The design of wave energy converters (WECs) has often been based on the principles of linear dynamics. Following from naval architecture practices for ships and offshore structures, reduced order models based on potential flow have been utilized as a primary design tool. While many researchers have acknowledged the potential importance of nonlinearities to the WEC dynamics, the usage of nonlinear models remains mostly relegated to consideration of survival conditions. Nonetheless, there remains substantial interest in the importance of nonlinear dynamics especially when considering power production and control of WECs.
In this study, we examine the dynamics of a heaving point absorber whose diagram is shown in Figure 1. Tests were conducted in the US Navy’s Maneuvering and Sea Keeping (MASK) basin. Previous work has focused on linear modeling of this device. Test data obtained by model-scale wave tank tests was exploited and a series of tests in which the WEC was deballasted to shift the waterline into a conical section of the WEC’s hull (“Waterline B” in Figure 1) were considered. In this configuration, the WEC’s dynamics should exhibit a high degree of nonlinearity. In this paper, WEC models obtained by linear and nonlinear system identification (SID) are compared and analyzed. The obtained SID results show that, contrary to the prevailing belief in the literature, nonlinear models provided only marginally better performance than linear ones, which indicates that linear models can still provide many benefits as powerful tools for controller design and state estimation.