Abstract
This study evaluates the feasibility, accuracy, and limitations of using 1/100-scale physical mooring systems to represent full-scale mooring behavior for wave energy converters (WECs) during small-scale tank testing. The work focuses on a point absorber WEC based on the Reference Model 3 device (Neary et al. 2014) deployed in conditions representative of the PacWave South test site and examines whether small-scale physical testing can reliably inform numerical modeling, design decisions, and future prototype development.
Four mooring configurations were designed at full scale: catenary chain, taut nylon, semitaut nylon, and semitaut polyester. These designs were then scaled using Froude scaling laws for implementation in the National Laboratory of the Rockies’ Sea Wave Environment Lab wave tank. Due to geometric constraints, the mooring layouts were approximated with a two-line planar (fore–aft) system, providing equivalent longitudinal stiffness for unidirectional wave conditions.
Because accurate small-scale mooring representation is inherently challenging, significant effort was dedicated to materials selection and line fabrication. Despite careful construction, line mass densities, diameters, and stiffnesses could only be matched to within approximately 10%–20% of scaled design values, and larger deviations occurred for nylon ropes requiring multiple springs. Additional challenges arose from the observer effect, as load cells and instrument wiring introduced added mass, damping, and unintended forces that influenced WEC motion and anchor loads.
A comprehensive testing campaign included free-decay tests, forced-displacement tests, 81 wave-run tests across three wave conditions, and three unique load cell configurations (freehanging, secured, and no load cell). Post-test load cell calibrations showed minimal calibration drift overall but revealed that one load cell experienced damage late in testing.
Numerical simulations using mid-fidelity numerical models were conducted to better understand the tank testing results. Specifically, the well-known open-source WEC modeling code WECSim coupled to MoorDyn was used.