Investigation on the influence of mooring material and configuration on energy conversion efficiency and extreme mooring tension in a novel multi-body floating-point absorber

Multi-body floating-point absorbers (FPAs) are promising wave energy converters, yet their performance and safety depend critically on the nonlinear dynamic stiffness of synthetic-rope moorings. This study integrates a static–dynamic stiffness formulation into a fully coupled time-domain potential-flow framework to evaluate hydrodynamic performance and mooring loads for a novel multi-body FPA. Two three-segment hybrid mooring systems—catenary and taut—are designed, each with three pre-tension levels (catenary: 1%, 2%, 4% MBL; taut: 10%, 20%, 30% MBL) and three middle-segment rope materials (nylon, polyester, HMPE), yielding 18 configurations (two mooring systems × three pretension levels × three intermediate-section materials). Empirical stiffness relations parameterized by mean tension and tension amplitude are solved iteratively, with an enhanced power-spectral-density procedure used to estimate tension amplitude in irregular seas. Performance is quantified by capture width ratio and mooring tensions under regular waves and irregular operational and survival seas, and water-depth effects are examined at 60, 80, and 100 m. Short-term extreme tensions are predicted using the Average Conditional Exceedance Rate (ACER) interpolation method with multiple random seeds and validated against a Gumbel fit. Results indicate that under three water depths, nylon middle segments provide higher capture width ratios and lower tensions than polyester and HMPE; catenary systems outperform taut systems in energy capture at comparable pre-tension; increased pre-tension systematically raises tensions; and shallow water (60 m) produces larger tensions than 80–100 m due to shorter suspended segments. For the studied device, ACER-based extremes agree with Gumbel estimates and remain below design limits, and a catenary mooring with 1% MBL pre-tension and a nylon middle segment emerges as the preferred configuration for balancing energy capture and safety.