Simplifying Damping Matrix Calibration in Coupled Offshore Systems Using Parameter Reduction Through Modal Decomposition

One of the challenges in (automatically) calibrating damping in numerical models is the large number of parameters that need to be tuned, especially when calibrating complex floater motions with cross-terms in the damping matrices. Coupled motion behavior is common in systems like floating offshore wind turbines (FOWTs) or side-by-side moored vessels. To overcome this challenge, a method is developed which harvests the intuitive knowledge inherently captured in the modal domain to apply in time domain. Linearization and modal analysis of the initial numerical model provides the relation between the unit body motions and the modes, which allows to apply damping on one mode only, such leading to a significant parameter reduction. Simplified test cases show that the method works for typical nonlinear non-classically damped systems with frequency dependent added mass and damping, even though the relationship between unit body motions and modes is only valid in linear(ized) condition. Successful automatic calibration of the combined heave-pitch mode of an FOWT using both linear and quadratic damping serves as proof of concept.