Abstract
Low-frequency (LF) motions of floating structures are commonly modeled as the response of an oscillator to a second-order wave excitation. We present here an empirical method that reliably estimates the oscillators parameters and quadratic transfer function (QTF) used in such models.The method is based on an active stationkeeping system that enables to accurately control external boundary conditions applied on the floating structure in a wave basin. The resulting system can be successively tuned to different frequency ranges of interest. Then, by deconvolution and optimization, LF damping and added-mass loads, as well as a response-independent wave excitation load, can be evaluated. From the wave elevation, and estimated load time series, the difference-frequency QTF is finally estimated by a cross-bi-spectral analysis, including a new treatment of statistical noise.The paper describes the proposed method in details, and illustrates it with the study of a ship-shaped floating unit in a sea-state of relevance for the fatigue design of mooring systems (steep waves, low return period).
Highlights
Large-volume moored structures have eigenfrequencies in the range 2–20 mHz, i.e.well below frequencies contained in the wave spectrum (50–200 mHz)
In the same way as linear wave loads are described by linear transfer functions, the 2nd order LF excitation on the floater is modeled by a difference-frequency Quadratic Transfer Functions (QTF) denoted H(2)
It has been shown earlier that the presented active positioning system is able to replicate tests obtained with a passive soft mooring system (Sauder and Tahchiev, 2020)
Summary
Large-volume moored structures have eigenfrequencies in the range 2–20 mHz, i.e.well below frequencies contained in the wave spectrum (50–200 mHz). In the same way as linear wave loads are described by linear transfer functions, the 2nd order LF excitation on the floater is modeled by a difference-frequency Quadratic Transfer Functions (QTF) denoted H(2) It is a complex function linking the hydrodynamic load to a pair of incoming wave components with complex amplitude and frequency (ai, fi)i={1,2} as follows. QTFs generally include an imaginary part (a phasing) which can be significant in shallow water, and exhibit amplitude variations over the bi-frequency (f1, f2) domain This phasing with respect to the incoming wave is important when it comes to capturing extreme loads in mooring lines, as these occur when large LF offsets (leading to a large ‘‘local’’ stiffness of the mooring system) are combined with large WF excitation
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