Estimating nonlinear coupled frequency-dependent parameters in offshore engineering
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The design of deepwater compliant offshore structures requires engineers to address many difficult challenges including defining and modeling the local offshore environment, specifying the associated combined global loading on innovative platform designs, and numerical simulation and model test verification of the platform response characteristics. The focus of this research investigation is the recovery of key parameters from time series measured during an industry type model basin test program using the reverse multiple input/single output technique. In particular, this study confronts critical problems of practical interest and extends the methodology to address the inclusion of nonlinear coupled systems in which the parameters of interest can be frequency-dependent. The analysis is developed around the nonlinear coupled equations of motions for a deepwater mini-TLP design and includes the consideration of nonlinear stiffness, quadratic damping, surge/pitch and sway/roll coupling and the frequency dependency of both the hydrodynamic added-mass and damping coefficients. A series of complementary model test measurements for the complete compliant model and the rigidly restrained hull by itself were used as the basis for the data in the system identification procedures. In addition, behavior of the hydrodynamic added-mass and damping coefficients as a function of frequency was simulated for the mini-TLP using an industry standard radiation-diffraction software package. These results were used in evaluating the accuracy of some of the key problem parameters. The results presented demonstrate the methodology as modified in this study is quite robust and yields predications that are more accurate for the parameters associated with the largest motions of the platform. Practical issues regarding the application of this approach, utilization of both force and moment measurements, and observed strengths and weakness in dealing with data regardless of its source are discussed. 2003 Elsevier Ltd. All rights reserved.