Abstract
Water-injection, oil production and water-supply pipelines are prone to pitting corrosion that may have a serious effect on their longer-term serviceability and sustainability. Typically, observed pit-depth data are handled for a reliability analysis using an extreme value distribution such as Gumbel. Available data do not always fit such monomodal probability distributions well, particularly in the most extreme pit-depth region, irrespective of the type of pipeline. Examples of this are presented, the reasons for this phenomenon are discussed and a rationale is presented for the otherwise entirely empirical use of the ‘domain of attraction’ in extreme value applications. This permits a more rational estimation of the probability of pipe-wall perforation, which is necessary for asset management and for system-sustainability decisions.
Highlights
Pipelines play an important if seldom-recognized role in modern society
Only pitting is considered, limited to the low-alloy steels and cast irons widely used for these pipe systems
The vertical axis of the Gumbel plot is usually defined in terms of the standardized Gumbel variable w, defined as w = (x − u)α, where x is the maximum pit depth having a cumulative distribution function (CDF), Fx (x), and a probability density function (PDF), fX (x), each defined by: FX (x) = FW [(x − u)α] with FW (w) = exp (−e-w )
Summary
Pipelines play an important if seldom-recognized role in modern society. They are used as part of offshore and on-land oil and gas production and transportation, drinkingwater supply systems and the conveyance of sewage to treatment plants. The prime focus is on the variability in the maximum pit depth encountered in the practical applications and, the prediction of the probability of wall perforation This has implications for asset management and for life extension and, by implication, for sustainability. The third application is for the external pitting corrosion of (usually unprotected) cast iron drinking-water mains as used extensively in many cities and urban areas world-wide These pipes are buried in various soils, ranging from sands to heavy clays, and, as will be seen, this has some effect on the variability of pit depth. A brief review is necessary of the modern understanding of the pitting-corrosion phenomenon This is given in the Discussion, couched in phenomenological rather than electrochemical terms, and shows that much of the conventional notion of the progression of pit depth must be revisited.
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