Cranes on Barges
The incident at Alphen aan den Rijn revealed a number of safety issues that occur when a crane is used on a barge. The collapse of the two mobile construction cranes which were standing on barges highlighted several serious shortcomings that should have been addressed during the preparations for the lift, as well as recommendations to improve risk management following the investigation. Although using cranes on barges is common practice, there are critical matters that need to be considered before the lift commences.
Mobile cranes are designed to be used on level ground. A mobile crane driver will be meticulous about getting the crane level when setting the outriggers. This is because the slew ring bearings on land-based cranes are limited, typically, in out-of-level capacity by two degrees. Once you exceed this figure at maximum load, slew ring failure is a strong possibility. In addition, the fleet angle of crane rope acting on the sheaves at the tip of the jib is also limited. The sheaves are designed for a vertical load, with only a slight out-of-plane allowance.
Although a typical flat top barge will have a level surface, the equilibrium of the barge will be affected by several factors such as the load, ballasting and wind/wave conditions. As soon as you introduce an off centre load onto a barge – say by lifting a load using a crane – the barge will want to tilt. This can be countered by ballasting the barge, but it is important to note that this is rarely as quick as the crane can take load. Any movement in the barge will result in the crane limits discussed above becoming close to being exceeded. In addition to the above, any angle in the barge deck can also result in a change in the lift radius increasing load on the crane components further.
Crane lifts on land require serious planning for a range of matters, including lift radius, ground conditions and load dimensions. However, when the lift calls for the crane to be on a floating platform, the risks and issues become even more complex. As well as needing to determine the reduced lifting capacity of the crane due to the factors listed above, the stability of the whole system also needs to be considered. The vertical centre of gravity is a significant factor in the stability of the barge and knowing where this is at all stages of a dynamic lift is paramount. Always ensure you have specialist assistance and that proper thought is given to the various elements that come into play when a crane is subject to the dynamics of a barge.
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