Fluid-Structure Interaction Simulations of a Pipeline Span Exposed to Sea Bottom Currents
Authors: Pontaza Juan P. , Menon Raghu , Swanson Richard , Jhingran Vikas , Hill Mike , Kopp Frans , Hoffman Joe 
Published: July 2010

Abstract
The paper is concerned with the numerical simulation of fluid-structure interaction (FSI) of a pipeline span exposed to sea bottom currents. The pipeline lies on the seabed at one end and departs the seabed to attach to a PLET (pipeline end termination) at the other end. The suspended span in-between is referred to as a ‘free span’. When exposed to sea bottom currents the free span may experience vortex-induced vibrations (VIV), which may cause fatigue damage to the pipeline. The length of the suspended span is 95 ft, and the gap to diameter ratio varies along the length from 0.0 at the seabed touch point to approximately 2.0 at the PLET. The sea bottom current at a reduced velocity of 3.2 is an extreme event for this span. The VIV response of the pipe span is predicted by coupling a three-dimensional viscous incompressible Navier-Stokes solver with a beam finite element solver. Parameters such as turbulence in the flow, proximity of the seabed, pipe sagging due to submerged weight, and pipe-soil interaction, are all accounted for in the FSI simulation. The motivation for such a considerable simulation effort is to gain insight into VIV of pipe spans in proximity of the seabed, and in the long-term to manifest this insight in a set of design guidelines for free spanning pipelines. Design guidelines for free spans are typically based on VIV amplitude and frequency responses for isolated pipes, with little regard to effects of seabed proximity. This may result in overly conservative designs and/or expensive span remediation recommendations, when in reality no span remediation is required.

1. Introduction
Vortex induced vibrations of structures continue to be one of the most challenging fluid-structure interaction (FSI) phenomena in deepwater drilling and production. A majority of these structures such as risers, tendons, umbilicals and spar hulls have circular cross-sections. In the presence of ocean currents, the boundary layer flow around these tubular structures separates and initiates vortex shedding. The resultant fluctuating forces exerted on the structure excite oscillations of the structure, known as vortex-induced motions (VIM) or vortex-induced vibrations (VIV). An important feature of these fluid-structure systems is ‘synchronization’ or ‘lock-in’, typically described as involving a synchronization of the body oscillation frequency with one of the natural frequencies of the structure. During synchronization, the vibration amplitude of the structure due to VIV may increase significantly; which could lead to fatigue damage of the structure and ultimately to fatigue induced failure. The range of synchronization is especially wide for low mass ratio structures, common in deepwater production.
In general, research efforts in this area have been focused on developing and sharpening tools to predict the risk of structural failure due to VIV. Due to the difficulty in modeling the fluid forces and the fluid-structure interaction, the analysis of VIV has remained a somewhat empirical science (a combination of theory aided by a large experimental dataset).
With the advent of high performance computing (HPC) infrastructure (e.g. clusters for parallel computing) and the improvement of methods and algorithms to numerically solve the governing equations that describe the behavior of the fluid and the structure from first principles, it is now possible to perform numerical simulations of the VIV phenomena. The objective of this paper is to give a brief overview of the current state-of-the-art in FSI simulation capability and to document a full three-dimensional FSI simulation of pipeline VIV using in-house developed capabilities.