As loads get bigger, there is an increasing interest in tandem,
triple and even quadruple crane lifts, Richard Krabbendam looks at some
of the safety implications.
Ever increasing dimensions and weights make it more and more necessary to consider the use of two or more cranes to lift one load. Such an operation frequently requires the preparation of a detailed rigging plan to clearly determine the limits within which the lift can be made safely. The main objective of this rigging plan is to investigate how possible overloading of the lift cranes can be avoided. One method of avoiding overloading cranes is the use of equalizer beams, but there are other methods as well. Careful examination of a crane’s capacity chart is essential before a lift is planned. Different countries prepare these in different ways: in the Netherlands for example, the crane capacity charts were all based on 66 2/3 % of tipping load, while in other countries such as Germany or the USA the crane capacity charts are usually based on 75 % or even 85 % of tipping. Nowadays, all capacity charts are based on 75% of tipping moment. One should also be aware of the fact that not only tipping of a crane but also strength of the boom determines the max. allowable lifting capacity. The strength of the crane boom is usually the capacity range at short radii, whereas the tipping area usually covers the larger radii. In some cases this is clearly marked on the capacity chart of the crane. When lifting with one single crane, this margin against tipping ensures that the crane can accept a certain overload. Under all circumstances one must try to avoid situations in which the crane’s rated capacity is exceeded, but as we all know in some cases it happens unexpectedly, e.g. when the horizontal level of the crane is disturbed by weak ground conditions, or when wind affects the load. When lifting with two or more cranes the load distribution over each crane can, however, be very critical and should be calculated before the lift is made. Most contractors include an increased safety margin against tipping of the crane when a tandem lift is executed. The Dutch safety rules state, for example, that the capacity of each crane should not exceed 75 % of the rated capacity when lifting a load with two cranes. This safety margin can be decreased, provided a detailed rigging study is prepared and submitted to the concerned authorities for approval.
Too strict margins?
During my career in heavy lifting I came across quite a few occasions where this general safety rule is ,in my opinion, too strict. Safety is an absolute must on jobsites, but it is wrong to apply rules if they are not relevant to a particular situation. For this reason I take the liberty of discussing some considerations which, in my opinion, make sense when lifting loads with two or more cranes.
One of the first things to do is evaluate the lift planned and work out what could go wrong and why!
This article is the first in a series of three in which we will look at the following situations:
However , all three cases have one determining factor and that is the position of the CoG of the load in relation to the lifting points. Depending on the location of the CoG and the lifting points it could be an easy and safe lift or extremely difficult and unsafe one. It is the task of the rigging engineer to determine the criteria by which a load can be considered safe and easy or when it is necessary to take special precautions. In all cases we want to achieve one goal: The lift must be made safely and we can only achieve that when the cranes are not overloaded.
The detailed rigging study should prove to us and our client how we have effectively and safely planned the lift and show that none of the cranes will be overloaded
Case Study
The CoG is just below the imaginary line between both lifting points “A” and “B”. What is the effect on each crane if the beam is not lifted horizontally, but one crane lifts faster than the other ? This can easily be calculated with a mathematical formula. If we tabulate the values they are as shown in Table 1.
As can be seen from this table, the load variation between lift point
A and lift point B at 15° is only 1.78 tonnes (approx. 3.2%)
If the distance between A and B decreases as per figure 2 and the distance of the CoG to the imaginary line AB increases, it results in a much larger load variation as can be seen from table 2:
Because of the inclined position, the CoG moves closer to point “B” and therefore the load in “B” increases
When the CoG is located exactly on the imaginary line AB, then there will be no load variation at all, if the beam is not lifted horizontally.
What have we learned from this exercise? The most important principle is that one cannot use the same safety criteria for different multi-crane lifts. Each project should be evaluated by itself and, if the location of the CoG in relation to the lifting points justify it, one could consider a less severe safety margin. There are also a number of practical rules that one must observe when lifting a beam with two cranes.
Danger!
Suppose we are lifting a pressure vessel with two cranes and crane
“B” lowers quicker than crane “A”. Consequently the transport saddle (2)
closer to point “B” touches the trailer turntable earlier then the
saddle (1) near point “A”, resulting in quite an increase of load in
crane “A”. See figure 4 below:
When one observes these the rules of simple mechanics and closely study what happens when a lift is made with more then one crane, one can make lifts safely at all times.
Remember: When the support points (=transport saddles 1 and 2) are not at the same position in relation to the C0G as the liftpoints (taillug A and lifting trunions B), you can be in for an unpleasant surprise as the load in one of the liftpoints can increase significantly. (Crane “A” load increases from 76 ton to 148.4 Ton, when crane “B” lowers the load on the trailer near saddle 1 first).
In heavy lift shipping, its is daily practise to make use of the combined strength of two heavy derricks or cranes on board of a ship. Two 250 tons derricks may be utilized up to their full combined capacity of 500 tons. The exercise we did proves that there is no reason why we should not do the same with mobile cranes. Of course all observations should be taken into account.
Let us now consider case 2), where a pressure vessel is lifted with a main lift crane and a tail crane.
The main lifting lugs “B” are placed 20.1 m away from the CoG. and located on the centerline of the vessel. It is this socalled offset of the tailpoint which could work in our advantage, provided we plan it right.
Case Study
Let’s take a look at the plotplan of the foundation, on which the pressure vessel must be erected.
The foundation is a concrete plint at groundlevel and let’s asume there is ample room to position the main lift crane and tailcrane. In order to prepare the most competitive bid, the rigging engineer should study the most economic erection method. I realize that there are a lot more factors that determine the crane capacity and type to be used , but to keep this case study relatively simple, let’s assume that the truckcrane solution offers the most economic option.
Selection of main lift crane.
Although the initial raising of this column will be carried out by means of a main lift crane and tailcrane, at the end, when the column reaches it’s vertical position, the main lift crane must be capable to lift the complete column of 150 tons by itself at the required radius onto the foundation. To select the main lift crane, one has to find a crane type which has sufficient lifting capacity and at the same time sufficient boom length to reach the required lifting height. The type of crane (crawler- or truckcrane) to be used is in most cases determined by other factors such as:
Let’s get back to our case study.
It is the task of the Rigging engineer to select the most economic cranes and positions of both cranes. We have allready decided, that the main lift crane will be a truckcrane. For the 150 tons pressure vessel, a 350 tons truckcrane with Superlift will be sufficient (i.e. the Demag TC-1200 with Superlift attachment would be a good choice). With a boom lenght of 48 m and 80 Tons counterweight of the Superlift attachment, the crane has a rated lifting capacity of 166 Tonnes at 12 m radius, which is just sufficient to carry out the lift.
Tailcrane selection.
The selection of the tailcrane is a bit more complex. For tailing work, one ussually selects a crawler crane, which can travel towards the main crane during the erection procedure. When a crawler crane is selected, the main consideration is the required lifting capacity at min. boom length and a good travel path for the crane. In case a truckcrane is selected, the lifting capacity of the tailing truck crane greatly depends on the position of the tailcrane in relation to the main lift crane.
By selecting the most economic position of lifting lugs and main lift crane as well as tailcrane, one can greatly influence the overall rigging cost for a particular pressure vessel.
Some possible crane positions are as shown in Fig. 2
Behind the vessel’s basering (same position as tailing crawler crane)
Both cranes at the side of the vessel as close as possible to each other
The best position for the tailcrane is selected by calculating the decrease of the tailload when the vessel is lifted from horizontal into vertical position. This tailload decrease is caused by the offset of the tailpoint. By means of a mathematical formula one can calculate the decrease of tailload in relation to the angle with the horizontal, when the vessel is erected from horizontal into vertical position. If we calculate the tailload and main lift load in relation to the angle of the vessel’s centerline with the horizontal, one get results as tabulated in table 1 below:
Table 1
As can be seen from above table, the decrease in tailload becomes significant when the vessel reaches an angle of approx. 70o with the horizontal. This tailload decrease can be used to our advantage and in some cases could lead to a smaller tailcrane compared to the one originally selected. When main lift crane and tailcrane are as positioned in position 1 of Fig.2, then crane “A” as well as crane “B” need to luff out much more then as shown in position 2 of Fig.2. Even position 2 is not the best position with regard to crane capacity.
A more significant reduction in tailload (thus crane capacity) can be achieved by change of position of main lifting lugs. The shape of this pressure vessel makes it relatively easy to install the main lifting lugs in the conical shaped transition zone approx. 4.2 m above the CoG.
See Figure 3.
Position 1 is not the ideal location for both cranes. When both cranes are placed as shown in position 2, one gets the best results. The optimal position can be established by comparing the crane capacity charts and decrease in tailload. Due to the position of the main lifting lugs we greatly reduce the tailload and therefor the capacity of the tailcrane. The capacity of the main lift crane remains the same as the total weight of the pressure vessel does not change. In case the offset of the tailpoint is increased, the decrease in tailload during erection is greater as well.
With main lifting lugs only 4.2 m above the CoG. the change in tailload and main lift load during the erection procedure are as tabulated in table 2.
Table 2
Instead of the required 200 Tons telescopic crane ( i.e. a Liebherr LTM-1200) for tailing , one can now do the job with a 70 Tons Telescopic crane (i.e. a Krupp KMK 4070), with significant cost reductions in mob./demob. cost as well as rental costs.
Finally, it should be emphasised that lifting jobs as described above allow a very small margin of error and can only be carried out under expert supervision and with detailed load calculations and rigging studies. At all times the lifting blocks should remain exactly above the lifting points. (Check verticality of the lifting tackle !!)
In the next issue I will discuss important points when lifting a heavy pressure vessel in the 400-500 tonnes range, with two main lift cranes and one tailcrane.
***********
Ever increasing dimensions and weights make it more and more necessary to consider the use of two or more cranes to lift one load. Such an operation frequently requires the preparation of a detailed rigging plan to clearly determine the limits within which the lift can be made safely. The main objective of this rigging plan is to investigate how possible overloading of the lift cranes can be avoided. One method of avoiding overloading cranes is the use of equalizer beams, but there are other methods as well. Careful examination of a crane’s capacity chart is essential before a lift is planned. Different countries prepare these in different ways: in the Netherlands for example, the crane capacity charts were all based on 66 2/3 % of tipping load, while in other countries such as Germany or the USA the crane capacity charts are usually based on 75 % or even 85 % of tipping. Nowadays, all capacity charts are based on 75% of tipping moment. One should also be aware of the fact that not only tipping of a crane but also strength of the boom determines the max. allowable lifting capacity. The strength of the crane boom is usually the capacity range at short radii, whereas the tipping area usually covers the larger radii. In some cases this is clearly marked on the capacity chart of the crane. When lifting with one single crane, this margin against tipping ensures that the crane can accept a certain overload. Under all circumstances one must try to avoid situations in which the crane’s rated capacity is exceeded, but as we all know in some cases it happens unexpectedly, e.g. when the horizontal level of the crane is disturbed by weak ground conditions, or when wind affects the load. When lifting with two or more cranes the load distribution over each crane can, however, be very critical and should be calculated before the lift is made. Most contractors include an increased safety margin against tipping of the crane when a tandem lift is executed. The Dutch safety rules state, for example, that the capacity of each crane should not exceed 75 % of the rated capacity when lifting a load with two cranes. This safety margin can be decreased, provided a detailed rigging study is prepared and submitted to the concerned authorities for approval.
Too strict margins?
During my career in heavy lifting I came across quite a few occasions where this general safety rule is ,in my opinion, too strict. Safety is an absolute must on jobsites, but it is wrong to apply rules if they are not relevant to a particular situation. For this reason I take the liberty of discussing some considerations which, in my opinion, make sense when lifting loads with two or more cranes.
One of the first things to do is evaluate the lift planned and work out what could go wrong and why!
This article is the first in a series of three in which we will look at the following situations:
- A long horizontal beam (weight=110 Ton) which is being lifted by two
cranes (i.e. one at each end), in which the centre of gravity (CoG) is
located close (1500 mm) to the imaginary line between both lifting
points. See figure 1 below.
- The erection of a pressure vessel from horizontal into vertical
position with one main lift crane and one tail crane. The centre of
gravity is somewhere in between the main lifting lugs and the tailing
lug.
- The erection of a pressure vessel from horizontal into vertical position with two main lift cranes and one tail crane. The centre of gravity is located closer to the two main lifting points then the tailing point. See figure 3.
However , all three cases have one determining factor and that is the position of the CoG of the load in relation to the lifting points. Depending on the location of the CoG and the lifting points it could be an easy and safe lift or extremely difficult and unsafe one. It is the task of the rigging engineer to determine the criteria by which a load can be considered safe and easy or when it is necessary to take special precautions. In all cases we want to achieve one goal: The lift must be made safely and we can only achieve that when the cranes are not overloaded.
The detailed rigging study should prove to us and our client how we have effectively and safely planned the lift and show that none of the cranes will be overloaded
Case Study
The CoG is just below the imaginary line between both lifting points “A” and “B”. What is the effect on each crane if the beam is not lifted horizontally, but one crane lifts faster than the other ? This can easily be calculated with a mathematical formula. If we tabulate the values they are as shown in Table 1.
| Loadvariation at angle of: | 15° | 30° | 45° | 60° | 75° |
| Lift point A: 55 Ton | 54.11 Ton | 53.07 Ton | 51.66 Ton | 49.21 Ton | 42.53 Ton |
| Lift point B: 55 Ton | 55.89 Ton | 56.93 Ton | 58.34 Ton | 60.79 Ton | 67.47 Ton |
If the distance between A and B decreases as per figure 2 and the distance of the CoG to the imaginary line AB increases, it results in a much larger load variation as can be seen from table 2:
| Loadvariation at angle of: | 15° | 30° | 45° | 60° | 64.5° |
| Lift point A: 110 Ton | 95.96 Ton | 79.76 Ton | 57.62 Ton | 19.27 Ton | 0.18 Ton |
| Lift point B: 110 Ton | 124.04 Ton | 140.24 Ton | 162.38 Ton | 200.73 Ton | 219.82 Ton |
Because of the inclined position, the CoG moves closer to point “B” and therefore the load in “B” increases
When the CoG is located exactly on the imaginary line AB, then there will be no load variation at all, if the beam is not lifted horizontally.
What have we learned from this exercise? The most important principle is that one cannot use the same safety criteria for different multi-crane lifts. Each project should be evaluated by itself and, if the location of the CoG in relation to the lifting points justify it, one could consider a less severe safety margin. There are also a number of practical rules that one must observe when lifting a beam with two cranes.
- Never allow the lifting tackles to become inclined during the lift.
- Always ensure that each crane’s boom head is exactly above the lifting point (the lifting tackle should remain in vertical position. No side forces should be applied on the boom). This can be achieved by setting i.e. crane “A” in free swing mode and control the operation with the other crane “B”.
Danger!
There is one other matter which cannot be overlooked and easily
overloads one of the two cranes if one does not know what happens, when
the beam or a pressure vessel is lowered on two supports.
Suppose we are lifting a pressure vessel with two cranes and crane
“B” lowers quicker than crane “A”. Consequently the transport saddle (2)
closer to point “B” touches the trailer turntable earlier then the
saddle (1) near point “A”, resulting in quite an increase of load in
crane “A”. See figure 4 below:
When one observes these the rules of simple mechanics and closely study what happens when a lift is made with more then one crane, one can make lifts safely at all times.
Remember: When the support points (=transport saddles 1 and 2) are not at the same position in relation to the C0G as the liftpoints (taillug A and lifting trunions B), you can be in for an unpleasant surprise as the load in one of the liftpoints can increase significantly. (Crane “A” load increases from 76 ton to 148.4 Ton, when crane “B” lowers the load on the trailer near saddle 1 first).
In heavy lift shipping, its is daily practise to make use of the combined strength of two heavy derricks or cranes on board of a ship. Two 250 tons derricks may be utilized up to their full combined capacity of 500 tons. The exercise we did proves that there is no reason why we should not do the same with mobile cranes. Of course all observations should be taken into account.
Let us now consider case 2), where a pressure vessel is lifted with a main lift crane and a tail crane.
***********
LIFTING A PRESSURE VESSEL WITH ONE MAIN CRANE AND A TAIL CRANE
In the previous article, I discussed the lifting with two cranes of horizontal pressure vessels or beams. I plan to continue with the discussion of erecting a pressure vessel from horizontal position into the vertical by means of one main lift crane and a tailcrane. Specific points which should be considered and can be advantageous to the lifting contractor include:- Decreasing tailload due to offset of tail lift point
- Position of tailcrane
- Capacity of tailcrane
- Type of tailcrane i.e. crawler crane or truckcrane
- Type and Capacity of main lift crane (crawler- or truckcrane)
- Position and type of lifting points (Lifting lugs or lifting trunnions)
The main lifting lugs “B” are placed 20.1 m away from the CoG. and located on the centerline of the vessel. It is this socalled offset of the tailpoint which could work in our advantage, provided we plan it right.
Case Study
Let’s take a look at the plotplan of the foundation, on which the pressure vessel must be erected.
The foundation is a concrete plint at groundlevel and let’s asume there is ample room to position the main lift crane and tailcrane. In order to prepare the most competitive bid, the rigging engineer should study the most economic erection method. I realize that there are a lot more factors that determine the crane capacity and type to be used , but to keep this case study relatively simple, let’s assume that the truckcrane solution offers the most economic option.
Selection of main lift crane.
Although the initial raising of this column will be carried out by means of a main lift crane and tailcrane, at the end, when the column reaches it’s vertical position, the main lift crane must be capable to lift the complete column of 150 tons by itself at the required radius onto the foundation. To select the main lift crane, one has to find a crane type which has sufficient lifting capacity and at the same time sufficient boom length to reach the required lifting height. The type of crane (crawler- or truckcrane) to be used is in most cases determined by other factors such as:
- Cost for mobilisation and demobilisation of the complete crane in relation to the work period on site.
- Other work that can be done with the crane on that specific jobsite?
- Whether there is a need for the crane to travel with load or not?
- How many times must the crane be relocated on site?
Let’s get back to our case study.
It is the task of the Rigging engineer to select the most economic cranes and positions of both cranes. We have allready decided, that the main lift crane will be a truckcrane. For the 150 tons pressure vessel, a 350 tons truckcrane with Superlift will be sufficient (i.e. the Demag TC-1200 with Superlift attachment would be a good choice). With a boom lenght of 48 m and 80 Tons counterweight of the Superlift attachment, the crane has a rated lifting capacity of 166 Tonnes at 12 m radius, which is just sufficient to carry out the lift.
Tailcrane selection.
The selection of the tailcrane is a bit more complex. For tailing work, one ussually selects a crawler crane, which can travel towards the main crane during the erection procedure. When a crawler crane is selected, the main consideration is the required lifting capacity at min. boom length and a good travel path for the crane. In case a truckcrane is selected, the lifting capacity of the tailing truck crane greatly depends on the position of the tailcrane in relation to the main lift crane.
By selecting the most economic position of lifting lugs and main lift crane as well as tailcrane, one can greatly influence the overall rigging cost for a particular pressure vessel.
Some possible crane positions are as shown in Fig. 2
Behind the vessel’s basering (same position as tailing crawler crane)
Both cranes at the side of the vessel as close as possible to each other
The best position for the tailcrane is selected by calculating the decrease of the tailload when the vessel is lifted from horizontal into vertical position. This tailload decrease is caused by the offset of the tailpoint. By means of a mathematical formula one can calculate the decrease of tailload in relation to the angle with the horizontal, when the vessel is erected from horizontal into vertical position. If we calculate the tailload and main lift load in relation to the angle of the vessel’s centerline with the horizontal, one get results as tabulated in table 1 below:
| Loadvariation | 0 ° | 15 ° | 30 ° | 45 ° | 60 ° | 70 ° | 80 ° | 85 ° | 90 ° |
| Liftcrane “B” |
67.6T
|
69.4T | 71.3T | 73.9T | 77.9T | 82.8T | 93.8T | 107.5T | 150T |
| Tailcrane “A” | 82.4T | 80.6T | 78.7T | 76.1T | 72.1T | 67.2T | 56.2T | 42.5T | 0.0T |
As can be seen from above table, the decrease in tailload becomes significant when the vessel reaches an angle of approx. 70o with the horizontal. This tailload decrease can be used to our advantage and in some cases could lead to a smaller tailcrane compared to the one originally selected. When main lift crane and tailcrane are as positioned in position 1 of Fig.2, then crane “A” as well as crane “B” need to luff out much more then as shown in position 2 of Fig.2. Even position 2 is not the best position with regard to crane capacity.
A more significant reduction in tailload (thus crane capacity) can be achieved by change of position of main lifting lugs. The shape of this pressure vessel makes it relatively easy to install the main lifting lugs in the conical shaped transition zone approx. 4.2 m above the CoG.
See Figure 3.
Position 1 is not the ideal location for both cranes. When both cranes are placed as shown in position 2, one gets the best results. The optimal position can be established by comparing the crane capacity charts and decrease in tailload. Due to the position of the main lifting lugs we greatly reduce the tailload and therefor the capacity of the tailcrane. The capacity of the main lift crane remains the same as the total weight of the pressure vessel does not change. In case the offset of the tailpoint is increased, the decrease in tailload during erection is greater as well.
With main lifting lugs only 4.2 m above the CoG. the change in tailload and main lift load during the erection procedure are as tabulated in table 2.
| Loadvariation | 0 ° | 15 ° | 30 ° | 45 ° | 60 ° | 70 ° | 80 ° | 85 ° | 90 ° |
| Liftcrane “B” | 119.6T | 120.7T | 121.9T | 123.4T | 125.7T | 128.2T | 133.3T | 138.5T | 150T |
| Tailcrane “A” | 30.4T | 29.3T | 28.1T | 26.6T | 24.3T | 21.8T | 16.7T | 11.5T | 0.0T |
Instead of the required 200 Tons telescopic crane ( i.e. a Liebherr LTM-1200) for tailing , one can now do the job with a 70 Tons Telescopic crane (i.e. a Krupp KMK 4070), with significant cost reductions in mob./demob. cost as well as rental costs.
Finally, it should be emphasised that lifting jobs as described above allow a very small margin of error and can only be carried out under expert supervision and with detailed load calculations and rigging studies. At all times the lifting blocks should remain exactly above the lifting points. (Check verticality of the lifting tackle !!)
In the next issue I will discuss important points when lifting a heavy pressure vessel in the 400-500 tonnes range, with two main lift cranes and one tailcrane.
***********
