This article reports research projects on the loss of stability of flexible pipes" tensile armours supported by PETROBRAS, focusing on their purposes, implementation strategy and results. The authors also postulate some principles which are important for the comprehension of this failure mechanism.
PETROBRAS has used flexible pipes in its subsea production systems since 1970s. Besides the experience as major purchaser, more than one hundred research programs related to the utilization of flexible pipes have been internally developed. Despite all investment and developed practices, some of their failure mechanisms are still misunderstood. This is indeed the case of the instability of flexible pipes' tensile armours. If a researcher tries to carry out a bibliographic search in article databases, he will find only a few papers on this subject and they will seldom lead to incontrovertible methods and conclusions. In fact, initiatives with apparently good methodological approach may fail if most fundamental comprehension of the investigated phenomena is not carefully built. Birdcaging, for example, has long been deemed to be the instability mechanism in which the tensile armours fail - even the current API 17B does not mention another process –, despite each tensile wire has degrees of freedom other than the radial. The concern on flexible pipe's tensile wire instability becomes critical as the use of large-diameter flexible risers in deepwater production and export systems becomes attractive. The combination of high external pressure and large cross-sectional area causes compressive end-cap effects; further, the compression itself tends to open gaps in the multilayer structure and to reduce both the pipe's bending stiffness and the radial stiffness associated to each tensile wire. Further, this matter is crucial in flexible pipes conducting gas, because internal pressure releases (shutdowns) often occur, then improper compressive conditions may be kept for time longer than ever tested.