During ship life, operating conditions may change, tanker may be converted into FPSO, and flag re... more During ship life, operating conditions may change, tanker may be converted into FPSO, and flag requirements may be modified. Generally these modifications have few impacts on existing structures; flag requirements only rarely are to be applied retroactively. Nevertheless in some cases modifications of operating condition may induce considerable consequences, making in the worst cases impossible any reengineering. For example converting a common tanker, built with plain steel of grade A into an Offshore Floating Unit able operating in cold region, may require a grade change corresponding to a grade B. It is obviously meaningless to replace all material just because material certificates. Steels used by shipyards have to fulfill Classification society’s requirements dealing with mechanical strength; generally shipbuilding corresponds to a small part of steelmaker’s production. For this reason steelmakers are reluctant to produce steels with mechanical properties corresponding exactly ...
Committee Mandate Concern for the collapse behaviour of ships and offshore structures and their s... more Committee Mandate Concern for the collapse behaviour of ships and offshore structures and their structural components under ultimate conditions. Uncertainties in strength assessment shall be highlighted. Attention shall be given to the influence of response to load combinations including accidents; fabrication imperfections; life-cycle effects; and user approach. Consideration shall be given to the practical application of methods. Introduction Determining the ultimate strength of ship and offshore structures involves the ability to predict and measure component, sub-system, and system structural maximum capacity beyond which the capacity diminishes. The purpose of this committee is to present a summary of recent work published after the time period covered by the 2018 committee that addresses these goals within the guidance of the Committee Mandate. Effective consideration of these factors requires definition of terms and description of the use of ultimate strength calculations and...
Fatigue failure has been identified in the 70s as a major offshore failure mode and as a possible... more Fatigue failure has been identified in the 70s as a major offshore failure mode and as a possible ship structure one, only when high stress concentration were existing. With the merchant and navy fleets ageing numerous fatigue cracking has been observed more and more often leading to consider necessary to make compulsory the design fatigue assessment. BV introduced design fatigue rules in 2000, at the same time than the semi-probabilistic format. The aim of this paper is to present these rules in the semi-probabilistic context. The classification rules context is first analysed presenting the approaches for hull girder and local component strength verification. The uncertainties link to the used data and methods are identified. Then the semi-probabilistic format and uncertain parameters are presented and the selected partial safety factors are detailed. The rule fatigue assessment procedure and criteria are discussed giving the selected formulations and associated safety margins. In...
A comparative study on fatigue strength assessment procedures used by the classification societie... more A comparative study on fatigue strength assessment procedures used by the classification societies has been performed by Committee III.2, 'Fatigue and Fracture', of the International Ship and Offshore Structures Congress (ISSC'2000). A pad detail on the longitudinal coaming of a Panamax container vessel was selected as an example. This detail was chosen because of the well-defined loading due to hull girder bending. Large differences in predicted fatigue lives were found, ranging from 1.8 to 20.7 years. The spreading of results is attributable to assumptions regarding loads, local stress determination and S-N curve. For comparison, a direct calculation of loads using the spectral method was performed. Also this calculation showed a relatively short fatigue life of 5.3 years, although the structural detail is considered not to be prone to fatigue failures.
Fatigue cracking is surely the more common failure mode of marine structures. The return experien... more Fatigue cracking is surely the more common failure mode of marine structures. The return experience shows that in the majority of the cases, cracks start at welded joints and so the verification methods developed since the 70s deal with welded joints. More recent cases on FPSOs have shown that cracks can also start from non welded areas and the development of the ULCS (Ultra Large Container Ships) points out the necessity of methods for the hatch corners which are non welded areas. To provide a solution for the design of these ships Bureau Veritas developed a local stress S-N curve formulation including as-welded, non welded and improved welded details. The formulation is based on the accumulated knowledge in fatigue verification approaches since the 70s, the marine return experience and the analysis of available published data. The paper presents the development of the formulation starting by the definition of a S-N curve for as-welded joints covering the low and high cycle domains, then the extension of to the stress release welded joints with different R levels and finally the generalisation to non welded and improved welded details. The non-welded detail S-N curves include the effects of the yield strength on the low cycles domain and of the mean stress level on the slope of the high cycles domain. The defined S-N curves are calibrated versus existing methods for welded joints and mechanical component fatigue verification and also versus test data. Finally, an illustration of a practical application on two ship deck details is given showing acceptable results.
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Papers by Guy Parmentier