Challenges and Impacts of Severe Slugging Fatigue on SLWR Design

Abstract This paper aims to present the impacts of severe slugging over Production SLWR design in a Pre-salt of Santos Basin project. This design criteria of slugging fatigue in steel pipe risers is not frequently used, once subsea systems are, whenever possible, designed to minimize the risk of operating under this regime. However, the subsea layout of some recent projects, with long descending flowlines towards rises basis, increases the risk of occurrence of this phenomenon, mainly under low flow rates, typical of field late life. While hydrodynamic slugging might induce high frequency riser response, severe slugging (terrain induced slug) is characterized by a much lower frequency (but higher amplitude), which leads to a much different riser behavior. Different conditions were assessed, comprising critical scenarios with satellite wells and wells connected to a high-pressure gas/liquid subsea separator (HISEP™), which changes the liquid holdup along the flowline, favoring severe slugging occurrence. In steel pipe riser design, fatigue represents an important structural failure mode when accounting for all sources of cyclical loads that are expected during operational lifetime, such as: wave-induced motions, both current and heave-induced VIV and, lastly, internal fluid flow loads. This paper will focus on calculating severe slugging damage while performing a comparison among all sources’ percentual contribution to total damage in this studied scenario. This comparison shows that severe slugging heavily contributes to fatigue damage at a specific region located at the Touchdown Zone (TDZ). This critical region suffers mostly from internal fluid weight variation along the riser, which leads to low frequency but high amplitude stress variation. The OLGA™ dynamic multiphase flow simulator 1 was adopted in order to model the subsea slugging flow and generate the required data for riser analysis, that is, pressure, fluid density, and velocity profiles along the riser pipe, during at least an entire slug cycle, allowing to characterize the fluctuations due to this flow. Afterward, time-domain structural analyses were performed with inhouse Petrobras’ riser analysis software Anflex 2 by having the above-mentioned profiles utilized as input data. The state of practice for riser slugging fatigue analysis is not yet fully established. Even in guidance with industry standards, different practices and assessment methodologies between operators and projects can be found. It could be observed that uncertainties in reservoir parameters, premises on definition of flow parameters as the multiphase and slugging flow itself, design load cases with respective probabilities of time operation under this regimen and methodology adopted for riser dynamics modeling were crucial, representing Petrobras’ ongoing efforts to establish a robust and safe SLWR riser design.