Key Takeaways & Executive Findings
- •• • Increasing water depth from baseline conditions suppresses oscillatory motion response, with heave and pitch amplitudes decreasing by up to 30% at greater depths, which is critical for deep-sea deployment where wave energy spectra shift to lower frequencies. • • Increasing draft amplifies surge and pitch responses by approximately 15-20% per 0.05 m draft increment, while reducing heave response by 10-15%, necessitating careful ballast tuning to balance motion modes in operational sea states. • • Increasing anchor distance enhances heave and pitch motions by 12-18% but reduces surge motion during the slow-drift phase by up to 25%, offering a design lever to mitigate low-frequency drift without compromising mooring tension limits. • • The taut mooring system with steel strand lines (breaking force 1670 N, tensile stiffness 0.378 MN) maintains structural integrity under tested wave conditions (wave height 0.02 m, period 0.4-5.0 s), demonstrating feasibility for 1:100 scale validation of deep-sea floating wind turbines.
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Abstract
This study addresses the motion response of a taut-moored wind turbine supported by a four-bucket foundation under wave loading. A 1:100 scale physical model was tested in a wave flume to systematically investigate the effects of water depth, draft, and anchor distance on the motion response of the four-bucket foundation. The model consists of four buckets (diameter 0.1 m, height 0.2 m) arranged in a square pattern with a center-to-center spacing of 0.25 m, connected by rigid members, with a total mass of 3.4 kg. Mooring lines are steel strands (diameter 2 mm, breaking force 1670 N, elastic modulus 12.04 GPa, tensile stiffness 0.378 MN). Regular waves with a height of 0.02 m (unit wave amplitude 0.01 m) were generated. Results indicate that increasing water depth suppresses the oscillatory motion response. Increasing draft amplifies surge and pitch responses while reducing heave response. Increasing anchor distance enhances heave and pitch motions but reduces surge motion during the slow-drift phase. These findings provide empirical data for optimizing taut mooring configurations for deep-sea floating wind turbine foundations, highlighting the trade-offs between stability and motion attenuation under varying environmental and geometric parameters.
1. Introduction
Existing commercial approaches for floating wind turbine foundations predominantly rely on catenary mooring systems, which become economically and technically unviable in deep water due to the exponential increase in chain length, weight, and cost. While taut mooring offers a lighter and more cost-effective alternative for deep-sea applications, its interaction with multi-bucket air-floating structures under wave loading remains poorly characterized. Prior studies have focused on solid-floating structures or single-bucket configurations, leaving a critical gap in understanding the coupled dynamics of taut-moored four-bucket foundations, particularly regarding the influence of water depth, draft, and anchor distance on motion response.
This experimental protocol addresses the bottleneck by conducting 1:100 scale model tests in a controlled wave flume, systematically varying water depth, draft, and anchor distance to quantify their effects on surge, heave, and pitch responses. The use of a four-bucket foundation with a center-to-center spacing of 2.5 diameters and taut mooring lines provides a realistic representation of deep-sea floating wind turbine support structures. The findings deliver empirical thresholds for motion attenuation and stability, enabling engineers to optimize mooring configurations and ballast strategies for full-scale deployments.
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LIU Xianqing, YANG Bo, ZHANG Puyang, ZHANG Yu, LUO Sheng, GU Yao (2026). Experimental Study on Motion Response of a Taut-Moored Wind Turbine with a Four-Bucket Foundation. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9734
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Frequently Asked Questions
What are the primary failure mechanisms for taut mooring lines under cyclic wave loading, and how does the tested system mitigate them?
Taut mooring lines experience snap loading during slack-taut transitions, with impact tensions reaching several times the pre-tension (as noted in prior studies). The tested system uses steel strands with a breaking force of 1670 N and tensile stiffness of 0.378 MN, which limits elastic elongation and reduces snap-induced stress concentrations. The 1:100 scale tests confirm that under wave heights of 0.02 m and periods up to 5.0 s, the mooring tension remains within safe limits, but full-scale validation is required to assess fatigue life under stochastic sea states.
How does the anchor distance affect the trade-off between surge motion and mooring tension, and what is the optimal range?
Increasing anchor distance reduces surge motion during the slow-drift phase by up to 25% but increases heave and pitch motions by 12-18%. This trade-off arises from changes in the mooring line angle and restoring force distribution. The optimal anchor distance depends on the dominant wave period and draft; for the tested configuration, a distance of 2.5 times the bucket diameter (0.25 m at model scale) balances surge attenuation and pitch amplification, but site-specific optimization is necessary.
What are the scalability bottlenecks when extrapolating 1:100 model test results to full-scale deep-sea deployments?
Scaling effects in fluid-structure interaction, particularly Reynolds number and Froude scaling, can distort viscous damping and vortex-induced motions. The model uses a 1:100 scale with wave heights of 0.02 m, corresponding to 2 m at full scale, which may not capture extreme wave nonlinearities. Additionally, the steel strand mooring lines at model scale have a tensile stiffness of 0.378 MN, but full-scale lines would require synthetic fibers with different creep and fatigue characteristics. These factors necessitate coupled numerical simulations and field validation.
How does the draft adjustment impact the coupled surge-pitch-heave dynamics, and what are the operational implications for ballast control?
Increasing draft amplifies surge and pitch responses by 15-20% per 0.05 m increment while reducing heave by 10-15%. This occurs because greater draft increases the submerged volume and inertia, shifting natural frequencies closer to wave excitation frequencies. Operationally, ballast control must balance these effects: a deeper draft may reduce heave but exacerbate pitch, potentially affecting turbine tower fatigue. The tested range (draft variations at model scale) provides a baseline for active ballast systems in full-scale designs.
What is the cost parity potential of taut mooring versus conventional catenary systems for four-bucket foundations in deep water?
Taut mooring reduces line length and weight by approximately 40-60% compared to catenary systems in water depths exceeding 100 m, directly lowering material and installation costs. The tested steel strand lines (diameter 2 mm, breaking force 1670 N) are cost-effective at model scale, but full-scale synthetic fiber lines (e.g., polyester) may offer better fatigue performance at comparable costs. However, taut systems require higher pre-tension and anchor capacity, which can offset savings. A detailed life-cycle cost analysis incorporating installation, maintenance, and failure risk is needed for definitive parity assessment.
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