Dogging and rigging activities

Multiple hoist lifting

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Use this page as a guide when working with multiple cranes.

The use of two or more cranes to move and position loads can be very hazardous. Multiple cranes should not be used when a single crane is capable of doing the job. However, there are occasions when multiple cranes are necessary, such as:

during construction and assembly of oil and gas rigs
bridge construction
large scale industrial projects such as power stations, smelters, and refineries.

Certification

A competent person should be assigned direct responsibility for coordinating and directing a multiple crane lift.

For very complex lifts, the advice of an experienced structural engineer may be required to properly plan the operation. But, a competent person must maintain immediate supervision at all times.

Planning and co-ordination

The importance of careful planning and a thorough briefing of all personnel involved in the lift cannot be overstated.

Many multiple crane lifts have failed through oversights, incorrect crane selection, incorrect siting of the cranes, and misunderstanding between the crew members.

If the weight of the load, its centre of gravity and weight of the lifting gear are not known in advance, then the job should stop until these facts are known.

Whenever possible, a dummy run/practice lift should be staged before the lift. This is to check that the cranes can perform all stages of the operation within radius while maintaining enough clearance from obstructions and power lines.

A dummy run/practice lift should also be done to confirm that the agreed communication method is understood and suitable.

Crane selection and siting

Whenever possible, select cranes of equal capacity and similar characteristics. This will make the synchronisation of crane movements easier to achieve.

Calculating load share directing a multi-crane lift

Where the load to be lifted is beyond the capacity of any of the selected cranes, equalising gear may be required to make sure that each crane supports its correct portion of the load.

Equalising gear is needed when the cranes are close together, such as for lifting large columns and similar objects. It also acts as a lever.

When the cranes have different capacity, the load to be lifted should be slung away from the centre of the equalising beam. This is so that the load taken by each crane is proportional to its capacity.

The load is slung closer to the end of the equalising beam supported by the larger-capacity crane, to increase its share of the load and reduce the part of the load carried by the second crane.

Sample Calculation

To calculate where to sling a particular load on the equalising beam when lifting with two cranes of unequal capacity:

mass of the column to be lifted = 19t
mass of the equalising beam = 1t
capacity of crane 'A' = 16t
length of the equalising beam = 8m

Directing a multi-crane lift

When directing a multi-crane lift, follow these rules:

assess the weather conditions and make sure the lift is conducted during stable low wind conditions
make sure that all crane hoist ropes remain vertical at all stages
only one motion should be undertaken at a time
avoid slewing movements wherever possible
use luffing-up in preference to luffing-down
make sure all crane movements are carried out at slow speeds
where available, use appropriate instruments to monitor the angle of the load, how vertical the rope is, and the force in each hoist rope
in pick-and-carry operations, make sure the axis of each crane remains fully aligned with each other
where you cannot observe all necessary locations, post doggers or riggers to observe and report on the progress of the lift.

Single crane, two hook operation

Crane to load orientation for mid-air rotation

Panel rotation stage one

Panel rotation stage two, 43 degree angle

Panel rotation stage two, 63 degree angle

In-plane and out-of-plane lifts

In-plane lifts are when load's lift point is located longitudinally in line with the crane boom. This is the preferred method when doing a lift because there will be no side loads on the boom and boom components.

Out-of-plane lifts are those which are not within the manufacturer requirements of in-plane. These should be avoided.