttgart Wind Energy (SWE), University of Stuttgart
Stuttgart, Baden-Württemberg, Germany

by site measurements, empirical joint probability functions and Monte
Carlo samplings, which are common methods for the reconstruction ofenvironmental conditions. In the early design stage, the site-specific joint
distributions of wind speed, wave height and wave period are notprovided for a 15 MW spar-type FOWT (Vigara et al., 2019). And theempirical functions are not applicable for the specific site. Meanwhile,
the Monte Carlo method generates a large scale of samplings and thehigh computational effort is not preferred for preliminary designs. Lackof environmental details heightens uncertainty in predictions, therefore
it is essential to identify the most important environmental parameters
for reliable and efficient mooring fatigue predictions.

ABSTRACTIn the early design phase for a 15 MW spar-type Floating Offshore Wind
Turbine (FOWT), lack of environmental details heightens uncertainty inmooring fatigue predictions. Sensitivity analysis (SA) is employed toinvestigate the importance of wind and wave on this FOWT, based on asite-specific model from EU H2020 funded project COREWIND.
Numerical simulations of wind-only, wave-only and wind-wave tests are
conducted in OpenFAST. The SA results prove that among wind speed,
significant wave height and peak wave period, wind speed is the mostinfluential parameter and wave impact is limited on mooring fatigue
loads. The wind-only mooring fatigue loads are generally higher than theintegrated wind-wave results, indicating that we can apply wind-only
tests for efficient and reliable predictions of mooring tension fatigue forthis site-specific FOWT. The findings of SA decrease the prediction
uncertainty and promotes efficient mooring designs.

Sensitivity analysis (SA) is used to investigate the impact of input
parameters on output variables of interest. Two classes of SA methods,
namely local and global SA are commonly employed. The local SA
solves the ‘inverse problem’ and follows the ‘one-factor-at-a-time’approach, while the global SA provides the ranking of importance andthe interaction among the input parameters. In this paper, we conduct SA
for mooring tension fatigue of the 15 MW spar-type FOWT, based on apreliminary model from EU H2020 funded project COREWIND.

KEY WORDS: Sensitivity analysis; wind and wave impact; mooring
tension fatigue; 15 MW Floating Offshore Wind Turbine (FOWT); spartype floater.

The current SA research on mooring tension fatigue for FOWTs include
the investigation of environmental parameter impact (Müller and Cheng
2018; Li et al., 2019; Müller 2020) and the comparison of different
mooring configurations (Campanile et al., 2018; Barrera et al., 2020; Panet al., 2021). For a 5 MW spar-type FOWT at the water depth of 320 m,
by comparing 5 different wind-wave results, it is found that mooring
fatigue is more sensitive to wind loadings (Li et al., 2019). For a 10 MW
semi-submersible FOWT, the global SA proves that wave height has adominant role on mooring fairlead tension fatigue, followed by mean
wind speed and wave period (Müller 2020).

INTRODUCTION
Over the lifetime of floating offshore wind turbines (FOWTs), complex
environmental conditions cause cyclic loadings and increase mooring
failure risks. Although there is little public data on mooring failures ofFOWTs, lessons can be learnt from the oil and gas industry that fatigue
is one of the main reasons for mooring failures (Ma et al., 2019). Whatis more, the fault tree analysis for FOWTs shows a high failure rate formooring system fatigue (Kang et al., 2019). Mooring failures are harmful
to mooring structural strength and result in unacceptable movements ofthe floater, which affects the wind power generation. Therefore, afeasible mooring design and a reliable fatigue prediction are critical formooring integrity and in-service performance of FOWTs.

Compared to the 5 MW and 10 MW FOWTs, this 15 MW FOWT has alarger size of wind turbine and environmental conditions are different.
The direct implementation of mooring fatigue conclusions from otherFOWTs is questionable. But the findings about the most importantenvironmental parameters for the 10 MW semi-submersible FOWT areconsidered in this paper, as the design basis provides distributions ofmean wind speed, significant wave height and peak wave period
separately. Besides, the significant influence of wind speed on mooring
tension fatigue is observed (Pan and Cheng 2022), but the question

Recent mooring fatigue studies on FOWTs investigate the effect ofmooring line dynamics (Azcona et al., 2017), wave nonlinearity (Xu etal., 2019) and mooring designs (Pan et al., 2021). Different proceduresare proposed for reliable mooring fatigue predictions (Müller and Cheng
2018, Barrera et al., 2020). In mentioned studies, sea states are described240