Experimental investigation of a dual-spar floating offshore wind farm with a shared mooring line: Extended campaign with multiple wave conditions, improved instrumentation, and reduced mooring stiffness

Shared mooring offers potential cost benefits to floating wind farms. However, it also introduces complex dynamic interactions between multiple floating structures and an interconnected mooring subsystem. From a scientific perspective, shared-mooring floating offshore wind turbines constitute strongly coupled multi-physics systems, where aero-hydrodynamic loading, mooring-line nonlinearities, and platform rigid-body dynamics interact across multiple time and length scales. Experimental investigation of such systems remains particularly challenging due to (a) the large number of variables that must be monitored over widespread areas in the laboratory, (b) the difficulty of setting up target configurations and pretensioning models appropriately, and (c) the complexity required to introduce external forcing in the form of waves, wind, and current. This paper presents an experimental study on a dual-spar floating offshore wind farm as a continuation and complement to Lopez-Olocco et al. (2023) and Liang et al. (2024). In the present paper, substantial improvements have been introduced in the experimental setup. Specifically, the use of pulleys to transmit mooring line tension has been eliminated, the tension is now measured at all fairleads, and the motions are measured on both floaters. In addition, the stiffness of the dual-spar system has been reduced by changing the characteristics of the shared line. Finally, the test matrix has been extended to include decay tests exploring coupled surge and sway modes, as well as bichromatic waves. The present and previous configurations are still comparable, but differences are large enough that they require distinct numerical modeling. In fact, the dramatic snap-slack events in the shared line found by Lopez-Olocco et al. (2023) are not found under similar wave conditions in the present reduced stiffness configuration. Also, first-order wave frequency peaks and near-zero troughs in the shared line tension PSDs have been explained here by the dynamics of relative spar motion. Overall, the results presented aim to enhance the understanding of the floaters and mooring line dynamics resulting from shared line configurations while providing benchmark data for researchers and industry. To this latter aim, multimedia of the experiments, MATLAB format figure files with all the curves in the paper, and the OrcaFlex model file with the static setup, are made available in the link https://short.upm.es/ixe17 for general use. • Scarce experimental research on shared mooring lines for floating wind turbines. • Large improvements introduced in the experimental setup wrt previous literature. • Shared line with reduced stiffness wrt previous experimental literature tested. • Dramatic snap-slack events in shared line in previous literature not existing here. • Results suggest that shared mooring can be a practical solution for cost effective mooring.