Publications

Equivalent Linearisation And Fatigue Reliability Estimation For Offshore Structures

No abstract is provided for this article.

Reliability of the conventional approach for stress/fatigue analysis of pitting corroded pipelines – Development of a safer approach

The remaining strength and the fatigue life of externally pitted corroded metallic pipeline are often assessed using an idealization to model the pit morphology, typically semi-ellipsoidal and cuboidal. Such idealized pit geometries are shown herein to underestimate Stress Concentration Factors (SCFs). By implication this leads to significant overestimates of fatigue life for pitting corroded pipes under sustained cyclic loading which could be the reason behind some of the recent unexpected pipeline failures. Further investigation clarified that this can occur only when there is little or no plastic deformation within the pit which is typical for pipelines constructed from brittle and quasi-brittle materials. Therefore, applying the conventional approach that uses simplistic geometries to model corrosion pits is not reliable for stress/fatigue analysis of pipelines with brittle or quasi-brittle materials such as cast iron or high-grade steel. However, for pipelines made of ductile materials, there is a specific internal pressure, termed critical operating pressure herein, below which there is no plastic flow within the pit and simple idealization of pit morphology results in underestimated SCFs. A semi-empirical equation is developed herein to allow this critical operating pressure to be calculated, based on which a protocol is outlined for correct estimation of the SCFs of pitting corroded pipelines. The conclusions of this study are supported by numerical results validated against novel full-scale burst capacity tests with both simplistic and complex-shaped pits.

Long-term Under-deposit Corrosion of Carbon Steel Pipelines under Stagnant Seawater

Statistical Characterization of Corroded Steel Plate Surfaces

Perforation of steel plate is of considerable interest for the longer-term safety and integrity of ships hulls and tanks and for nuclear containment systems. This paper reports on the preliminary random field characterization of the surface of two sample corroded steel plates as a trial towards generating numerically the surface topology of larger areas of steel plate. Of particular interest is the maximum depth of penetration and its dependence structure across the plate area. The potential applications and the background to the modelling of corrosion loss and pitting are reviewed briefly to set the scene for appropriate surface characterization. Contrary to the assumptions often made for the statistical characterization of maximum pit penetrations, they are highly dependent. This makes representation using extreme value probability distributions questionable. An alternative probability-based form of representation is proposed.

A micromechanics model for environmental stress corrosion in GFRP

Understanding the mechanisms of environmental stress corrosion is very important for assessing the durability and damage tolerance predictions of composites using glass as the main reinforcement. A mechanistic model describing these mechanisms for unidirectional GFRP in tension is described. The model is based both on the chemical behaviour of glass, and in particular that of glass flaws, and on more recent models of stress corrosion. These were combined with fracture mechanics, the shear lag theory, and a probability model for flaw size. The results are very encouraging. The model shows that it is possible to obtain mechanisms of GFRP breakdown, which corresponds to observed experimental behaviour.

Coating Life Prediction for Water Ballast Tank

Corrosion is the key factor responsible for the degradation of ship structures and in no place is this truer than water ballast tanks. Coating protection system is a continual research interest for classification societies and coating industries. Up to now, most coating performance analyses are qualitative not quantitative. Coating life prediction is almost always based on experience and various assumptions, due to unavailability of practical data support systems. This paper describes a preliminary impartial investigation of coating life carried out with interviewees from the Australian Defence Science and Technology Organization (DSTO), shipyards, coating supplier and an independent expert. Plate surface, edges and welds in ballast tank were considered and the influences of dry film thickness (DFT) and surface preparation (SP) are addressed. The investigation gives some insight into the life of practical coating systems for water ballast tanks. Coating life is proposed to be representable by a normal distribution when corrosion breakdown is below 10% of plate thickness, which is of practical implication.

Probabilistic remaining life estimation for deteriorating steel marine infrastructure under global warming and nutrient pollution

The longer-term serviceability and structural safety of steel infrastructure exposed to seawater conditions may be affected by global warming and by seawater nutrient pollution. These may affect abiotic and biotic (microbial) corrosion. A model for long-term corrosion is developed from data obtained from steel piling exposed for 33 years in a seawater harbour. The effects on corrosion losses on the structural reliability of steel sheet piling as used in harbours world-wide were investigated as a function of seawater temperature rise from global warming and of seawater nutrient pollution. The results show that structural reliability is more sensitive to likely nutrient pollution than to predicted increases in seawater temperature, noting also that global warming also could increase nutrient pollution from anthropological sources.

Probing and monitoring multiple forms of localised corrosion occurring concurrently on pipeline steels in marine environments

An electrode array probe, with a novel approach, has been specifically designed to enable the in-situ field monitoring of multiple forms of localised corrosion of steel exposed to marine environments. The initiation and propagation of localised corrosion within the crevice area as well as at the steel weldment were visualised and evaluated over one-year field exposure at two different marine locations in Australia. The results show the varied initiation and propagation behaviour of crevice corrosion and weldment corrosion on steel, which provide experimental evidence for ‘bi-modal’ behaviour and the change-over of corrosion modes in extended periods of marine exposure.

Monte-carlo methods for structural reliability analyses

Elementary monte-carlo sampling as a technique to artificially replicate an experimental program has been used successfully for relatively simple problems, provided the failure probability to be calculated is relatively large. The present paper reviews the elementary monte-carlo method and describes a little known, but improved approach known as importance sampling (a).

Modelling immersion corrosion of structural steels in natural fresh and brackish waters

The multi-phase mean-value model previously proposed for modelling the marine immersion corrosion of low carbon and low alloy structural steels is examined for application to fresh and brackish waters. Use is made of field data for brackish and fresh waters available in the literature, supplemented with data or estimates for water temperature, pH, hardness and nutrient levels. It is shown that the data exhibits consistency with the model and that it is a function of average water temperature. Corrosion in brackish and fresh waters corrosion depends on water hardness, pH and nutrient levels, with higher pH levels and lower water hardness associated with higher aerobic levels of corrosion but these are not significant for anaerobic corrosion. In the anaerobic phases 3 and 4 of the model, the available data and associated trends are interpreted as showing that elevated levels of nutrients produce higher rates of corrosion. Conversely, these phases showed very low rates of corrosion for fresh waters with very low nutrient levels. Consistent with basic corrosion theory and with laboratory observations, salinity by itself is not a clear distinguishing characteristic. The model provides a new approach to interpreting the available data for corrosion in fresh and brackish waters. It permits plausible explanations for previous apparently inconsistent observations for corrosion in brackish waters. Finally, it reinforces the need for full and detailed reporting of corrosion testing programs, including details of precise timing, location, orientation and environmental conditions including means and variations in water temperatures, DO, salinity, pH, water hardness, carbonates and various nutrients.

Trends in long-term corrosion of aluminium alloys in marine, industrial and urban environments

No abstract is provided for this article.

Optimum Design of Reinforced Concrete Tanks

Lower bound methods have had little application in shell design. The paper presents optimum lower bound solutions for a circular cylindrical tank unrestrained at the top, and either fixed or hinged around the base. Both the optimization method of Rozvany and of Marcal, Prager, and Shield give the same results. A numerical comparison of optimized lower bound design, elastic design, and uniform isotropic yield-line analysis is made. Savings in reinforcement volume of 20% to 25% can be obtained in certain cases. The effects of prescribed minimum amounts of reinforcing and of cracking below working load are analyzed.

Corrosion of working chains continuously immersed in seawater

No abstract is provided for this article.

Structural Aspects of the Newcastle Earthquake

On 28 December, 1989 the city of Newcastle was struck by a magnitude 5.6 earthquake killing 12 people and causing losses estimated at $1000 million. It was the first time a major Australian city has been so extensively damaged as a result of an earthquake, and the first time that deaths have occurred. With a few exceptions, most of the damage was to older loadbearing masonry construction or to infill masonry in modern framed construction. There was relatively little structural damage to modern buildings, industrial facilities and lifelines. This paper reviews the damage to structures and brings out the main points to be considered in future design and construction.

Statistical Characterization of Pitting Corrosion—Part 2: Probabilistic Modeling for Maximum Pit Depth

It is proposed that for immersion corrosion conditions extreme pit depths are those resulting from immediate pit initiation and pit growth without significant metastable activity. This is ...

Optimal Fiber-Reinforced Plate Corners

The plates that will be analyzed herein have their fibers stressed either in tension on compression in each face of the plate, and it is assumed that the matrix material of the plate has no inherent strength except in shear. The problem analyzed in this paper concerns the optimal fiber layout in a corner of a fiber reinforced plate supported by two simply supporting edges meeting at an angle 20 > 90°. It would appear that there are two distinct possibilities for the optimal fiber layout in the corner, depending on the assumptions made, and the interpretation of the boundary conditions. An attempt is made in this note to study the implications of these alternatives.

A convenient approach for estimating time-dependent structural reliability in the load space

A procedure, formulated in the space of the load processes, is described for estimating the reliability of structures subject to multi-parameter time-varying loading. For most realistic reliability problems the load process space is of low order. As a result, the required multidimensional integration is significantly simplified. The proposed approach also has well defined steps. As a result, there is increased transparency and reduced problems of integration instability and non-convergence. Both loads and resistances are described in terms of random variable parameters and time dependent structural resistances can be considered. Numerical examples are given to illustrate the proposed method. Example applications are given for a fixed base rigid-plastic portal frame subjected to time dependent loads and resistances. Linear and non-linear limit state equations and Normal and non-Normal distribution of the random variables are considered and compared, in some cases, to the results evaluated using Monte Carlo simulation.

Wave loading effect in offshore structural reliability

The reliability analysis of multi-membered structural systems such as steel tubular offshore platforms (jacket-type) has proved to be a challenging task. One matter of particular importance is the modelling of applied wave loading. For the structural analyses used in reliability estimation, one single wave loading system only is most commonly considered. Real wave trains, however, reach the various structural components at different times. This may have an effect on the sequence of member failure and hence on structural failure. This can, in turn, affect the calculated probability of failure for the structure as a whole. The importance of this possibility is investigated in the paper for a reasonably realistic but idealized offshore structure.