A Study on Preventive Measures for Fatigue Cracking at Welded Joints in Cargo Tank Structure of Chemical Tanker

Abstract

In the cargo tank structure of the chemical tanker, fatigue cracking at the cruciform welded joint between the transverse bulkhead and the double bottom, which is the most important part under severe stress conditions, can lead to a serious accident.

In this study, the shapes of the weld toes of cruciform welded joint specimens, which were assumed to be the weld part between the transverse bulkhead and double bottom, were measured using a weld bead laser scanner, which was a handy measuring device, to estimate the stress concentration factors. These shapes were also measured using conventional silicon mold. It was shown that the stress concentration factors for each measurement were almost the same.

In addition, fatigue tests using the cruciform welded joint specimens were carried out and compared with the results obtained in previous studies. Each fatigue strength for full-penetration cruciform welded joint of duplex stainless steel plate and that of austenitic stainless steel plate was determined by summarizing all these results. Furthermore, from the comparison between the fatigue strength obtained from the fatigue tests and the fatigue design diagram for ship hull structure provided by the classification society Nippon Kaiji Kyokai (ClassNK), The maximum allowable stress concentration factor of the examined full-penetration cruciform welded joint was determined.

As a result of them, we have put into practical use a technique for measuring of the shape of welded joints in cargo tank structure of chemical tanker under construction at shipbuilding site using the weld bead laser scanner and estimating the stress concentration factor.

If the estimated stress concentration factor is less than the maximum allowable stress concentration factor, it can be immediately determined that fatigue cracks will not initiate. It is a breakthrough in terms of quality control and assurance in shipbuilding.