Internal flow induced vibration (FIV) is regarded as a potential threat to safe operation of subsea rigid tie-in spools with significant freespanning and exposed to turbulent multiphase internal flow. Common industry practice of assessing FIV is to apply the conservative likelihood of failure (LOF) approach, which may lead to undesirable high project cost. In this paper, a higher-level approach involving power spectrum density (PSD) of the random excitation loading coupled with finite element analysis is presented. This approach has been successfully applied to a recent Equinor-operated large subsea field development project in Barents Sea - Johan Castberg - for optimized assessment of FIV of a number of rigid production spools, which do not pass the LOF criterion. It is concluded that FIV due to multiphase internal flow is not expected to generate additional fatigue damage to any of the considered production spools, resulting in significant project cost saving by avoiding installation of mitigation.
Internal Flow Induced Vibration (FIV) is regarded as a potential threat to safe operation of topside and subsea pipe works, leading to fatigue failure (Energy Institute, 2018). The vibration mechanism can manifest itself in different forms, requiring different approaches to be applied for performing the corresponding engineering assessment. An example of a system exposed to FIV may be subsea rigid tie-in spool, exposed to turbulent multiphase internal flow. The flow generates broadband random excitation force, causing random vibrations in the spool. It is therefore essential to have a reliable analysis approach to capture the effect of such an internal flow induced vibration. Common industry practices have been to apply a very conservative screening exercise to evaluate the likelihood of failure (LOF) defined on the basis of general categorized level of flexibility of a piping system. A reference can be made to Energy Institute (2018).
