Designing a floating crane installation

5 January 2006

Lifting calculations are complicated when the job takes place on water, as David Duerr explains

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 be made, it is important to consider the likely size of the barge.

      The useful radius applies to the useful physical reach of the crane only. Lifting capacities are always determined with respect to the normal operating radius of the crane as measured from the centre of rotation.

      Making the first check of the crane’s capacity may be easy or virtually impossible. Some crane manufacturers publish load charts for barge service. These charts show lifting capacities for conditions in which the crane is out of level by various angles. If charts are available, then one need only make an estimate of the maximum angle by which the crane will be out of level.

      If barge service load charts are not available, estimating the lifting capacities is very difficult, if not impossible. Depending on the boom length and operating radius, the barge service capacities may be on the order of 90% of the land service capacities or less than 50% of the land capacities.

      As an example, the curves in the illustration (Figure 2) represent the lifting capacities of a Manitowoc 2250 crane fitted with a 76m boom. The upper curve is a graph of the lifting capacities when operating on land. The lower curve shows the lifting capacities at a crane list of 3 degrees. There is no simple method by which a crane user can determine the lifting capacity of a crane that is out of level. In this case, the user must work with the manufacturer to determine rated capacities. Again, an estimate will have to be made of the out of level angle of the crane, but the difficult job of determining the safe capacity is left to those best equipped to do that engineering work.

      The selection of a barge suitable for a crane installation requires the investigation of two areas. First is the structural adequacy of the barge’s deck and internal structure to safely support the loads from the crane. Second is a set of marine performance issues.

      Flat deck steel barges of the sort most commonly used for crane work are constructed as follows: The deck comprises plate that is stiffened with ribs that run along the length of the barge. This deck structure spans between transverse trusses or bulkheads within the barge (Figure 4). The bulkheads are stiffened plate walls that divide the barge into a series of watertight internal compartments. The trusses are structural frames that provide support to the deck, sides, and bottom.

      Consider a truck crane on a barge. All loads from the crane will be impressed on the deck at the outriggers. The typical deck construction will rarely be able to carry these loads from a large crane. Therefore, either the crane must be positioned such that the outriggers land on hard points, such as the intersection of transverse and longitudinal bulkheads, or steel grillages will have to be installed on deck to transfer the outrigger loads to the internal structure.

      Crawler cranes present a similar challenge. On a barge deck, the crawler loads will probably be carried at a few hard points (Figure 5). The best advantage can usually be gained by setting the crane with the crawlers in the longitudinal direction. The spacing of the transverse trusses is commonly in the range of 1.5 to 3m, so the crane can usually be located such that the crawlers bear directly over three or more trusses.

      The outrigger or crawler track loads are calculated in roughly the same way as for a crane installation on land. Once the crane’s reactions are known, the barge structure can be analysed by a naval architect or structural engineer.

      The second set of issues that affect the barge selection falls under the category of marine performance. The barge’s width, or beam, has a significant effect on the barge’s performance in service. As discussed above, the beam affects the useful radius of the crane (Figure 7). In this respect, one might think that smaller is better. On the other hand, the beam affects the barge’s stability in such a way that larger is better. The required barge stability is ultimately dictated by its affect on the crane’s performance and, in some jurisdictions, by local laws or regulations.

      Another important consideration is how the crane is secured to the barge. In many places, there is no guidance in the regulations that defines the magnitude of forces that should be used to design the crane securements. We can, however, develop a reasonable design load from the way the barge lists and trims while lifting.

      Here, we must return to the marine analyses performed to determine the barge’s orientation throughout the lift range of the crane. The slope of the barge deck will result in a horizontal force acting in the plane of the deck that is created by the vertical weight of the crane plus its lifted load and the angle from horizontal of the deck. A design force can be calculated by conservatively ignoring the benefit of friction between the crane and the deck. Developing a design vertical load to the securements will most likely be more subjective. A suitable design load must be determined by empirical means.

      If the barge crane is to be used in an environment where significant barge motions may occur, dynamic loads must also be considered. Unless the crane is to be used offshore, however, such dynamic loading is unlikely and, as an operational restriction, should be avoided.

      A fundamental need in the engineering of a barge installation of a mobile crane is the establishment of the load chart. Most crane users intuitively recognise that the normal load charts cannot be used when the crane is mounted on a floating barge. First, the barge will continually move as the crane lifts and swings its load, thereby failing to provide the firm and level support that is required by the normal load chart. Second, the movement of the barge and crane can result in the development of dynamic loads that are not considered in standard crane design.

      Consider first the out-of-level support. The typical crane operator’s manual requires that the crane be leveled to within 1% (0.57 degree). A crane mounted on a floating barge will experience continuous changes in trim and list (Figure 8). This usually makes impossible keeping the crane level to within 1%. So we ask the question: How do we develop mobile crane load charts for barge service? The short answer is: We don’t.

      As previously noted, crane load charts normally cannot be developed by the crane user. The most significant reason for this is that most crane users do not have enough information about the crane to accurately assess its structural and mechanical limitations. Many components of the crane will be stressed differently when the crane is out of level. Determination of the crane’s capacity requires detailed knowledge of the strength of all of these components.

      Some crane manufacturers publish barge service load charts for some of their models. If such charts are available, the planning of a barge installation is much more efficient. If such charts are not available, then the manufacturer must be contacted for assistance.

      Crane and barge combination

      Typically, the contractor will propose a crane and barge combination and perform the necessary analyses to develop a table of data for each lift to be made. This table must include the radius and load for each lift and the angles by which the crane is out of level both side-to-side and front to back. This information is provided to the manufacturer for evaluation. This process will be time consuming, particularly if the first setup is found to be inadequate. A number of iterations may be needed to arrive at a crane and barge combination that is acceptable to the manufacturer for the proposed lifts.

      There is one last point that must be made regarding barge service load charts. The standard barge charts are based on out of level only. The lifting capacities do not account for any dynamic loads other than those normally considered in crane design. If a barge-mounted crane is to be used in exposed waters, wave action may cause barge motions that will create additional dynamic loads. The possibility of dynamic loads may require additional derating. The crane manufacturer should be consulted for guidance.

      Once the crane is secured on the barge and barge service load charts have been established, the time has come to make the lifts. As with conventional lifting on land, planning is the primary key to success and, at this point, the planning that is unique to marine work has been completed. A few final points to remember are worth listing here.

      1. The barge will move. As the load is lifted and swung, the constantly moving centre of gravity will cause the barge’s list and trim to change.

      2. Movement of the barge will affect the crane’s radius. The crane operator must be aware of what to expect so that he can compensate.

      3. Movement of the barge may cause the load to sway more than is normal on land. This, too, must be recognised by the operator so that he can adjust the crane’s movements to control the load.

      4. And last, you can’t do it alone. A safe mobile crane installation on a barge cannot be engineered without the assistance of the crane manufacturer to establish the crane’s rated capacities. This may be as simple as requesting barge service load charts or it may be as complex as asking the manufacturer to develop special charts for one installation.

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