Slug Induced Vibrations Modelling

Abstract

In this paper, the authors present an alternative methodology to improve (or at least to reduce) the uncertainty level within the prediction of the life time of subsea structures. The focus is made on rigid spool & jumper (or any piping system) prone to internal intermittent multiphase flow (currently named slug flow) and any potential Flow-Induced Vibrations (FIV) phenomenon. As it will be more exhaustively detailed here below, the proposed modelling aims at recovering at the best both (i) the multiphase flow kinematics (i.e. both gas pocket & liquid slug) to be expected in the flow loop and (ii) the resulting loads seen by the structure. The considered multiphase flow solver is based on the well-known Unit Cell Model (UCM, refer to Nicklin et al. (1962), Wallis (1969)) and coupled with usual commercial Finite Element (FE) solvers to recover the expected vibratory levels within the mechanical system. With rigorous purpose, a step-by-step validation process is presented within this paper to progressively validate the different step changes in regard to the current Best Practices (as e.g. reminded by Payne (2015) or Ancian (2016)). Both Computational Fluid Dynamics (CFD) model and experimental database, as extracted from the literature, have been considered to assess the ability of the proposed methodology to recover the expected multiphase flow kinematics and the loads induced by a Taylor bubble flowing within a rigid spool. Once validated, the multiphase flow solver has been coupled to a Finite Element (FE) model to properly assess the Flow-Induced Vibrations (FIV) of the spool resulting from such intermittent slugging solicitations. As here below underlined, the presented comparisons with the Industry Standards suggest (i) the need to challenge the recommended practices to ensure safe and reliable design and (ii) to properly manage the safety Design Fatigue Factor (DFF) to be considered within engineering phases.