HYBRID DESIGN CONCEPT USING HIGH-STRENGTH CAST STEEL INSERTS FOR TUBULAR JOINTS OF OFFSHORE STRUCTURES

In a joint project of a German working group supported by the ECSC and the Studiengesellschaft fur Stahlanwendung e.V., the fatigue behavior of large-scale hybrid tubular joints with inserts manufactured from t he high-strength cast steel GS-12 MnMo 7 4 welded into tubular members formed from the fine-grained steel StE 500 were compared to the behav ior of large-scale welded tubular joints. The latter were made from me dium-strength fine-grained steel StE 355 and high-strength StE 690. In addition, data from hybrid joints with cast steel inserts of medium-s trength GS-8 Mn 7 welded into StE 355 tubulars is available for compar ison. The tests were carried out under variable amplitude loading in a rtificial seawater. The results were evaluated for the failure criteri a fatigue life to crack initiation (a = 1 mm) and through crack. With medium-strength (R-p0.2 > 355 N/mm(2)) hybrid tubulars, where by the u se of cast steel inserts the welds were removed into areas of lower st ress concentration fatigue lives higher than a factor of 100 were achi eved compared to the welded nodes, even those made from StE 690. Howev er, by the use of high strength (R-p0.2 > 500 N/mm(2)) cast steel inse rts and tubular members of corresponding strength, the fatigue life to crack initiation was improved by a factor of two despite a thickness reduction compared to the medium-strength design. Post-weld treatments of the welded tubulars without cast steel inserts like shot-peening, TIG-dressing or their combination resulted only in a slight increase o f fatigue life. The results of this investigation do not only show how to improve the fatigue life by a new design using cast steel inserts, but indicate also how to revise design codes from the point of damage calculation (damage sum of 0.50 for welded nodes and 0.25 for cast st eel inserts instead of the conventional value of 1.00), as well as con sideration of fatigue life to initiation of a technically detectable c rack with a; defined depth, e.g., a = 1 mm.