Hari B. Kanegaonkar

Non-Gaussian stochastic response of nonlinear compliant platforms

Abstract A moment function method is presented to estimate the stochastic response of compliant offshore platforms with nonlinearity in stiffness based on non-Gaussian closure. For guyed towers with clump weight, the nonlinearity in stiffness is of the softening type. The random wave loading is expressed in terms of a rational spectrum, making the system Markovian. Using Itos rule for stochastic differentiation, differential equations for moments up to the fourth order are developed. The system of equations is closed by considering the fifth and sixth cumulants to be zero. For stationary response, differential equations become algebraic equations. The moments are obtained by solving the system of nonlinear algebraic equations. It is observed that the Gaussian closure method is inadequate for defining the complete probabilistic characteristics of the response.

Fatigue analysis of offshore platforms with uncertainty in foundation conditions

Abstract Foundation flexibility is an important consideration, in designing offshore structures against fatigue, since it has a significant effect on the systems dynamic response. However, a considerable amount of uncertainty is expected in the estimation of foundation flexibility due to the natural nonhomogeneity of the soil as well as laboratory and in-situ testing errors. The proposed method explicitly uses of the statistical nature of uncertainty in the soil stiffness to estimate the variability in the instanteneous and fatigue response using the First-Order Second Moment technique, with emphasis on uncertainties in the dynamic shear modulus along with random wave loading. Uncertainties in the response are estimated considering the randomness in the eigen values, eigen vectors, and the mechanical transfer function. Fatigue life is estimated at different levels of uncertainty for the dynamic shear modulus. It is observed that the uncertainty in the dynamic shear modulus of the soil has more significant effect on the fatigue life of the joints close to the base. The variability in the fatigue response increases with a reduction in the mean dynamic shear modulus. The response is more sensitive to changes in the dynamic shear modulus at its lower values.

Nonlinear Random Vibrations of Compliant Offshore Platforms

A procedure is presented for nonlinear random vibration analysis of deepwater guyed tower platforms subject to wave loading. The nonlinear stiffness provided by the guylines is approximated by an analytical function and wave load is expressed as an output of white noise passing through a linear filter. The differential equation of motion is expressed in terms of a set of first order stochastic equations. Using Ito’s rule of stochastic differentials and averaging operations, a system of ordinary differential equations involving moments of the load and the response are obtained. The equations are solved in the time domain by a numerical method and hierarchy closure is obtained by Gaussian and non-Gaussian closure techniques. The response is modeled as a mixture distribution. It is shown that the response is non-Gaussian at higher sea states.

STOCHASTIC FATIGUE RESPONSE OF JACKETS UNDER INTERMITTENT WAVE LOADING

A combination of spectral and probabilistic analysis techniques is proposed for estimating the fatigue damage of jackets subjected to intermittent wave loading. It is shown that a Gaussian response assumption is unconservative at higher sea states.

Mooring line fatigue in an offshore guyed tower

Abstract A probabilistic method is presented for the fatigue analysis of the mooring lines of a guyed tower. The wave loading is idealized as the first component of a two-dimensional Markov process. Using Itos rule of stochastic differentials, differential equations for moments up to the fourth order are obtained, and these are solved using numerical techniques for both Gaussian and non-Gaussian methods. The displacement response is modelled as a mixture distribution. The probability distribution of guyline tension is then estimated. The probability density for peak guyline tensions is estimated by mapping a Gaussian process into the non-Gaussian process of guyline tensions using the double inversion technique and estimating level crossings. The tension fatigue is estimated using Palmgren-Miners rule. It is shown that the fatigue damage estimated using non-Gaussian closure is greater than that estimated using Gaussian closure.