Static Strength Assessment of the A-Frame with Cap Fly Jib

Figure 1. Photo of pontoon crane with A-Frame.

The purpose of work is to investigate strength assessment of the A-Frame with cap fly jib regarding to heavy lift pontoon crane.

Pontoon crane structure consists of a pontoon and the lifting appliance. The lifting appliance consists of the A-frame mounted on the pivots, two cap fly jibs mounted on the top shafts of A-frame and is withheld by the main stay cables. The length of the A-frame is approximately 50 m. Max length of the A-frame with fly jib in vertical position is approximately 70 m.

Heavy lift pontoon crane can be seen in Figure 1.

Figure 2. Finite element model of the A-Frame.


Loads and Constraints

Five load cases are regarded, as shown in the Figure 3.

The nominal load 800t (2x400t, Load case 1) and the maximum top jib symmetric testing load is 880t (2x440t, Load case 2). Load case 3 considers a load that increases from 100 to 2000 tons in 20 steps, with non-linear analysis, in order to assess the risk of buckling. Load cases 4 and 5 are nominal cases like case 1 (800t), where the jib angle (relative to the A-frame) is increased to 5 degrees for load case 4 and to 10 degrees for load case 5.

Figure 3. Initial load conditions: a) internal pressure; b) nodal temperatute.



Figure 4. Presure distribution at long section cut of WVC.

Figure 5. Presure pulsating in low frequency domain.

Occured Results

After checking of FEM analysis results and verified with analytical solutions the following conclusions have been made:

  1. Utilization factor (0.8 < 1.0) is Okay for the current WVC design.
  2. Obtained max stress level is not exceed allowable value (185 MPa).
  3. Axial translation of the WVC is about 5.0 mm and that is Okay for thermal load compensators which will be simoultaniously installed with pipelines.

Von Mises Stress distribution level and axial translation in WVC can be see in Figures 6 and 7.

Figure 6. Von Mises Stress distribution [Pa].

Figure 7. Axial (Ox) translation distribution [m].

The finite element model (FEM) is made using plate elements for all main load-carrying structures and solid elements. The total number of elements used for the A-frame structure amounts to approximately 250000. Element size ranges from 50-150 mm in general. Plate thickness generally ranges from 10-60 mm.

The mainstay cables are modelled as rod elements (diameter 78, 86 and 110 mm) and are connected to the jib and A-frame with similar RBE3 spiders.

General representation of FEM can be seen in Figure 2.

FEM Description