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Shira L Broschat - One of the best experts on this subject based on the ideXlab platform.

  • a systematic study of the lowest order small slope approximation for a pierson moskowitz Spectrum
    IEEE Geoscience and Remote Sensing Letters, 2011
    Co-Authors: Yanqiu Wang, Shira L Broschat
    Abstract:

    In an earlier work, a model for calculating scattering from rough surfaces at very low grazing angles was presented. The utility of this model depends on the lowest order small slope approximation (SSA1). In order to ascertain when this new model, called the SSA+, is applicable, we present a systematic study of the accuracy of the SSA1 bistatic scattering cross section for 1-D surfaces for a Pierson-Moskowitz roughness Spectrum and the Dirichlet boundary condition. Different frequencies, angles of incidence, and wind speeds are combined to examine 112 different cases. The ranges of scattering angles for which the SSA1 is accurate are given, and an analysis of when the SSA+ can be used is presented. All data and plots are available online.

  • numerical studies of the small slope approximation for rough surface scattering using a pierson moskowitz Spectrum
    Journal of the Acoustical Society of America, 1995
    Co-Authors: Shira L Broschat, Eric I Thorsos
    Abstract:

    The small slope approximation (SSA), introduced by Voronovich in the mid‐1980s [A. G. Voronovich, Sov. Phys. JETP 62, 65–70 (1985)], has been shown to be a very promising method for modeling wave scattering from rough surfaces. The theory gives a systematic series which is manifestly reciprocal at each order, reduction to perturbation theory is intrinsic to the formulation, and reduction to the Kirchoff approximation occurs under the appropriate conditions when the first two terms of the series are retained [E. I. Thorsos and S. L. Broschat, 2082–2093 (1995)]. In this paper numerical results are presented for scattering strengths for one‐dimensional, pressure release surfaces satisfying a Pierson–Moskowitz power law Spectrum. Results are given for incident angles varying from grazing to normal over the full angular range of scattering. Comparisons with Monte Carlo integral equation results show that the SSA is extremely accurate over a large range of scattering angles including low forward grazing angles.

Yanqiu Wang - One of the best experts on this subject based on the ideXlab platform.

  • a systematic study of the lowest order small slope approximation for a pierson moskowitz Spectrum
    IEEE Geoscience and Remote Sensing Letters, 2011
    Co-Authors: Yanqiu Wang, Shira L Broschat
    Abstract:

    In an earlier work, a model for calculating scattering from rough surfaces at very low grazing angles was presented. The utility of this model depends on the lowest order small slope approximation (SSA1). In order to ascertain when this new model, called the SSA+, is applicable, we present a systematic study of the accuracy of the SSA1 bistatic scattering cross section for 1-D surfaces for a Pierson-Moskowitz roughness Spectrum and the Dirichlet boundary condition. Different frequencies, angles of incidence, and wind speeds are combined to examine 112 different cases. The ranges of scattering angles for which the SSA1 is accurate are given, and an analysis of when the SSA+ can be used is presented. All data and plots are available online.

Khalid Moin - One of the best experts on this subject based on the ideXlab platform.

  • OMAE2009-79347 RELIABILITY ANALYSIS OF COMPLIANT OFFSHORE TOWER UNDER EARTHQUAKE LOADS
    2020
    Co-Authors: Danish Syed, Nazrul Hasan, Khalid Moin
    Abstract:

    ABSTRACT Articulated offshore tower with universal joints in the intermediate level leads to a multi-hinged configuration that can be used for a variety of deep water application. They are flexibly linked to the sea-bed by a universal joint and comply with the oscillatory environmental loads causing large fluctuating seismic demands at the articulating joints. This paper investigates the dynamic response and the reliability assessment of articulated joint (s) of such structures under seismic sea environment. The analysis includes the influence of sea bed shaking on the water-particle kinematics by using Californian earthquakes. The sea state is characterized by DNV version of Pierson Moskowitz Spectrum. The dynamic equation of motion is derived using Lagrangian approach, taking into the account the nonlinearities associated with structure and loads. A limit-state function for seismic demand for a universal joint has been derived. Using the derived limit-state function and the responses obtained after time-domain seismic analysis, reliability assessment of the articulated joint has been carried out, using efficient MPPbased probabilistic methods. Design point, important for probabilistic design of articulated joint, located on the failure surface has been worked out. Stochastic sensitivity analysis has been performed to assess the relative importance of design parameter on the stochastic response of articulated joint

  • reliability analysis of compliant offshore tower under earthquake loads
    ASME 2009 28th International Conference on Ocean Offshore and Arctic Engineering, 2009
    Co-Authors: Syed Danish Hasan, Nazrul Islam, Khalid Moin
    Abstract:

    Articulated offshore tower with universal joints in the intermediate level leads to a multi-hinged configuration that can be used for a variety of deep water application. They are flexibly linked to the sea-bed by a universal joint and comply with the oscillatory environmental loads causing large fluctuating seismic demands at the articulating joints. This paper investigates the dynamic response and the reliability assessment of articulated joint (s) of such structures under seismic sea environment. The analysis includes the influence of sea bed shaking on the water-particle kinematics by using Californian earthquakes. The sea state is characterized by DNV version of Pierson Moskowitz Spectrum. The dynamic equation of motion is derived using Lagrangian approach, taking into the account the nonlinearities associated with structure and loads. A limit-state function for seismic demand for a universal joint has been derived. Using the derived limit-state function and the responses obtained after time-domain seismic analysis, reliability assessment of the articulated joint has been carried out, using efficient MPP-based probabilistic methods. Design point, important for probabilistic design of articulated joint, located on the failure surface has been worked out. Stochastic sensitivity analysis has been performed to assess the relative importance of design parameter on the stochastic response of articulated joint.Copyright © 2009 by ASME

P H Taylor - One of the best experts on this subject based on the ideXlab platform.

  • local and far field surface elevation around a vertical cylinder in unidirectional steep wave groups
    Ocean Engineering, 2004
    Co-Authors: Eugeny Buldakov, Eatock R Taylor, P H Taylor
    Abstract:

    Abstract The problem of diffraction of a unidirectional incident wave group by a bottom-seated cylinder is considered. We assume the amplitude of the incoming wave to be small in comparison with other linear scales of the problem, and develop the corresponding second-order perturbation theory. We use the Fourier transform to treat time variation and separate spatial variables when solving the non-homogeneous second-order problem. The resulting set of non-homogeneous Bessel equations is solved numerically. Solutions for various types of incoming wave Spectrum are obtained including the Gaussian Spectrum and the Pierson–Moskowitz Spectrum. To validate the method, problems with gradually decreasing bandwidth of Gaussian Spectrum are solved and it is shown that the corresponding solution approaches that for the monochromatic case. The Pierson–Moskowitz Spectrum with a set of realistic physical parameters is used as an example of extreme wave interaction with an offshore structure. The corresponding first- and second-order solutions are obtained and the effect of non-linearity on the solution is discussed with the emphasis on the growth of maximum free-surface elevation on the cylinder’s surface and generation of high frequency free radiated waves.

  • Local and far-field surface elevation around a vertical cylinder in unidirectional steep wave groups
    'Elsevier BV', 2004
    Co-Authors: Eugeny Buldakov, Eatock Taylor R, P H Taylor
    Abstract:

    The problem of diffraction of a unidirectional incident wave group by a bottom-seated cylinder is considered. We assume the amplitude of the incoming wave to be small in comparison with other linear scales of the problem, and develop the corresponding second-order perturbation theory. We use the Fourier transform to treat time variation and separate spatial variables when solving the non-homogeneous second-order problem. The resulting set of non-homogeneous Bessel equations is solved numerically. Solutions for various types of incoming wave Spectrum are obtained including the Gaussian Spectrum and the Pierson-Moskowitz Spectrum. To validate the method, problems with gradually decreasing bandwidth of Gaussian Spectrum are solved and it is shown that the corresponding solution approaches that for the monochromatic case. The Pierson-Moskowitz Spectrum with a set of realistic physical parameters is used as an example of extreme wave interaction with an offshore structure. The corresponding first- and second-order solutions are obtained and the effect of non-linearity on the solution is discussed with the emphasis on the growth of maximum free-surface elevation on the cylinder's surface and generation of high frequency free radiated waves. © 2003 Elsevier Ltd. All rights reserved

Nor Hafizah Ramli Sulong - One of the best experts on this subject based on the ideXlab platform.

  • stochastic response of intact and a removed tendon tension leg platform to random wave and current forces
    Arabian Journal for Science and Engineering, 2017
    Co-Authors: D. O. Oyejobi, Mohammed Jameel, Nor Hafizah Ramli Sulong
    Abstract:

    This work analysed the dynamic response of intact and a removed tendon tension leg platform (TLP) in simultaneous action of random wave and current loads in normal and less severe environments. Two different sea states of extreme severe sea state but less probable and less severe state and most probable are considered in this study. The artificial random wave is simulated by Monte Carlo simulation using Pierson–Moskowitz Spectrum. Wave diffraction effect is not considered since the ratio of characteristic dimension to wavelength is less than limit. The coupling in all degree of freedoms and various degrees of nonlinear effects is considered. The inertia, mass, damping matrices of equation of motion and hydrodynamic force vector are formulated and solved numerically using Newmark integration scheme. The statistical results show that removal of one tendon increases surge and tendon tension values, while heave and pitch are not adversely affected in both environments. The response values of the TLP in extreme severe sea state are quite higher compared to less severe sea state. The percentage increase in all degree of freedoms and tendon tension is <5 % when one tendon is removed as compared to intact tendon TLP. It could be concluded that maximum and minimum tension in tendon constraints is equally passed by the TLP tendons.