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In-depth | CLASSIFICATION New rules for high strength steel


A different approach to rules governing new materials used in containership construction has emerged at Germanischer Lloyd.


G


ermanischer Lloyd has issued a new set of supplementary rules covering the application of new


steel used in containership construction that are said to offer a different approach to rules set out by rival class body ClassNK. Te new classification and construction


rules cover steel with yield strength of 460N/mm2


, a higher tensile material


which is being introduced as part of upper hull construction in the very largest of containerships, where lower tensile materials would demand plate thicknesses that are too weighty, too costly, and too difficult to weld. GL’s new rules apply to steel plates of up


to 100mm in thickness. GL says that, based on fracture


mechanics, there are two approaches to brittle fractures. One, which it attributes to ClassNK, has rules based on the notion that brittle fractures occurring in plates are arrested, by virtue of the structural properties exhibited by ‘YP47’, the higher tensile steel developed by Japanese steel mills in association with ClassNK. . The German society says that it has,


rather, based its own rules on avoiding initiation of brittle cracking altogether. Te approach looks to analyse internal defects in the structure using ultrasonic techniques at the time of construction.


Unless otherwise agreed, the surface


finish of the products shall be subject to standard EN 10163 specifications relating to the surface finish of hot-rolled steel plate, Class A, or equivalent standard. Te


“The fracture tests GL has in mind are already well established in the aero, energy and nuclear industries, but they will represent a change of philosophy for shipowners.





manufacturer may, at his discretion, remove surface defects by grinding, provided that the depth of material ground away does not exceed 3mm in relation to the nominal thickness of the product and provided also


that at least 93% of the nominal thickness remains. Defects shall not be repaired by welding.


Life expectations GL is not advocating a blanket limitation on defects as such, however, as this would lead to production costs that could not be reasonably met by yards. Rather, it is looking to identify defects, in say a weld seam, at the time of construction, in order to establish design life expectations. Ultrasonic test results will be properly


documented in such a way that the performed testing can be retraced at a later date. Ten years after construction, GL would check specific weld seams to see if defects had reached a critical level. Normally, cracks in steel are rounded, and grow over a 20-30 year period, but even in the case of sharp cracks, GL postulates that a crack close to the steel’s surface apparent aſter four years will not become critical until 16 years have elapsed, meaning that its 10 year check includes a six year safety margin. GL recommends that a second check should be carried out 15 years aſter a ship has been built. It says that the fracture tests it has in mind are already well established in the aero, energy and nuclear industries, but that they will represent a change of philosophy for shipowners. NA


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The Naval Architect April 2009 17


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