HAM Solutions for Vortex-Induced Vibration of a Circular Cylinder

The vortex-induced vibration (VIV) phenomenon is result of fluid-structure interaction which occurs in many engineering fields. The study of VIV of a circular cylinder is of practical importance (such as in marine cables and flexible risers in petroleum production). In this paper, one classical phenomenological VIV model — the motion of the cylinder is modeled by a simple linear equation, and the fluctuating nature of the vortex wake oscillation is modeled by a van der Pol oscillator, is analyzed. Firstly, the homotopy analysis method (HAM), a powerful technique for highly nonlinear problems, is developed to solve the coupled fluid-structure dynamical system with the convergence of the homotopy series solutions being demonstrated. Based on the HAM solutions, some properties of the fully nonlinear classical coupled VIV model are presented. All the results proved that the proposed HAM scheme has potential to be an effective analytic technique to study the VIV problems.

Vortex Induced Motion of TLP With Consideration of Appurtenances

Offshore floating platform configurations often consist of geometrically simple and symmetrical shapes which are made complicated by the presence of appurtenances such as helical strakes, tendon porches, steel catenary riser (SCR) porches, pipes, chains, fairleads and anodes on the surface of the hull. Previous studies mainly on spars show that these hull external features affect the Vortex Induced Motion (VIM) performance of the platform significantly. This is to be expected since VIM is controlled by the flow separation on the hull surface and the resulting vortex shedding patterns. Scale effects may also play a role in model tests for bare cylinders or hulls with bare cylindrical columns, whereas previous studies have shown less Reynolds dependence when appurtenances are modelled. This study investigates the effect of hull appurtenances on VIM of a multi-column floating platform, i.e. a Tension Leg Platform (TLP) designed for Southeast Asian environment. Significant difference in VIM behaviors is expected between spars and TLPs since the column aspect ratios are very different and TLPs do not have helical strakes that are commonly fitted on spars. Model testing and Computational Fluid Dynamics (CFD) simulation are used in this VIM study, with the former being the emphasis of this paper. Descriptions of the respective experimental and numerical methodologies are presented and the comparison of the results is made. Further work required to improve the model test set-up and the CFD simulation are suggested. From this study, it is shown that the effect of appurtenances on TLP VIM simulation is important and must be taken into account to obtain realistic results.

Flexible Flapping Dynamics of Parallel Elastic Plates in a Uniform Flow: Application to Energy Harvesting Devices

A new high-order finite element Coupled Field with Explicit Interface (CFEI) code has been developed for simulating flapping motion of a thin flexible body in a uniform flow with strong add-mass effects. In the first part, we study the flapping dynamics of a single cantilevered plate for wide range of mass ratios and maintaining relatively low bending rigidity through our direct fluid-structure simulations. As a function of mass-ratio, the flapping dynamics reveals three distinct regimes: (i) fixed-point stable; (ii) limit-cycle flapping; and (iii) chaotic flapping. The changes associated with regime transition with increasing mass ratio are analyzed by vortex wake patterns and tip-displacement responses. Dependencies of stability predicted by the theoretical analysis are confirmed by the nonlinear fluid-structure simulations. In the second part, two parallel cantilevered plates will be investigated as a function of spacing between the parallel plates to assess the flapping motion and the net energy transfer. The flow-induced vibrations of this kind of coupled system have a potential to extract energy from the surrounding fluid flow for generation of electric power.

VIV Assessment of Deepwater Lazy-Wave Risers

The steel lazy wave riser is an emerging solution for deepwater applications in harsh conditions. The addition of buoyancy to provide the unique “lazy wave” shape reduces the dynamic stresses at the touchdown zone resulting in improved performance due to vessel motions and waves. However as the buoyant region cannot be easily fitted with Vortex-Induced Vibration (VIV) suppression, VIV becomes a critical aspect of the design. The present study focuses on understanding the global response of a deepwater lazy wave riser with a combination of computational fluid dynamics (CFD) and semi-empirical software analysis. An industry first full scale CFD simulation with different buoyancy region geometries is presented and analyzed to understand the field response and provide guidance on important aspects of design. Results show a different response than what was expected based on previous testing of similar systems, introducing a new parameter related to the aspect ratio of the buoyancy modules.

An Improved Computational Fluid Dynamics (CFD) Model for Erosion Prediction in Oil and Gas Industry Applications

In recent years, the study of flow-induced erosion phenomena has gained interest as erosion has a direct influence on the life, reliability and safety of equipment. Particularly significant erosion can occur inside the drilling tool components caused by the low particle loading (<10%) in the drilling fluid. Due to the difficulty and cost of conducting experiments, significant efforts have been invested in numerical predictive tools to understand and mitigate erosion within drilling tools. Computational fluid dynamics (CFD) is becoming a powerful tool to predict complex flow-erosion and a cost-effective method to re-design drilling equipment for mitigating erosion. Existing CFD-based erosion models predict erosion regions fairly accurately, but these models have poor reliability when it comes to quantitative predictions. In many cases, the error can be greater than an order of magnitude. The present study focuses on development of an improved CFD-erosion model for predicting the qualitative as well as the quantitative aspects of erosion. A finite-volume based CFD-erosion model was developed using a commercially available CFD code. The CFD model involves fluid flow and turbulence modeling, particle tracking, and application of existing empirical erosion models. All parameters like surface velocity, particle concentration, particle volume fraction, etc., used in empirical erosion equations are obtained through CFD analysis. CFD modeling parameters like numerical schemes, turbulence models, near-wall treatments, grid strategy and discrete particle model parameters were investigated in detail to develop guidelines for erosion prediction. As part of this effort, the effect of computed results showed good qualitative and quantitative agreement for the benchmark case of flow through an elbow at different flow rates and particle sizes. This paper proposes a new/modified erosion model. The combination of an improved CFD methodology and a new erosion model provides a novel computational approach that accurately predicts the location and magnitude of erosion. Reliable predictive methodology can help improve designs of downhole equipment to mitigate erosion risk as well as provide guidance on repair and maintenance intervals. This will eventually lead to improvement in the reliability and safety of downhole tool operation.

A Cartesian Explicit Solver for Complex Hydrodynamic Applications

In order to efficiently address complex problems in hydrodynamics, the advances in the development of a new method are presented here. This method aims at finding a good compromise between computational efficiency, accuracy, and easy handling of complex geometries. The chosen method is an Explicit Cartesian Finite Volume method for Hydrodynamics (ECFVH) based on a compressible (hyperbolic) solver, with a ghost-cell method for geometry handling and a Level-set method for the treatment of biphase-flows. The explicit nature of the solver is obtained through a weakly-compressible approach chosen to simulate nearly-incompressible flows. The explicit cell-centered resolution allows for an efficient solving of very large simulations together with a straightforward handling of multi-physics. A characteristic flux method for solving the hyperbolic part of the Navier-Stokes equations is used. The treatment of arbitrary geometries is addressed in the hyperbolic and viscous framework. Viscous effects are computed via a finite difference computation of viscous fluxes and turbulent effects are addressed via a Large-Eddy Simulation method (LES). The Level-Set solver used to handle biphase flows is also presented. The solver is validated on 2-D test cases (flow past a cylinder, 2-D dam break) and future improvements are discussed.

Experimental Investigation on the Suppression Device of VIV of a Flexible Riser

Experimental investigations were conducted on a flexible riser with and without helical strakes. A uniform current was obtained by towing a riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The results of the bare riser show that the characteristics of the synchronization of the VIV for a flexible riser have many orders, and the excited mode jumps from one to another abruptly. During the high order synchronization regime, the VIV response decreases with the increased order of the synchronization. The experimental results also indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the riser increases with strake pitch and height.

A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

A Numerical Investigation Into the Impact Pressures of Different Base Forms Using SPH Method

In this paper, the impact pressures of two different base forms are comparatively studied using Smoothed Particle Hydrodynamics (SPH) method. It is summarized from previous works that the improved weakly compressible SPH model shows better performances than incompressible SPH model in numerical simulations of free surface flows accompany with large deformations and strong discontinuities. Such advantages are observed in numerical accuracy, stability and efficiency. The weakly compressible SPH model used in this paper is equipped with some new correction algorithms, among which include the density reinitialization algorithm and a new coupled dynamic Solid Boundary Treatment (SBT) on solid boundaries. The new boundary treatment combines the advantages of both the repulsive boundary treatment and the dynamic boundary treatment, intending to obtain more stable and accurate numerical results. A benchmark test of dam breaking is conducted to prove the reliability of the numerical model used in this paper. Two representative cases, among which one has one cavity and the other one has three cavities, are numerically investigated and compared to support the conclusion that the base form with cavities generally experience lower local and overall impact pressures than the base form of flat plate. It is found that with the application of cavities on the bottom, the peak values of the boundary pressure near central bottom significantly decrease, leading to smaller force load and better structural stability. The mechanisms of such phenomenon might be the pressure absorption effect conducted by the cavities.

An Experimental Investigation on Cylinder VIV Trajectories Under Different Model Scale

In this paper, model tests were carried out to investigate two degrees of freedom VIV of horizontally-laid cylinders with diameters of 5cm, 11cm, 20cm and length 120cm and compared their vibration trajectories. The test results showed that the in-line and cross-flow vibration frequency of different scale cylinders demonstrate “multi frequency” phenomenon, that is, the in-line vibration frequency is not only twice but also once or four times as much as the cross-flow vibration frequency in some scale, natural frequency and reduced velocity conditions. Also, the cross-flow multi-frequency vibration phenomenon occurred. The trajectory of the vibration cylinder differentiated from the traditional “8” shape accordingly. The vibration trajectory, especially of small-scale cylinder, changed in most conspicuous manner. Through the initial research and analysis, it was found that in addition to in-line and cross-flow natural vibration frequency and the flow velocity, the shape of cylinders was also one of the main causes leading to different vibration trajectory forms.