USEFUL STRUCTURAL LIFE ASSESSMENT OF DOCKSIDE CONTAINER CRANES

Kenton Lee, SEAssociate, Liftech Consultants Inc.

Feroze R. Vazifdar, SEVice President, Liftech Consultants Inc.

Simon L. H. WongEngineering Manager, Hongkong International Terminals

Reprinted from Ports ’01Proceedings of the Conference

American Society of Civil EngineersHeld April 29–May 2, 2001

Norfolk, VA

© 2001 Liftech Consultants Inc.

12.doc 1

USEFUL STRUCTURAL LIFE ASSESSMENTOF DOCKSIDE CONTAINER CRANES

Kenton Lee, ASCE Member, Associate, Liftech Consultants Inc., 3666 GrandAve., Oakland, CA 94610, klee@jwdliftech.com

Feroze R. Vazifdar, ASCE Member, Vice President, Liftech Consultants Inc.,3666 Grand Ave., Oakland, CA 94610, fvazifdar@jwdliftech.com

Simon L. H. Wong, Engineering Manager, Hongkong International Terminals,Kwai Chung Container Port Container Port Road South, Kwai Chung,New Territories, Hong Kong

BACKGROUND

What is to be gained by a useful structural life assessment of older cranes? What is �usefullife?�

The useful structural life is the remaining time the crane can be operated with an acceptable riskof failure. When first asked to consider acceptable risk, the usual response is an acceptable riskis no risk. Unfortunately, all structures have a reliability of less than 1.00.

For cranes, the most common acceptable risk of a single structural detail failing is about 1 in 50,using the damage tolerant design philosophy. The consequences of the failure of one detailmay be limited with periodic inspection.

This paper presents the methods used to develop a structural inspection program and to evaluatethe useful structural life using statistical analysis and the principles of fracture mechanics.

THEORETICAL OVERVIEW OF STRUCTURAL LIFE EXPECTANCY

The prediction of fatigue crack growth is based on statistical data and the principles of fracturemechanics. Cracks grow from initial discontinuities, usually at welded joints. The crack sizeincreases with each cycle of loading, until the crack reaches a critical size and the member failssuddenly without warning. The study determines the actions needed to reduce the risk of suchfailures to acceptable levels and if such actions are economic.

Crack prediction based on the statistical approach is not perfect. It provides a method ofimproving the reliability of the structure.

The reliability can be estimated once the current condition of the structure and the operationaldemands are known. The useful life may be extended by improving details and increasing theintensity and frequency of the structural inspection. However, the cost of the improvementsmay not be justified, in which case other options need to be considered. Other options rangefrom changing the use or refurbishing the structure to scrapping the cranes.

SEQUENCE OF TASKS

A structural life assessment of a container crane follows the steps listed below.

© 2001 Liftech Consultants Inc.

12.doc 2

1. Perform a condition survey.

2. Perform a fatigue cumulative damage analysis of the cranes and determine the non-destructive testing (NDT) inspection intervals.

3. Make an initial estimate of the useful life of the cranes based on current maintenancelevels.

4. Develop a structural maintenance program and provide procedures for the repair ofstructural defects.

5. Develop the final estimate of the useful life of the cranes based on new maintenancelevels.

6. Explore options to refurbish, scrap, sell, or relocate the cranes based on economics.

1. Condition SurveyThe engineer�s visual assessment provides valuable information regarding the crane�soperations and the present crane condition. The condition survey provides a comparisonbetween the �as-built� condition of the crane and that shown on the manufacturer�s drawings.The survey also provides a means for the engineer to assess the condition of fracture criticalmembers (FCMs) and determine whether they have any welded attachments that couldaccelerate fatigue crack growth.

Fracture critical members are tension members or tension components of members whosefailure could lead to collapse of the crane, collapse of the trolley, or dropping the load. Weldedattachments to FCMs can severely accelerate fatigue growth in an otherwise acceptable design.Of special significance is the elimination of wrap-around weld details on the fracture criticalmembers and connections. Wrap-around welds are prohibited by AWS.

The fatigue life can be shortened by 2.5 times if a fillet weld is too near an edge of a member.See BS 7608; 1993. The edge weld changes the detail from class F to G. A crack may initiateat a poor weld detail and grow into the parent metal of a fracture critical member. Since fatiguecracks grow perpendicular to the principal stress, the crack will grow across the member. Asimple weld connecting a walkway or electrical box can lead to a serious failure.

During the condition survey, the engineer takes extensive photographs of each joint. Thephotographs will be included in the structural inspection manual that will be used by the NDTinspector to understand what to inspect and to report his findings.

2. Cumulative Damage Analysis and Estimating Inspection IntervalsCurrent specifications for cyclically loaded structures adopt a damage tolerant designphilosophy. This means that if fatigue cracks were to occur in any given member, theremaining structure should be able to safely carry the load until a routine periodic inspectiondetects the crack. Therefore, the periodic inspection interval should be long enough to make theinspection economically feasible, but short enough to detect the crack before it reaches anunstable state. The cumulative damage analysis provides a method to estimate the inspectionintervals.

For the HIT cranes, Liftech used BS 7608 (BSI 1993) for the cumulative damage assessment.

© 2001 Liftech Consultants Inc.

12.doc 3

A computer model was generated for each crane based on data provided on the structuraldrawings and on field information gathered during the condition survey. The fatigue and loadspectrums were generated based on the theoretical vessel operation, the trolley loading, and thenumber of cycles of operation. The fatigue spectrum describes the vessel loading andunloading operation for the trolley. The load spectrum describes the trolley loading and thenumber of cycles of operation during the life of the crane.

Miner�s rule is used to calculate the cumulative fatigue damage produced by the spectrum. Thecumulative fatigue damage is defined as:

CD = Σni x ∆σ3

Where:CD = Cumulative Fatigue damageN = The number of cycles∆σ = The calculated stress range

The computed cumulative damage is compared to the cumulative damage that the detail canwithstand reliably as determined from tests.

K2 = N x ∆σTestm

Where: m = 3 for most detailsN = The number of cycles that the test sample withstood at two standard deviationsbelow the mean, or a design reliability of 0.9773, when subjected to a constant stressrange of∆σ Test. Α reliability of 0.9773 means there is a 2.27% chance of failure.

The ratio between the calculated and allowable cumulative damage is the relative cumulativedamage, R.

R = CD/K2

Liftech�s statistical method uses principles of fracture mechanics to determine inspectionintervals for cranes.

Using the relative cumulative damage, R, the desired reliability factor, d, and an importance factor, I, theinspection interval in cycles is determined.

n = ( ∆(d-2))N/(RxI) in number of cyclesWhere:

∆ = Reciprocal of the antilog of the standard deviation of log N.d = reliability factor; d = 2 provides a reliability of 0.9773 and is commonly used forcranes.N = Design no. of cycles. N = 2,000,000 cycles is commonly used for older cranes.I = Importance factor as shown in the chart below.

IMPORTANCE FACTORI Description

4.0 Non-redundant, tension (FCM)2.0 Redundant, tension; non-redundant, compression (FCM)1.0 Redundant, compression (non FCM)

© 2001 Liftech Consultants Inc.

12.doc 4

Typically, Liftech recommends inspecting the crane when the relative cumulative damagereaches 0.6. This provides a reliability of 0.999, a significant improvement over 0.9773.

Liftech�s inspection program concentrates the inspection effort where it is most important.Only a few fracture critical members with high relative cumulative damage may require morefrequent inspection, while all other joints could be inspected less frequently. Through cost-effective inspections, reliabilities can be greatly increased, and the life of a crane can beextended well beyond its original planned life.

3. Initial Assessment of Useful LifeThe initial estimate of the useful life, prior to a NDT inspection of the cranes, is based on thecurrent condition of the cranes and on predicting the number of fatigue cracks in a crane. First,the relative cumulative damage is calculated as discussed above.

R = CDact /CDdes

CD act = calculated cumulative damage due to the actual operation to date. CD act is proportionalto Σnactx MLFact

3

CDdes = K2, the allowable cumulative damage. CDdes is proportional to Σni x MLFdes3

Knowing R, and assuming a Gaussian distribution for the test data, calculate the reliabilityfactor d.

d = Ln(R)/Ln(∆+2) in number of standard deviations below the meanWhere:

∆ = Reciprocal of the antilog of the standard deviation of log N from test data.Most connections on a crane are classified according to BS 7608 as class F/F2 details. For thesedetails, ∆ = 0.6.

Finally knowing d, the reliability, r, is calculated.

The predicted number of cracks is as follows:Predicted fatigue cracks = (1-r) x Nj

Where:r = reliability of the joint at d standard deviations below the mean.Nj = no. of joints in the crane subjected to fluctuating stresses.

The estimated remaining structural life, prior to NDT inspection, is based on the relativecumulative damage, the crane�s design life, and the number of years in operation.

Estimate of remaining useful life = (1/R-1) x YopWhere:

Yop = years in operation

Typically, the estimate of remaining useful life will increase after the crane is inspected andcracks are repaired. So, the initial assessment provides the useful life of the crane, at its currentmaintenance levels, prior to repairing the cracks.

The initial estimate of the useful life provides options to the owner. If the predicted number ofcracks is excessive, the owner may scrap the cranes and forego a costly NDT inspection. If theestimate of the useful life is between eight to ten years, he may decide to proceed with the NDTinspection, crack repairs, and refurbishment of the cranes.

© 2001 Liftech Consultants Inc.

12.doc 5

For HIT�s cranes, the two Paceco cranes had an initial useful life assessment of one to twoyears. However, HIT decided to proceed with the remaining investigation.

4. Structural Maintenance ProgramLiftech�s structural maintenance includes preparation of a NDT inspection manual for eachcrane. The manual addresses the structural details to be inspected: whether the detail is fracturecritical or non-fracture critical; the method of locating each detail; the required method ofinspection � Visual, Magnetic Particle, Ultrasonic, or Radiographic; the inspection interval foreach weld detail; the inspector�s qualifications; the required reporting procedure for the defectfindings; and the repair procedure. An NDT inspector inspects the crane; the Engineer preparesa repair procedure or redesigns the connections, and the cracks are repaired.

If a crack is detected in time and repaired before it becomes unstable, the metal in the vicinityof the repaired crack is rejuvenated, and the reliability of the repaired joint would be the sameas new. However, as the cranes age, the cracking pattern becomes more unpredictable, and thefrequency of cracking increases nonlinearly. Thus, closer inspection intervals are required asthe crane ages. If properly maintained, the cranes mechanical and electrical components shouldreach obsolescence long before the structure.

5. Final Assessment of the Useful Life of the CraneThe NDT inspection of the crane provides the actual number of fatigue cracks that havedeveloped in the crane.

Based on the actual number of cracks, the revised reliability, r, and the revised relativecumulative damage, R, are computed. The remaining life at current maintenance levels is thenrecalculated.

When the cracks in the crane structure are repaired, the life typically improves by at least oneinspection cycle, between three and six years.

6. Determination of Economic Benefits by the OwnerOnce the useful life data is determined, the crane owner can make an economic assessment ofthe cranes. Refurbishment could include new drives, outreach extension, and crane raise. If theremaining useful life is low, the owner may decide to scrap the cranes, relocate them to a lighterduty port, or sell the cranes.

HIT CASE STUDY

In 1999, Liftech Consultants Inc. was retained by Hong Kong International Terminals to assessthe structural useful life of eight dockside cranes manufactured by Paceco/MES, IHI, andHitachi. The cranes had been operating for 14 to 28 years. HIT wished to operate the cranesfor an additional 10 years if the structures have low risks of catastrophic failure.

Quay Crane DataThe cranes had trolleys with the trolley travel drive machinery on the trolley frame. ThePaceco/MES cranes had truss type booms. The IHI cranes had twin plate girder booms, withthe trolley rails on the outside of the plate girders. The Hitachi cranes had twin rectangular boxgirder booms with the trolley rails on the inside of the girders. See below.

© 2001 Liftech Consultants Inc.

12.doc 6

Crane I.D. Yr Com-missioned

SWL underspreader

Outreach/Back-reach/Gage

No. of lifts basedon Twistlock Count

Paceco 63Paceco 71

1972 32.7 LT (revisedfrom 35 LT)

36m/9.14m/24.38m

2,600,000

2,650,000

IHI 41IHI 43IHI 61IHI 64

1976 40 LT 36.6m/9.14m/24.38m

2,100,0002,200,0002,300,0002,400,000

Hitachi 42Hitachi 62

1985 35 LT 36.6m/9.14m/24.38m

1,500,0001,450,000

Condition SurveyIn general, the cranes were found to be in good condition. Hitachi 62 was in the best condition.Some corrosion and other non-fatigue related problems were found and will not be discussed inthis paper.

Indications of cracks were on all cranes and were noted in the NDT inspection manuals.

Cumulative Damage Analysis and Inspection Interval EstimatesLiftech performed cumulative damage analysis of the three different types of cranes based onthe crane operating data provided by HIT and the assumed fatigue design criteria.

In addition to an annual visual inspection of the cranes, the following table shows therecommended inspection interval for a few sample components of the Paceco/MES cranes.Similar tables were also generated for the other cranes. The table identifies non-fracture criticaland fracture critical components. The inspection interval is either the number of containermoves or the number of years from the latest inspection, whichever occurs first.

NDT INSPECTION INTERVALS FOR PACECO/MES CRANESCOMPONENT FCM/NFCM INSPECTION INTERVAL

LESSER OFNO. OF MOVES YEARS

FRAMELandside Trolley Girder Connection FCM 300,000 3Landside Trolley Girder Support Beams FCM 600,000 6Waterside Trolley Girder Support Beams FCM 1,200,000 12Portal Beam NFCM 2,400,000 24BOOMDiagonal @ Upper chord NFCM 300,000 3Forestay FCM 600,000 6Braces @ Upper Chord NFCM 1,200,000 24

Only a few of the crane structural components require inspection once every three years. Theremaining components need to be inspected at 6, 12 or 24-year inspection intervals. This is

© 2001 Liftech Consultants Inc.

12.doc 7

economic. A three-year inspection interval for all joints is excessive and a six-year inspectioninterval is probably excessive for some joints and inadequate for others. Using the inspectionintervals shown above, the required down time to inspect the cranes is also significantlyreduced.

Estimated Fatigue Crack Frequency vs. Actual Fatigue Cracks Documented in NDTInspectionBased on the crane operating data provided by HIT and the assumed fatigue design criteria forthe MES, IHI, and Hitachi cranes, the expected frequency of detectable fatigue cracks wascalculated at current maintenance levels, prior to an NDT inspection.

An NDT inspection was then performed. The inspection provided data on the actual crackingpattern for the cranes. The table below compares the predicted number of fatigue cracks foreach set of cranes at current maintenance levels vs. the actual fatigue cracks detected during theNDT inspection. Fatigue cracks are those cracks that originated at FCM weldments as a resultof cyclical container loading of the crane structure.

Crane I.D. Predicted no. of fatigue cracks Fatigue cracks detected during NDT inspectionPaceco 63 33 to 36 13Paceco 71 35 to 38 7

IHI 41 3 to 5 2 to 3IHI 43 3 to 5 5IHI 61 4 to 6 12IHI 64 6 to 8 11

Hitachi 42 0 to 1 2Hitachi 62 0 to 1 3

The actual fatigue cracking pattern for IHI 41 and 43, Hitachi 42 and 62 mimics the predictedpattern quite closely. There is a significant variation in the other cranes.

Since the relative cumulative damage varies as the cube of the stress range, a small variation inthe stress range magnifies the relative cumulative damage significantly. For the Paceco/MESand the Hitachi cranes, the average moving loads used in the analysis are 74 kips and 70 kips,respectively. A 10% reduction in this weight would account for most of the variation in thecracking pattern. The test data had a large scatter which accounts for some of the statisticalvariations. The combination of both effects probably accounts for the sharp difference in thecracking patterns.

Useful Structural Life AssessmentThe table below compares the estimated future structural life for each crane at reliabilities of97.73%, prior to NDT inspection, after NDT inspection, and after all repairs are completed.

Structural Life ExpectancyPrior to NDT

inspectionBased on NDT

inspection resultsAfter all repairs are

completedCrane I.D. Years Years Years

Paceco 63 0 to 1 6 to 7 11 to 12

© 2001 Liftech Consultants Inc.

12.doc 8

Paceco 71 0 to 1 10 to 11 15 to 16IHI 41 12 to 13 16 to 18 20IHI 43 12 to 13 10 to 12 15 to 17IHI 61 10 to 11 5 to 7 10 to 12IHI 64 8 to 9 6 to 7 11 to 12

Hitachi 42 15 to 16 10 to 11 15 to 16Hitachi 62 16 to 17 8 to 10 13 to 14

In the case of the structural life expectancy of the cranes after repairs are complete, we haveincreased the life by approximately one inspection period, except for IHI 41, where thestructural life expectancy is limited to the twenty-year design life. This is reasonable, since therepaired areas are now rejuvenated and the metal in the vicinity of the repairs has an improvedreliability.

When all recommended repairs are complete, we estimate the useful structural life of the cranesat between 10 and 20 years as shown in the table above. We expect the useful structural lifewill significantly exceed the mechanical and electrical obsolescence of the cranes.

RecommendationsOf special significance are repairs to eliminate all wrap-around weld details on the fracturecritical members and their connections. The fracture critical members are the forestay,backstay, landside A-frame brace, trolley girder, boom, both trolley girder support beams, andportions of the trolley structure.

Other areas that need attention are welded attachments to the trolley girders and other FCMs onthe Paceco/MES, IHI, and Hitachi cranes. These connections need to be modified so the welddetail is transformed from a fatigue class G to a fatigue class F2, or better. Modifications wereprovided in our report to HIT.

OWNER�S PERSPECTIVE

Liftech has demonstrated that the results of the statistical approach compares well with actualfindings. The NDT inspection program generated by them concentrates on the FCM members.It provides an economic and time saving alternative to inspecting all joints at the minimuminspection interval. At the same time, it addresses the critical joints whose failure could result incatastrophic consequences.

At the time of preparing this paper, the recommended structural repairs are 90% complete. Astructural maintenance program for the cranes is now in place, that will extend the life of alleight QCs by an additional ten years.

With the forecast of increasing throughput, HIT wants to improve the berthing facility wherefive panamax QCs operate to handle post-panamax vessels. Two IHI cranes, which have thelargest number of weld defects, are being considered for transfer to other operationally lessdemanding terminals within the HPH group. For the remaining six QCs, HIT is consideringretrofitting the electrical controls and drive systems at a rate of two cranes per year.

© 2001 Liftech Consultants Inc.

12.doc 9

RELATED MATERIAL

British Standards Institution, BS 5400: Part 10: 1980, Steel, Concrete and Composite Bridges,Code of Practice for Fatigue, 1980.British Standards Institution, BS 7608: 1993, Code of Practice for Fatigue Design andAssessment of Steel Structures, 1993.American Welding Society, ANSI/AWS D1.1:2000, Structural Welding Code-Steel.Jordan, M.A., Nondestructive Evaluation of Crane Structures, American Association of PortAuthorities, 1989.Jordan, M.A., Structural Maintenance of Dockside Container Cranes, American Association ofPort Authorities, 1999. Recommendations

Maddox, S.J., Fatigue Strength of Welded Structures, Abington Publishing, Cambridge, 1991.

© 2001 Liftech Consultants Inc.

© 2001 Liftech Consultants Inc.

© 2001 Liftech Consultants Inc.