Potential Mooring System Optimization Using Polymer Spring Component – Floating Offshore Wind Turbine Application
Authors: Aryawan Iwan , McEvoy Paul , Kim Seojin , Faria Rui Pedro 
Published: April 2023

The renewable energy industry through offshore wind has been growing rapidly with increasing project capacities. To accommodate this huge energy transition demand, the projects are moving towards open sea and deeper water. In line with this trend, various design development and innovation on Floating Offshore Wind Turbine (FOWT) technology are progressing to improve the feasibility as well as the economy of the projects.

Mooring system is part of major costs components in Floating Offshore Wind (FOW). Reducing mooring loads is desirable to create more room for optimization of the mooring configuration. This could lead to opportunity for saving on the capital costs, more environmentally friendly solutions through reducing the mooring footprint, as well as better mooring integrity. The innovative polymer spring component has been introduced in the mooring industry that enables significant reduction on the mooring loads if it is properly designed. In this paper, the applicability of polymer spring component in FOW applications is further investigated through mooring case studies utilizing comprehensive design analysis.

For this purpose, the VolturnUS-S semi-submersible floating platform design is selected. This reference floating offshore wind turbine is developed by the University of Maine – USA to support the International Energy Agency (IEA) 15-MW reference wind turbine.

In total, there are eight individual mooring designs analyzed in this study, which are rooted from two very different base mooring systems. The first base mooring system is a ‘conventional’ steel wire with ground chain semi-taut mooring system. While the second one is based on a ‘more novel’ nylon-based taut mooring system with three different declination angles. For each base mooring configuration above, an alternative mooring configuration is proposed by having the polymer spring as part of the mooring component. Dynamic mooring analysis is performed for both the base and the alternative mooring configurations using the same design load cases, and the results are compared.

The mooring assessment is performed using fully coupled dynamic time-domain analysis tools Orcaflex with the hydrodynamic database of the platform generated by the Orcawave. All the proposed mooring systems are designed in compliance with the applicable industrial design codes for the Ultimate Limit State (ULS), while meeting the target allowable platform offsets. It should be noted that this paper focuses on optimizing the mooring system for the ULS scenario only. It is acknowledged that Fatigue Limit States (FLS) may be governing in some projects. The expansion of this work to FLS, however, is outside the scope of this paper but will be explored in future papers.

Offshore Ulsan, eastern sea of South Korea is selected as the basis for the operational location for all the case studies presented in this paper. Ulsan and its offshore region are being intensively promoted as one of the hubs for the FOW projects. Hence, the results of this mooring study may provide useful input for the developers of these wind farms to further optimize their mooring systems.

The benefits of the polymer spring as a mooring component are demonstrated through the case studies. Promising prospects of combining the polymer spring with synthetic nylon rope for FOW mooring system applications are highlighted. At the end of this paper, conclusions and recommendations for future work are drawn.