In wellhead fatigue analysis, results are quite often sensitive to input parameters that are associated with considerable uncertainty. Such parameters may, e.g., be the cement level in the annulus between surface casing and conductor, the soil and template support, friction and down weight on the landing shoulder, etc. A common approach in deterministic design to deal with such problem is to use worst case assumptions on sensitive parameters. However, this may result in unreasonable short fatigue lifetime estimates.
In this paper, it is shown how existing fatigue design equations using the concept of design fatigue factors can be augmented with characteristic design values for additional uncertain variables such that the overall safety level obtained is comparable to the recommended target levels for typical uncertainties in load effect calculations. Results from sensitivity analysis and SRA are compared to investigate to what extent sensitivity analysis results can be utilized as part of the design analysis. The intention of the proposed approach is to be reasonably conservative but to avoid being overly conservative.