Actvity_27190_Analysis_on_the_Structure_of_the_SLEDGE_Barge.pdf

Analysis on the Structure of the SLEDGE’s Barge

Activity 27190

August 31, 2011

Surface Forces Logistics Center – Engineering Services Division – Naval Architecture and Marine Engineering Branch – Naval Architecture Section

By: Vincent Wickenheiser

Reviewed By: Tim McAllister

Purpose:

The purpose of this study is to analyze the current structural arrangement on the barge for the SLEDGE to determine the cause and possible solutions to the excessive transverse hogging evident when it was dry-docked in June 2011.

The transverse hog was reported as 4 inches up along the length of centerline of the barge.

Analysis:

The barge’s transverse structure consists of bulkheads and trusses located every 8 feet. The primary elements of concern are the trusses located at frames 30, 46, 54, and 62. When built, these trusses consisted of vertical beams spaces 7 feet apart, with diagonal beams providing rigidity (Figure 1). During the life of the barge some of these diagonals have been removed to provided easier access through the barge and provide more storage space. The removal of these diagonals is the cause of the hogging.

Figure 1: As Designed Truss Structure

To analyze the structure the Nominal ABS Design Pressure was applied to the bottom of the hull. This pressure represents a large force applied evenly over the bottom of the hull. The structure was analyzed under this load assuming that the trusses did not have any diagonal beams (See Appendix A). The removal of the diagonals creates a scenario where the channel beams along the top and bottom of the truss become separate structural elements, instead of a joint structural element as part of a large “I-beam.” The top and bottom members become overstressed in bending and this allows racking and longitudinal deflections in the truss. The truss structures are not sufficiently rigid relative to the bulkheads, and shed their load to the bulkheads. This is evident in the buckled plate visible on the bulkhead at frame 38. The buckling of the bulkheads further reduces the rigidity of the hull, increasing the hogging deflection.

Results:

The ideal recommended repair is basically a return to the original structure. This includes:

• Frame 62: Install the two missing diagonals. This includes removing the two added vertical beams on the port side and the small diagonals associated with them.

• Frame 54: Install the one missing diagonal

• Frame 46: Move the compressor located on the port side and install the one missing diagonal.

• Frame 30: Install the missing diagonal on the port side between the longitudinal bulkhead and the outboard vertical beam.

This option would provide the best results in terms of structural strength. The barge would be restored to its original structural design (much like Figure 1).

However, if access is still an issue there is a second option that is shown in Appendix B. This second option would involve adding “access panels” that contain a cutout that would allow relatively easy access through the truss. The access panels can be installed to replace the vertical beams (See sections 7-A and 7-B from Appendix B) or be installed to be placed next to the vertical beam (See section 7-C through 7-E from Appendix B). The access panel itself is comprised of quarter inch plate with a face plate around the opening and flanges along the sides to provide some rigidity. These panels would reduce the unstiffened span in the truss from 7’ to 3’. Diagonal beams would still be installed to complete the trusses. Overall, this second option is not as structurally optimal as the first, but may be more desirable from an access point of view.

Both of these solutions still have a problem with the compressor components located at frame 46. The compressor is component in the way of the diagonal element that would normally be located just port of centerline. A diagonal beam can be installed that does not run directly corner-to-corner like the normal diagonals, but this would have a much reduced effect in adding stiffness to the structure (see section 7-D in Appendix B). The ideal solution is to move the compressor component and install the diagonal, corner-to-corner.

During the installation, it is recommended that the barge is straightened to its original shape. This can be done through the use of weights located on centerline and loosening the keel blocks to allow the hull to bend back into its original shape. The maximum load on the deck should not exceed 320 pounds per square foot when weights are loaded on the barge.

Conclusions:

• The removal of the diagonals reduced the required stiffness of the transverse truss structure. This resulted in an overstressing of the transverse web frame, channels, and transverse bulkheads along with the excessive hogging noted in the June 2011 drydocking.

• The web frame configuration should be returned to the original design, or to one of the modified designs as shown in Appendix B. The modified designs provide acceptable alternatives to the diagonal beam replacements. These alternatives were provided to improve access through the truss frames.

Appendix A –Calculations

Appendix B – Alternatives Sketches