Wall thickness often presents a considerable influence in offshore pipeline capital expenditure (CAPEX). This influence is enhanced in design of ultra deepwater trunk lines of large diameter, where any wall thickness increase provides a huge impact on project costs. In ultra deepwater scenarios, thicker pipelines may eventually implicate not only in higher costs, but may also compromise the project feasibility due to installation load constraints related to laying vessels availability.
One potential way to reduce the pipeline wall thickness is to calibrate fitness-for-purpose safety factors through application of structural reliability methods, instead of utilizing the standardized safety factors presented in international codes. Since mid-nineties, several offshore pipeline design codes have been allowing the calibration of safety factors by structural reliability analysis. The purpose of such an allowance is that structural reliability methods would eliminate some eventual conservatism presented in the safety factors proposed by codes. Although this enables the achievement of optimized safety factors, more than fifteen years have passed and only few pipeline projects have taken advantage of the benefits of safety factor calibration.
This paper evaluates which potential benefits are available through safety factor calibration, particularly for wall thickness reduction purposes in ultra deepwater pipeline design. Calibrated safety factors are presented for some scenarios related to ultra deepwater export pipelines, considering “system collapse criteria” limit state. The calibrated safety factors are compared with the standardized safety factors presented by international pipeline design codes. The potential for safety factor reduction by the utilization of linepipes with more stringent manufacturing tolerances and the consideration of the thermal ageing imposed by coating application are also discussed.