Ship Design or Fabrication Problem? Analysis Trumps Conjecture in VLCC Bulkhead Failure


Project Info

We need a welding expert! The call came to our office from one of the world’s largest energy companies; we’ll call it the XYZ Co. One of their supertankers, a VLCC (Very Large Crude Carrier), experienced a bulkhead failure during heavy seas while taking on ballast water. The crew saw that a center-line tank wasn’t filling with ballast water at the expected rate. Later the crew discovered that bulkhead perimeter welds had failed allowing water to leak into an empty adjacent compartment.

Was the bulkhead failure due to a ship design, materials, or a construction problem? Since the ship was almost new and presumably designed to meet ABS (American Bureau of Shipping) Rules, the owners wanted an independent forensic investigation of the failure to determine its cause. Since weld fractures allowed the leak, XYZ suspected that the welding was done improperly. Our office suggested that since the weld failures might be a result, and not the cause of the bulkhead failure, a team effort involving both metallurgy and ship structure experts would be prudent. The client agreed and a team was sent to the Far East to inspect the ship.

At the ship, the condition of the bulkhead was photographed and thoroughly documented by the team. Samples of the structural steel and failed welds were excised and shipped back to the U.S. for analysis. In addition to the failed welds, many of the bulkhead’s reinforcing girders were found to have buckled between web stiffeners. Which happened first: the buckling or the weld failures?

Subsequent materials testing indicated that the steel met ABS requirements. However, significant weld defects were found including poor pre-weld fit-up, too small fillet welds, and incomplete weld penetration.

 

 

 

 

 

 

As is our normal practice in forensic engineering, background information was obtained regarding the design and specifications for the ship. A subtle and not immediately apparent difference between the design and the construction of the ship was that stiffeners that reinforce the wide flange webs of the reinforcing girders had not been built according to the design plans. Corners of the as-designed rectangular stiffeners had been “snipped,” or diagonally cut off leaving them trapezoidal. After a detailed stress analysis was performed, it was found that the snipped stiffeners lowered the buckling load to not quite one-half of the load that would have been safely carried if the ends had not been snipped. The reason for snipping the ends of the stiffeners is unknown; perhaps it was done to permit easier girder fabrication.

The answer to the question of failure cause was then clear. Deviations from the ship’s design lowered the load-carrying ability of the bulkhead. Web stiffeners were not built according to the plans, weakening the girders and in turn, the bulkhead. Although the welds were not properly made, they were not the primary cause of the bulkhead failure. The structural analysis made this apparent.

RTI’s marine experts have decades of experience in the handling, securing, and transport of many kinds of cargoes throughout the world, including the inspection and damage survey for the ocean and river vessels that carry them. RTI is well placed to provide such services and can respond to requests for warranty work and more detailed investigations into causes of damage that may arise.

In keeping with RTI’s pioneering spirit; The Imaging Sciences Group has continued to develop new and innovative solutions. Our Immersive Interactive allows for an immersive exploration experience in a 3D environment without the need for heavy computer systems to run it. Here is a live demo of an oil tanker similar to the one discussed in this blog. This demo is very simple, however our solutions can be very detailed to suit your needs. Click Here to experience it right from your web browser!

 

Categories

  • Marine
  • Metallurgical Engineering