Super duplex stainless steel tubes in grade SAF2507 have long been used for umbilical application in deep water oil and gas subsea systems. One of the oldest subsea systems installed with super duplex stainless steel tube SAF2507 is at the Åsgard oilfield located in the Norwegian Sea about 200 km offshore of the north part of Norway (figure 1). The Åsgard field comprises of the Midgard, Smørbukk and Smørbukk South deposits, which were discovered in 1981, 1984 and 1985 respectively.
Corrosion evaluation of the tubes was presented at EUROCORR 2016, Montpellier, France, September 16th 2016 by Cathrine Holager, Materials Engineer (Aker Solutions) and authored by Karin van Thoor (Aker Solutions) and Sophia Ekman (Sandvik Materials Technology R&D) with the title of "Corrosion Assessment of the 17-year old Åsgard Smørbukk Umbilical".
In addition to the previous report, this following article will report on the investigation of the fatigue properties of the tubes in two sections of the umbilical tubing installed in this field. Samples were taken from different areas of the dynamic umbilical, i.e. the splash zone and the most dynamic zone, and evaluated by fatigue testing.
High frequency fatigue tests were performed on the samples and showed that fatigue properties did not differ significantly. The tests showed insignificant reduction in the fatigue strength of the tubes used both in the most dynamic part and splash zones. In addition, mechanical properties were evaluated to understand the effect of deformation history from fabrication, service life and sample preparation. No significant effect, besides a slight hardness increase compared to the delivery condition, was observed.