Condition assessment of pressure water pipelines is a relatively new technology, although it has been well developed in the oil and gas industry for decades. There are two main types of assessment: direct and indirect. Direct condition assessment involves physical measurements of the pipe itself, whilst indirect methods measure some external factor, such as soil corrosivity.
Although condition assessment can frequently relate to steel, the majority of the assessment effort is focused on cast iron pipes, since they represent the largest proportion of pipes in Australia, and many are approaching the end of their reliable service life.
One of the most common methods of direct condition assessment in the oil and gas industry is the use of “÷intelligent pigs’. However the graphitic network of a typical cast iron pipe, and the resultant corroded mass, represent a significant barrier for the traditional “÷intelligent pigs’ used in the oil and gas industry.
Indirect methods of condition assessment rely on techniques that attempt to make an assessment of an asset by measuring a surrogate parameter, which can be extrapolated to give a measure of remaining pipe life.
Actual “÷break’ or failure analysis remains an industry standard. The input of break data into various economic models is still currently the major replacement trigger for watermains in Australian water utilities. Realistically, this is appropriate for small diameter reticulation mains where the costs of direct condition assessment methods are not justified. For critical, larger diameter watermains, however, knowledge of future failure regimes is becoming increasingly important. Whilst a number of indirect technologies have been available for some time, the most usual parameter for buried ferrous watermains is an assessment of soil corrosivity. This is itself a parameter in which strong evidence exists for a direct causal relationship with cast iron and steel pipe performance.
Historically a number of corrosivity measurements have been widely used. These all had limitations and now have largely been disregarded for quantitative purposes. A recent approach is to assess soil corrosivity in quantitative terms, such as a Linear Polarisation Resistance (LPR) measurement. Whilst LPR is not new, its use to predict future pipe performance certainly is. The soil sample has to be prepared with a moisture content at wilt point (the point at which a plant starts to wilt due to limited moisture) before
being subjected to galvanostatic or electrochemical tests. These results are then fed into a set of algorithms, which can be used to predict pipe performance. The algorithms, LPR analysis and predictions are currently intellectual property and as a result are not as yet industry wide practice. Further research and development is currently underway to refine this technique, but at the moment it is the major indirect technique that is capable of predicting future pipe performance because it is fundamentally based on a corrosion rate assessment, rather than a simple qualitative assessment.
Acoustic and transient pressure methods have also been under development for some years, but like most of these indirect techniques, the major issue to be addressed is the ability to detect pitting attack with sufficient sensitivity. However, they can be used to provide an economical broad assessment of a pipeline segment. However, simply blindly applying LPR technology to a buried steel or cast iron pipeline will not necessarily provide an accurate prediction of the future performance of that pipeline. A careful review of the history and circumstances of each watermain being assessed will be required before its future performance can be predicted with sufficient accuracy.
Condition assessment of non-metallic pipe is limited to AC pipe in situations where there are considerable quantities of this pipe in service. Acoustic and transient pressure techniques are having some success, but require further research and validation. In many cases the water industry practice of “÷hot’ or “÷under pressure’ tapping results in the acquisition of a small section of the pipe that can be assessed to provide information on the rate of internal and external deterioration. The rate of internal deterioration in a non-bitumen coated AC pipe system can be quite uniform, but externally there can be considerable variability. However, in general AC pipe is nowhere near as variable as external corrosion of a metallic pipe. These aspects can simplify the assessment of AC pipe systems in comparison with metallic pipe systems.
There is not yet a high demand or major need for detailed condition assessments of PVC or PE, although some general information and performance can be assessed by reverse engineering techniques and testing of mechanical properties of the plastic materials. The reverse engineering technique looks at the operating conditions of the pipeline under assessment to obtains data on operating pressures, and the level and extent of pressure surges to establish if the pipeline is operating within its design capability. This is particularly the case for pumped mains subjected to surge and fatigue, which can result in premature and unexpected failures of plastic pipes. Further development of indirect methods is an area of ongoing active research.
The above is taken from a previously published article by Greg Moore, and has been reprinted with the permission of the Australian Corrosion Association.