The Effect of Mooring Line Parameters in Inducing Parametric Resonance on the Spar-Buoy Oscillating Water Column Wave Energy Converter

Abstract

Although it is widely accepted that accurate modeling of wave energy converters is essential for effective and reliable design, it is often challenging to define an accurate model which is also fast enough to investigate the design space or to perform extensive sensitivity analysis. In fact, the required accuracy is usually brought by the inclusion of nonlinearities, which are often time-consuming to compute. This paper provides a computationally efficient meshless nonlinear Froude–Krylov model, including nonlinear kinematics and an integral formulation of drag forces in six degrees of freedom, which computes almost in real-time. Moreover, a mooring system model with three lines is included, with each line comprising of an anchor, a jumper, and a clump weight. The mathematical model is used to investigate the highly-nonlinear phenomenon of parametric resonance, which has particularly detrimental effects on the energy conversion performance of the spar-buoy oscillating water column (OWC) device. Furthermore, the sensitivity on changes to jumper and clump-weight masses are discussed. It is found that mean drift and peak loads increase with decreasing line pre-tension, eventually leading to a reduction of the operational region. On the other hand, the line pre-tension does not affect power production efficiency, nor is it able to avoid or significantly limit the severity of parametric instability.

Numerical investigation of parametric resonance due to hydrodynamic coupling in a realistic wave energy converter

“…Relying on the fact that for this system each line has a relatively high tension when compared with their mass, it is possible to model each mooring line segment as a rigid and inelastic line. Consequently, for each line, two equations are written for the vertical and horizontal force equilibrium, one for the torque balance, and two for imposing geometrical constraints [16].…”

A Real-Time Detection System for the Onset of Parametric Resonance in Wave Energy Converters

“…The nonlinear hydrodynamic model calculates the Froude-Krylov force on the exact wetted surface of the WEC at each time step, rather than using the time-invariant mean wetted surface (as is the case for linear hydrodynamic models). This type of nonlinear Froude-Kylov force model has subsequently been used in [29][30][31][32][33][34] to investigate parametric resonance in WECs. Other nonlinear hydrodynamic modelling approaches include only modelling the restoring force nonlinearly, rather than the entire Froude-Krylov force [35][36][37][38][39][40].…”

“…The device in this study is a cylindrical spar-buoy, providing a generic representation of a heaving point absorber WEC. The selection of this type of WEC stems from the numerous reports of parametric pitch/roll for axisymmetric heaving point absorbers [17][18][19][20][29][30][31][32][33][34]39,41,45,46,[48][49][50]. The particular geometry used in this study is a representation of a classic spar platform from the literature [62][63][64][65], comprising a simple vertical cylinder whose natural period of pitch motion is about twice the heave resonant period.…”

Experimental Investigation of the Mooring System of a Wave Energy Converter in Operating and Extreme Wave Conditions

“…The design phase and survivability analyses of a WEC crucially require accurate numerical modeling, in synergy with the experimental investigation of the hydrodynamic loads acting on the device [8]. In [9], the experimental results of a 1:20 model scale of oscillating water column (OWC), installed as a prototype in Galway Bay, are reported.…”

“…Although slack mooring systems with submerged buoys and clump weights are common in the traditional ocean engineering field, wave energy applications require a different design in order to withstand extreme loads while guaranteeing efficient operation and power absorption. Slack moorings for wave energy applications are mainly used for axisymmetric buoys working as point absorbers, extracting energy from the heave motion, such as [8,14]. However, thanks to their geometrical symmetry and working principle, such devices are insensitive to the wave direction.…”