Hybrid design concept using high-strength cast steel inserts for tubular joints of offshore structures2

In a joint project of a German working group supported by the ECSC and tile Studiengesellschaft für Stahlanwendung e. V, the fatigue behavior of large-scale hybrid tubular joints with inserts manufactured from the 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 behavior of large-scale welded tubular joints. The latter were made from medium-strength fine-grained steel StE 355 and high-strength StE 690. In addition, data from hybrid joints with cast steel inserts of medium-strength GS-8 Mn 7 welded into StE 355 tubulars is available for comparison. The tests were carried out under variable amplitude loading in artificial seawater. The results were evaluated for the failure criteria fatigue life to crack initiation (a = 1 mm) and through crack. With medium-strength (R(p 0.2) > 355 N/mm2) hybrid tubulars, where by the use of cast steel inserts the welds were removed into areas of lower stress concentration , fatigue lives higher than a factor of 100 were achieved compared to the welded nodes, even those made from StE 690. However, by the use of high-strength (R (p0.2) > 500 N/mm2) cast steel inserts 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 of 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 steel inserts instead of the conventional value of 1.00), as well as consideration of fatigue life to initiation of a technically detectable crack with a defined depth, e.g., a = 1 mm.