Flow induced vibration (FIV) can go undetected in subsea pipework, potentially leading to fatigue failures. Although FIV screening methods have been developed, these tend to be conservative for multiphase pipe flows and are typically only validated for simple single bends. This paper investigates the usage of Computational Fluid Dynamics (CFD) to predict realistic forcing functions that could be used to analyse stress and fatigue in complex combinations of bends and tees, typically seen in subsea flow-lines, manifolds and jumpers.
When simulating multiphase flow through a complicated pipe configuration it is important to define the slug length and velocity at the inlet to correctly predict force magnitudes and frequencies. A novel approach is presented which uses a quasi-three-dimensional (Q3D) CFD method to obtain horizontal slug flow inlet boundary conditions for subsequent force prediction. The results of this approach are compared with physical tests of slug flow through a single bend and predictions based on more established slug flow correlations.
The Q3D CFD method produced slug flow inlet boundary conditions that matched correlations and test data well. The conclusion is that Q3D CFD provides improved horizontal slug flow inlet boundary conditions compared to simplistic approaches such as defining time varying square waves based on correlations. The forcing frequencies predicted using the Q3D CFD approach showed an improved distribution compared to a boundary condition based on correlation, and the magnitudes of the forces on the pipe bend compared well with published test data.
It is well-established that the flow of fluids through process pipework generates fluctuating forces caused by turbulence, cavitation, flashing, multiphase and acoustic effects. Under certain conditions, these forces can be sufficient to cause catastrophic fatigue failures. In the North Sea vibration and fatigue account for 21% of all pipework failures (1).
When a system is operational and accessible then these issues are typically assessed by a combination of field vibration measurements and finite element stress analysis. However, if the pipework is inaccessible (e.g. subsea), its duty is to be changed or it is yet to be constructed then predictions of the fluid forces (typically referred to as the forcing function) are required.