Scale model behaviour and float geometry influence on an oscillating water column system

Abstract

A numerical model based on a 1:60 scale Lazy-S Mooring Line has been developed using the dynamic analysis software OrcaFlex, to determine the software’s capabilities when analysing discretized scale models, and to quantify the effect that varying float surface geometry has on mooring line system damping. Static analysis was performed, followed by dynamic analysis, based on experimental data from physical testing in the University of Plymouth 35m Sediment Flume tank, against two distinct float shapes with equivalent hydrostatic properties but aspect ratio variance perpendicular to fluid flow. The impact of scale modelling on static line forces, effective tension and driving frequency were evaluated as a comparative base between the experimental and numerical findings, and the intricacies of scale modelling evaluated in the context of the OrcaFlex software. Evaluation of Static Line Forces (SLF) finds that systems comprised of taut lines at scale can misinterpret line tension as constant and may not exhibit expected static and dynamic tension behaviour, theorized as due to the nodal method OrcaFlex adopts for calculating line tension. Dynamic analysis of driving frequency amplitude suggests an agreement of behaviour between data sets – i.e, a reduced surface area results in a greater driving frequency of tension loading, particularly in the Heave direction of Oscillating Water Column (OWC) motion – based on the original defined float types and a proposed plate of 30% greater drag area. Drag area was determined to have a reduced influence at higher frequencies due to motion lag within the system such that, as OWC motion scales past a certain threshold, optimisation of float geometry will yield diminishing returns. Consequently, evaluation of results suggests that numerical scale modelling in OrcaFlex is an appropriate method for modelling behaviour as a function of changing hydrostatic properties, but that consideration must be made when modelling taut scale systems to limit potential numerical discrepancies.