Quantification of Nonlinear Fluid/Structure Interactions via Higher-Order Statistics

In-Seung Park & Hyungsuk Yoo (PhD students), Sapna Mehta (MS student), Tiffany E. Monroe (UG student)

1) Modeling and prediction of nonlinear fluid/structure interactions from random excitation-response data.
2) Reduction of the amount of raw experimental data to quantify fluid/structure interactions
3) Detection of short-duration phase-coupling between first- and second-order random wave components resulting in large amplitude waves.
4) Recovery of second-order force spectra from wave-excitation motion-response model basin test data.

Directly supports modeling efforts related to: nonlinear analysis; platform and riser responses; waves, wind, and current;and theme structure experiments.

The approach rests upon the following:
Third-order (i.e., cubic) Volterra models. Advances in higher-order statistical signal processing (developed largely outside offshore technology) to determine Volterra kernels up to third-order. Orthogonal search techniques to generate both sparse and orthogonal models. Experimental data bases generated at the OTRC model basin. The modeling capabilities developed in this project will provide a means of linking the theoretical, numerical, and experimental work being carried out within the fluid/structures thrust area. In addition, higher-order wavelet transform techniques are used to detect evidence of short-term phase-locking of first- and second-order components in laboratory-generated irregular seas. Utilization of higher-order statistical processing to recover first- and second-order force spectra from model basin test data.

Cross-Bispectral Analysis Applications (Stransberg, Trondheim, Norway)
Second-order effects (Naess, Trondheim, Norway)
Second-order effects in irregular waves (Stansberg, Trondheim, Norway)
Skewness of random surface waves (Winterstein, Stanford, USA)

Unique features include: Third-order (i.e., cubic) modeling capability, Valid for nonGaussian random excitation, Sparse models requiring less raw experimental data, Applicable to both experimental and numerical data. Use of wavelet-based (as opposed to Fourier based) higher-order "spectra" to detect momentary phase-locking. Recovery of second-order force spectra from model basin test data, thereby allowing comparison with second-order force spectra determined from diffraction codes.

October 1994 to September 1997

Improved understanding and capability to: Identify and quantify nonlinear (up to third-order) fluid/structure interactions; Model such interactions, and predict nonlinear responses based on such models; Quantify nonlinear spectral transfer of energy from various frequencies in the excitation to other frequencies in the response; Generate orthogonal models to facilitate decomposition of an observed response power spectrum into its constituent linear, quadratic, and cubic components; Generate sparse models to reduce the amount of raw data, and hence financial costs, required to quantify fluid/structure interactions. New techniques to quantify nonlinear wave interactions in laboratory-generated irregular seas. This cross-disciplinary project is unique in that we are transferring to offshore technology state-of-the-art nonlinear system modeling and prediction capabilities, which were developed principally outside the offshore technology sector.