Fatigue analysis of a riser in Orcaflex

The purpose of this project was to suggest options for life extension of a riser.

Jee’s client had an installed umbilical riser which they wished to continue to use beyond its calculated fatigue design life. Jee was asked to verify the initial analysis of the riser using Orcaflex and, if the life calculations were lower than required, suggest ways that the life could be extended for 4 different components within the umbilical.

Jee reviewed the input data from the previous analysis and was able to identify several areas where the model was unduly conservative, and suggest suitable alternative approaches.

An Orcaflex model of the riser system was created and a number of cases with different wave loadings and directions were run. MathCAD calculations were also undertaken to calculate bending and tension stress factors for the metallic components within the umbilical. These items of data were then combined, along with suitable SN curves for the different components and the typical annual wave loading distribution, to calculate the fatigue lives for the components.

Jee predicted a longer fatigue life than had been initially calculated, thus extending the service life of the riser.

Benefits to the client included:

• Confidence to extend the use of the riser
• Having detailed analysis of the fatigue damage seen in each component meant fatigue lives could be calculated for both service and structural components