Predicting fatigue life of steel tube umbilicals is challenging especially in deep and ultra-deep water regions such as West Africa and Gulf of Mexico. This is due to high top tensions and bending moments that increase the internal friction impact on the umbilical fatigue performance. Accounting for the additional stresses induced by internal friction in a consistent manner in umbilical fatigue analysis is non-trivial. Although it is the subject of many papers (Sævik, Hoffman, Kavanagh, Ekeberg, Sødahl, and Bury), there is currently no standard industry approach.
This paper aims to assess the effects of the stresses induced by internal friction on the overall umbilical fatigue life. Several methodologies for umbilical fatigue life assessments are discussed in this paper, including:
Traditional Straight tube analogy (friction free);
Weibull distribution approach and;
The effects of internal friction induced stresses on umbilical fatigue performance predicted by each method are compared and discussed.
This study shows that the internal friction induced stresses have great impact on the overall umbilical fatigue performance. Fatigue life can be reduced by a factor of 10 if internal friction is considered in the calculations. The significance of internal friction and its negative impact on fatigue performance can also increase with water depth.