Fatigue Design Criteria for Small Super Duplex Steel Pipes

Stainless steel pipes ranging in sizes from approximately 10 to 100 mm OD are used extensively in umbilicals for the control and monitoring of underwater installations for oil and gas production. Umbilicals are subjected to tensile loads as well as variable amplitude loading from wave and current actions. Fatigue is therefore a critical issue in the design of umbilical components. Sea water resistant high strength super duplex steel with ultimate strength of typically 800 to 900 MPa is used to save weight and reduce the wall thickness. Some umbilicals installed by Statoil have design pressure up to 1035 bar, which in combination with large dynamic loads from floating production units makes fatigue design of the umbilicals a challenging issue. While the fatigue performance of butt welded pipes for pipelines and risers are established and implemented in design guidance and codes, the experimental basis for design of small diameter piping made of high strength materials is not well documented in the open literature. However, unpublished data from in-house investigations indicate that small pipes in super duplex steel perform significantly better than larger diameter pipes in lower strength materials. It is therefore apparently scope for a “thinness effect”, i.e. a bonus effect that could be applied to the data for large diameter pipes in current codes to account for the higher S-N curves for small stainless steel pipes. This paper reviews some of the fatigue data for piping and compares these data with experimental evidence from a joint industry project. Tentative fatigue design guidance for small diameter super duplex steel piping is presented. Questions concerning special issues such as the possible influence of wall thickness, mean stress and pre-straining due to reeling are discussed.