Abstract
As more satellite wells are tied into existing subsea infrastructure, fluid flow velocities in subsea piping can be significantly higher than was anticipated during design. These higher velocities may lead to increased flow-induced vibration (FIV) as well as erosion due to sand transport. FIV is caused by flow-induced turbulence and associated pressure fluctuations, typically generated at piping geometry, such as bends or tees. These flow-induced fluctuating forces pose structural integrity concerns for subsea piping in terms of cyclic stressing and, over time, a threat of fatigue failure.
Currently, a high-level screening approach for FIV exists as part of the Energy Institute (EI) Guidelines on the Avoidance of Vibration Induced Fatigue Failure (AVIFF). However, the EI AVIFF guideline does not provide a framework for the direct estimation of vibration levels, stress levels, or fatigue life. To address this gap, we have developed a comprehensive screening procedure for FIV in subsea piping. Assets which fail a screening based on published guidelines (e.g., EI AVIFF guidelines), may undergo a more detailed screening based on numerical simulation. The paper gives an overview of such a screening, based on computational fluid dynamics (CFD) and structural finite element analysis (FEA).
CFD is a specialized field requiring specific expertise and a significant amount of computational resources. The CFD calculates the unsteady signature of flow-induced forcing on the piping for a given flow condition (i.e., multiphase flow). This forcing is then applied to a structural Finite Element (FE) model of the piping, with the output being displacement, acceleration, stress (among possible others) and ultimately a fatigue life estimate.
FIV and erosion measurements are much more costly subsea than they are on topsides. Hence, advanced simulation techniques are valuable tools in determining the integrity status as well as the safe operating limits of subsea piping.
