On the reliability of standard wave spectra in structural response analysis

The present study is intended to elucidate the reliability of the most common standard models of one-dimensional wave spectral densities applied in the design of fixed offshore structures. The results presented refer especially to flexible offshore platforms of the gravity type. The study is based on a comparative analysis of typical structural response quantities derived, by applying wave spectra obtained from wave measurements and standard wave spectra. The analysis includes1008 empirical wave spectra estimated from wave measurements at three different locations on the Norwegian Continental Shelf.     

Speckle contrast of the sum of N partially correlated speckle patterns

In this paper a general method is presented for calculating the theoretical speckle contrast of a sum of correlated speckle patterns, motivated by the need to suppress the presence of speckle in laser projection displays. The method is applied to a specific example, where correlated speckle patterns are created by sequentially passing light through partially overlapping areas on a diffuser, before being projected onto a screen. This design makes it possible to find a simple expression for the correlation between speckle patterns. When the set of correlations involves symmetry, it is shown that the expression for the speckle contrast becomes simpler. The difference in performance between discretely and continuously varying speckle patterns is also investigated. In an example with speckle reduction by a rotating sinusoidal grating, it is found that continuous variation gives a speckle contrast that is 0.61 times the contrast obtained by discretely summing the maximum number of independent patterns.     

Safety of active implantable devices during MRI examinations: a finite element analysis of an implantable pump

The goal of this study was to propose a general numerical analysis methodology to evaluate the magnetic resonance imaging (MRI)-safety of active implants. Numerical models based on the finite element (FE) technique were used to estimate if the normal operation of an active device was altered during MRI imaging. An active implanted pump was chosen to illustrate the method. A set of controlled experiments were proposed and performed to validate the numerical model. The calculated induced voltages in the important electronic components of the device showed dependence with the MRI field strength. For the MRI radiofrequency fields, significant induced voltages of up to 20 V were calculated for a 0.3T field-strength MRI. For the 1.5 and 3.0OT MRIs, the calculated voltages were insignificant. On the other hand, induced voltages up to 11 V were calculated in the critical electronic components for the 3.0T MRI due to the gradient fields. Values obtained in this work reflect to the worst case situation which is virtually impossible to achieve in normal scanning situations. Since the calculated voltages may be removed by appropriate protection circuits, no critical problems affecting the normal operation of the pump were identified. This study showed that the proposed methodology helps the identification of the possible incompatibilities between active implants and MR imaging, and can be used to aid the design of critical electronic systems to ensure MRI-safety.     

Time domain modeling of self-excited aerodynamic forces for cable-supported bridges: A comparative study

Prediction of the wind-induced dynamic response of suspension bridges, emphasizing self-excited forces, is discussed in this paper. The self-excited forces have been modeled by two commonly applied unsteady models and an unsteady model introduced and explained in this article. A novel frequency-independent approximation of the self-excited forces, which for the suspension bridge considered provides results as accurate as those from the unsteady models, is also presented. An integration method that may reduce the number of time steps necessary to avoid amplitude and phase distortion of the self-excited forces has been introduced and applied successfully in a comprehensive case study.     

Along-wind response of suspension bridges with special reference to stiffening by horizontal cables

This paper outlines the application of the theory of stochastic dynamics to evaluate the forced response of suspension bridges subjected to gusty winds. The main emphasis is placed on the analysis of the along-wind response and complete formulae for the root-mean-square response are worked out. The concept of gust response factors is also discussed. The effects of lateral cables as a means to reduce the along-wind response is dealt with. Numerical results of four suspensions bridges with centre spans in the range 525–1250 m are presented, including a discussion of the general trend of the along-wind response.     

Spectral analysis of nonlinear wave load effects on offshore platforms

This paper deals with stochastic theory of nonlinear wave forces emphasizing their effects on the structural response of offshore towers. The analysis of forces and response is limited to second-order statistics expressed in terms of power spectral densities. Numerical examples are included to demonstrate the potential of the theory and to quantify the effects of nonlinear forces on the structural response.     

Interrelations in multi-source geohazard monitoring for safety management of infrastructure systems

In this paper a methodology is put forward for enhancing safety by linking multi-source monitoring to multi-hazards threatening infrastructure systems. An application is demonstrated with a conceptual model comprising two systems. One system includes the infrastructure, exemplified by a reservoir and its dams, along with settings for geohazards. The other system contains the associated monitoring possibilities. Interrelations are quantitatively explored by applying an existing method of systems theory, extended to include multiple systems. This is used to investigate geohazard-triggering potential, interaction intensity and dominance. It is further used to explore the reaction of monitoring components to geohazard action as well as safety values of the monitoring system and its components. The interrelations established have a general relevance for the consideration of geohazards threatening reservoirs and dams, and the associated monitoring. This is explained with a case study. Moreover, the interrelations can be used for defining hazard chains in multi-hazard assessment, planning of monitoring programmes and detecting precursory pathways within a monitoring system. The methodology constitutes the basis for comprehensive safety and risk management embracing, multi-hazard assessment as well as structural health monitoring.     

2D Modeling of nanoscale DG SOI MOSFETs in and near the subthreshold regime

We have developed a 2D model for double-gate SOI MOSFET based on a solution of the Laplacian for the device body utilizing conformal mapping techniques. The model yields explicit expressions for the subthreshold and near-threshold electrostatics of the device, including the perpendicular electrical field and the potential distributions along the silicon/insulator interfaces and at the center gate-to-gate axis. From these expressions, we derive information on the threshold conditions, the potential barrier topography, and the electron density distribution in the device under different biasing conditions. This model constitutes a framework for very precise, scalable, compact models of nanoscale MOSFETs.