Sau-Lon James Hu

H2 active vibration control for offshore platform subjected to wave loading

Abstract Offshore platforms are usually located in hostile environments. These platforms undergo excessive vibrations due to wave loads for both normal operating and extreme conditions. To ensure safety, the displacements of the platforms need to be limited, whereas for the comfort of people who work at the structures, accelerations also need to be restricted. This article is devoted to developing a proper procedure on applying H2 control algorithm for controlling the lateral vibration of a jacket-type offshore platform by using an active mass damper. In comparison with earlier studies, a number of improvements in problem formulation, wave force filter design, and control algorithm implementation are made. The present paper also numerically demonstrated the effectiveness of H2 active control. As expected, it significantly outperforms the corresponding passive control that uses a tuned mass damper.

A systematic linear space approach to solving partially described inverse eigenvalue problems

Most applications of the inverse eigenvalue problem (IEP), which concerns the reconstruction of a matrix from prescribed spectral data, are associated with special classes of structured matrices. Solving the IEP requires one to satisfy both the spectral constraint and the structural constraint. If the spectral constraint consists of only one or few prescribed eigenpairs, this kind of inverse problem has been referred to as the partially described inverse eigenvalue problem (PDIEP). This paper develops an efficient, general and systematic approach to solve the PDIEP. Basically, the approach, applicable to various structured matrices, converts the PDIEP into an ordinary inverse problem that is formulated as a set of simultaneous linear equations. While solving simultaneous linear equations for model parameters, the singular value decomposition method is applied. Because of the conversion to an ordinary inverse problem, other constraints associated with the model parameters can be easily incorporated into the solution procedure. The detailed derivation and numerical examples to implement the newly developed approach to symmetric Toeplitz and quadratic pencil (including mass, damping and stiffness matrices of a linear dynamic system) PDIEPs are presented. Excellent numerical results for both kinds of problem are achieved under the situations that have either unique or infinitely many solutions.

Pole-Residue Method for Numerical Dynamic Analysis

AbstractSystems of second-order linear ordinary differential equations (ODEs) to arbitrary input functions appear in many fields of physics and engineering. Numerical methods for solving these kinds of problems have been mainly performed in the time and frequency domains. In contrast, this paper develops an efficient, seminumerical pole-residue method implemented in the Laplace domain. In this article, a key concept and development is on how to compute the poles and residues of the output from those of the input and system transfer functions. Once the poles and residues of the output are known, the corresponding time history of the output is readily obtained. Even though the correctness of the new method has been verified by using a step-by-step time-domain solution, the accuracy of the new method in theory is higher than that of any time-domain approach, partially because the output obtained from the new method is a continuous function of time.

Cross Modal Strain Energy Method for Damage Localization and Severity Estimation

A cross modal strain energy (CMSE) damage diagnosis method, which is capable of localizing the damages and estimating their severities, is presented. The numerical study uses measurements synthesized from a finite element model of a laboratory-scaled offshore platform. Several damage scenarios are investigated, including: single/double damaged element(s), with/without measurement noise. Numerical results suggest that good performance on both damage location and severity assessment can be achieved by implementing the CMSE method.Copyright

PROBABILISTIC WAVE SPECTRUM AND FATIGUE ESTIMATION

This study introduces a new way to characterize a long-term random ocean wave by using a ‘probabilistic’ wave spectrum. Based on this probabilistic wave spectrum, an efficient and accurate method for estimating the fatigue damage of offshore structures is developed. A numerical example involving 8136 archived wave spectra measured at the Gulf of Mexico is provided to demonstrate the use, and to show the accuracy, of the proposed method.

Kalman-based underwater tracking with unknown effective sound velocity

In underwater tracking based on range measurements, the transit time between a pinger and a receiver is often measured, then it is converted into a slant range by multiplying an effective sound velocity (ESV). Clearly, the accuracy of the calculated slant range depends largely on the accuracy of the ESV. Even with a small percentage ESV error, the resulting range measurement can have significant error particularly when the transit time is large. The ESV between a pinger and a receiver certainly is location dependent, and is very difficult to determine accurately because the ESV would be affected by many factors, such as water temperature, pressure and salinity. Even with a very good knowledge of the sound velocity profile (SVP), the calculated ESV based on the ray tracing theory, or an equivalent, might still involve non-negligible error. However, a common assumption adopted by many published papers related to Kalman-based underwater tracking is that the ESV is not only a known quantity, but also location independent. This paper proposes a Kalman-based underwater tracking model that treats the location-dependent ESV as a state variable. Instead of using the calculated range as measurement, it employs the transient time directly as measurement. Both simulation and filed data have been used to demonstrate the efficiency and superiority of the proposed method. It has been shown that while using the proposed model and approach, the tracking accuracy can be significantly improved and the estimated ESV agrees well with its true value.

Effect on Kalman based underwater tracking due to ocean current uncertainty

Although it was recognized that an unmanned underwater vehicle (UUV) in reality might experience changing ocean current along its mission trajectory, most published papers related to the single beacon underwater navigation assumed a constant ocean current over the whole duration. When this assumption is against the reality, those underwater tracking models not only will fail to adequately describe the change of ocean current over the duration, but also may introduce large error to their position estimation. In this paper, by including adequate ocean current uncertainty in the process model at each discrete time, the estimated ocean current can exhibit time-varying and location-dependent characteristics. The effect on the accuracy of underwater tracking due to the inclusion of the ocean current uncertainty is investigated through a kinematic model that treats the unknown effective sound velocity (ESV) as a state variable. In addition to the Kalman filter, the performance of the corresponding Rauch-Tung-Striebel (RTS) smoother is also studied. Both simulation and filed data are used to study the effect on the accuracy of underwater tracking.

Computing Transient Response of Dynamic Systems in the Frequency Domain

AbstractFrequency-domain methods are usually more efficient computationally than time-domain methods to compute the responses of linear dynamic systems. However, a common drawback of frequency-doma...

Model and Algorithm Improvement on Single Beacon Underwater Tracking

In range-based underwater tracking, the accuracy of the calculated slant range depends largely on the accuracy of the effective sound velocity (ESV). The ESV between a pinger and a receiver certainly is location dependent, and is very difficult to determine accurately. However, a common assumption adopted by many published papers related to single beacon underwater tracking is that the ESV is not only a known quantity, but also location independent. This paper proposes a novel Kalman-based single beacon tracking model that treats the ESV as a state variable. For applying this novel model, a solution method by treating transit time directly as measurement is developed. Traditional Kalman filters for single beacon underwater tracking often update state variables at a uniform rate. However, because the time of arrival measurement arrives at irregular time intervals, implementing a Kalman-based filter for updating state variables should carry out at nonuniform sampling rates. In addition to the realtime filter tracking, this paper also implements the Rauch–Tung–Striebel (RTS) smoother for postprocessing (reconstruction). This paper proposes a hybrid timing algorithm that can achieve both accuracy in filtering and efficiency in smoothing. Through numerical examples of using simulated and field data, both the extended Kalman filter and RTS smoother results show that while implementing the proposed model and algorithm, the tracking accuracy can be significantly improved and the estimated ESV agrees well with its true value.

A New System Identification Method Operated in the Pole Domain

A recent article [1] showed that the poles and residues of responses could be easily obtained from those of the loadings and system functions. Following the same theoretical principle, a new system identification method operated in the pole domain has been developed and tested in this article. In this method, the poles and residues associated with the input signals and output signals were first extracted by using the multi-signal Prony-SS method. Sequentially, the system poles were simply the output poles minus the input poles. The remaining work was to compute each corresponding system residue, which in theory is equal to the output residue (at the system pole location) divided by a quantity obtained from the input function calculated at the system pole location in the Laplace domain. In the numerical study, a 5-DOF shear building model was chosen to demonstrate the effectiveness of the proposed method. Two kinds of loading were investigated: (1) a sinusoidal decay signal, and (2) a signal composed of two sinusoidal components. From both scenarios, one concluded that the proposed method could accurately identify the system poles and residues, and thus the corresponding system transfer function.