This study summarizes a review of the dissection and post-dissection material testing of a recovered flexible riser with a damaged outer sheath and localized wire corrosion, and the subsequent analytical fatigue assessment of the riser based on findings from that dissection. Flexible pipe technology and experience is still relatively young when compared to the operational experience of rigid pipeline systems. This drives the continued effort to understand performance characteristics and to maintain integrity management and good practice guidance, particularly as ongoing projects continue to extend the environmental, structural, and service life requirements of such flexible pipe systems.
Incidents relating to outer sheath damage and annulus flooding account for most flexible pipe damage and failure statistics. The ultimate consequence of annulus flooding is principally a failure resulting from either reduction in armour capacity through corrosion or accelerated corrosion-fatigue.
The fatigue of armour wires is in many cases the limiting factor for the design life of flexible risers. Fatigue design of armour wires is, in general, based on full scale tests in which a given pipe design is subjected to a fatigue load history simulating the accumulated damage of the design life. These methods are costly and time consuming, thus this study aims to quantify the reduction in fatigue life due to metal loss/corrosion in the armour wires using advanced numerical analysis.
To evaluate the damage, Wood led a dissection of the recovered flexible pipe and collected wire samples for material and dimensional analysis. Wood observed an increase in pipe circumference around breach location; noteworthy as the sections of the outer sheath around the breach location were thinned out or missing completely, indicating a likely bird-caging effect of the tensile armour wires.
A layer-by-layer dissection was then performed such that each of the layers constituting the makeup of the flexible riser was removed individually and a condition assessment of each layer recorded. All tensile wires were retained following dissection for examination. Selections of wires were sampled for mechanical properties (yield strength and UTS) and metallography, dimensional measurement and testing.
Once the amount of material loss in the armour wires was measured, a local analysis of the flexible pipe was performed to quantify any increased wire stress and annualized fatigue damage between the original design wire size and the measured wire size.
Finally, integrity management and life extension recommendations for the current risers connected to the facility were made for future risers, all identical in size and service type to the recovered pipe.