Fatigue life of tubular joints in offshore steel structures under stochastic loading is studied. Fatigue test series with various types of stochastic loading that are realistic in relation to offshore structures have been carried through on both full-scale tubular joints and smaller "welded test specimens. The test results obtained show a significant, difference in fatigue life, determined by the Miner sum, between constant amplitude and variable amplitude fatigue tests. The present paper concentrates on the results obtained on the full-scale tubular joints, with a special emphasis on the effect of repair-welding on the fatigue life.
In the North Sea, a large number of offshore steel structures have been constructed over the last two decades. Soon after oil and gas exploration and production began in the North Sea in the mid 1960"es, it became apparent that the steel structure design developed for offshore activities in the Gulf of Mexico was not adequate, when transferred to the more rigorous North Sea environment. In particular, fatigue cracks evolved as a result of wave action during severe winter storms. Thus, it was evident that there was a great need for better understanding of the fatigue phenomenon, so that safer structures could be built. This lack" in the understanding of the fatigue behaviour of offshore steel structures has resulted in comprehensive investigations carried out both in Scandinavia and on a wider European basis, involving all the countries located around the North Sea. In the Danish part of the North Sea with oil and gas production, water depths are rather moderate, around 40–50 m, but frequent storms also create a rough environment in this area. At present, the Danish oil and gas fields in the North Sea have a total of about 35 fixed steel platforms.
