Session: 06-08-01 Model Tests - I

Submission Number: 175579

Robust Identification of 1 DOF Hydrodynamics From Free Decay Tests 

Potential flow models are indispensable tools for rapid evaluation and early stage design, yet they routinely misrepresent damping, especially when viscous effects are significant. Inaccurate damping propagates into biased predictions of decay rates, response amplitudes, fatigue loads, and control/stability margins, ultimately affecting design choices and risk assessments. This motivates a data driven approach to recover effective damping with quantified credibility.

We propose a two stage identification procedure for a 1-DOF system. First, an optimization routine fits a nonlinear 1 DOF system with added mass and combined linear and quadratic damping. In the second stage, a local sensitivity analysis is performed around the optimum parameter set to assess parameter uncertainty and diagnose possible ill-conditioning. This analysis is based on the Fisher Information Matrix (FIM), which quantifies how sensitively the model output responds to variations in each parameter. The Singular Value Decomposition of the FIM is then used to evaluate the conditioning of the estimation problem: small singular values indicate parameter correlations or poor identifiability, allowing us to determine whether linear and quadratic damping can be meaningfully separated.

From a study of multiple free decay records, we observe that the identified coefficients are local, effective parameters tied to the specific decay history; there is no universal pair of linear and quadratic damping constants for nonlinear decay. Added mass is consistently well identified, largely through its influence on the resonance period. In contrast, separating linear from quadratic damping is often ill conditioned and data dependent; we therefore suggest selecting free decay records that are representative of the intended sea state, e.g., by matching the decay’s velocity distribution to that of the target sea state before inferring damping parameters.

Presenting Author: Oronzo Dell'edera MOREnergy Lab, Politecnico di Torino

Presenting Author Biography: Graduated in Aerospace Engineering from the Politecnico di Torino, where I also earned a Ph.D. in Mechanical Engineering with a focus on high-fidelity modeling of floating structures. My background is in offshore energy systems, including Wave Energy Converters and Floating Offshore Wind Turbines.

Authors:

Oronzo Dell'edera MOREnergy Lab, Politecnico di Torino
Francesco Niosi MOREnergy Lab, Politecnico di Torino
Filippo Giorcelli MOREnergy Lab, Politecnico di Torino
Bonaventura Tagliafierro Uppsala University
Giovanni Bracco MOREnergy Lab, Politecnico di Torino
Bruno Paduano MOREnergy Lab, Politecnico di Torino