C. Guedes Soares

Uncertainties in probabilistic numerical analysis of structures and solids-Stochastic finite elements

The main sources of uncertainties involved in the analysis of structures and solids are shown and the tools available to deal with them. While trying to cover the complete modeling process, ranging from the problem formulation via the mathematical model all the way to the numerical approximation, we have tried to expose areas in need of further research. The techniques and methods involved in stochastic modeling are explained in somewhat more detail, as they are newer and less known than those used for the deterministic modeling.

Risk assessment in maritime transportation

Abstract A review is presented of different approaches to quantify the risk in maritime transportation. The discussion of several accident statistics provides a global assessment of the risk levels and its differentiation in ship types and main types of ship losses. Early studies in the probability of ship loss by foundering and capsizing are reviewed. The approaches used to assess the risk of structural design are addressed. Finally a brief account is given of recent development of using formal safety assessments to support decision making on legislation applicable internationally to maritime transportation.

Reliability of maintained, corrosion protected plates subjected to non-linear corrosion and compressive loads

A model is proposed for the non-linear general corrosion wastage in a plate, which is able to describe an initial period without corrosion due to the presence of a corrosion protection system, and an exponential increase in wastage up to a steady-state value. The influence of adopting this model instead of a linear one is demonstrated by studying the reliability of a corrosion-protected plate subjected to compressive loads, and maintenance actions. The paper examines how the plate collapse strength will vary in time as a result of generalised corrosion and plate repair. In the presence of general corrosion, the plate thickness will decrease at a random rate as a function of time and will affect the net plate section resisting the applied load. The collapse strength against compressive loading is described by a function of time and the resulting reliability is quantified. The sensitivity of the reliability estimates with respect to several parameters is also studied. The reliability is predicted by a time-variant formulation and the effects of repair actions in the reliability assessment are shown.

Representation of double-peaked sea wave spectra

Abstract About 1000 measured wave spectra from the North Atlantic and 6000 from the North Sea, were analysed so as to determine the frequency of occurrence of double-peaked spectra in sea states of different intensity. This type of spectrum did not occur so often close to coast and in high sea states. A four-parameter theoretical formulation was proposed to represent double-peaked spectra and was shown to provide an excellent fit to measured spectra. The average values of the spectral parameters describing the two peaks did not show any clear dependence on significant wave height.

Simulation modelling of repairable multi-component deteriorating systems for ‘on condition’ maintenance optimisation

Abstract The main purpose of this work is to model continuously monitored deteriorating systems by using Monte Carlo simulation and embedding the resulting model within an ‘on condition’ maintenance optimisation scheme that aims at minimising the expected total system cost over a given mission time. The simulation model is first introduced by considering a non-repairable single component subjected to stochastic degradation. The modelling is then generalised to multi-component repairable systems. To find the optimal degradation thresholds of maintenance intervention, the cost optimisation procedure employed is a simple search in the space of the maintenance thresholds. The sensitivity of the results to some of the driving cost parameters has also been examined.

Model uncertainty in the long-term distribution of wave-induced bending moments for fatigue design of ship structures

The uncertainty of the long-term distribution of the wave-induced wave bending moments in ships has been assessed, relating it to ocean areas, ship routes, ship speeds and ship types. Weibull distributions have been fitted to the calculated values of long-term distributions and their shape factor has been related to the ship length. The number of load cycles in a ship lifetime has been calculated, and characteristic values were derived for different levels of probability. A two-parameter Weibull distribution defined by a characteristic value and a shape factor has been proposed as a load model to be adopted in design rules for the assessment of fatigue damage in ship structures. The model uncertainty of the two parameters defining the distribution is assessed as a function of the method of fitting the long-term distribution, of ship speed and of the climatological data.

Uncertainty modelling in plate buckling

Abstract A brief account is given of the basic principles of uncertainty modelling with first-order second-moment methods. The main features of the behaviour of plates subjected to compressive loads are considered and various prediction methods are compared. The governing parameters of plate behaviour are used to construct probabilistic models for the laboratory test situation, for the analysis and for the design of plates. The uncertainty of the strength predictions is quantified in the different cases. Special attention is given to the implications of problem formulation and to the methodology of uncertainty modelling.

Analysis of directional wave fields using X-band navigation radar

The aim of this work is to present an approach to describe complex sea states including the ones consisting of superpositions of swell and wind sea components, using a nautical radar in X-band as a remote sensing technique. In the present article, the inversion method to obtain the spectral representation of the wave fields is described. The method is applied to analyse data obtained from simulation techniques, as well as from measurements obtained during oceanographic campaigns in the Bay of Biscay and North Sea.

Design equation for the compressive strength of unstiffened plate elements with initial imperfections

Abstract A review is made of equations that have been proposed for the design of ship plates subjected to compressive loads. The effect of the various parameters is quantified showing that a design method should account explicitly for plate slenderness, residual stresses, initial distortions and boundary conditions. A new equation that includes all of the variables is derived and calibrated with existing results of experiments and of numerical calculations. The uncertainty associated with the use of equation is also quantified.

An Application of the Peaks Over Threshold Method to Predict Extremes of Significant Wave Height

The paper describes an application of the Peaks Over Threshold (POT) method to significant wave height data of Figueira da Foz, Portugal. The method is briefly explained and justified. The exponential distribution is shown to be adequate for modeling the peaks of clustered excesses over a threshold of 6 m. Estimates of return values are given. The exponential character of the data is theoretically justified in the Appendix.