LIFTING A PRESSURE VESSEL WITH ONE MAIN CRANE AND A TAIL CRANE (1)

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)

Let’s look at Figure 1, which shows a pressure vessel of 150 tons in weight and an overall lenght of  approx. 36.6 m. The  lifting point “A” at the base ring  (skirt) of the vessel is positioned  3.0 m above the centerline of the vessel.

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 (3 m) 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.

Erection of heavy pressure vessel Train 5 Qatar

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?

The above points will help to evaluate which crane type suits the job best.  In general one could say that truckcranes  are ideally to travel from one jobsite to another for relative short workperiods on each jobsite. The  mobilization and demobilization costs for a crawler crane  are in general higher than a truckcrane and  therefore this crane type is ussualy applied on jobsites with longer workperiods. Another great advantage of  the crawler crane is that it can eassily travel on site from one location to another even when loaded.

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 length 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 tail crane compared to the one originally selected. When  main lift crane and tail crane 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 tail crane. 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. 

The next article will deal with lifting a large and heavy pressure vessel with two main lift cranes and one tail crane

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