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

Influences of Mooring Line Failure on Dynamic Response of Floating Wind Turbine under Extreme SeaLoads
Zhenju Chuang, Shiqi Liu，Aobo Zhang, Zhen He

Naval Architecture and Ocean Engineering College, Dalian Maritime University.
Dalian, Liaoning, China

ABSTRACTsubstructure characteristics, the mooring system plays a critical role inthe design of structure motions and natural periods (Barrera et al.,2020). Moreover, the mooring system is an essential part of floating
structures to ensure production system survival capability and stability(Barrera et al., 2019).

This article aims at proposing an effective methodology forinvestigating the dynamic response of a floating wind turbine undermooring line breakage in extreme sea loads. An aero-hydro-cable-servo
numerical model of NREL 5 MW OC4-DeepCwind semi-submersible
type floating wind turbine with three mooring lines is chosen for a fully
coupled time domain analysis. The FOWT is assumed to be exposed to
100-year extreme sea loads in the Norwegian Sea (Haltenbanken).
Three dimension radiation/diffraction theories are applied to calculate
first-order wave force, second-order mean drift force, and slowly
varying drift force of semi-submersible platform. Morison’s equation isused for small diameter structures to account for fluid drag loads.
Nonlinear time-domain finite element methods are employed to analyze
the behavior of mooring lines. Transient response of platform motions,
tension variation of the mooring lines are quantitatively studied. Results
show that the failure of the mooring line has a significant impact toreduce the platform ’ s stability performance. FOWT is observed to bedrifted away up to 800 meters under such condition. This is fatally
dangerous to the entire wind farm. Collision risk is high and redundant
mooring lines can be set up.

Mooring systems constitute an important element in the cost
breakdown, typically about 8% of the capital expenditure of a typicalmarine platform installation (Garrad, 2012), and therefore they have alarge impact on the cost of energy. For the past 20 years, more than 150
mooring lines have been replaced (due to accidents or preventive action)(Majhi et al., 2013; Maslin, 2013 ). As for FOWT, mooring line
breakage will lead to significant changes to the platform location andorientation, which may cause damages to electric cable, powertransmission system, and even collision between each FOWTs at thewind farm (Ma et al., 2020). Such accidents will seriously endanger thenormal operation and economic benefits of offshore wind farm(Hallowell et al., 2018).
Research activities have been carried out to investigate the importanceof mooring system integrity management. A series of guidance
documents and standards about position mooring systems are publishedfrom the main associations and class societies, such as API (AmericanPetroleum Institute, 2005; American Petroleum Institute, 2010), ABS(American Bureau of Shipping, 2014), DNV (Det Norske, Veritas,2018) and IACS W22 (International Association of Classification
Societies, 2010). All these guidelines and standards require redundancy
to line break during the mooring design stage (Ren et al., 2015). Li et al.
(2018) carried out time-domain simulations to investigate the transient
response of a SPAR-type floating offshore wind turbine in scenarioswith fractured mooring lines. The focus is on the motions of a floating
platform, the tensions in the remaining cables, as well as the energy
harvesting performance with or without the emergency, shut down at
the moment of cable fracture. Bae et al. (2017) checked motion
responses and mooring line tension of the semi-submersible FOWT
with a single broken mooring line. Yu et al. (2019) studied the coupledhydrodynamic response of a TLP platform after tendon one-time failure
and progressive failure of several tendons under extreme tropical

KEY WORDS: Floating wind turbine, mooring line failure, extreme
sea loads, dynamic response.

INTRODUCTION
Limited fossil fuel reserves and ever-increasing demand for energy
have resulted in the rapid development of renewable energy sources.
Wind energy due to its great potential is the most rapidly growing one.
The combination of the hydrodynamic loading from waves and current,
the aerodynamic effects of wind, structural dynamics of the support
structure, and the nonlinear effects of the controller together make thedesign of Offshore Wind turbines a very challenging exercise (Moratóet al., 2017). Floating offshore wind turbines (FOWT), in comparison
with fixed foundations, have a higher level of design complexity.
Despite the system stability is mainly driven by the floating152