Lifting calculations are complicated when the job takes place on water, as David Duerr explains SOURCE: CranesToday
The demands of heavy construction occasionally bring the lifting contractor to the water’s edge or beyond. This may be part of constructing a structure, such as a bridge, over a body of water or one segment of a marine transportation project, shifting cargo from one vessel to another.
Crossing this boundary creates the need for a floating crane or derrick. While existing marine heavy-lift vessels can sometimes be employed, it is often necessary to perform the lifts using barge cranes that consist of temporary installations of mobile cranes. It is this form of the floating crane that is of interest here.
The design of a mobile crane installation on a barge can be divided into four tasks.
* First is selection of the crane. The crane’s normal load charts will not apply on a barge, so the crane selection initially has to be made on some assumed derating.
* Second is the selection of the barge. The barge must be strong enough to support the crane, and must possess a certain level of stability afloat.
* Third is the design of the crane installation on the barge. This work applies principles from naval architecture and structural engineering.
* Fourth is the development of load charts that are applicable to the specific installation.
The main crane selection issues sound, on the face of it, exactly like those for selecting a crane for a land-based lift. The crane must have the vertical and horizontal reach to place the load, and it must have the capacity to safely lift the weight. However, the effects of the marine environment alter both of these areas.
First, let’s consider reach. In addition to the usual concerns about boom interference and the like, the barge itself also impacts on the performance of the crane. The barge crane is almost always lifting its load onto or off of some adjacent structure, such as another vessel or a dock. Thus, the useful radius of the crane is measured from the edge of the barge (see Figure 1, above right).
The width of the barge figures significantly in the evaluation of the barge with respect to its stability and resistance to listing as the crane operates. This is discussed below. For now, we must just keep in mind that the ability of the crane to do its job is affected by the length and width of the barge. Thus, when checking the layout of the lifts to bemade, it is important to consider the likely size of the barge. Continue reading “Designing a floating crane installation” »