640 publications from this institution
Overview checking occurs whenever an engineering design is reviewed by another engineer without specifically carrying out checking calculations or following through the design process in detail. Results are reported herein of a survey examining the effectiveness of overview checking. The respondents were requested to assess whether a structural design should be considered “undersized”, “correct” or “oversized”. Using the responses as a basis, a decision model is proposed for the effectiveness of overview checking as a function of error magnitude and of checker's experience. It is concluded that correct assessment of a design as “safe” is not a function of experience. However, it appears that more experience is beneficial in selecting economical designs.
No abstract is provided for this article.
No abstract is provided for this article.
In Part 1 of this paper, it was shown that statistical models using pooled data are of poor quality with very wide variability. The present paper considers probabilistic corrosion modelling based on corrosion mechanics principles and notes the effect of environmental and other factors that should be included in model development. These include temperature, dissolved oxygen, salinity, calcium carbonate and pH, water velocity, and marine growth. A new multi-phase, nonlinear mean value model is proposed for the corrosion of mild- and low-alloy steels under “at-sea” immersion conditions. Comments are made about calibration of the model and the further research required for complete model development is discussed.
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.
A previous study of the immersion corrosion of mild and low-alloy steels in natural coastal seawater found a high degree of consistency between the corrosion losses observed at different testing sites when allowance was made for average seawater temperature and sites with obvious pollution and velocity effects were excluded. However, some sites showed early corrosion rates considerably below the trend. Carefully conducted laboratory experiments using real seawater also show low trends. It is proposed that the more severe early corrosion of steels in natural coastal immersion conditions is caused by elevated nutrient levels in seawater. Not only does this increase the rate of growth of biofilms and of marine fouling as normally assumed but it also raises the rate of metabolism of bacteria. It is contended that the latter is the principal cause of increased early corrosion at higher nutrient levels. A detailed analysis of an extensive range of field data provides support for this contention. It was found that low rates of early corrosion usually are associated with pristine waters that have much lower nutrient contents compared to typical coastal waters. The implications carry over to the various parameters describing the previously proposed corrosion loss model. It is found that elevated nutrient levels increase the early corrosion rate but reduce the maximum corrosion that occurs under aerobic conditions, and reduce, slightly, the time to transition to anaerobic conditions.
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.
Keywords IMPERIAL COLLEGE STRUCTURAL, RELIABILITY, ASSESSMENT, STRUCTURES, MAJOR, MONTE CARLO, METHODS, SAMPLING... Show All
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.
No abstract is provided for this article.
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.
In the phenomenological model for the general marine corrosion of mild (and high strength low alloy) steel introduced previously, the corrosion process eventually enters a state governed predominantly by the activity of sulphate-reducing bacteria (SRB). This state has been modelled by two consecutive phases, the first describing a transient behaviour and the second the longer-term quasi-steady-state corrosion response. In earlier papers these have been described only by bounding parameters, calibrated to comprehensive field data. Herein a plausible idealized mathematical model is proposed for the corrosion losses within these phases. The model is based on an idealization of the distribution of nutrients within the corrosion product layer and its depletion by SRB as the anaerobic conditions necessary for their metabolism develop. With the gradual depletion of nutrients, the activity of the SRB reduces to a quasi-steady-state process governed by the rate of external nutrient supply and the loss of (protective) corrosion product. It follows that high levels of nutrients, such as in offshore oilfields and in nutrient-polluted waters can be associated with high rates of corrosion under anaerobic conditions. Also, in warmer (tropical) waters, the metabolism of SRB is enhanced. Both phenomena are consistent with field observations. A preliminary calibration of the models for phases 3 and 4 is attempted using the limited available field data.
A phenomenological model for the probabilistic description of the corrosion of steel under given marine conditions, using weight loss as a function of time is considered herein. The model uses accepted theories for short term surface corrosion and employs modern understanding of the action of bacterial colonization of the surfaces of specimens, including the development of anaerobic conditions. Kinetic, diffusion, nutrient and anaerobic components of the model are identified and mathematical descriptions given.
Carbon steel pipelines are used extensively in the oil and gas industry for sub-sea applications. Usually they are cathodically protected and have exterior surface protective coatings. Where these are ineffective or not used, the evolution of weld zone corrosion with time may be of interest. Experience has shown that generally the heat-affected weld zone is more severely corroded by pitting than either the weld zone itself or the parent metal. However, quantitative data are scarce, particularly for longer-term exposures. Herein observations are reported of the pitting corrosion of API X56 Spec 5 L grade pipeline steel exposed continuously to natural Pacific Ocean seawater for 3.5 years, and extended to much longer exposure periods by comparison to similar steels exposed for up to 30 years. It is shown that the corrosion mass loss, maximum pit depth and pit depth variability are not simple linear functions of exposure time as often assumed in practice but are more complex functions. These functions are consistent with those observed previously for mild steels in various marine exposure conditions. However, considerable differences were noted in severity. The reasons for this are discussed and further work is outlined.
This paper reports on some early observations in a more extensive field programme aimed at elucidating basic in-situ immersion corrosion behaviour characteristics for mild and low alloy steel exposed in coastal seawater regions. Five sites are described - two in Newcastle Harbour and three in Lake Macquarie (two in the entrance and one in Fennel Bay). Results are described for coupons recovered after one, three and six months exposure. Overall corrosion rates for the six months were found to be in the range 150 - 300 micrometres/year. Seasonal factors such as rainfall appeared to have a significant influence on the corrosion behaviour, particularly for the specimens at Fennel Bay. For the present observations, corrosion rates were found to increase with salinity, dissolved oxygen and water velocity but reduced with increasing biofouling. These results are not inconsistent with those previously reported - the present investigation is an attempt to obtain results systematically. (a) For the covering entry of this conference, please see IRRD abstract no. E200447.
Although it is well known that moisture ingress in glass fiber reinforced polymers (GFRP) enhances the phenomenon of stress corrosion cracking in the fibers, and that this reaction is likely to proceed more rapidly at the weakest sites in the glass fiber surface, a fundamental law that would permit the valid extrapolation of stress rupture curves to long service lives is yet to be developed. As a result, design guidelines for glass fiber reinforced polymers components have been developed mainly on a prescriptive rather than of a performance basis. Based on the well established knowledge on the chemical behavior of glass and, in particular, that of glass flaws, a model that combines fracture mechanics, shear lag theory, and a probability model for flaw size is developed to describe the behavior of GFRP composites. The predicted results, although limited to rather idealized situations, are very encouraging. They suggest that, with only modest assumptions about material properties, it is possible to obtain mechanisms of GFRP breakdown, which correspond with observed experimental behavior.
Available evidence suggests that design checking can significantly reduce the incidence of structural failures resulting from human errors in the design process. Since “real-world” experimentation is not possible, it is of interest to develop a model to simulate the effects of human error on a typical design task and also to model typical checking procedures. The development and analysis of these models is described in the present paper. The design task considered is that of member design for a rafter of a steel portal frame building. It was found that checking efficiencies between 0.6 and 0.9 are most effective in increasing structural reliability and that, often, only two separate design checks are necessary to virtually eliminate the incidence of structural failure due to human error.
Corrosion of mild steel in contact with both seawater and particulate media has important practical implications. It is considered herein for coupon exposures up to 2 y in 0.05 mm and 0.425 mm (nom.) granular media in natural Pacific Ocean seawater and in seawater dosed with calcium nitrate ostensibly to stimulate microbiological corrosion. The observed pit depths were consistent with an empirical bi-modal corrosion prediction model and overall lowest for smaller particle sizes and greatest without particles. Contrary to expectations, nutrient dosing tended to reduce pit depths. The observations are discussed in light of earlier observations and for relevance to practical implications.
