Steel Tube Umbilicals (STUs) are integral to many offshore production systems and must be designed to endure demanding fatigue environments over the course of their service life. It is therefore fundamental that the design methods employed to calculate the fatigue life of STUs are suitably validated against real test data.

This paper compares the predicted STU fatigue life derived using UFLEX2D analysis with that observed by flex cycling a manufactured sample to failure. Characteristics such as tube helical geometry, internal pressure, tension, bending, and friction stick-slip behaviour are considered. Calculated strains are presented alongside strain gauge measurements taken from the sample, and fatigue lives derived using DNV-RP-C203 are compared against the actual numbers of cycles for tube through wall-thickness cracks to develop.


Oceaneering Umbilical Solutions (OUS) continues to invest in improving engineering technology and has key focus on developing accurate analysis techniques for designing reliability into its STU products. The past 2 years have seen a number of new initiatives including participation in the Umbilical Phase II JIP with Marintek to support development of the non-linear FE analysis code UFLEX2D and providing sample umbilicals to the project for test and validation work.

More recently work has been carried using UFLEX2D to compare the theoretically predicted fatigue life of an STU against a full-scale flexure test to failure. This paper provides a description of the analysis approach and results of the test.

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