Publications

Gradient estimation for applied Monte Carlo analyses

For many practical applications of system analysis by Monte Carlo simulation, an important piece of information is the sensitivity of the outcome to input information. One matter of interest is the sensitivity of the outcome of the variables controlling it, that is the gradients with respect to the (usually implicit) performance function. Typically, analytical first (and higher) derivatives are not available and the usual numerical approach requires major computational effort. The present paper proposes estimation of gradients using a simplified (linear) response surface, which is a function of the random variables, and which is constructed after the Monte Carlo simulation has been performed. It uses all, or a subset of, the simulation run outcomes. No extra simulation runs are required. There are few restrictions on the form of the performance function and the gradients are estimated at small computational cost. The present paper is restricted to normal random variables.

Structural Design and Design Checking

The process of structural design has received very little attention from the research community despite its centralness in day to day structural engineering. Much of the research which has been done has been carried out in Australia over a number of years and with the cooperation of many structural engineers. The present paper reviews the reasons for being engaged in this relatively unusual research activity and provides an overview of the work to date. A review is given of the incidence and type of errors made in basic design tasks such as calculations, table look up and table interpolation, and how these errors relate to results obtained for more comprehensive tasks such as loading determination and member design. One of the outcomes of the work has been to suggest areas in certain current design codes which are prone to misinterpretation by practitioners. Preliminary results related to design computation checking and inspection are also reviewed. This work potentially has major implications for quality assurance programmes. (Author/TRRL)

A numerical study of damage caused by combined pitting corrosion and axial stress in steel pipes

Localized pipe wall damage accounts for many failures. Numerical modelling of pipes under increasing axial load and constant internal pressure when there is corrosion pits on the exterior surface of the pipe is reported herein. It is shown that for the assumed ideal elastic–plastic material the shape and volume of the plastic field depend on pit depth and its geometry. Pipe wall fracture around a pit can be associated with a critical plastic section. The results reported herein should be relevant for estimating of the risk of perforation and of loss of contents for steel pipes under different loading.

Long-term external pitting and corrosion of buried cast iron water pipes

Improved understanding of the development with time of pitting and corrosion of the external surfaces of buried cast iron water pipes is crucial for the management of water supply distribution networks. The present study reports observations of surface topography of cast iron pipes exposed between 34 and 129 years in clay soils. Pit depths, surface areas and widths associated with deeper pits are reported, including as functions of exposure period. Pitting appears to progress in a step-wise manner as also earlier reported for the corrosion of steels. While there is a relation between pit depth and lateral size of pits through the amalgamation of pits to form plateaus into which new pits tend to form, the predominant progression of material loss through corrosion and pitting is into the metal rather than laterally. The practical implications of these observations are discussed.

Mathematical modelling of the diffusion controlled phase in marine immersion corrosion of mild steel

In the mathematical modelling of the ‘general’ corrosion of mild and low alloy steels and of some other metals oxygen diffusion through the corrosion products to the corroding surface controls the rate of corrosion at least during some period in the corrosion–time behaviour relationship. Under slightly different theoretical assumptions various early authors derived expressions for this phase. It was usually assumed the derived relationship applied from initial immersion. Only very limited laboratory support for the derived relationships exists. They have been applied to field data for atmospheric corrosion but with constants and exponents modified empirically. In the present paper the theory is reviewed and extended. It is argued that diffusion control cannot apply from first immersion and that this provides considerably greater freedom to mathematical modelling. Allowance is made for non-uniform corrosion product density and permeability and for loss of corrosion product. Recent detailed ‘at-sea’ experimental observations obtained on the Eastern Australian seaboard are applied to test the theory for mild steel. It is shown that the most likely competing mathematical relationships produce relatively similar results. The derived relationship is fitted to literature data to deduce its temperature dependence.

General multi-dimensional probability integration by directional simulation

n-Dimensional integration by Monte Carlo directional importance sampling using analytical or numerical integration for each simulated direction is discussed. A suitable general class of sampling distributions is considered. The class is denoted as the class of mixed ūgm-centred standard Gaussian and D -truncated ūgm-centred standard Gaussian distributions, where ūgm is a position vector and is a pragmatically chosen subset of R n. The relative efficiency of different choices of the sampling distribution from the class is studied by way of examples for probability integration in particular. In addition, the same integrals are formulated in different variable representations by the substitution method before the directional Monte Carlo integration is carried out. Moreover, it is demonstrated how sensitivities of the integrals with respect to parameter variations or integration boundary variations can be obtained during the same Monte Carlo simulation primarily set up for obtaining the integral. The novelty compared with previous reports on the topic of integration in the standard Gaussian space is the demonstration that the combination of simulation and numerical integration can be applied for general integrals without any significant change of efficiency.

<i>Technical Note:</i> Rust Removal from Steel Coupons After Short-Term Marine Immersion

The quantification of mass loss, surface topography, depth of pitting, and localized corrosion for steels subject to marine corrosion requires the removal of rusts, preferably without causing additional mass loss, pitting, or other damage. Conventional procedures using inhibited hydrochloric acid or Clarke's solution are shown to remove short-term marine rusts but also to cause new corrosion including pitting and related rust products. Both increase with longer exposure to the cleaning solutions. Water washing with soft water left calcareous materials on the metal surface. Ultrasonic removal did not cause additional corrosion but was effective only for very light rusts. Although relatively slow, electrolytic cleaning (electrolysis) produced clean surfaces without new corrosion.

Concrete Delamination Caused by Steel Reinforcement Corrosion

Practical experience and laboratory observations indicate that corrosion affected reinforced concrete structures are prone more to cracking and delamination of the concrete than to the loss of structural strength. A review of research literature suggests that insufficient theoretical work on corrosion induced concrete delamination has been undertaken. This paper proposes a theoretical method to predict the time to concrete delamination caused by steel reinforcement corrosion. The method is based on fracture mechanics and uses crack opening in concrete as the criterion for its delamination. An analytical model is derived to determine the crack width in concrete. A numerical example is given to illustrate the proposed method and a comparison with both test results and an empirical model indicates a good agreement. It is found in the paper that the corrosion rate is the most significant single factor that affects the time to concrete delamination. The proposed method can be used for the prediction of time to concrete delamination for reinforced concrete structures located in chloride-laden environments. In principle, this provides a means for judging timely intervention and as such has the potential to prolong the service life of corrosion affected concrete structures.

Simulation in Time-Invariant and Time-Variant Reliability Problems

No abstract is provided for this article.

Service life of pitted pipes subject to pressure fluctuations and hydrogen embrittlement

For estimating the likely service life of brittle material pipes with exterior corrosion pits and subject to hydrogen embrittlement crack propagation from the pits is of interest. Similarity of pattern of fatigue cracking from these pits under pressure fluctuations to that under monotonically increasing pressure is referred to. This similarity permits the effect of hydrogen on the cracking rate to be formulated. The depth of cracks initiated from surface pits is estimated as a function of pipe service time, based on the proposed formulation for Hydrogen-Assisted Fatigue Cracking.

Gaussian upcrossing rate solution for structural serviceability

An analytical solution for the structural serviceability problem of concrete creep response is developed herein. The solution is based on stochastic crossing theory, using the upcrossing rate approach for nonstationary stochastic processes. Both the creep effect and applied load are modelled as Gaussian stochastic processes. Since the creep effect is nonlinear in time the resultant structural response is a nonstationary process. Several simple but plausible simplificatons are made to render the solution analytically tractable. An example is given to illustrate the approach and the analytical results have been verified by simulation.

On the ALARP approach to risk management

There is an increasing trend by regulatory authorities for the introduction of the as low as reasonably practicable (ALARP) approach in dealing with risk management of proposed or existing complex hazardous systems. For these, decisions about acceptability or tolerability of risks and consequences can have very significant financial, economic and other consequences for the proponents. Conversely, there may be very significant social and socio-economic implications. ALARP as a guide to achieving a satisfactory outcome has a certain intuitive appeal for the practical management of industrial and other risks. However, as suggested herein, there are a number of areas of concern about the validity of this approach. These include representativeness, morality, philosophy, political reality and practicality. An important, and in some respects fundamental, difficulty is that the risk acceptance criteria are not fully open to public scrutiny and can appear to be settled by negotiation.

DISCUSSION. STRUCTURAL RELIABILITY ASSESSMENT FOR MAJOR STRUCTURES.

Keywords IMPERIAL COLLEGE STRUCTURAL, RELIABILITY, ASSESSMENT, STRUCTURES, MAJOR, MONTE CARLO, METHODS, SAMPLING... Show All

Effect of Immersion Depth on Marine Corrosion of Mild Steel

Marine corrosion data for mild steel coupons exposed at various immersion depths at various geographical locations are reviewed. Analysis of the data confirms that oxygen concentration and water temperature are important parameters influencing corrosion weight loss as a function of time and that there is no obvious effect of depth per se. However, it does not follow that these parameters govern corrosion behavior at all time, since it is known that longer-term corrosion behavior is governed by anaerobic conditions. The recently proposed multi-phase corrosion-time model for marine immersion corrosion is then adapted to allow for variation in dissolved oxygen concentration. It is shown that the model is consistent with the data. The data is applied to obtain a preliminary calibration of the model. It is argued that for very long-term corrosion the influence of anaerobic conditions is likely to be important, although relevant data are very scarce.

Maximum Pit Depth Variability in Water Injection Pipelines

No abstract is provided for this article.

Structural reliability analysis of corroded infrastructure and life prediction modeling

Representativeness of small coupons for corrosion of large plates

Earlier work (Melchers and Jeffrey 2002) showed that coupon size and shape was not a major contributor to the variability in corrosion loss estimates. However these conclusions were based on the relatively small coupons used typically in corrosion field studies. In order to estimate whether such small coupons produced realistic results for much larger plates, mild steel plates 1.2 m x 0.8 m x 3mm thick were exposed for up to 2.5 years in immersion, tidal and splash zone conditions at Taylors Beach, NSW. The plates were recovered at 6, 12, 18, 24 and 30 months. At each stage the plates recovered were cleaned, dried and guillotined into 12 equal rectangles suitable for fitting into a mechanical 3D scanner. The digitised scan results show a high degree of similarity of corrosion across a surface, and a similar pattern of corrosion loss with time of exposure. Estimates for variability of corrosion loss were made from the digitized data. Differences between immersion, tidal and splash zone corrosion were observed. The results validate the use of smaller coupons for field studies and loss and variability estimates.

Structural Deterioration Modelling Issues for Reliability Based Management

This paper discusses various issues in developing a structural deterioration model for naval surface ships for use in a reliability based management method. Over time a naval ship structure will reduce in strength due to deterioration processes that will gradually increase the risk of failure. Increased maintenance is then required to ensure an acceptable level of structural integrity. Due to increasing economic pressures to reduce the cost of ownership, and to extend the life of a platform, there is a need to develop cost effective maintenance plans. Structural reliability techniques are currently being developed to achieve this. The various aspects and information needed to develop this approach are discussed. The dominant deterioration mode on an ageing structure is corrosion with paint coating breakdown. The operational profile of the ship is a dominant factor in the amount of deterioration that occurs. Naval operations will affect coating breakdown and corrosion. This information is discussed with regards to the probabilistic data required for the development of models of these processes. Typical survey data is also presented with a view to determine the critical areas for research focus. Finally, a way ahead is developed for the use of this modelling in developing maintenance planning of naval structures.