S.K. Bhattacharyya

Parameter identification of a compliant nonlinear SDOF system in random ocean waves by reverse MISO method

Abstract The determination of the drag and inertia coefficients, which enter into the wave force model given by Morisons equation, is particularly uncertain and difficult when a linear spectral model is used for ocean waves, and the structure is compliant and has nonlinear dynamic response. In this paper, a nonlinear System Identification method, called Reverse Multiple Inputs–Single Output (R–MISO) is applied to identify the hydrodynamic coefficients as well as the nonlinear stiffness parameter for a compliant single-degree-of-freedom system. Four different types of problems have been identified for use in various situations and the R–MISO has been applied to all of them. One of the problems requires iterative solution strategy to identify the parameters. The method has been found to be efficient in predicting the parameters with reasonable accuracy and has the potential for use in the laboratory experiments on compliant nonlinear offshore systems.

Coupled dynamics of SeaStar mini tension leg platform

Abstract The role of mini tension leg platforms (TLP) in oil exploration and production in marginal deepwater fields is becoming increasingly important. SeaStar is a mini TLP that combines the simplicity of a spar and favourable response features of a TLP. In this paper, the results of a detailed numerical investigation of the coupled dynamic behaviour of SeaStar are reported with special attention to platform-tether coupling. The numerical study has been carried out using a finite element computer code developed for the nonlinear dynamic analysis of compliant offshore platforms in the time domain using Morison type wave loading. A typical SeaStar platform has been considered at two water depths, 215 m and 1000 m. Experimental investigation was conducted for a scaled model corresponding to 215 m water depth for validation of the numerical model. The model-prototype correlation studies have been conducted using the numerical model. Response amplitude operators of motion and tether tension are presented for the two typical water depths and conclusions drawn.

Parameter Identification of a Large Floating Body in Random Ocean Waves by Reverse MISO Method

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the Reverse Multiple Inputs-Single Output (R-MISO) method, to a single-degree-of-freedom system with linear and cubic nonlinear stiffnesses. The system mass is split into a frequency independent and a frequency dependent component and its damping is frequency dependent. This can serve as a model of a moored floating system with a dominant motion associated with the nonlinear stiffness. The wave diffraction force, the excitation to the system, is assumed known. This can either be calculated or obtained from experiments. For numerical illustration, the case of floating semi-ellipsoid is adopted with dominant sway motion. The motion as well as the loading are simulated with and without noise assuming PM spectrum and these results have been analyzed by the R-MISO method, yielding the frequency dependent added mass and radiation damping, linear as well as the nonlinear stiffness coefficients quite satisfactorily. @DOI: 10.1115/1.1493201#

System Identification of a Coupled Two DOF Moored Floating Body in Random Ocean Waves

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily.

Wave propagation in semi-infinite plane anisotropic thin circular shells

Abstract The problem of propagation of elastic waves in semi-infinite thin circular cylindrical shells has been investigated. The focus of the paper is to bring out the effect of a finite end which forms a boundary of the shell. The shell theory used is a linear theory due to Sanders incorporating a plane anisotropic material model. The frequency equation is viewed as an “end problem”, leading to an eigenvalue structure of the system which encompasses both propagating and attenuating modes for a prescribed input excitation frequency at the finite end. The existence of the attenuating modes implies trapping of the part of the energy in the end zone of the shell and thus essentially represents the dynamic counterpart of the well known static Saint Venant effect. The eigenvalue structure is investigated and illustrative numerical examples worked out. The amplitude ratios associated with the various modes of propagation have also been investigated.

ON THE APPLICATION OF SIMILITUDE TO INSTALLATION OPERATIONS OF OFFSHORE STEEL JACKETS

Abstract The role of similitude in scaled simulation of major installation operations of offshore steel jackets, namely, loadout, launching and upending has been studied. The physical modelling can be looked upon both as an adjunct to numerical modelling using computers and also as an independent tool of investigation. The problems of design of models and of experiments are discussed and the prediction equations based on similitude are given.

Damping Characteristics of Heave Plates Attached to Spar Hull

Damping elements are used in ships and offshore structures as response reduction devices for maintaining the hydrodynamic response within acceptable limits. The use of such elements so far has been based on past experience or using empirical based design approach. An investigation of the wave excitation forces acting on these structures with and without damping elements may give an insight into the damping effects of such elements. Also understanding the flow pattern and the magnitude of flow variation around these elements may be useful to improve the design methods. In the present study, measurement of damping forces and response were carried out to establish the relationship between wave frequency amplitude and damping ratio. It was observed that the damping varies with wave period non-linearly and linearly with wave height. Also it was observed that increase in added mass combined with viscous damping is a key factor for the effectiveness of heave plates in reducing motion response of spar platforms. Of the various configurations tested, the spar with double heave plate at the bottom has higher damping compared to other configurations.Copyright

On two solutions of fifth order Stokes waves

Abstract It has been demonstrated numerically that there are two real solutions of the dispersion relation of fifth order Stokes waves given by Skjelbreia and Hendrickson (Fifth order gravity wave theory, Proc. 7th Coastal Engng Conf. , The Hague, 1960, pp. 184–196) which is widely used and recommended in offshore codes of practice. The additional solution predicts a triple crested wave and it always seems to coexist with the conventionally accepted solution. Some properties of this solution are given.

On experimental investigation of load-out, launching and upending of offshore steel jackets

Abstract Load-out, launching and upending are major sequential installation operations of the offshore steel jackets. Due to vital engineering importance of these high risk operations, a thorough scientific understanding of their mechanics is desirable and a physical simulation approach using scaled models is a powerful method to achieve this end. This paper presents such an approach in the context of a case study which is oriented towards gaining adequate understanding of the modelling principles, model design and simulation of operations. It also highlights the role of decisive parameters as they affect the operational performance. Some of the experimental techniques of interest are also presented.

Perforated-ball velocity meter measurements of the water wave kinematics in nonlinear, long and short crested waves

Direct measurement of the wave kinematics in laboratory and field has always occupied a central place in experimental research in water waves. In this paper, a laboratory scale, multi-stage three component Perforated-ball Velocity Meter (PVM) has been used to measure the wave kinematics in linear as well as nonlinear waves, in long crested as well as short crested random waves. Special attention is devoted to measurement of the near surface kinematics, especially in regular nonlinear waves. The laboratory measurements in deep and shallow water wave flumes and wave basin have been conducted using the PVM and the measured kinematics have been compared with the calculated kinematics based on several semi-empirical methods as well as ultrasonic current meter measurements. The comparisons have been found to be excellent at all water depths.