Residual fatigue life assessment of structure with unknown loading history 1

[rather_be_riding]

Someone floated a project the other day which involved refurbishing a used item of plant for a customer who has no loading history for it. The plant structure can be considered to be fatigue sensitive and some anecdotal evidence suggests that maybe at some point someone was actually concerned about this but there's really nothing to work with.

For the purposes of debate, lets say that the structure all checks out and welds are subjected to NDT and turn out to be in good condition. How do you go about determining the remaining fatigue life? It could be 90% of its way through its design life or 10%; at either point, the structure should still be in good condition. I recognise this is probably a somewhat unusual case and maybe no one has a great answer but even a good text reference would be appreciated.

Side note: I was unsure if this was better here or in structural but figured that maybe the aero engineers might have valuable input and they'd be more likely to monitor this forum.

 

[HTURKAK]

I just screened your posts and still i do not see any clue for the type of ( a used item of plant ). The loading history is essential data . If not available, you may look for experience with comparable equipment operating under similar conditions,speak with operator for the loading history. You need to conduct fatigue assessment study to determine the remaining fatigue life.

I will suggest you to look to the Part 14 – assessment of fatigue damage of ASME FFS-1 (Fitness-For-Service )

 

[rb1957]

Why is this $1m crane sitting there out of service ?

"DTA" = Damage Tolerance Analysis

another day in paradise, or is paradise one day closer ?

 

[CWB1]

Without the loading history, all the rest is pointless at this stage anyway.

JMO, but its better NOT to have either the history or past analysis of a critical structure for liability's sake until your own work is completed. Both could be flawed and not reflect the true state of the structure, so if you rely on either (or it appears you did) rather than do your own thorough inspection and analysis before signing off then you'll look rather reckless if it fails.

 

[rb1957]

ok, I'm on the opposite side of that. If you have data it's a starting point. How you validate that data is one thing, How suspicious you are of it is another. Is it "factual" ?

With no data you're left with conservative assumptions, or possibly industry "best" practice.

Of course if this is looked at in hindsight (and/or in court) then nothing is going to be good enough !!

still waiting a reply "why is this $1m asset sitting ?"

another day in paradise, or is paradise one day closer ?

 

[rather_be_riding]

@All - having had an opportunity to discuss the issue with the end client, it turns out 3 of the 4 engineers involved had a VERY different idea to the one who approached my client with the project in the first place so this is all academic. However, I threw this out there for the purpose of the discussion just as much as for this particular case.

@LittleInch - the regulator appears to be too worried about whether people are working in high vis and whether trestles are load rated to concern themselves with actually understanding lifting equipment. Such is life. Luckily there will always be a few companies out there who want to do the right thing for some reason.

@FacEngrPE - this applies to almost all lifting equipment in my opinion.

@HTURKAK - lifting equipment was noted but the specifics don't matter - I was intending to float a generalised question to best 'fish' out suggestions from a broad range of experiences. Which as it turns out, has yielded your reference which I'll chew through at my leisure down the track. All approaches do appear to rely on the existence of a load history at first glance however. It was as very quick glance though so don't judge too harshly if you're aware of a specific area which is particularly pertinent.

@rb1957 - I'll answer this one more generally. These cases may come up in or out of service. A common one I've come across involves cranes which have been taken from service and resold without history or even knowledge of their used status to a third party. Sometimes the unsuspecting third party finds out they've been screwed and then they go looking for solutions. The load history in that case is truly completely unknown and could range from unused through to absolutely shagged. Unused mystery lifting equipment is surprisingly common in my experience as well. Sometimes it's left around a site from the previous users and the new user/owner/occupant gets a grand idea to use it. Sometimes, it was a headache that was too difficult to deal with at the time; production was ramping down maybe and the associated handling equipment could be substituted or eliminated rather than maintained or repaired. Then at some point a rejig in lines or processes requires new means of handling something in the area the old equipment is in so they look at repurposing. A very common example would be crane runways as well. They're often reused for multiple crane installations. Sometimes they're checked for alignment if you're lucky. Rarely are they reviewed for fatigue and standard inspections don't look at runways in any particular detail if at all because everyone is too busy getting excited about the mechanical aspect. If you're lucky, they might get some love every 25 years or so. Thanks for the acronym. The DT should have been obvious.

@CWB1 - I understand your train of thought; however lets assume there's some reasonable level of confidence in the history for the sake of arguing over it. At the end of the day, if I sign off on it, I HAVE to rely on that history if I'm going to offer a non-restrictive use of the equipment which is what we really expect to see happen in the vast majority of cases. If I don't trust the history, it's either out of service or subjected to crack growth analysis and much more expensive inspections designated in line with what that yields. The third option might be rb1957's lovely suggestion of testing components which could be very useful and isn't an approach I've previously tried. Technically, if we limit our discussion briefly to cranes specifically, they're required to be assessed for remaining working life on an annual basis by comparison of the loading history with the design classification. At some point, it's essential to rely upon the history, even if you've looked over the crane and detected no signs of fatigue damage because we're talking about the future condition of the item not just how it was at the time of inspection.

 

[rather_be_riding]

@rb1957 - jinx on that. It's either rely on it or go nuclear on cost. Sorry about the slow reply - it's rather early here

 

[rb1957]

ok, so there isn't a piece of equipment sitting around, this is more a general query about fatigue design and maybe some horrible practices ?

It is an unfortunate outcome of fatigue design that having defined a safe life, then the structure is unsafe to operate beyond that life without further investigation. Yes, the structures look sound (or rather "look" sound) but fatigue defects (undetectable fatigue defects) may exist, so continued operation is at peril.

How a used crane can be sold without knowledge of it's previous life, with the intention of being used as a crane, is quite beyond me.

Although, I would have thought that cranes would've been designed for low stress levels, and therefore low/no fatigue effects.

Two approaches to extend the fatigue life, without going "all in" on a through DTA ...
1) repeated "limit" loads which would validate some period of operations (like an annual check-up) ... would require some analysis to validate the approach.
2) design the crane structure to be fail-safe, and for the failures to the clear and obvious. The crane "could" operate with a failed member for a limited period of time, whilst the replacement part was being brought on-site.

My industry has by and large left fatigue behind, but now safety depends on various inspectors and maintenance techs ... but it seems to work ...

another day in paradise, or is paradise one day closer ?

 

[FacEngrPE]

This is a bit off topic - a robust inspection program with knowable inspectors is an essential part of reducing the risks of operating cranes. Well trained operators and riggers are also essential. Companies that chose to ignore this will eventually incur the wrath of their OHS regulators.
However if your company is a supplier of an organization that has decided safe crane operation is really important , and has made it a contract requirement as the US Navy does, you will get in trouble with them first. The NAFAC Navy Crane Center site is worth a read.

The NCC site has several useful safety videos, which are generally applicable.

 

[rather_be_riding]

@rb1957 - Originally the question matched a specific piece of equipment I received an enquiry about but after I managed to speak to someone who actually knew what was going on, it didn't fit the profile. As you can see it's pretty much either a case of it's too hard/expensive or figure out your damage tolerance and inspect to suit. By now it's definitely more a general query to see if anyone approaches things differently to me. Your sample approach is the outstanding option and one day the right job will come along and I might be able to give it a whirl.

RE: used lifting equipment. It's not overly hard really when you think about it. Buy cheap crane, cut and shut, sell aforesaid crane to unsuspecting buyer who makes widgets and knows precisely nothing whatsoever about cranes bar the fact that they need one and it needs to lift a certain load. Get money, try to be far away before anything happens. A lot of the time I imagine no one ever knows anything more about it and everyone continues on merrily. It's not even all that hard to do it if you're honest about it being second hand really. Most people approach stuff like this on the basis that if it's not cracked now, it's OK for as long as they want to use it. I regularly have to argue with people who think a test load proves the competence of a structure beyond all doubt.

Completely wrong on the stress level/fatigue front in the majority of cases. Of course where people are aiming for the higher classifications, you're right but those are the exception rather than the rule. Once again this is a function of cost too. You can sell a C3 crane to someone and conscientiously avoid asking whether there's any possibility that they might really need a C8 crane for the rather continuous duty role they have in mind. You'll be cheaper than the supplier who quotes a C8 crane. Over time, this seems to have the effect of causing everyone to avoid up-speccing rather than resulting in issues for the disingenuous. When cracking starts appearing after 5 years, you might be outside warranty and you can assist the client with great sincerity explaining how they have used it far in excess of its design intent and no one can do anything about that now but you'll do them a great price on a higher classification crane because you can see they're in a tough spot.

Side note - it wasn't my intention to whine about less than professional practises. It just tends to come up when you have to consider what's likely to happen in the real world and how much you can trust anything at face value.

@FacEngrPE - no argument here. Just wasn't the point of the post. I spend a fair amount of time trying to advocate for better quality inspections. It's difficult on the main because of the cost drivers. No-one loves paying for PM. Fixing stuff that ain't broke (yet) isn't very sexy.

 

[CWB1]

lets assume there's some reasonable level of confidence in the history for the sake of arguing over it. At the end of the day, if I sign off on it, I HAVE to rely on that history if I'm going to offer a non-restrictive use of the equipment which is what we really expect to see happen in the vast majority of cases. If I don't trust the history, it's either out of service or subjected to crack growth analysis and much more expensive inspections designated in line with what that yields.

Depending on the work being quoted, I dont see how using that info is possible. If you're simply recertifying a crane that has been continuously operated and maintained under the original O&M program then sure, history is useful and important. Removing and rebuilding (OP said refurbishing) the structure OTOH is a different circumstance entirely bc its outside the original engineer's scope of work - new install + O&M. In that instance you will be 100% design responsible and cannot claim flaws in the original design or O&M records caused failure. Even if there are no failures, if you are discovered to have taken significant shortcuts by relying on the original design (without being the original designer) or O&M history then they are well within their rights to sue or get regulators involved bc that's a pretty flagrant violation of the standard of care.

As to how shops refit cranes, its not a particularly expensive or lengthy process for simple traveling bridges bc realistically - theyre simple machines.

 

[rb1957]

buying a used crane without history ... caveat emptor. Someone buying a crane should know what they have to do to recertify the crane, if not refer PT Barnum.

I assume cranes have some design life (I suspect to be based on a number of max load lifts). History would only help if thoroughly approved/stamped or some such. I've recently done a project where I showed the remaining life in a plane based on the airplane operation records. Airplane records (logbook) are taken as fact. If the crane's records are not held in the same regard as an aircraft's logbook, then they are of little value other than to say number of lifts completed.

another day in paradise, or is paradise one day closer ?