"EXPERIENCE, SERVICE, SOLUTIONS"

CASPER, PHILLIPS & ASSOCIATES

Accident Repair

In December, 1995 the boom latch malfunctioned on a container crane located at the Port of Cristobal, Panama. Subsequent events caused the hoist rope to fail as the boom was being lowered. The boom free fell until stopped by the forestays. Impact was large enough to lift the back legs about a meter off the ground.

The shock load was transmitted up the forestays, down the backstay to a rear A-frame, and finally into a compression strut that braces the rear A-frame. This compression strut buckled under the extreme overload. The crane would have collapsed in a heap had not the machinery house arrested the failure by bracing the strut at mid length.

Concurrent with the strut failure, another shock wave traveled out to the boom tip where whip action snapped the boom tip platform loose. Additional damage included broken gantry wheel bearings and a cab arm that snapped in two.

This type of accident has happened many times since the beginning of the container industry. When it does happen, the terminal operator usually decides that the boom latch serves no useful function and, instead, is a safety hazard that is best eliminated.

CP&A was retained to direct the repair with instructions to restore normal terminal operations as soon as practical. Local construction resources were extremely limited. We had to forego use of a crane. None available. Tools and materials were also difficult or impossible to find. Fortunately, the Port's own work force supplied adequate manpower to provide able assistance to the repair specialists we brought in.

One suggestion for a repair scheme was to simply take down the buckled strut and either repair or replace with new. This was not practical because all upper works members, including A-frames and backreach truss, are pin connected. If the compression strut was temporarily removed, the remaining linkage would collapse. Strut repair using flame bending (i.e., thermal upset) techniques was a far better choice for several reasons:

Could be done without a crane.
Preserved original geometry without need to know as-built length of original strut.
Avoided time and cost to brace the unstable linkage.
Special rigging needed for the flame straightening could be done with the Port's own work force.

The one factor against thermal upset repair was the extremely severe local buckling. This example is probably the most severe buckling that has ever been flame straightened. Our repair expert, Dan Holt, was not fully confident that it could be done. He was especially concerned that the crimping was so severe it had cracked the steel plates. Later, we discovered this was true and it greatly complicated the repair process.

In theory, the thermal upset process is simple, predictable and produces a superior quality repair. If anything it improves original metallurgy by pseudo normalizing and also by eliminating some residual stress. Otherwise, it is metallurgically benign. The only other concern is that it must be directed by an expert or it may cause additional local buckling.

The process works because as steel plates are heated, they will thicken if in-plane expansion is restrained. Upon cooling, the new thickness is preserved and in-plane dimensions are shortened thus holding steel volume constant. It is somewhat of a "black art" but not so much so once the basic physics and structural engineering principles are understood.

Attached accident repair photos depict the original buckled shape, in situ repair in progress and finished repair. As mentioned above, the crimped steel had cracked. We drilled crack stopper holes to keep the cracks from running out of control. Cracks were repeatedly welded and then recracked as the crimps were pulled out. Once the plates were restored to a flat shape, the cracks were permanently rewelded and NDT inspected by UT and MT. Holes were covered with steel straps which were seal welded to preserve the original air tight corrosion proofing.

The broken boom tip platform was repaired by taking the platform down, reconstructing the damaged parts in the Port's shop and reinstalling the platform. Without a crane, some ingenious rigging was needed. Repair specialists from Marine Technical Services directed the Port's work force for this work as well as for wheel bearing replacement and cab arm repair.