Session: Mooring

Paper Number: 164603

164603 - Design and Assessment of Hybrid-Catenary Mooring Systems Using Steel Wire and Polyester Ropes for a 10MW Semi-Submersible and Spar-Buoy Wind Turbine 

Abstract: 

Introduction

This study investigates the cost-effectiveness of using steel wire ropes (SWR) and synthetic polyester ropes (PR) in floating offshore wind turbine mooring systems compared to traditional steel chains. The research focuses on identifying potential cost savings and assessing their dependence on floater type, water depth, and met-ocean conditions. The study encompasses the comprehensive design and evaluation of three mooring system designs: a conventional catenary system with steel chains, a hybrid catenary system with chains and steel wire ropes, and a hybrid catenary system with chains and polyester ropes. These designs are analyzed for a semi-submersible floater at 200m water depth under harsh weather conditions and a spar buoy floater at 300m water depth under similar conditions. Both structures are supporting the DTU 10MW reference wind turbine [1]. The objective is to compare on the basis of specific KPI’s the material cost of the mooring system, the floater motions, the mooring line tension and lifetime, the tower and hub acceleration, and the blade root and tower base combined bending moments.

Methodology

The analysis comprises several key steps. First, a detailed hydrodynamic analysis of the chosen floating structures is performed at their respective water depths. This step involves solving the diffraction and radiation problems to determine hydrodynamic loads, the hydrodynamic added mass, and damping coefficients and mean drift forces and moments. These parameters are crucial for accurate mooring system design and are obtained using both in-house [2] and commercial software [3] ensuring validation and accuracy. Second, the three mooring system designs are thoroughly evaluated using quasi-static and dynamic analysis techniques. The quasi-static analysis establishes the initial dimensioning of the systems to achieve the specified offset. Dynamic analysis, conducted both in the frequency and time domains, assesses the mooring line behavior under various load cases (DLCs) according to IEC standards. A state-of-the-art hydro-servo-aero-elastic tool [4], [5] is employed for the global time-domain coupled analysis, considering the coupled interactions of the wind turbine, floater, and mooring system. The analysis considers multiple limit states (fatigue, ultimate, and accidental) to ensure design robustness following ABS-195 [6] provisions.

Results

Analysis of the three mooring system designs indicated that all are viable solutions for both 10 MW spar-buoy and semi-submersible floater configurations. KPI comparisons confirmed that the coupled floating structures exhibited similar dynamic behavior across all three mooring system designs. Both SWR and PR proved effective for station-keeping in hybrid rope-chain mooring systems. Consistent across all designs, chains exhibited fatigue-driven behavior, while ropes (SWR and PR) demonstrated ultimate-driven characteristics. However, a notable difference emerged in fatigue loading: for the semi-submersible floater at a quasi-static design offset of 15 meters, both SWR and PR hybrid systems reduced fatigue loading compared to the conventional all-chain system. Conversely, for the spar-buoy floater at a design offset of 25 meters, both SWR and PR hybrid systems resulted in increased fatigue loading on the mooring chains compared to the conventional system, with this effect being more pronounced for the PR system.

Conclusion

The analysis indicates that the suitability of rope-based mooring systems may depend on various interacting parameters including design offset, imposed pretension, and water depth. Cost analysis highlighted the potential economic benefits of hybrid systems, particularly concerning reduced material costs and potentially reduced installation costs. Further investigation into a wider range of environmental conditions, water depths and key parameters may be necessary to establish more generalized design guidelines.

References

[1]Bak, C., et al., “Description of the DTU 10 MW Reference Wind Turbine, “DTU Wind Energy Report-I-0092, 2013

[2]Mavrakos, S.A., “Users manual for the software HAMVAB”. School of Naval Architecture and Marine Engineering, Athens, Greece, 1995

[3]ANSYS AQWA Theory Manual, ANSYS Inc. Southpointe 2600 ANSYS Drive Canonsburg, PA 15317, 2015

[4]Riziotis V., Voutsinas S., “GAST: A general aerodynamic and structural prediction tool for wind turbines”, EWEC, Dublin Castle, Ireland, 1997

[5]Manolas, D.I., et al. “Hydro-Servo-Aero-Elastic Analysis of Floating Offshore Wind Turbines”, Fluids, 5(4), 200, 2020

[6]ABS 195, “Guide for building and classing floating offshore wind turbines”, 2021

Presenting Author: Dimitrios Manolas iWIND Renewables

Presenting Author Biography: Mechanical Engineer (2007, NTUA) with a PhD degree in wind turbines' servo-hydro-aero-elasticity (2015, NTUA), working in the wind energy sector since 2009. Partner at iWind Renewables SA since January 2018, with special interest in aero-elasticity and wind turbine analysis and design, onshore and offshore. Part-time professor teaching wind turbines' aero-elasticity in ICARE (HUST University, Wuhan-China) and in EUREC (NTUA, Athens-Greece) Master programs since 2016.
Developed the comprehensive aeroelastic analysis tool hGAST, the frequency domain hydrodynamic solver freFLOW and the fully non-linear hydrodynamic solver hFLOW. Authored 43 scientific articles in refereed journals and conference proceedings. Involved in 13 national and EU-funded R&D research projects related to energy systems and in more than 100 commercial projects.

Authors:

Dimitrios Manolas iWIND Renewables
Dimitrios Konispoliatis National Technical University of Athens, School of Naval Architecture and Marine Engineering
Spyros Mavrakos National Technical University of Athens
Maurizio Meleddu Teufelberger-Redaelli Tecna S.p.A., Italy