A.P. Teixeira

Structural reliability of two bulk carrier designs

The paper aims at quantifying the changes in notional reliability levels that result from redesigning a traditional single-hull bulk carrier to become a double-hull structure. The probability of failure is calculated using a first-order reliability method. The evaluation of the wave-induced load effects that occur during long-term operation of the ship in the seaway is carried out. The still water loads are defined on the basis of a statistical analysis that accounts for the ship type. The ultimate collapse bending moment of the midship cross section is used as the basis for the reliability formulation. It is shown that by introducing the double-hull in the bulk carriers their level of structural safety can be increased up to values typical of tankers. Additionally, the time-dependent degrading effect of corrosion on the ultimate moment is also taken into account in the reliability assessment of bulk carriers.

Adaptive surrogate model with active refinement combining Kriging and a trust region method

The reliability analysis of engineering structural systems with limit state functions defined implicitly by time-consuming numerical models (e.g. finite element analysis structural models) requires the use of efficient solution strategies in order to keep the required computational costs at acceptable levels. In this paper, an adaptive Kriging surrogate model with active refinement is proposed to solve component reliability assessment problems (i.e. involving one single design point) with nonlinear and time-consuming implicit limit state functions with a moderate number of input basic random variables. The proposed model, in the first stage, uses an adaptive Kriging-based trust region method to search for the design point in the standard Gaussian space and predict an initial failure probability based on the first-order reliability method as well as sensitivity factors for the input basic random variables. This initial prediction is then verified or improved efficiently in a second stage using Monte Carlo simulation with importance sampling based on a Kriging surrogate model defined iteratively around the design point using an active refinement algorithm. A convergence criterion that detects the stabilization of the failure probability prediction during the active refinement process is also proposed and implemented. The usefulness of the proposed adaptive Kriging surrogate model in terms of accuracy and efficiency for reliability assessment of engineering structural systems is shown in the paper with two relevant numerical examples, involving a highly nonlinear analytical limit state function in two-dimensions and an advanced nonlinear finite element analysis structural model in a larger dimensional space.

Incorporation of human factors into ship collision risk models focusing on human centred design aspects

This paper presents an approach that more adequately incorporates human factor considerations into quantitative risk analysis of ship operation. The focus is on the collision accident category, which is one of the main risk contributors in ship operation. The approach is based on the development of a Bayesian Network (BN) model that integrates elements from the Technique for Retrospective and Predictive Analysis of Cognitive Errors (TRACEr) and focuses on the calculation of the collision accident probability due to human error. The model takes into account the human performance in normal, abnormal and critical operational conditions and implements specific tasks derived from the analysis of the task errors leading to the collision accident category. A sensitivity analysis is performed to identify the most important contributors to human performance and ship collision. Finally, the model developed is applied to assess the collision risk of a feeder operating in Dover strait using the collision probability estimated by the developed BN model and an Event tree model for calculation of human, economic and environmental risks.

Elasto-plastic Behaviour of Plates Subjected to Heat Loads

Abstract The collapse of steel plates subjected to thermal loads representative of fire conditions is analysed taking into account the temperature dependence of the steel properties. A brief description of the temperature dependence of steel properties is provided as well as the approach taken to account for their change during the heating process in the numerical algorithm. A series of calculations of load temperature curves is performed for plates of different aspect ratio, slenderness, initial imperfections and boundary conditions in order to established how they affect the temperature and the collapse load of the plates. It was observed that the maximum load carrying capacity of the plates is often reached at temperatures ranging from 100°C to 200°C, a situation in which the yield stress of the material has not decreased too much yet. The effect of the elastic support of the plates is important until collapse is reached, but afterwards it can be ignored.

Strength of plates subjected to localised heat loads

Abstract This paper presents results from a parametric study to quantify the effect of localised heat loads on the collapse of steel plates. This study was carried out using a general-purpose non-linear finite element code, which takes the thermal loads into account. The load–shortening behaviour of square plates under the combined effects of localised thermal loads and biaxial in-plane compression was obtained for different breadth to thickness ratios. In particular, the effects of the size of the heated area and its temperature differential are quantified. It was observed that the plate strength decreases rapidly when the heated area increases beyond 50% of the total area of the plate. Finally, design curves are proposed to predict the collapse strength of the plates subjected to localised heat loads.

Parametric study on the collapse strength of rectangular plates with localized imperfections under in-plane compression

The effects of localized imperfections on the collapse strength of plates are studied by means of finite element analysis performed by different authors. First a benchmark study is performed to assess the accuracy of the results obtained using different codes on the same case study. Afterwards, a parametric study on the effect of the localized imperfections on the collapse strength is performed. Finally, the effect of combining local and global imperfections was studied. It is found that the addition of a localized imperfection changes the collapse strength of the plate, which depends on the amplitude and size of the localized imperfection change.

RELIABILITY OF LOAD BEARING STEEL PLATES SUBJECTED TO LOCALISED HEAT LOADS

A reliability formulation is presented for thermally insulated load bearing plates subjected to the localized heat loading. The limit state function is defined in terms of stresses, which account for the additional in-plane compressive loads that the plate will sustain before collapse. The collapse strength of plates is determined with a non-linear finite element code that accounts for the elasto-plastic behavior and for the changes in material properties with temperature. A first order second moment approach is presented to quantify the uncertainty of the heat loads and to describe the importance of the governing variables in the limit state function. An example of the reliability analysis of a steel plate is provided using a time independent first order method.

Design equations for plates subjected to heat loads and lateral pressure

A brief description is presented of the strength behaviour of rectangular plates subjected to fire-induced intense heat loads of such a magnitude as to degrade the mechanical properties of the material. Design curves are proposed to predict the collapse of the plates under the biaxial state of stress induced by heat loads. Interaction curves are also derived for the case of plates that are initially subjected to lateral pressure and later to intense heat loads.

Maintenance Planning of an Offshore Wind Turbine Using Stochastic Petri Nets With Predicates

Operation and maintenance (O&M) activities have a significant impact on the energy cost for offshore wind turbines. Analytical methods such as reliability block diagrams and Markov processes along with simulation approaches have been widely used in planning and optimizing O&M actions in industrial systems. Generalized stochastic Petri Nets (GSPN) with predicates coupled with Monte Carlo simulation (MCS) are applied in this paper to model the planning of O&M activities of an offshore wind turbine. The merits of GSPN in modeling complex, multi-state and multi-component systems are addressed. Three maintenance categories classified according to the size and weight of the components to be replaced and the logistics involved, such as vessels, maintenance crew and spares and, the associated delays and costs are included in the model. The weather windows for accessing the wind turbine are also modeled. Corrective maintenance based on replacements and age imperfect preventive maintenance are modeled and compared in terms of the wind turbine’s performance (e.g. availability and loss production) and of the O&M costs.

Reliability analysis of a ship hull structure under combined loads including slamming loading

During the last decades, much attention has been paid to long-term structural reliability analyses applied to design and maintenance planning of ships. However, many extreme loads occur in short time periods in particular severe sea conditions. In this paper, a random process model of the ship hull girder subjected to still-water, wave- and slamming-induced loads is formulated based on the random process theory covering a short time period during extreme seas in the short time duration of slamming. The correlation of the peak value of slamming- and wave-induced loads is considered according to the fact that the wave and the velocity of the ship are the dominating causes of the slamming. Through the analysis of the stochastic process of limit state function, the reliability is evaluated by means of up-crossing analysis and parallel system reliability method. In a numerical example, the influences of the damping ratio, slamming rate and correlation coefficient on the failure probability are analysed, considering different failure modes. The results show that the buckling failure of the deck under compression is the most dangerous case and the damping ratio influences the impact of the slamming load on the reliability.