This paper focuses on a theoretical approach to access the fatigue life of flexible pipes. This methodology employs functions that convert forces and moments obtained in time-domain global analyses into stresses in their tensile armors. The stresses are then processed by well-known cycle counting methods, andS-Ncurves are used to evaluate the fatigue damage at several points in the pipe’s cross-section. Finally, Palmgren-Miner linear damage hypothesis is assumed in order to calculate the accumulated fatigue damage. A study on the fatigue life of a flexible pipe employing this methodology is presented. The main points addressed in the study are the influence of friction between layers, the effect of the annulus conditions, the importance of evaluating the fatigue life in various points of the pipe’s cross-section, and the effect of mean stresses. The results obtained suggest that the friction between layers and the annulus conditions strongly influences the fatigue life of flexible pipes. Moreover, mean stress effects are also significant, and at least half of the wires in each analyzed section of the pipe must be considered in a typical fatigue analysis.

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Several feasibility studies on the use of large diameters steel catenary’s risers for higher water depths have been made by PETROBRAS. It should be mentioned that the structural design criteria, traditionally employed for such risers under extreme and operational conditions, were based on the API-RP-2RD. This design criterion makes use of single safety factor on von Mises stress. Recently, a new design standard DnV-OS-F201 for metallic risers has been proposed. This standard, based on limit state design principles, are presented on the LRFD (Load and Resistance Factor Design) format, allowing different riser design alternatives to take into account the environmental conditions. This paper presents the main feasibility study outlines of an 18” Steel Lazy Wave Riser (SLWR), attached to the border of a spread-moored FPSO at 1800 water depth, for an extreme design condition, considering both the API and DnV criteria. All numerical analysis were accomplished by the PETROBRAS’s in-house computer code ANFLEX.

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During drilling operations, the wellhead system and top hole casings shall be designed to support dynamic loads from the connected riser through the BOP stack/LMRP. As dynamic motions are associated to stress variations, fatigue becomes a major concern for designers. The accumulation of damage at the wellhead and close regions is dependent on several aspects, such as the riser components, the interactions soil-conductor and conductor-surface casing, and of course the environmental conditions. Consequently, fatigue analysis involves complex numerical models and requires the simulation of a huge number of loading cases. The present paper aims to estimate the fatigue damage at critical components of the top hole casings and at the wellhead. Two different approaches were investigated. In the first, a global model is analyzed in the time domain (TD), and the Rainflow cycle counting method is used to calculate fatigue damage. The global model includes the drilling riser, wellhead, casings, and interactions between components and with soil. In the second, the same model is analyzed in the frequency domain (FD), and the Dirlik method is used to calculate fatigue damage. Additionally, to allow a better evaluation of stresses at complex geometry regions, forces and moments obtained using the TD methodology were combined with load-to-stress transfer functions, defined by means of a local model and symbolic regression (SR) analysis. The local model includes a detailed 3D model of the pressure housings, and soil-to-casing interaction. The obtained results indicate that the pressure housings are not sensitive to fatigue, and also that the analyses performed are feasible, contributing to reduce computational costs in wellhead fatigue assessments.