Despite in the offshore area the use of high strength steels is increasing, in fatigue design codes for welded joints no distinction between low, medium and high strength steels is made; besides limited results are available about fatigue behavior of high strength steels welded joints. To contribute to cover this lack of information the fatigue behavior of TENARIS X70 steel welded risers was experimentally investigated and the results are hereby presented. Main finding was that the S-N curves and the cumulative damage limit generally recommended in the Standards could be too strict when high grade steel welded joints are involved.
In the development of deepwater oil and gas reserves it is continuously facing the challenging on one hand of supporting more severe laying / in service conditions and on the other hand of containing and reducing costs. One of key components is the risers system, which becomes an important factor as water depth increases; in fact both its strength and costs are quite sensitive to water depth, and in particular in ultra deep water environments (greater than 2000 m) the requirement of reducing riser weight is gaining importance. In this context the technological evolution in the risers system seems to point out a trend towards an increasing use of high strength weldable steels. As a consequence the availability of high grade weldable steel risers, up to grade API X100, with a wall thickness/out diameter (WT/OD) ratio adequate to the expected collapse performance becomes a strategic interest. Though a main critical aspect of welded steel risers can be the fatigue strength of welded joints, in fatigue design codes for welded joints design no distinction between low, medium and high strength steels is made and therefore the potential benefits in use such high grades is not considered in the fatigue design.
