World Port Development June 2014 - Container Crane Supplement8

c o n t a i n e r c o n t a i n e r c r a n e s u r v e y

STS Cranes - ‘Thorough’

Structural Examination

Most operators do not implement theirstructural maintenance practices and oftendon’t inspect in a regular intelligent way. Inthis article Michael Jordan, CEO at LiftechConsultants explains the current situation…

hree in the morning on a cool, foggy,and windless day, the structural engineer arrives at the ship-to-shore

crane. One boom support hanger is fractured.The low profile boom is extended over theship, which is now unable to depart as plannedbecause the boom is in the way.Why did thehanger fail? It was overload tested just weeksearlier. So how could it not be strong enough?Why didn’t both hangers fail like they did onthe Port Elizabeth low profile crane? Howcan we roll the boom back and free the ship? The hanger failed when a fatigue crack grewto a critical size.The size is critical when thework done by the deflecting cracked structureexceeds the work absorbed by the cleavingsteel, resulting in brittle fracture. It is a stabilityproblem, like when a rolling cart passes thecrest of a hill, and rolls down the other side.This failure could have been avoided if thepersonnel involved in the “thorough” examinationunderstood the issues.The overload test onlydeals with the yield strength of the structure.It cannot test the fatigue strength after thereis more use. It only tests the present condition.Later, after more load cycles, which makecracks grow, the lifting capacity is less. Cracksgrow. Once the “cart” rolls over the crest,even a little: BANG! Oops! The hanger breaks.The “cart” looked so benign sitting on thecrest. But once it has rolled over the crestthere is no stopping it.Who knew that theremight be a problem not revealed by theoverload test? The structural engineer knew.The boom did not fall because the structuralengineer anticipated the danger and, althoughnot required by any code or specification, hedesigned the boom support hangers so if onefailed the remaining waterside hanger couldcarry the entire weight of the boom. He spentan insignificant amount on the structuralcost, saved the stakeholders a bundle, andprobably saved the operator’s life - a very bigbenefit for a very small cost.

How do we safely move the boom? The boomhad rotated so much that the walkwaysinterfered with the hanger. But there was nostructural damage to the boom or theremaining hanger.The situation just lookedbad.Answer: Cut off the walkways and rollthe boom back, riding on only one hanger.When the engineer explained the solution,those standing next to him on the boomwere unconvinced. Even though he told themthe crane is designed for this event, they allquickly climbed down to the wharf and stoodclear to watch what they thought would be adisaster.The walkways were cut loose andthe engineer, who was never before allowedto operate the crane because the lawyerswouldn’t allow it, retracted the boom and wenthome to bed.A nuisance but no disaster! Butthere is one more event worth mentioning.When the engineer climbed to examine thefailed plate, he discovered wet paint on thehanger right where the brittle failure occurred.Why was there wet paint? The original painthad been removed and the crane had beeninspected by magnetic particle examinationand then repainted the day before! So whatwent wrong? There was a load test and therewas an inspection. But the players, althoughcompetent and conscientious, simply didn’tunderstand fatigue and brittle fracture.Theythought the overload test proved the cranewas safe, a common misconception.

The phenomena of brittle fracture in steelare complex. Fatigue crack growth is alsocomplex. Both brittle fracture and crackgrowth are understood by many engineers,but not by many maintenance personnel. Solet’s learn.The Fatigue Criteria sketch explainswhat happens in simplified terms.As loadsare applied, cracks grow and they grow exponentially until the component fails.Thesketch shows a simplified, condition of anotched bar subjected to only five load cycles,a fictitious situation but sufficient to explainthe principles.The stress pulls the atoms, thedots, apart.They are trying to hold on buteventually the bond between the atoms atthe root of the notch break.The crack growsuntil sudden brittle failure: BANG! Oops!Understanding fatigue damage and failure isvery simple except for a myriad of difficulties.If you have ever split a log for kindling andwatched a piece fly off, you saw a brittle fracture.A small tap will send the piece on a speedyflight.Where did the kinetic energy comefrom? Certainly the tap wasn’t enough.Theenergy was stored in the elastically flexedpiece and was just waiting for the chance todepart.This is brittle fracture.We hope thefollowing explanations are enlightening.

One difficulty is acceptable risk.The allowablefatigue damage is based on thousands of fatiguetests, but only on tests. No mathematician orcomputer program can accurately predict theperformance of a particular detail subjectedto a particular experience.The test resultsare the best we can do now.We have a lot ofstatistical data but the scatter is large.Wemust deal with probabilities; our work is not anexact science. One problem is, like snowflakes,no two welds are the same.They may be verysimilar but they are not the same. So we relyon statistical analysis of test data. A largeamount of test data has been generated.Welds are classified according to the details.For each class allowable stresses are determinedbased on reliability. For fracture critical components, that is, those whose failure would

T

Boom or bust

STS Cranes - ‘Thorough’

Structural Examination

So what are the difficulties?

Brittle fracture

Container Crane Supplement - June 2014 World Port Development 9

c o n t a i n e rc o n t a i n e r c r a n e s u r v e y

be a catastrophic, the chance of failure is onein fifty for each detail, if the structure is usedas assumed and never inspected.These areunacceptable odds.The odds must be improvedby “thorough inspections.” Since the risk isnever zero we must live with some chance of a catastrophe.The crucial question is:Whatchance of catastrophic failure is acceptable?The code allowables are based on one failurein fifty, if there are no “thorough” inspections.These are unacceptable odds.They must beincreased.The most economical way is by

intelligent, economic periodic inspection.Many standards require periodic inspections,but do not specify an acceptable risk. Liftechuses a risk of failure of 1 in a 1000 for non-fracture critical components, thosewhose failure would be a nuisance and one in 100,000 for fracture critical components,those whose failure would cause a catastrophe.These reliabilities can beachieved through the use of fracture mechanics analysis and competent periodicinspection.

The situation is exacerbated by many misnomers.The term “fatigue” implies thatthe steel gets old and tired like an animal.Steel does not get old and tired.What we callfatigue is simply crack growth. Crack growthstarts at an initial discontinuity.All weldedstructures contain discontinuities, especiallyat the toe of the welds.When fluctuatingcyclical stresses are applied the cracks simplygrow.The atomic bonds between atoms break.This transfers more stress to remaining atomsand the more atoms with broken bonds, thefaster the crack grows. Once the crack is largeenough, the structure fails suddenly withoutwarning, just like the hanger failed.This is shownon the figure - Fatigue Criteria.An exampleof the how the misleading term, fatigue, causesconfusion is evident by the wording in section6.3 of the Code of practice for the use ofcranes Part 2: Inspection, maintenance andthorough examination -Cargo handling cranesof BS 7121-2-9:2013, an excellent guide and astep in the right direction:“Steel structuressuffer from fatigue, a process which can createcracks that propagate over time.” Fatiguedoes not create cracks, the cracks, althoughundetected, are there initially. Fatigue is thegrowth of cracks. Fatigue does not mean thesteel is tired and aging.The British Standardcorrectly calls attention crack initiation atundetected weld discontinuities. Clearly,undetected weld discontinuities can and docause serious failure.Another misunderstoodterm is “fatigue life.” The structure does notdie or even need to be considered unreliablebecause its use is longer than the fatigue life.The issue is cumulative damage and theresulting reliability, which depends on theapplied load and the number of applicationsand the fatigue strength of the weld detail.The acceptable cumulative damage can bedetermined.The reliability can be determinedusing fracture mechanics and statistical analysis.

Most cranes come with a recommendedinspection regimen and, based on Liftech’sresearch, most operators don’t follow theserecommendations.They modify to plan withoutsufficient knowledge to make cost effectivedecisions. Some details that should be inspectedare not and some that aren’t worth beinginspected are. Considering the cost ofinspections, attempts to not perform the fullregimen are reasonable. Just as for earthquakesand high winds, the chance of a catastrophefatigue failure is low and is very unlikely to

Misnomers

Thorough examination

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occur during a particular person’s watch. Butthey do occur, sooner or later. Failure is unlikely.The damage, however, may be great. In effect,there is zero chance of an infinite loss.

To complete the picture we should look atthe more easily understood mode of failure:ductile plastic failure. The most commonexample of ductile plastic failure is the tensionbar strength test.The mode is very differentfrom the brittle fatigue failure mode.The atomsresist the applied load through shearing stresses.The atoms slide by each other and beforecollapse the distortion is great. Imagine abent mild steel bar.At first it is elastic, thedisplacement is temporary.When the load isremoved the bar returns to its originalshape. If the applied load is increased beyondthe elastic limit the bar bends plastically. Itremains bent even after the load is removed.The failure is stable and slow, no sudden collapse.The yielding bar gives a warning thatthe bar is failing.The criteria for this mode issimple, strength is the primary variable. If thebar passes the load test, it is ok. It is strongenough and we can depend on its strength.When subjected to loads, the displacementdoesn’t grow with time.

The third sketch presents the acceptableenvelope.The envelope is bounded by both thestatic strength limit and the fatigue limit.Theallowable load and cycle boundaries will varydepending on each situation. Some “thoroughexamination” recommendations with comments:☛ Follow the spirit of the British Standard withthe following amplifications and exceptions,when the exceptions are supported bycompetent engineering analysis.

☛ Use a 1 in 1,000 chance of nuisance failures and a 1 in 100,000 chance of a catastrophic failure.☛ If you have a limited budget, which mosthave, concentrate on avoiding catastrophesand tolerate nuisance failures. Put your moneywhere it does the most good. Make decisionsbased on rational engineering analysis.☛ Educate inspectors and maintenance personnel about fatigue phenomena so theyknow what to look for, how to look, and whichindications are important and which are not.☛ Use visual inspection; but in addition,always use some magnetic particle or similarNDT methods to examine the surfaces atweld toes. Inspect full penetration weldsusing ultrasonic examination.Visual inspection

isn’t useless but remember by the time youcan see a crack, unaided, the number of loadapplications before failure is often five percentof those already experienced. So a visible crackmeans failure will be sooner rather than later.☛ Determine inspection intervals based on youracceptable risk.Use an acceptable risk approachand not on approach based on predicted crackgrowth size studies. Base the interval oncrack initiation and the ability to find cracks.☛ Determine reliability based on fracturemechanics, and not age, to determine if thestructure is safe to use. Some of our cranesthat have been well maintained remain reliable for twice the “design life.”☛ Identify the fracture critical components onthe structural drawing and show the ratio of thecalculated cumulative damage to the design allowable cumulative damage for each component.☛ Keep well documented records of inspectionsand findings. If you have a large number ofcranes, create a database so you will knowwhich details are problematic.

We and others will have more suggestions andcomments.We hope this discussion is the firststep in our journey to provide safer and morereliable structures at reasonable costs.

Ductile shear failure

The acceptable envelope

More to come