A model for predicting dynamic umbilical stresses due to friction, elastic bending and tension has been developed and compared with strain gauge data from full scale dynamic testing. The results have been used to develop effective stress and fatigue analysis procedures that can be applied for both extreme and fatigue design. These procedures have been applied for both extreme and fatigue design cases to demonstrate the importance of different physical effects. Different applications in terms of tension and geometry may require different procedures for fatigue analysis. Alternative procedures are therefore proposed considering effects from umbilical geometry and water depth. As the oil industry continues to move into deeper waters the challenges related to umbilical design increases. Hence, methods and procedures need to be continuously developed to meet these challenges.


This paper reports a summary of the work performed between 1998 to-date in the UFLEX R&D project. The R&D project was designed to develop a FE-based fully non-linear stress analysis program for analysis of complex umbilical crosssections. The project has consisted in the following (see ref. /1/ and /2/ for further details):

  1. Model development including the mathematics, the numerics and software development.

  2. Small-scale material testing for input data.

  3. Full-scale umbilical testing for validation.

  4. Development of new methodology to implement in umbilical design.

The software was designed to include the following functionality:

  • Arbitrary geometry modeling including helical elements wound into arbitrary order.

  • The helical elements may include both tubes and filled bodies.

  • Elastic, hyper-elastic, and elastic-plastic material models included.

  • Initial strain can be modeled.

  • Contact elements, including friction, are implemented.

  • Tension, torsion, internal pressure, external pressure, bending and external contact loading (caterpillars, tensioners, etc.) are all loads that can be analyzed for.

The small-scale testing included:
  • 21 load vs. deformation tests on metals, polymers etc., i.e. materials used in umbilical cross-section designs.

  • 122 friction factor tests, i.e. friction factors of interacting materials inside umbilical cross-sections.

The full-scale umbilical testing has to-date included:
  • 11 mechanical behavior tests to compare the model's prediction of:

  • Axial strain vs. axial force.

  • Axial strain vs. torsion.

  • Torsion vs. torsion moment for various axial force levels.

  • Moment vs. curvature for different tension levels.

  • One mechanical behavior test on a strain gauge monitored umbilical to compare the model's prediction of stress components including the friction induced component.

Finally, the learnings have been used to explore ways of effectively implementing friction stresses in the extreme and fatigue evaluation of dynamic riser umbilicals.

UFLEX Model Description and Validation

Details of the UFLEX model can be found in /1/. The UFLEX model has been validated through extensive full-scale umbilical testing. Some of these results are presented in the following.

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