Risers and flowlines are an integral part of deepwater oil and gas field developments around the world. Risers, which serve as the interface between floating platforms and subsea flowlines, are subjected to low-stress high-cycle fatigue loading due to platform motions and vortex induced vibration (VIV). Flowlines are increasingly required to withstand high-stress range fatigue due to high pressure and high temperature (HP/HT) conditions causing lateral buckling along the flowline. Risers and flowlines are generally made by steel tubulars which are joined by girth welds for most subsea applications. Therefore, the quality of the girth welds is critical to the fatigue performance of risers and flowlines.
Fatigue design of risers and flowlines is based on the SN fatigue approach. However, that approach does not address the potential for weld flaws to affect performance. Fracture mechanics based engineering critical assessments (ECAs) provide the technical basis for Non-Destructive Evaluation (NDE) and critical flaw acceptance criteria (FAC). The FAC should address maximum allowable flaw sizes at the fabrication stage to ensure that initial girth weld flaws do not grow excessively and cause unstable fracture or through wall failure over the entire service life.
Where there is variability and/or uncertainty, ECAs use conservative assumptions and safety factors. However, as HP/HT developments are becoming more common, FAC resulting from ECA tend to be smaller. The specification of FAC plays an important role in the success of the project in terms of quality, cost and schedule. A more stringent FAC will have more weld rejections, which results in slower fabrication and higher cost, or may even become too small to be detected using automatic ultrasonic testing (AUT). In addition, weld repair will adversely affect the quality and increase probability of failure as girth weld failures are often found initiated at weld repairs. The result is questions about assumptions and safety factors applied.
As part of this reliability-based assessment, this paper considers two design examples to address reliability based ECA flaw acceptance criteria and safety factors of risers and flowlines. The first example is a deepwater steel catenary riser (SCR) subjected to fatigue loads due to vessel motion, wave fatigue and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads.
This paper offers valuable insights into a balanced approach for inputs selection in ECA by deriving a reliability based FAC and comparing it with the approach outlined in DNV-OS-F101 (Reference 1). It demonstrates FAC can be significantly increased by using reliability based ECA, and as such it will result in faster fabrication and reducing the project cost and schedule. This is of particular interest when considering fatigue performance and life extension of risers and flowlines, asset integrity management, and their relationships with project cost and schedule.
Instead of the fit-for-purpose ECA which calculates fatigue life with known girth weld flaws, this paper discusses how to determine allowable initial flaw sizes to satisfy riser and flowline fatigue requirements by deriving a probability density function of the critical flaw acceptance criteria using a reliability based Monte-Carlo approach. This paper provides an approach which is beneficial not only for detailed design but also for tendering purposes during the very early stages of projects, with less conservatism.