spotchecksafety

Ever increasing dimensions andweights make it increasinglynecessary to consider the use of twoor more cranes to lift a single load.Such an operation frequently

requires the preparation of a detailed riggingplan to determine clearly the limits withinwhich the lift can be made safely. The main objective of this rigging plan is

to investigate how possible overloading ofthe lift cranes can be avoided. One methodof avoiding overloading cranes is the use ofequaliser lifting beams, but there are othermethods as well. Careful examination of a crane’s capacity

chart is essential before a lift is planned.Different countries prepare these in differentways: for example in the past in theNetherlands, crane capacity charts were allbased on 66²/³ percent of tipping load, whilein other countries such as Germany or theUSA, the crane capacity charts were usuallybased on 75 percent or even 85 percent oftipping. Nowadays, all capacity charts arebased on 75 percent of tipping moment.One should also be aware of the fact that

not only the tipping of a crane but alsostrength of the boom determines themaximum allowable lifting capacity. Thestrength of the crane boom is usually thecapacity range at short radii, whereas thetipping area usually covers the larger radii.In some cases this is clearly marked on thecapacity chart of the crane. When lifting with a single crane, this

margin against tipping ensures that the cranecan accept a certain overload. Under allcircumstances one must try to avoidsituations in which the crane’s rated capacityis exceeded, but as we all know, in somecases it happens unexpectedly, for instancewhen the horizontal level of the crane isdisturbed by weak ground conditions, orwhen wind affects the load. When lifting with two or more cranes,

the load distribution over each crane canhowever be critical and should be calculatedbefore the lift is made. Most contractorsinclude an increased safety margin againsttipping of the crane when a tandem lift isexecuted. The Dutch safety rules state, forexample, that the capacity of each crane

www.heavyliftpfi.com November/December 2012 59

Lifting heavy loads withtwo or more cranesAs loads get bigger, there is an increasing interest in tandem,triple, and even quadruple crane lifts. Richard Krabbendam,who makes a welcome return to HLPFI’s writing team, looksat some of the safety implications in the first of a series ofthree articles.

Lifting a 520-ton column with twotruck cranes, a Demag TC-3000,

and CC2000 crawler crane.

59-69HLNovdec12v1spots_HL template 07/11/2012 10:28 Page 59

spotchecksafety

should not exceed 75 percent of the ratedcapacity when lifting a load with two cranes.This safety margin can be decreased,provided a detailed rigging study is preparedand submitted to the concerned authoritiesfor approval.

Too strict?During my career in heavy lifting, I cameacross quite a few occasions where this generalsafety rule is, in my opinion, too strict. Safetyis an absolute must on jobsites, but it is wrongto apply rules if they are not relevant to aparticular situation. For this reason, I take theliberty of discussing some considerationswhich, in my opinion, make sense whenlifting loads with two or more cranes.One of the first things to do is evaluate

the lift planned and work out what could gowrong and why.This article is the first in a series of three

in which we will look at:l A long horizontal beam (weight =110 tons) which is being lifted by twocranes ‘A’ and ‘B’ (one at each end), inwhich the centre of gravity (CoG) islocated close (1,500 mm) to theimaginary line between both liftingpoints. See figure 1.

l The erection of a pressure vessel(weight = 150 tons) from a horizontalinto vertical position with one main liftcrane and one tail crane. The centre ofgravity is somewhere in between themain lifting lugs ‘B’ and the tailing lug‘A’. See figure 2

l The erection of a pressure vessel (weight= 520 tons) from a horizontal into verticalposition with two main lift cranes and onetail crane. The centre of gravity is locatedcloser to the two main lifting points thenthe tailing point. See figure 4. The above three lifting cases are quite

different from a safety aspect. The first israther straight forward and does not, in mostcases, require any special precautions.However, all three cases have one

determining factor and that is the position ofthe CoG of the load in relation to the liftingpoints (A+B). Depending on the location ofthe CoG and the lifting points it could be aneasy and safe lift, or an extremely difficultand unsafe one. It is the task of the riggingengineer to determine the criteria by which aload can be considered safe and easy, or whenit is necessary to take special precautions. Inall cases we want to achieve one goal: the liftmust be made safely and we can only achievethat when the cranes are not overloaded.The detailed rigging study should prove

to us and our client how we have effectively

November/December 2012 www.heavyliftpfi.com60

Load variation at angle of: 0º 15° 30° 45° 60° 64.5°

Lift point A: 110 tons 94.66 tons 79.76 tons 57.62 tons 19.27 tons 0.18 tons

Lift point B: 110 tons 125.34 tons 140.24 tons 162.38 tons 200.73 tons 219.82 tons

As can be seen from this table, the load variation between lift point A and lift point B at 15°

is now 30.68 tons (approx. 32.24 percent)

TabLE 2

Load variation at angle of:0º 15° 30° 45° 60° 75°

Lift point A: 55 tons 54.11 tons 53.07 tons 51.66 tons 49.21 tons 42.53 tons

Lift point B: 55 tons 55.89 tons 56.93 tons 58.34 tons 60.79 tons 67.47 tons

As can be seen from this table, the load variation between lift point A and lift point B at 15°

is only 1.78 tons (approx. 3.3 percent)

TabLE 1

59-69HLNovdec12v1spots_HL template 07/11/2012 10:28 Page 60

spotchecksafety

and safely planned the lift and show thatnone of the cranes will be overloaded.

Case Study

The CoG in Figure 1 is just below theimaginary line between both lifting points ‘A’and ‘B’. What is the effect on each crane ifthe beam is not lifted horizontally, but onecrane lifts faster than the other? This caneasily be calculated with a mathematicalformula. If we tabulate the values they are asshown in Table 1 (only 3.3 percent extra loadin point B).If the distance between A and B decreases

as per figure 3, and the distance of the CoGto the imaginary line AB increases, it resultsin a much larger load variation as can be seenfrom Table 2 (now 32.24 percent more loadin point B).Because of the inclined position, the

CoG moves closer to point ‘B’ and thereforethe load in ‘B’ increases.When the CoG is located exactly on the

imaginary line AB there will be no loadvariation at all, if the beam is not liftedhorizontally.What have we learned from this exercise?

The most important principle is that onecannot use the same safety criteria fordifferent multi-crane lifts. Each projectshould be evaluated by itself and, if thelocation of the CoG in relation to the lifting

Remember: When the support points (= transport saddles 1 and 2) are not at thesame position in relation to the CoG as theliftpoints (taillug A and lifting trunions B),you can be in for an unpleasant surprise, asthe load in one of the liftpoints can increasesignificantly. (Crane ‘A’ load increases from76 tons to 148.4 tons, when crane ‘B’ lowersthe load on the trailer on saddle 2 first).In heavy lift shipping, it is a daily practice

to make use of the combined strength of twoheavy derricks or cranes on board of a ship.Two 250-ton derricks may be utilised up totheir full combined capacity of 500 tons.The exercise we did proves that there is noreason why we should not do the same withmobile cranes. Of course, all observationsshould be taken into account. HLPFIPlease note, this article is intended for guidance only.Whilst every care has been taken to ensure theaccuracy of the contents, no responsibility will beaccepted by the publishers for any errors.

points justify it, one could consider a lesssevere safety margin. There are also anumber of practical rules that one mustobserve when lifting a beam with two cranes.l Never allow the lifting tackles to becomeinclined during the lift.

l Always ensure that each crane’s boom headis exactly above the lifting point (the liftingtackle should remain in vertical position.No side forces should be applied on theboom). This can be achieved by settingcrane ‘A’ in free swing mode andcontrolling the operation with crane ‘B’.

Danger!

There is one other matter that cannot beoverlooked and easily overloads one of thetwo cranes if one does not know whathappens, when the beam or a pressure vesselis lowered on two supports.Suppose we are lifting a pressure vessel

with two cranes and crane ‘B’ lowers quickerthan crane ‘A’ (see figure 4). Consequentlythe transport saddle (2) closer to point ‘B’touches the trailer turntable earlier then thesaddle (1) near point ‘A’, resulting in quite anincrease of load in crane ‘A’. See figure 5(from 76 tons to 148.4 tons).When one observes these rules of simple

mechanics and studies closely what happenswhen a lift is made with more then onecrane, one can make lifts safely at all times.

www.heavyliftpfi.com November/December 2012 63

www.jumbo-offshore.com www.heavyliftspecialist.com

Richard Krabbendam has been a heavy lift specialistduring his whole working career after which he formedKrabbendam Advies Service. A Master of MechanicalEngineering from Delft University of Technology, he hasworked with BigLift and Mammoet, and was a co-founder ofITREC. He helped to set up Jumbo Offshore and was involvedin the development of its super heavy lift carrier fleet, the J-Class, which uses two 900 tonne mast cranes for subseainstallation works. Since his retirement from Jumbo he hasbeen working as a freelance trainer/engineering consultant.

59-69HLNovdec12v1spots_HL template 07/11/2012 10:28 Page 63