Accidental Limit State Assessment of Cost-Effective Mooring Systems for Floating Offshore Wind

Abstract The mooring system is a major cost component in the Levelized Cost of Energy (LCOE) for Floating Offshore Wind (FOW) farms. To achieve an acceptable LCOE, mooring and anchoring designs must be both fit-for-purpose and cost-effective. Several strategies can be employed to minimize mooring system costs from both procurement and Transport and Installation (T&I) perspectives. These strategies include reducing the number of lines, shortening line lengths, optimizing component sizes, lowering pre-tension, simplifying line configurations, and selecting cost-effective anchoring systems. Although a combination of these approaches is possible, reducing the number of mooring lines is often considered the most attractive option for achieving significant cost reduction. A three-line mooring configuration is highly attractive for reducing mooring system costs, which in turn lowers the LCOE. When combined with a smaller footprint, this configuration can further reduce the LCOE. A three-line taut mooring system for FOW applications is made more feasible by using load reduction devices such as polymer springs. However, a critical question remains: Is the integrity of this three-line mooring configuration adequate from the Accidental Limit State (ALS) perspective? This paper addresses this vital aspect of the design from both theoretical and practical viewpoints, ensuring a comprehensive assessment of its feasibility and safety. The paper incorporates an assumed quick-release mechanism for all three-line options for the power cable connection to the platform, designed to protect it during ALS events. A conservative cost estimate for this component has been included in the procurement costs, based on industry insights. The study begins with a review of the acceptable accidental limit state and redundancy criteria specifically defined for floating offshore wind systems, highlighting why these criteria differ from those utilized in the oil and gas industry. The behavior of three-line mooring systems under ALS conditions will be compared to that of typical six-line mooring systems widely proposed in the current floating offshore wind industry. In total, six distinct mooring system configurations are evaluated in this study through comprehensive case studies. These configurations consist of three types of mooring systems: all-chain catenary, semi-taut polyester, and taut polyester. Each type will be assessed in two different configurations: a three-line setup and a six-line setup. The mooring systems examined in the six case studies are designed to comply with the Ultimate Limit State (ULS) and Fatigue Limit State (FLS) design codes for operational conditions Offshore Ulsan, South Korea. Accidental limit state assessments are conducted to evaluate the integrity of these systems following a single line failure. The costs of restoring these six mooring systems to their original states after such specified accidents are analyzed and compared. The study concludes by comparing the actual costs of three-line and six-line mooring systems considering procurement, transportation and installation (T&I), and ALS recovery costs over the entire lifespan of a 500MW wind farm.