The structural behavior of the flexible pipe tensile armors inside the end fitting (EF) is different from the one observed in the pipe body. The actual design methodology shows that the end fitting of the riser top section (at the interface with the floating unit) is critical with respect to fatigue, as stress concentration that occurs in this region during the end fitting assembly leading to significant plastic strains. Aiming at improving the structural performance of flexible risers, Petrobras designed and patented a novel concept of flexible pipe end fitting in which its assembly can be carried out without the need of folding the tensile armor thus avoiding the introduction of plastic strains and residual stresses in this layer. In order to assess and quantify the benefits of this new concept, in this work, a comparative study between the current end fitting concept used by the industry and the new technology proposed by Petrobras was conducted. This study comprises not only the stress distribution along the wire inside the end fitting but also the fatigue performance. For this purpose, finite element analyses have been carried out considering the entire end fitting mounting process and also the operational loads. These analyses were calibrated with results from midscale tests in which a longitudinal slice of the real end fitting was simulated. An instrumented tensile armor wire was embedded in the resin epoxy and pulled out from a test device. The results obtained show that the using of the proposed technology results in a considerably end fitting fatigue performance improvement.
The offshore operational experience has shown that the flexible pipe end fitting (EF) may be the weakest point in terms of fatigue resistance of the tensile armor layers. The findings observed in dissections from risers which were in operation or subjected to qualification tests reveal that there is a key point with respect to fatigue inside the end fittings. This point corresponds to the section in which the wires of the tensile armors are subjected to significant plastic strains caused by the end fitting assembly process. Thus, this study aims to propose an analytical approach to predict the fatigue resistance of flexible pipes focusing on their terminations. A local analytical model is employed to predict the stresses induced in tensile armor of the pipe due to the end fitting assembly and also during operation. These stresses are used in conjunction with a previously detailed approach that is employed to compute the fatigue resistance of flexible pipes. The methodology is validated by comparing its results to several real scale prototype tests.The results obtained during validation showed that the application of this methodology in calculating the fatigue life has low variability and therefore the failures observed in the qualification tests are well predicted. This methodology has been successfully used by Petrobras in flexible risers life extension.
This work focuses on a methodology to predict the fatigue life of flexible pipes with wires broken in their tensile armors. Initially, the mechanical behavior of these pipes is discussed. Relying on this discussion, a simple set of equations is proposed in order to calculate the stresses in the armors of these pipes. These equations employ pre-estimated linear coefficients to convert forces and moments that act on the pope into stresses. These stresses are then processed by well-known cycle counting methods and S-N curves are finally used to evaluate fatigue damage at several points in the pipe’s cross section. The use of this methodology is exemplified by the assessment of the fatigue life of a 6” flexible pipe in which 0 up to 5 wires of its outer tensile armor are broken. The results indicate a substantial reduction in the fatigue life of the pipe with the increasing number of wires broken.
This paper studies the structural response of a 6.0″ flexible pipe under pure tension considering two different situations: the pipe is intact or has five wires broken in its outer tensile armor. A three-dimensional nonlinear finite element model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each wire of the tensile armors and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests are also presented in order to validate the theoretical estimations. The theoretical and experimental results indicate that the imposed damage reduced the axial stiffness of the pipe. High stress concentrations in the wires near the damaged ones were also observed and, furthermore, the stresses in the inner carcass and the pressure armor are affected by the imposed damage, but, on the other hand, the normal stresses in the wires of the inner tensile armor are not.
A Sub-Surface buoy hybrid riser system is considered as a solution for deepwater export systems. Besides the in place static and dynamic loads, the installation operation was analyzed. The development includes the reduced model tests in a (so far) unique Deep Water Ocean Basin. A reduced model in an adequate scale was designed and constructed. It was submitted to an almost full equivalent depth and a comprehensive equivalent environment. The later corresponds to representative Campos Basin currents together with critical high crossing waves, which were made to excite resonance of the FPSO (Floating Production Storage and Offloading) hull. Typical moored FPSOs’ motions are much more than other platforms like Semi-Submersibles or TLPs and therefore the SSB is very adequate, since it resists easily to the cited critical environment conditions. Besides describing the system main characteristics, the paper describes the model testing in detail and present main results. Some problematic aspects has been are clearly appreciated for the first time, anticipating that what could happen in full scale. This therefore, requires a design improvement as also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.