The common proverb that “The Proof of the Pudding is in the Eating” makes it clear that it does not matter how fancy the decoration is, the true test of a pudding is how it tastes. Everybody knows that sticky toffee pudding does not have the looks, but it tastes great.
The same is true of the engineering calculations and analyses that form the basis of most engineering consultancy. The ultimate test, of course, comes when the resulting recommendations are implemented in real-world situations. There are, however, a number of potential drawbacks to implementing advice without some verification, and these drawbacks can be more significant than a disappointingly bland finish to the meal, leaving you wishing you had chosen the cheese board!
For example, engineers are typically cautious and, therefore, may complete calculations with multiple safety factors and contingencies. Consequently, their advice will be very safe, but it may be unsustainably costly. Alternatively, errors in inputs, or the use of inappropriate methods, may lead to unsafe results and, hence, to advice that does not prevent failures.
Some case studies that illustrate the benefits of supporting consultancy advice with practical testing are discussed below.
Case 1: Complex Defects, Geometries and Materials
Pipeline inspection plays a vital role in identifying defects in pipelines, thereby contributing to the prevention of failures. Any structure containing a defect is by definition imperfect and will not have the level of safety and reliability it had as a new, undamaged structure. There are many ways to assess the significance of defects, ranging from relatively simple empirical methods to complex finite element analyses to probabilistic statistics-based approaches. We must remember that with all methods, there can be significant uncertainties, no matter how “fancy” the methods may seem.
Material properties have a huge influence on the strength of a structure containing a defect, yet information is often limited, simple assumptions are made and minimum specified properties are used. The load acting on the defective structure is another key factor. Often, this value is assumed based on the maximum pressure allowed during operation, ignoring additional sources of loading such as temperature, bending and ground loading, or local stress concentrations from deformations such as denting, ovality or roof topping. And of course there will also be uncertainty in the dimensions of the damage, regardless of the measurement technique. Finally, it is important to remember that, regardless of the inputs, the methods used have inherent uncertainties and even with perfect inputs could not be expected to replicate reality exactly.
We also regularly come across cases – be they defect combinations or materials and loading conditions –that fall outside the range of past experiences or proven assessment methods. In these situations, predicting performance with any confidence becomes very difficult, possibly making it necessary to develop case-specific approaches. A good example of just such a case was the advice given to an operator who found that a number of fittings from valve stations had become heavily corroded. While the assessment of corrosion in straight pipe is well understood, the tolerance of tees and bends to damage is less clear. These days, a widely used approach to predicting performance with complex geometries and loading conditions is the use of finite element analysis. In this case, laser scanning was used to generate accurate geometries for the components. Representative material was not available for mechanical testing, so some good-practice assumptions were made regarding stress strain behavior. Burst pressures were calculated showing the components to be safe for the operating pressure. Subsequent burst testing of the fitting in the worst condition highlighted some issues. The actual failure pressure was close to the predicted pressure and well above the operating pressure – which is good. However, the failure location was not correctly predicted.
This case illustrates the challenges, particularly regarding material properties and loading conditions, in assessing complex features as well as the benefits of testing to prove and refine methods.
Figure 1: Finite element model of a corroded tee fitting with equivalent plastic strains.
Case 2: Effective and Efficient Rehabilitation
Maintaining pipeline integrity as systems age leads to more and more pipelines needing rehabilitation as coatings break down and corrosion develops. To go beyond simply advising recoating and into supporting operators with the best choice, it is important to balance a variety of constraints, including coating adhesion, abrasion resistance, cost of application, surface preparation requirements, expected future service, etc. Selection based on experience, understanding service conditions, careful review of manufacturers specifications, evaluation of coating formulation and application procedures can point in the right direction. However, committing to extensive rehabilitation work only to find that the product selected does not perform as expected is undesirable. To help an operator with exactly this challenge, we recently completed a desktop study to identify candidate coatings. A laboratory test program that included accelerated cyclic corrosion test conditions and abrasion resistance testing to simulate many years of service was then implemented for the selected coatings. Running in parallel to the laboratory work, field trials were performed to assess the real-world application and usability of the coatings.
The accelerated laboratory testing allowed a direct comparison of coating performance. The field trial provided additional practical experience and validated the accelerated testing. Considering the data holistically, from the theory to the application, confident recommendations were made for effective and cost-efficient rehabilitation coating choices.
Case 3: Change of Service
Changes of service from oil to gas to water have long been a feature of the upstream and downstream pipeline industry. The widespread adoption of hydrotesting to prove pipelines are fit for purpose was driven by experiences with changing from oil to gas service in the 1950s. There has, however, been increasing interest in repurposing pipelines for the low-carbon future, either to transport carbon dioxide (CO2) captured from power stations or, more recently, to deliver hydrogen (H2) generated with renewable energy. Any change of service may result in new and unexpected threats to pipeline integrity. To help an operator demonstrate that they could maintain pipeline integrity following a future switch to CO2 transport, we completed a theoretical study considering the likely issues with pure CO2 and CO2 with various typical impurities from carbon-capture systems. This study resulted in draft specifications for acceptable gas compositions in relation to the presence of impurities such as water (H2O), nitrogen oxides (NOx) and sulfur oxides (SOx). To test and improve these specifications, a comprehensive testing program was completed in which typical line-pipe steel was exposed to expected and extreme combinations of CO2 and impurities. This process confirmed the process controls needed as well as the reliability requirements, instilling confidence in the consultancy recommendations and ensuring smart decisions and reliable actions.
Testing is a critical element in the suite of services needed to ensure credible and practical advice. It allows consultants to put theories and advice into practice in an environment that is much safer, more forgiving and more controllable than the real world, thereby demonstrating the validity of the approach, reassuring all stakeholders or identifying weaknesses that can then be addressed. Testing also provides the opportunity to go beyond normal conditions and investigate extreme cases.
Just as any chef worth his salt will taste his food as he works to make sure his customers get a delicious meal, consultants must be able to put their work to the test to provide operators and regulators with the confidence they need. Expertise backed by testing ensures that we can all dig into a hearty crumble rather than surviving on tasteless, wobbly blancmange.