The reliability analysis of offshore structures under wave and wind actions is considered using second order random wave theory. To represent non-Gaussian properties of the resulting wave kinematics, the Hermite moment transformation is used. Further, the so-called sample-specific linearization method developed already (to be used in conjunction with the directional simulation method and the linear wave theory) will be extended to take into account both (1) non-Gaussianity of wave/wind load due to nonlinear load processes and also (2) the non-Gaussianity of wave kinematics due to the nonlinear wave theory. This allows an out-crossing approach to be used to assess the structural probability of failure and the involving out-crossing rate (which is not generally available for non-Gaussian processes) is required to be estimated. Using the proposed procedure, simple structures are analyzed in one- and multi-dimensional cases and the results for structural probability of failure are compared with those obtained using simple linear wave theory. Outcomes show that the use of nonlinear wave theory may affect the results considerably.
Finely ground limestone, perhaps as part cement replacement, has been shown to reduce concrete permeability and enhance strength. Whether this holds also for coarse limestone aggregates is of both practical and environmental interest, particularly the effect on corrosion of steel reinforcement. This is confirmed herein with exposure tests over 14 years for a range of concretes exposed to a high chloride environment. Importantly, these concretes also showed delayed initiation of reinforcement corrosion and lower long-term corrosion losses compared with similar concretes made with coarse igneous or siliceous aggregates. The observations are attributed to enhanced bonding between cement-hydration products and the limestone aggregate surfaces. These findings are important both for reducing environmental impacts and greenhouse gasses and for extending the durability of reinforced concrete structures.
<abstract> <p>An important structural component for cavity brick and masonry-veneer construction are wall ties. Typically, they are galvanized steel, sufficiently strong to provide continuity for transmission of direct and shear forces. However, field observations show they are prone to long-term corrosion and this can have serious structural implications under extreme events such as earthquakes. Opportunistic observations show corrosion occurs largely to the internal masonry interface zone even though conventional Code requirements specify corrosion testing for the whole tie. To throw light on the issue electrochemical test for 2 grades of galvanized ties and 316 stainless steels combined with three different mortar compositions are reported. Most severe corrosion occurred at the masonry interface and sometimes within the masonry itself. Structural capacity tests showed galvanized ties performed better than stainless steel ties in lieu of stainless steel R4 class ties presenting significantly greater relative losses of yield strength, ultimate tensile strength and elongation structural capacity compared to R2 low galvanized and R3 heavy galvanized tie classes.</p> </abstract>
Seabed 'parking' of steel pipelines is a common practice in the oil and gas industry. However, despite the use of corrosion inhibitors and biocides internal corrosion remains an issue. So-called 'wet parking' may show severe pitting corrosion, often at the 6 o’clock position and usually attributed to microbiologically influenced corrosion, since high concentrations of various bacteria often are detected. The usual response, dosing with biocides, may not be not effective. It also can have environmental consequences if biocide is eventually or accidentally released. Current practices and options for parking of offshore pipelines are reviewed briefly. Recent findings for internal corrosion inside water injection pipelines are then considered as these have implications for 'wet' parked systems. After initial oxygen controlled corrosion longer term corrosion can occur under anaerobic conditions and this has implications for corrosion during periods of stagnant conditions and for corrosion under deposits of rusts and other debris. These corrosion mechanisms can be enhanced by microbiologically influenced corrosion but only if necessary nutrients are available. Acceptably low levels of corrosion can be achieved inside 'wet' parked pipelines with the use of seawater with low concentrations of particulate matter and low concentrations of microbiologically-critical nutrients.
Samples of new and 70year old pre-corroded OPC concrete were exposed for up to 48months in 6 sewers throughout Australia. Corrosion losses at each site followed the bi-linear trend originally proposed by Wells and Melchers [1]. During an initial phase (lasting <2years) negligible loss of material occurs however once the surface pH=6 losses commence and accumulate linearly at a rate that is likely to remain constant over time. Corrosion rates were found to be sensitive to humidity but insensitive to concrete alkalinity. A first pass model which predicts the rate of concrete sewer pipe corrosion from a knowledge of local average sewer gas temperature, humidity and H2S concentrations was also developed. The equation predictions were in good agreement with rates determined from field observation and historical data.
No abstract is provided for this article.
This paper reports pitting corrosion loss data of AA5005-H34 aluminium alloy immersed in natural seawater for up to 2 years. It is shown that the data for mass loss, maximum pit depth and the average value of 15 deepest pits as a function of exposure time are not closely consistent with the classical power-law function. Instead, the data show a greater affinity to the early part of a bi-modal trend. The uncertainty of the pit depth data was analysed using extreme value theory. The results are that scatter in the data sets increases with exposure time. This is considered to be the result of the pit depth data population being non-homogeneous, characterised by a mixture of deep pits with differing pitting morphologies. The results of this study suggest that longer-term data and homogeneous data population are likely to be more reliable for future corrosion loss prediction purposes.
No abstract is provided for this article.
To derive the extreme value statistics for maximum depth of pits in pitting corrosion, it is conventional to use the Gumbel distribution. However, this has been shown recently to be not entirely appropriate. Modern pitting theory considers that the pit population consists of two major sub-populations, ‘stable pitting’ that can produce extreme depth pits in longer-term exposures, and ‘meta-stable’ pitting. It is not appropriate to mix these two different populations. Also, because of lack of sufficient data at any one exposure time, it is conventional to combine data from different exposure times. Unfortunately the pit depth growth relationship commonly used for this purpose does not accord with actual observations. The reason is that for longer-term exposures sulphate-reducing bacterial activity is the main corrosive agent. Under these conditions some plausible assumptions show that the Frechet extreme value distribution is more appropriate than Gumbel to represent the maximum pit depth. A simple example demonstrates that the difference in maximum pit depth predicted by these two distributions is considerable. This could have serious practical implications for predicting long-term pitting corrosion failure. Also, the question of coupon size to optimize the amount of stable pitting is discussed.
This paper presents several aspects of a method for reliability assessment of reinforced concrete (RC) slab bridges with corroded reinforcement. It is assumed that corrosion will lead to a reduction in cross-sectional area of the reinforcing steel and/or a reduction in bond strength. Two types of corrosion are considered: general and localized corrosion. The method includes a non-linear finite element structural model and probabilistic models for traffic loads, corrosion propagation, bond characteristics, material properties, element dimensions and reinforcement placement. Reliability is estimated in terms of the reliability index using the first-order reliability method (FORM). For illustrative purposes, bridge reliabilities are calculated for a deteriorating, simple-span RC slab bridge, for ultimate strength and serviceability limit states.
Microbial induced corrosion (MIC) of reinforced concrete sewer piping and manholes is a significant issue in Australia and overseas costing water authorities hundreds of millions of dollars annually. It is anticipated that as the country's sewer infrastructure ages the problem will become more severe. Over the last 4 years an ARC and industry funded research project has been undertaken with the aim of building a mathematical model to predict the corrosion of concrete as a function of exposure time and environmental and operational conditions. After almost 4 years of field trials in Sydney, Melbourne and Perth sewers a detailed understanding of the evolution of the corrosion process has emerged and a phenomenological model has been developed. The present paper describes the study findings and their implication for pipe service life prediction.
A physically based mathematical model that can predict the moment‐rotation relationships of bolted extended end‐plate eave connections, using the connection dimensions as inputs is presented. The model recognizes the bounds within which the relationship must lie, viz. the initial (elastic) stiffness, the plastic moment capacity and the strain‐hardening stiffness of the connection. Comparison is made to a series of test results for a range of bolted end‐plate moment connections.
As oil and gas production increasingly moves to deeper waters the operating platforms of choice are FPSOs (Floating production storage and off-loading) ('floaters'). Station keeping is a crucial operational requirement since failure can have disastrous implications. Typically a highly redundant mooring system, consisting of chain and wire rope or synthetic rope is used. The chains have links made from 70 mm diam steel (420 mm link length) to 185 mm (1.1 m link length). Their design employs high 'factors of safety', based largely on North Sea operational experience. This reflects the high variability in loading conditions and in structural capacity as a result of fatigue and corrosion. Concern has been expressed that present design requirements may not be sufficient for operations in the Tropics. As a result, a major international Joint Industry Research project was established to study corrosion loss and pitting and also wear of mooring chain and of wire rope. The experimental work is based largely in Australia. It will provide data for revision of design and operational guidelines using modern structural reliability principles and state-of-the-art understanding and models for corrosion metal loss and pitting. The paper outlines the issues and provides the framework for the on-going project work.
The structural service life of brittle material pipes with exterior corrosion pits is likely to depend on crack initiation and crack development and this may be influenced by pressure loading fluctuations and the possibility of material hydrogen embrittlement. Recently developed methods are used to estimate the cracking pattern, the failure state of the crack development from external pits and the rate of Hydrogen-Assisted Cracking under fluctuating loadings. The effect of hydrogen from the surrounding environment on the cracking rate is formulated using a generalized form of Paris' law. The depth of cracks initiated from surface pits is estimated as a function of pipe age. A realistic example is presented and the results discussed.
Conventional Monte-Carlo integration for high reliability systems is extremely inefficient. It is possible to use a rather simple technique, known in the literature as importance sampling, to dramatically improve sampling efficiency (or the in the integral for a given amount of sampling). This technique, unlike some other variance reduction techniques, can be applied in a systematic manner to problems with one or more limit state functions, including unions and intersections of limit states. These applications as well as the theory, are outlined in this report. (Author/TRRL)
Despite being exposed to the harsh sea-spray environment of the North Sea at Arbroath, Scotland, for over 63 years, many of the reinforced concrete precast beam elements of the 1·5 km long promenade railing are still in very good condition and show little evidence of reinforcement corrosion. In contrast, railing replacements constructed in about 1968 and in 1993 are almost all badly cracked as a result of extensive corrosion of the longitudinal reinforcement. This is despite the newer concrete appearing to be of better quality than the 1943 concrete. Statistics for maximum crack width for each of the three populations, based on measurements made in 2004 and in 2006, are presented. In situ and laboratory measurements show that the 1943 concrete appears to have high permeability but it also shows high electrical resistivity. Chloride penetration measurements show the 1943 and 1993 concretes to have similar chloride profiles and similar chloride concentrations at the reinforcement bars. This is inconsistent with the 1943 beams showing much less reinforcement corrosion than their later replacements and casts doubt on the conventional practice for durability design focusing on reducing concrete permeability through denser concretes or greater cover.
A methodology is described for the assessment of the service life of liquid carrying metallic pipelines subjected to pitting corrosion. The estimate of pipeline life is based on the loss of liquid through pit holes during transportation. The growth of corrosion pits is modelled by a two-parameter exponential function having time dependency and a decreasing rate of pit growth. Parameters which are related to corrosion, pipeline dimension and liquid flow are treated as probabilistic variables. The first-order reliability method is used to estimate the probability of failure and the relative contribution of the various uncertain parameters to it. Failure is defined in terms of a maximum allowable degree of loss or ingress of fluid. Numerical results are given for a typical example pipeline. A sensitivity study was also carried out for the example pipeline to reveal the effect of the level of variability of some of the random variables on the failure probability. The results show that the probability of failure increases nonlinearly with time and that the contribution of pit hole size and pitting corrosion parameters are very significant for long service lives.
