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[2004] Setting Limit State Requirements for Ship Structure Assessment

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Author Admin 작성일04-11-04 10:29 Hit65,926 Count Comments0 Count


Proceedings of OMAE-FPSO 2004
OMAE Specialty Symposium on FPSO Integrity
Houston, USA 2004

Setting Limit State Requirements for Ship Structure Assessment

Author(s): Paul A Frieze and Jeom Kee Paik

Ship classification societies set requirements (rules) for the sizing of ship scantlings and, by default, for ship-shaped FPSOs in the absence of appropriate internationally agreed ISO standards. Class rules have developed independently over many years, based on a mixture of in-house studies and experiences. The requirements are based on a working stress design (WSD) approach. Thus, buckling is dealt with by application of a simple plasticity correction factor to elastic critical buckling stress. Fully effective plating is assumed between longitudinal stiffeners irrespective of their spacing, i.e., buckling effective widths are ignored. Consequently, neutral axes are incorrectly located rendering the determination of component and hull girder strength problematical. Combining the consequences of this with the differing requirements for each class results in varying reliability levels for nominally identical ships sized by different classes. Also, reliability levels determined for vessels appear to be importantly less than those demanded by the oil and gas industry for its offshore structures including FPSOs.
During the last few decades, the emphasis in design and strength assessment of land-based and offshore structures has moved from WSD to limit state design (LSD), as reflected in, for example, BS 5400, ISO 2394 and the ISO 19900 series. LSD offers a rational way to assess ship hull and component strength and a sound basis for determining their reliability. To enable the development of internationally agreed LSD ship structure assessment requirements, ISO/TC 8/SC 8/WG 3 was set-up. However, to provide a suitable framework for this development, general limit state requirements are required similar to those in ISO 19900. This paper describes some of the basis and development of these general requirements, to be known as ISO 18072-1.

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