The Effect of Tube Friction on the Fatigue Life of Steel Tube Umbilical Risers - New Approaches to Evaluating Fatigue Life using Enhanced Nonlinear Time Domain Methods
Authors: Kavanagh W.K. , Doynov K. , Gallagher D. , Bai Y. 
Published: October 2008

Abstract
A critical component in the wet tree development of deep water fields is the umbilical riser. Failure of the umbilical can result in partial or total loss of production, with resulting severe economic consequences. In many recent deep water projects, steel tubes for control and injection lines have become the preferred solution over traditional thermoplastic hose based designs. The fatigue design of these steel tube umbilicals (STUs) with accurate modeling of the steel tube interaction friction stresses is critical for any development using STUs.
Simplified conservative approaches where the nonlinearities of the tube friction are ignored have frequently been used for STU fatigue design. This paper presents the theoretical basis for an enhanced non-linear time domain fatigue model for deepwater STUs. In developing this model, the authors have proposed an analytical fatigue design approach that accounts for the non-linearity of tube friction using slip theory combined with time-domain global and local analytical methods.
A worked example is presented, where an ExxonMobil West of Africa field development STU has been analyzed using both the enhanced nonlinear time domain fatigue model and an alternative simplified model. The enhanced non-linear time domain fatigue model, has been used to evaluate the influence on fatigue life of several parameters, including friction, water depth and STU cross-section geometry, some of which become increasingly important in deeper water.
The merits of the enhanced nonlinear fatigue model over the simplified model are critically evaluated, as are the lessons learned on the importance of critical design parameters and analysis assumptions affecting STU fatigue life. Conclusions are also presented on the benefit of further enhancement of the analytical approaches commonly used to design for umbilical fatigue.

STU Fatigue Models
In dynamic floating production steel tube umbilical risers, cyclic stresses occur in the metal tubes within the umbilical due to the wave-induced dynamic response of the umbilical and the motion of the floating vessel. These dynamic stresses are a combination of the axial and bending cyclic stress variations in tubes, induced by the global motions of the umbilical, combined with the effect of friction between tubes and surrounding structure in the umbilical cross section.
Cross-section models and fatigue analysis techniques used to design these tubes frequently make simplified assumptions about how to calculate and combine cyclic stresses in these tubes to determine fatigue life. Such assumptions typically include :regular wave analysis of global riser response to fatigue scatter diagram seastates compared to alternative irregular wave time or frequency domain approachesnon-phase-consistent calculation of tube fatigue stresses under global umbilical bending and tensionfatigue calculations based on simplified tube friction/slip or no-friction modelapproximate cycle counting based on a regular wave response and single dominant frequency instead of an alternative which captures actual seastate frequency content and multiple amplitude cycles, for example a rainflow counting irregular wave approachin-plane fatigue analysis based on analysis of umbilical response to global motions in the same planeprediction of friction stresses using static friction coefficient rather than separate static and dynamic friction coefficients.