Is this assessment or design?

Home / Away from a collision course / Is this assessment or design?

Plant equipment integrity design and assessment methods and standards aren’t just linked to different life stages. The methods are essentially different in their concepts. This difference is sometimes misunderstood, and a strange mixture of design and assessment data then results in ambiguous integrity conclusions. Let’s consider some logic for engineering analysis tools selection.

Rubik's cubes

As promised, this blog is about engineering philosophy, and we begin from a highest level problem: What is the ultimate purpose of engineering activities? Getting things right? Yes, but the ‘right‘ may stand for various and controversial criteria, like budget savings in this year, or so.

The ultimate success criterion for engineering activities is understanding and addressing the reality for a continued safe operation of the equipment. The ‘Reality‘ in our context is the equipment actual load bearing capacity in the environment of real operational loads and damage processes. This is a key element of the design ‘Quality‘, as not ensuring the equipment integrity will always cancel cost/benefit aspirations and aesthetic emotions.

Example. You will soon be buying Christmas gifts for your beloved ones. There is a plenty of shelving in department stores. You will be analyzing cost/benefit and aesthetic features of various gifts, and will end up with few. So, what if your choice will break down in the day after Christmas, as soon as it enters the ‘operation’? Your perception of the object quality will be ruined, right? Those guys should be thinking about people using it! Did they? No, but they still sold their item. You bought the low cost, not the reliability. On the other hand, a higher cost doesn’t necessarily mean a better reliability either.

This situation is sometimes visible in plant design decision making, as the cost/benefit motive is given more priority than the reliability motive. In fact, a ‘reliable’ solution is substituted by a simply ‘cheap’ solution, by implicitly assuming that the reliability level of both alternatives will be broadly similar. But this is never true! Any changes to system design always trigger some changes to some of system properties. When choosing among design options, an engineer’s function is quantifying their impact on the plant integrity/reliability – that is understanding and addressing the Reality, otherwise:

warning sign

To prevent the collision (equipment failures) from happening, we firstly do design calculations. The design purpose is providing the equipment with reasonable safety factors against typical/dominant operational loads. In fact, the design stage uses a simplified abstraction of operating conditions to predict/build in a certain margin of the equipment load bearing capacity with reference to chosen simplified load cases.

The main issue with this approach is the limited number of loads considered, and the typically static formulation of the loads. In reality, most ‘live’ operating loads are fluctuating in time, which fact is responsible for a time dependency of real operational damage mechanisms. Is a wind load fluctuating in time – sure. Is a car chassis load fluctuating – naturally. Is a pipe internal pressure fluctuating – yes, not even mentioning water hammer instances. This are all examples from our everyday life. But does it mean that the design coverage and comprehensiveness are inferior to what they should be?

Yes and No. Design calculations don’t ever aim including all operating loads, as this is not practically feasible. Predicting load spectra is unlikely to be precise enough to describe the future damage progress confidently. Next figure shows an operational stress spectrum as measured. Could we predict that in design? Alas, it is possible, but very unlikely.

stress histogram

Assessment methods are intended to solve this problem – more detailed and specific damage prediction methods, but notably, based on operational evidential data: on real load spectra and on real material resistance characteristics. Design and Assessment calculations are not counterparts, but colleagues. What was reasonably designed must be reasonably assessed in a real operating environment in order to remove design stage uncertainties, obviously, if risks warrant.

What needs to be memorized about basic design calculations (as implied by majority of standards): Most design calculations evaluate static or equivalent-to-static load cases. A simplistic beam theory prevails to describe structural stresses. Stress raiser effects are approximated by discrete factors, hence error accumulation. Specified minimum material strength is used to compare to. Safety factors are applied on top of allowable stresses (or so) determined in the above way. As a result, a time dependent damage accumulation in operation and equipment resource can’t be determined in principle, if design data and calculations are used. But this goal is not ever aimed in design.

In practice, engineering personnel often tends to use design bricks to address assessment purposes, and this situation doesn’t bring us closer to the Reality at all. Few examples:

  • We have made a static FEA and determined peak stresses, then compared the stresses to a specified yield strength. The safety margin is 30%, hence the pipe will never fail. Question: ‘Is your load case actually static one?’
  • We have figured out what is the design case minimum required wall thickness of a vessel nozzle. The corrosion wastage is not yet there, so we have an X years remnant life. Question: ‘How confident are you in the load/resistance parameters used in design calcultations, e.g. is that the actual stress in the nozzle and the actual strength of the material?’
  • The pressure vessel was designed to such a country standard, hence for an assessment we must use other standards by that country, and also, material characteristics used in that country. Question: ‘Do you mean that integrity or that particular vessel at your facility is governed by the country of the vessel design, or maybe rather by actual loads it experiences versus its material strength characteristics as built?’

In this way, assembling a confident and realistic assessment framework is far not a trivial task. It usually requires advanced engineering analysis and as much evidential data as possible. At Quanty, we have a very specific knowledge and experience in dealing with these very problems, as we are looking beyond standards from our practical science foundation.

Why do we need to supplement a design by an assessment? – As soon as we concern about equipment resource and remnant life. What is a level of concern which justifies assessment investments? – This is failure risk driven. A cost-of-risk analysis will help in decision making.

Do you know what is the weakest point of both design and assessment predictions nowadays? The largely missing probability terms. Not knowing the probability of failure versus time is equivalent to not knowing the cost-of-risk versus time. This is why we advocate the practicality of probabilistic assessments of equipment integrity, which we of course can do for you.

Share your thoughts