Raghu G. Menon

Fairing Evaluation Based on Numerical Simulation

It is well established that strakes are effective at suppressing vortex-induced vibrations (VIV). Fairings are an attractive alternative to helical strakes, because they are a low drag VIV suppression solution. The paper presents an evaluation of a fairing design, based on numerical simulations — to be complemented at a later stage with current tank testing. This paper documents the computational fluid dynamics (CFD) and finite element analysis (FEA) of the evaluation: (1) 3-D CFD in the laboratory scale: 4.5 inch pipe, 3 ft/s current speed, (2) 3-D CFD in the full scale: 14 inch riser, 4 knots current speed, and (3) 3-D FEA of the full-scale fairing module latching mechanism, under service loads corresponding to 4 knots current speed. The analysis results show that the fairing design (1) is effective at suppressing VIV, (2) yields a low drag coefficient (0.52 at Re ∼ 106), and (3) its latching mechanism is adequate for use in calm sea states with 4 knots current speeds.Copyright

On the Numerical Simulation of Fluid-Structure Interaction to Estimate the Fatigue Life of Subsea Pipeline Spans: Effects of Wall Proximity

Subsea pipelines laid on uneven terrain typically have segments of unsupported spans, referred to as “free spans”. Alternatively, subsea pipelines lying on loose and fine gravel or sand may develop free spans due to sea bottom being scoured out due to current action. This paper is concerned with the numerical simulation of fluid-structure interaction (FSI) to predict the response of free spans exposed to sea bottom currents. When exposed to sea bottom currents these spans may experience vortex-induced vibrations (VIV), which may cause fatigue damage to the pipeline. The VIV response of the pipe span is predicted by coupling a three-dimensional viscous incompressible Navier-Stokes solver with a nonlinear 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. We pay special attention to the effect of seabed / wall proximity on VIV. Design guidelines for free spans are typically based on VIV amplitude and frequency responses for isolated pipes, with little regard to effects of seabed / wall proximity. This may result in overly conservative designs and/or expensive span remediation recommendations, when in reality no span remediation is required. Two examples of field applications are presented.Copyright

Numerical Simulations of Flow Past an Aspirated Fairing With Three Degree-of-Freedom Motion

We simulate the flow past circular cylinders that are outfitted with plain and aspirated short aspect ratio fairings. Aspirated fairings aim to reduce the mean drag by means of suction holes and blow holes strategically placed around the circumference of the fairing. Several designs are presented here, including centered and off-centered suction, and the designs assessed in terms of mean drag reduction and vortex-induced vibration suppression. We consider flow conditions that correspond to a high Reynolds number of 106 , and allow for three degree-of-freedom motions where the structure is allowed to respond to flow-induced cross flow forces, in-line forces, and spanwise moments. The computations are performed using a parallelized Navier-Stokes in-house developed solver using overset grids.Copyright

Prediction of Vortex-Induced Vibration Response of a Pipeline Span by Coupling a Viscous Flow Solver and a Beam Finite Element Solver

This paper describes a fluid-structure interaction (FSI) modeling approach to predict the vortex-induced vibration response of a pipeline span by coupling a three-dimensional viscous incompressible Navier–Stokes solver with a beam finite element solver in time domain. The pipeline span is modeled as an Euler–Bernoulli beam subject to instantaneous flow-induced forces and solved using finite element basis functions in space and an unconditionally stable Newmark-type discretization scheme in time. At each time step, the instantaneous incremental displacement is fed back to the fluid flow solver, where the position of the pipeline is updated to compute the resulting instantaneous flow field and associated flow-induced forces. Numerical predictions from the FSI model are compared to current tank experimental measurements of a pipeline span subject to uniform free-stream currents.

Flow-Induced Vibrations of High Gas Rate Well Jumpers: Tees vs. Bends

Subsea well jumpers are steel pipe sections that connect the flow path between the tree and the manifold. They are designed with bends to accommodate limited expansion due to variations in temperature and pressure. The unsteady change of flow direction in these bends induces fluctuating forces with broadband frequency content. If the fluctuating forces are of significant amplitude and/or induced in the right frequency range, they may lead to flow-induced vibrations (FIV) of the well jumper. FIV poses structural integrity concerns for subsea piping in terms of cyclic stressing and, over time, a threat of fatigue failure. A comprehensive FIV screening approach, based on computational fluid dynamics (CFD) and structural finite element analysis (FEA), and associated fluid-structure interaction (FSI) is used to estimate well jumper fatigue life due to FIV — for high gas rate production conditions. Although the FIV screening approach itself is novel, the main contribution of this work is in comparing FIV response and fatigue life of a well jumper outfitted with the traditional impact tees Vs. one outfitted with short radius bends. We find that the resulting flow-induced stress cycling is similar in magnitude in both geometries, but with different frequency spectra — giving the well jumper with short radius bends a 2x increase in fatigue life, relative to the well jumper with impact tees. We also perform a sand erosion analysis comparison between the two well jumpers, and find that the well jumper with short radius bends has a 3x diminished erosion rate, relative to the well jumper with impact tees.Copyright

Prediction of VIV Response of a Flexible Pipe by Coupling a Viscous Flow Solver and a Beam Finite Element Solver

The paper describes a fluid-structure interaction (FSI) modeling approach to predict the VIV response of a flexible pipe by coupling a three-dimensional viscous incompressible Navier-Stokes solver with a beam finite element solver – in time domain. The flexible pipe is modeled as an Euler-Bernoulli beam subject to instantaneous flow-induced forces and solved using C1 conforming finite element basis functions in space and an unconditionally stable Newmark-type discretization scheme in time. At each time step the instantaneous incremental displacement is fed back to the fluid flow solver, where the position of the pipe is updated to compute the resulting instantaneous flow field and associated flow-induced forces. Numerical predictions from the FSI model are compared to experimental measurements of a flexible pipe subject to uniform free-stream currents.Copyright

Coupled VIV and Heat Transfer Analyses: Design of VIV Suppression Configuration for the Independence Hub SCRs

Independence Hub (IH), a semisubmersible production platform in the deepwater Gulf of Mexico is currently under construction and will produce gas and condensate from multiple subsea developments. The platform, installed in some 8000 feet of water, is located in a region of the Gulf that is well known for its severe environment, particularly persistent loop currents that could last several months out of a year. The current profiles are such that there is a significant risk of fatigue failure of the risers due to Vortex Induced Vibration (VIV) and adequate mitigation through installation of VIV suppression devices, such as strakes and fairings, are required. An additional challenge that impacts the topsides facility and flex joint design is the low arrival temperature of the produced gas due to Joule-Thomson (JT) cooling. In the case of a bare riser (one without any VIV suppression), the heat transferred from the ambient seawater to the produced gas is sufficient to exceed the target arrival temperature. However, installing VIV suppression devices along the span of the riser may insulate it, which in turn has the potential to decrease the arrival temperature significantly. This paper will discuss coupled VIV and heat transfer analyses carried out to assess the risk of both VIV and low arrival temperatures for the IH production risers. The heat transfer analyses were carried out rigorously using 3D Computational Fluid Dynamics (CFD) simulations of the riser with and without VIV suppression devices. Helical strakes and short fairings were analyzed and optimum suppression configurations for two production risers that meet both the VIV and thermal requirements are presented. The optimum configuration was also subject to the requirement that the suppressed risers be S-layable. A hybrid suppression design based on both strakes and fairings are presented that meets the fatigue and arrival temperature requirements while minimizing installation risks using the S-lay method.Copyright