The ‘ensemble’ up-crossing rate technique consists of averaging the rate at which a random load process up-crosses a deterministic barrier level over the resistance distribution at successive time points. Averaging over the resistance makes the assumption of independent up-crossings less appropriate. As a result, first passage failure probabilities may become excessively conservative in problems with other than extremely low failure probabilities. The ensemble up-crossing rate technique has a significant potential in simplifying the solution of time variant reliability problems under resistance degradation. However, little is known about the quality of this approximation or its limits of application. In the paper, a Monte Carlo simulation-based methodology is developed to predict the error in the approximation. An error parameter is identified and error functions are constructed. The methodology is applied to a range of time-invariant and time-variant random barriers, and it is shown that the error in the original ensemble up-crossing rate approximation is largely reduced. The study provides unprecedented insight into characteristics of the ensemble up-crossing rate approximation.
The alkali–aggregate reactivity (AAR) of concrete, long known for mass concrete, can also induce corrosion of steel in reinforced concrete structures. Several examples are given for which the origin of observed reinforcement corrosion and loss of concrete cover originally was attributed to chloride-induced or to carbonation-induced reinforcement corrosion. Critical reviews of these cases, using available information, suggest that, more likely, the observed crack patterns and concrete deterioration are the result of long-term AAR-induced concrete matrix expansion and loss of concrete strength and that these effects occurred prior to the eventual initiation of reinforcement corrosion. This proposition is supported by finite element and other stress analyses of various concrete–steel ensembles. They show that concrete expansion produces tensile stresses localised at and near exterior concrete surfaces or relative to the reinforcement. The locations of high-stress and -strain zones so produced correlate with field observations of long-term concrete cracking and delamination. The present interpretations highlight that AAR may be a significant contributor to initiation and subsequent long-term development of reinforcement corrosion in structurally reinforced concretes.
Corrosion of cast iron pipes buried in wet soils has long been associated with severe localized effects often attributed to microbiological influences and also with the chemical and physical properties of the soils. Despite more than 70 y of research effort, correlations have remained poor. Recently, the reasons for this have been elucidated, as reviewed briefly herein. Also, interpretation of data from actual cast iron pipes buried in a variety of soils for up to 129 y shows that two critical factors governing depth of local corrosion penetration are the type of soil and the compaction of that soil around the pipe. The latter influences the size of wet air-voids in the soil at the pipe surface. These cause differential aeration and associated severe localized corrosion. The other critical factor is the time of wetness of the soil/metal interface. This is related both to atmospheric precipitation (e.g., rain) and to soil permeability as governed by compaction. Microbiological corrosion has no influence unless essential nutrients continue to be available. These new views on an old subject permit the development of corrosion penetration models for longer-term corrosion.
Structures are usually composed of many members and connections, each with individual properties. When the member properties and the loadings applied to the structure are specified in statistical terms, the determination of structural system reliability becomes a problem of multi- fold integration. Since direct integration is feasible only for extremely simple structures, several alternative approaches to the problem have been suggested, either through simplifying the integration process itself or through simplified reliability theory. These approaches are discussed in the context of use of computer programs and computer usage. (Author/TRRL)
No abstract is provided for this article.
Water injection or water flooding is widely used as a secondary recovery method to increase the production of oil usually from mature oil wells. It requires the injection of water into the reservoi ...
Forensic investigations were performed on chain links recovered from two FSO mooring systems in West Africa and Indonesia. Although these links were in tropical seawater for only 7 years, they experienced extremely severe pitting corrosion not previously seen in any available records. The material presented has direct application to ensuring the integrity of mooring systems as it provides insight into the mechanical behaviour of highly corroded chain links, statistics of the observed corrosion rates in these locations and guidelines for examining chain links. Corrosion on the surface of the links was mapped using 3D photogrammetry. The observed long-term corrosion rate is significantly higher than the average corrosion rate of 0.4 mm/year given in API RP-2SK. Analysis of the elementary composition of the rusts indicates that the large pits observed were most likely associated with Microbiologically Influenced Corrosion (MIC). The obtained results represent a breakthrough in the field of corrosion of mooring chains deployed in tropical seawater, corroborating the latest models that have been developed taking into account MIC. This work was conducted under the scope of the SCORCH JIP which involves ExxonMobil, Chevron, BP, Total, Shell, BHP Billiton, ConocoPhillips, Petrobras, Statoil, BREE, DNV, ABS, Bureau Veritas, INPEX, SOFEC, InterMoor, KBR, SBM, Franklin Offshore, Vicinay Cadenas, Bridon, Arcelormittal, Arrium and Vryhof Anchors.
This chapter describes the background to the estimation of service life for use in lifecycle cost analyses of infrastructure, including deteriorating infrastructure. Since loadings, material properties and dimensions are not known with certainty, service life estimates must be based on probabilistic analyses. The theory most useful for this, the theory of structural reliability, is outlined briefly and related to service life estimation through stochastic process theory. This requires the estimation of the probability of failure on first load application and the so-called ‘out-crossing rate’. Methods to estimate these include Monte Carlo methods and moment methods. Both are revised briefly and future trends for computational methods indicated.
AbstractThe conventional wisdom in corrosion testing is that the size of the coupon and its perimeter/area ratio (the surface area of the perimeter expressed as a percentage of the total surface area) are important parameters and that these will have an effect on the results obtained. In order to test this proposition, sets of mild steel coupons of the same exposed areas but having different perimeters were immersed in a marine environment. Corrosion mass loss was observed for area/perimeter ratios in the range 3·52-6·03%. Mass loss observations were taken at various times to 12 months. Sets of rectangular coupons of different sizes (50 × 100 mm and 100 × 200 mm) were also immersed. The results indicate that any variation caused by shape or size falls within the expected variability of corrosion at a test location.
No abstract is provided for this article.
No abstract is provided for this article.
Solving some integro-differential equation we find the Laplace transform of the first passage time for filtered Poisson process generated by pulses with uniform or exponential distributions. Also, the martingale technique is applied for approximations of expectations and distributions of the first passage times. The approximations accuracy is verified with the help of Monte-Carlo simulations.
Keywords NEWCASTLE UNIVERSITY EARTHQUAKES, CITIES, DAMAGE, SEISMOLOGY, SEISMIC, RISKS, ANALYSIS, BEHAVIOUR, BUILDINGS, DETERIORATION, MASONRY, COLLAPSE, FAILURES NEWCASTLE, NEW SOUTH WALES AUSTRALIA... Show All
The corrosion of old buried cast iron water mains is a problem for the Australian Water Industry. To better manage their assets the industry requires a way of predicting the remaining service life of their pipes. In order to do this a predictive model of long-term corrosion loss with time is required. This paper describes ongoing work being conducted at The University of Newcastle to develop corrosion loss models for cast iron pipes in soils, as a function of exposure time and soil environment. A preliminary corrosion model was calibrated in a previous work, using a limited set of data collected from recent field work. This data base was extended in the current work by including data (of maximum pit depth and some soil properties) from historical condition assessments within the Hunter Water network. Previous studies by the authors (and others) indicate the significance of soil moisture content on the long-term corrosion loss and rate. At most sites soil moisture was only measured at a single point in time (or not at all), so the long-term average value of soil moisture (required for model calibration) was uncertain. To better estimate the average long-term soil moisture content, a simple climate-soil moisture model was developed. This paper presents the collected field data, describes the method for estimating long-term moisture content and presents the updated model calibration based on the additional data.
Due to their good corrosion resistance, copper and copper alloys such as 90:10 Cu-Ni are used extensively in high-quality marine and industrial piping systems and also in marine, urban, and industrial environments. Their corrosion loss and pitting behaviour tends to follow a bi-modal trend rather than the classic power law. Field data for 90:10 copper nickel immersed in natural seawater are used to explore the effect of water pollution and in particular the availability of critical nutrients for microbiologically induced corrosion. It is shown, qualitatively, that increased dissolved inorganic nitrogen increases corrosion predominantly in the second, long-term, mode of the model. Other, less pronounced, influences are salinity and dissolved oxygen concentration.
1: Introduction. 2: Sources of risk. 3: Modelling of systems. 4: Performance of system elements. 5: Human error and human reliability data. 6: System evaluation. 7: Risk acceptance criteria. Appendix A. Index.
The corrosion of old buried cast iron water mains is a problem for the Australian Water Industry. To better manage their assets the industry requires a way of predicting the remaining service life of their pipes. In order to do this a predictive model of long-term corrosion loss with time is required. This paper describes ongoing work being conducted at The University of Newcastle to develop corrosion loss models for cast iron pipes in soils, as a function of exposure time and soil environment. A preliminary corrosion model was calibrated in a previous work, using a limited set of data collected from recent field work. This data base was extended in the current work by including data (of maximum pit depth and some soil properties) from historical condition assessments within the Hunter Water network. Previous studies by the authors (and others) indicate the significance of soil moisture content on the long-term corrosion loss and rate. At most sites soil moisture was only measured at a single point in time (or not at all), so the long-term average value of soil moisture (required for model calibration) was uncertain. To better estimate the average long-term soil moisture content, a simple climate-soil moisture model was developed. This paper presents the collected field data, describes the method for estimating long-term moisture content and presents the updated model calibration based on the additional data.
The design of prestressed concrete bridge beams usually assumes that the full capacity of the tendons can be achieved under ultimate load, based on the assumption of sufficient deformation capacity of the prestressing wires. Whether this is achieved also in older bridges is of increasing interest in remaining-life assessments since, especially in aggressive marine environments, corrosion of steel is known to cause loss of wire ductility. Results are reported herein of load tests to destruction for three full-sized and deteriorated prestressed concrete bridge beams recovered from a 45-year-old bridge exposed to an aggressive marine environment. The two beams with the greatest superficial deterioration showed progressive and premature failure of the prestressing wires. The beam with little superficial deterioration also showed progressive failure and failed to reach the ultimate load capacity based on current design theory and actual material properties. Possible reasons for the observed behaviour and the practical implications are discussed.
