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Offshore structures have been built in steel or concrete. With steel, new design concepts are necessary as the water depths increase. The so called hybrid' design of tubular joints with cast steel inserts moves the fatigue critical welds away from a highly stressed region. The object of the present research project was an investigation of this design concept for higher strength steels and reduced wall thickness under offshore-typical corrosive and fatigue loading. Compared to previous investigations with StE 355 for the tubulars and GS - 8 Mn 7 for the cast insert (wall thickness 38 mm), this investigation employed the high- strength steel StE 500 for the tubulars and the new cast steel GS - 12 MnMo 7 4 (wall thickness 28 mm). Large scale hybrid tubular joints were fatigue-tested under constant and variable amplitudes in artificial seawater. The results were: Despite of the reduced wall thickness the fatigue life to crack initiation was increased by a factor of two. Supplimentary inves tigations of tubular joints without cast inserts showed, that TIG- dressing plus shotpeening of the welds result in improved safety against crack initiation, albeit at high cost. The results of the present investigation can be employed for the design of tubular joints, also for other geometries with the help of finite-element-models developed. Even for greater water depths the construction of offshore structures made of steel employing the hybrid design of higher strength materials remains a technical and economical alternative to concrete structures. This concept can also be used to advantage in other fields of engineering for example in mining construction, steel buildings, bridges and vehicles.