Germanischer Lloyd (GL) reported that cracks have been observed in the side longitudinals of a series of panamax-containerships in way of the parallel midbody from the bottom up to about the waterline. The side shell of all ships had been built to the same detailed structural design, with angle bars as longitudinals, and with stiffeners welded to the top of these angle bars in way of webframes. The cracks appeared on these ships after about 10 years of service and seem to be caused mainly by fatigue. A similar kind of fatigue failure was observed on tankers and was attributed to low cycle fatigue from extreme draught differences between ballast and fully laden conditions, although these operational conditions donít apply to containerships.
GL performed a structural analysis and the results showed that the structural details of these older ships still comply with the latest rules. A full-scale measurement on board of one of the ships in the series was initiated and several theoretical investigations were performed, including a detailed F.E. strength calculation. It has been shown that some wave configurations cause warping stresses in the midbody. Shorter waves specifically create a torsional moment distribution over the length of the ship causing warping stresses as if there was warping restraint near the midship section. These stresses are large enough to significantly reduce the fatigue life of the structural elements.
It is recommended that good workmanship, with smooth welding seams, is essential to reduce risks. Additionally, the detailed structural design has to consider the fatigue requirements including the warping stress components. Recommended measures include the option not to weld stiffeners to the top of the longitudinals, use soft toe brackets in way of watertight bulkheads and to use bulb bars or T-bars instead of angle bars for the longitudinal profiles. The local stress level can be reduced using larger profiles, possibly of mild steel, and by increasing the shell plating of the bilge strake.