Owen H. Oakley

NUMERICAL PREDICTION OF BARE AND STRAKED CYLINDER VIV

The prediction of deepwater riser Vortex Induced Vibrations (VIV) is one of the most challenging areas in the offshore industry. Numerous experimental and numerical studies have been performed in an effort to improve the understanding and prediction of cylinder VIV behavior. This paper presents the numerical simulation of rigid circular sections, both bare and fitted with strakes, using a second order accurate finite element computational fluid dynamics (CFD) method. Two turbulence models are examined: the Spalart-Allmaras Reynolds Averaged Navier Stokes (RANS) and the Detached Eddy Simulation (DES). Pragmatic high Reynolds number simulations of fixed and moving cylinders are presented and compared with laboratory experiments. Flow visualization provides insights on how strakes mitigate VIV. Comparisons between RANS and DES results are also presented and discussed.Copyright

Simulation of Riser VIV Using Fully Three Dimensional CFD Simulations

Fully three dimensional computational fluid dynamics (CFD) solutions are combined with structural models of a tensioned riser to predict riser vortex induced motion. The use of three dimensional CFD solutions overcomes many of the shortcomings of combining a series of strip or two dimensional simulations to calculate the fluid forces on the riser. Three dimensional vortex structures are treated correctly and straked risers and variations in angle of attack can be studied directly. The proposed method uses finite element methods that are tolerant of sparse meshes and high element aspect ratios. This allows economical solutions of large fluid domains while retaining the important features of the large fluid vortex structures which drive risers. Long risers can be treated with readily available computers and examples of simulations of riser with L/D over 1400 are given and compared with previously published experimental data. These examples are used to illustrate several points regarding the effects of the treatment of the riser structure as well as the efficacy of rotating frame or pinned riser experiments used to simulate sheared currents. The method can also be extended to sheared currents whose heading varies with depth.

Fully Coupled Fluid-Structure Interaction for Offshore Applications

CAD integrated tools are accelerating product development by incorporating various aspects of physics through coupling with computational aided engineering (CAE) packages. One of the most challenging areas is fluid-structure interaction (FSI) of low mass bodies such as flexible marine risers/cables with vortex-induced vibrations. The focus of this work is on the application of a new Multi-Iterative Coupling (MIC) procedure to couple AcuSolve (fluid solver) with Abaqus (structural solver). The proposed new scheme has superior stability and convergence properties as compared to conventional explicit staggered schemes, especially for low mass-density ratios of structure to fluid. Demonstrations and validation of the scheme are presented and discussed along with additional challenges associated with FSI in production environments. The addition of an FEA solver enables the modeling of the nonlinear aspects of flexible riser VIV, namely, contacts with gaps, multi-body dynamics, seabed interaction, geometric and/or material nonlinearities.

Modeling Vortex Induced Motions of Spars in Uniform and Stratified Flows

This paper examines the vortex induced motions (VIM) of a spar type floating production platform in uniform and sheared currents. The large draft of modern production platforms means that in some of the extreme current events the flow past the platform is highly non-uniform along the hull. We discuss the simulation of these stratified flows associated with hurricane events and loop currents and the implications for experiments and numerical simulations. Model testing options are reviewed along with the potential effects of buoyancy due to temperature and salinity variations in the current. Comparisons are made between experimental test results and numerical simulations of VIM at small scale and projections are made to full scale behavior using computational fluid dynamics (CFD) and detached eddy simulation (DES).

Benchmarking of Truss Spar Vortex Induced Motions Derived From CFD With Experiments

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.Copyright

CFD HIGH L/D RISER MODELING STUDY

Fully three dimensional fluid flow simulations are used with a simple structural model to simulate very long risers. This method overcomes many shortcomings of methodologies based on two dimensional flow simulations and can correctly include the effects of three dimensional structures such as strakes, buoyancy modules and catenary riser shapes. The method is benchmarked against laboratory and offshore experiments with model risers of length to diameter ratios up to 4,000. RMS values of vortex induced vibration motions are shown to be in good agreement with measurements. The resources needed to model ultra deep water drilling and production risers are estimated based on current computer technology.

CFD TRUSS SPAR HULL BENCHMARKING STUDY

Blind comparisons of computational fluid dynamics (CFD) predictions are made against large scale experiments of a truss spar hull. The overall objective of the work is to try and mature CFD modeling capability related to motion and loads for offshore platforms. Specific goals include the testing of newly developed meshing techniques and seeking an understanding of how spar appurtenances interact. The key comparisons are illustrated in comparisons of the simple case of a spar hull with strakes alone against a second, far more complex case, involving multiple appurtenances. The latter exhibits very different vortex induced motion (VIM) behavior. The CFD predictions were made with knowledge of the as-built model details, but without access to the experimental results. The estimates were found to be in close agreement with the experiments or slightly conservative. We continue to observe that the better the model fidelity, the better will be the comparison with the benchmark. The computations show that high quality predictions are now feasible, given access to sufficient computational hardware and accurate meshing of the body in question. BACKGROUND

Analysis of Turbulent Flows and VIV of Truss Spar Risers

Complex flows through riser arrays, such as the case of risers located in the truss section of a truss spar, are very difficult to describe and analyze. It is especially difficult predict and correct Vortex Induced Vibration (VIV) response using traditional tools that were meant to analyze single risers rather than arrays of risers. Computational Fluid Dynamics (CFD) offers the designer the capability to properly analyze these complex problems, increasing the reliability of the design. In this study, a full scale truss spar with vertical risers is modeled using CFD. The VIV response of the risers is predicted and the effect of risers is correctly captured and compared with experiments.

Numerical Prediction of VIV and Comparison With Field Experiments

Computational fluid dynamics are employed to simulate vortex-induced vibrations of risers. The current method uses a three dimensional model with an optimized mesh to analyze high aspect ratio risers. A riser with an aspect ratio in excess of 4,000 is modeled and compared with field experiments. Results show good agreement with experiments and the ability of the method to predict observed harmonic content. With the computational resources that can now be brought to bare, simulations of deepwater risers are possible for design validation and guidance of empirically based tools that currently do not model high harmonics.Copyright

CFD MODELING OF CORRUGATED FLEXIBLE PIPE

The objective of this paper is to present Computational Fluid Dynamics (CFD) modeling of fully developed turbulent flow through a flexible corrugated pipe and to investigate the pressure drop reduction potential of liners. This work also aims to establish a framework to be used in large scale numerical simulations of the offshore transfer of cryogenic fluids. A 3-D CFD approach is considered more appropriate than 2-D axisymmetric one, since the wavy corrugation profiles lead to a great deal of internal turbulent structures for high Reynolds number over Re > 10 6