This presentation was held at the 2020 NAFEMS UK Conference "Inspiring Innovation through Engineering Simulation". The conference covered topics ranging from traditional FEA and CFD, to new and emerging areas including artificial intelligence, machine learning and EDA.

Resource Abstract

Recent riser analyses performed on completion workover risers (CWOR) deployed within a marine riser (MR) (i.e. a pipe-in-pipe riser), has found that the drag coefficient (Cd) is highly sensitive and crucial to fatigue life calculation in CWOR riser analyses. In the subsea industry, a range of 1.0 to 1.2 is commonly used for both open water and pipe-in-pipe risers. Nonetheless, the recommended practise (DNV-RP-C205) indicates that for circular cylinders close to a fixed boundary, such as the seabed, the Cd can reach up to a value of 2. This suggests that for a pipe-in-pipe riser, the Cd is expected to be higher and more complex than the common practise, (i.e. 1.0-1.2).

This paper describes the CFD analysis performed to investigate and calculate the actual Cd values for completion workover riser (CWOR) operating within a marine riser (MR). The analysis simulates the drag force as the CWOR travels across the annulus gap filled with fluid within the MR. A range of velocities and gaps commonly found between the MR and CWOR were investigated in the CFD analysis. The domain motion was captured using dynamic meshing and the K-Omega Shear Stress Transport (ko-SST). Turbulence model was used to simulate the transient flow pattern in the annulus.

Displacement versus Cd diagram is presented in this paper and the effects of the drag force on fatigue life are discussed for a range of relative velocities and radial gap distances. Results show that the drag force increases exponentially as the CWOR approaching the boundary. The CFD analysis indicates that the average Cd of the CWOR ranges from 0.5 to 42 depending on the relative velocities and radial gaps simulated. Global riser analysis sensitivity studies has found that the fatigue life is insensitive when the Cd is greater than 20. The Cd calculated from the CFD model was compared to the values cited in DNV RP C205 for circular cylinders close to a fixed boundary, and found to have a good correlation, thus validating the reliability of the model.

Currently, most riser analysis packages only allow a constant Cd to be input in the analysis model. This paper highlights the importance of non-linear drag coefficients and can be used as reference for future software developments so that a more representative fatigue life can be simulated for riser pipes oscillating in confined fluid environments.