Offshore systems need long pipelines to transport liquids between the ocean surface and the seabed floor in deep water. These long pipelines, the so-called marine risers are exposed to strong ocean currents and may experience vortex-induced vibration (VIV) with large amplitudes due to strong fluid-structure interaction. The aspect ratio (length to diameter) of these structures can reach up to 5000 in such deep waters. Moreover, the highly unpredictable and severe ocean conditions such as high velocity currents and wind can affect the riser and floating vessel system. Due to the strong interaction of vortex dynamics and structural motions, the fluid-structure interaction can cause high amplitude vibrations along the riser which may hinder the offshore operation and production. While experiments are expensive to perform, computational modeling and simulation can indeed help in understanding the coupled fluid-structure dynamics. To model the coupled physics accurately, one needs high computational resources for such large-scale fluid-structure simulation. In this study, a state-of-the-art petascale supercomputer of National Supercomputer Center (NSCC) of Singapore was utilized. The study deals with understanding the dynamics of the riser under uniform flow current and validating the in-house fluid-structure solver for VIV of long flexible risers.

The problem consists of a flexible cylinder with the aspect ratio (L/D) of 481.5. The non-dimensional parameters in the study are Reynolds number (Re) of 4000, non-dimensional axial tension of 51063, non-dimensional bending stiffness of 2.1×107 and mass ratio of 2.23. All these parameters are taken with respect to the MARINTEK experiment (oe.mit.edu/VIV/) conducted on the flexible riser.

For the high velocity ocean currents, the Reynolds number (Re) can reach up to millions. For validation purpose, we have used Re=4000. To model the turbulence phenomena, the hybrid LES technique based on delayed detached eddy simulation (DDES) has been employed for fluid-structure interaction of the riser. The parallel variational fluid-structure solver relies on the incompressible Navier-stokes equation solver, the semi-discrete time stepping and a non-linear interface force correction algorithm for strong added mass effects. The equations are solved in a partitioned staggered manner to couple the Navier-Stokes, the turbulence and the flexible structure. To scale the fluid-structure solver for large scale computations, a version of Krylov subspace iterative solvers was utilized to solve the equation system using distributed memory parallel algorithm based on the hybrid MPI and OpenMP strategy. In the current computation, the finite element mesh contained around 12 million grid points. It was carried out using 600 cores of Intel Xeon dual socket E5-2690v3 CPUs with message passing interface (MPI) parallelization. A time step size of 0.1 units was taken and the simulation ran till 8000 time steps. The data was collected for every 100^th step in the process.