Session: 09-02-04 Wind Energy: Moorings II

Submission Number: 156277

Experimental Comparison of Constant Tension Mooring and Catenary Mooring System for a Semi-Submersible Floating Wind Turbine 

The semi-submersible floating wind turbine has garnered increasingly interest in engineering applications due to its remarkable stability and adaptability across various water depths. The mooring system is a crucial component of the semi-submersible floating wind turbine, and its reliability, footprint and cost are vital for the large-scale deployment. Nonetheless, designing mooring systems for semi-submersible floating wind turbine in regions with water depths less than 100 meters remains challenging. Catenary mooring design necessitates the use of excessively long and heavy mooring lines, with clump weights installed on the touchdown segment to ensure adequate restoring force. Taut and tension leg mooring designs face challenges with peak load and fatigue damage to mooring lines. An innovative alternative, constant tension mooring, offers the advantages of a reduced footprint and lower cost, is well-suited for semi-submersible floating wind turbine. This design dissipates kinetic energy by altering the physical length of the mooring line between the anchor and the floating foundation, thereby addressing the peak load problem while maintaining a compact mooring arrangement.

To explore the dynamic characteristics of constant tension mooring, a specially designed constant tension mooring system featuring suspended counterweights for a 16MW semi-submersible floating wind turbine is tested and compared with the traditional catenary design. The constant tension mooring, engineered for equivalent restoring stiffness, incorporates multiple suspended counterweights, mooring lines, supports, and pulleys. The design achieves a 70% reduction in total mooring line length and an 80% reduction in mooring radius compared to the catenary mooring, thereby offering potential cost savings. Comparative model tests of the constant tension and catenary designs are conducted under rated, cutout, and extreme sea states, taking into account the turbulent wind, wave and current loads under actual operational conditions. The results indicate that the constant tension design effectively mitigates platform motion and nacelle acceleration at low frequencies, such as wind load frequency. Moreover, it exhibits a reduced motion response near the natural period compared to the catenary system, primarily due to the substantial damping contribution from the counterweights, supports and pulleys. Finally, the survivability of the constant tension mooring design is evaluated through an analysis of response under a 50-year extreme sea state, assessing platform tilt, horizontal drift, nacelle acceleration, and mooring loads to ensure compliance with design requirements.

Presenting Author: Yipin Wang Shanghai Jiao Tong University

Presenting Author Biography: Yipin Wang
PhD Candidate of Shanghai Jiao Tong University
Research Interests: Offshore floating wind turbine; Structural vibration control; Tension leg platform