André Luís Condino Fujarra

URANS Calculations for Smooth Circular Cylinder Flow in a Wide Range of Reynolds Numbers: Solution Verification and Validation

The flow around circular smooth fixed cylinder in a large range of Reynolds numbers is considered in this paper. In order to investigate this canonical case, we perform CFD calculations and apply verification & validation (V&V) procedures to draw conclusions regarding numerical error and, afterwards, assess the modeling errors and capabilities of this (U)RANS method to solve the problem. Eight Reynolds numbers between Re = 10 and Re=5×105 will be presented with, at least, four geometrically similar grids and five discretization in time for each case (when unsteady), together with strict control of iterative and round-off errors, allowing a consistent verification analysis with uncertainty estimation. Two-dimensional RANS, steady or unsteady, laminar or turbulent calculations are performed. The original 1994 k-ω SST turbulence model by Menter is used to model turbulence. The validation procedure is performed by comparing the numerical results with an extensive set of experimental results compiled from the literature.

Mitigation of Vortex-Induced Motion (VIM) on a Monocolumn Platform: Forces and Movements

A great deal of works has been developed on the spar vortex-induced motion (VIM) issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, a meticulous study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are motion amplitudes in both in-line and transverse directions, forces and added-mass coefficients, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.

Experimental Analysis of a Vertical and Flexible Cylinder in Water: Response to Top Motion Excitation and Parametric Resonance

Experiments with a vertical, flexible, and submerged cylinder were carried out to investigate fundamental aspects of risers dynamics subjected to harmonic excitation at the top. The flexible model was designed aiming a high level of dynamic similarity with a real riser. Vertical motion, with amplitude of 1% of the unstretched length, was imposed with a device driven by a servomotor. Responses to distinct exciting frequency ratios were investigated, namely, ft:fN,1 = 1:3; 1:1; 2:1, and 3:1. Cartesian coordinates of 43 monitored points positioned all along the span were experimentally acquired by using an optical tracking system. A simple Galerkins projection applied for modal decomposition, combined with standard Mathieu chart analysis, led to the identification of parametric resonances. A curious finding is that the Mathieu instability may simultaneously occur in more than one mode, leading to interesting dynamic behaviors, also revealed through standard power spectra analysis and displacement scalograms.

State-of-Art on Vortex-Induced Motion: A Comprehensive Survey After More Than One Decade of Experimental Investigation

After one decade of experimental investigation, the Vortex-Induced Motion – VIM phenomenon deserves a comprehensive survey concerning the advances related to its understanding, mainly under the consideration of the fundamental aspects that keep it in a close relationship to the dynamic behavior of the same phenomenon acting on slender bodies, the well known Vortex-Induced Vibration – VIV. A considerable amount of results can be found in the literature, although there are few works dealing with a general view of the problem. Probably, the main reason for such a large amount of works with no interaction between themselves and, consequently, without a common understanding about VIM might be due to its technological origin, featured by huge platforms with a variety of geometrical details, which ends up placing the researches more on the field of the faithful reproduction of the features in small-scale and less on the global understanding of the phenomenology regardless the floating system, e.g. a spar platform, a monocolumn or even a semi-submersible or a tension-leg platform. Obviously, no one should disagree that there is part of the research that must keep a faithful relationship with the full scale, however, in most of them it is possible to identify the common fundamentals concerning the fluid-structural interaction. The aim of the present work is to address a comprehensive evaluation of the experimental investigations during the past decade on the VIM, trying to gather a general understanding about its phenomenology including some comparisons to VIV. As a result, some relevant aspects are pointed out for a more prospective way of research.Copyright

Analysis Methodology for Vortex-Induced Motion (VIM) of a Monocolumn Platform Applying the Hilbert–Huang Transform Method

Vortex-induced motion (VIM) is a highly nonlinear dynamic phenomenon. Usual spectral analysis methods, using the Fourier transform, rely on the hypotheses of linear and stationary dynamics. A method to treat nonstationary signals that emerge from nonlinear systems is denoted Hilbert–Huang transform (HHT) method. The development of an analysis methodology to study the VIM of a monocolumn production, storage, and offloading system using HHT is presented. The purposes of the present methodology are to improve the statistics analysis of VIM. The results showed to be comparable to results obtained from a traditional analysis (mean of the 10% highest peaks) particularly for the motions in the transverse direction, although the difference between the results from the traditional analysis for the motions in the in-line direction showed a difference of around 25%. The results from the HHT analysis are more reliable than the traditional ones, owing to the larger number of points to calculate the statistics characteristics. These results may be used to design risers and mooring lines, as well as to obtain VIM parameters to calibrate numerical predictions.

Vortex-Induced Motion: Model Testing of a Monocolumn Floater

The vortex-induced vibrations - VIV have been studied for several fields of engineering due to its occurrence in different structures, such as electrical cables, industries chimneys and offshore risers. Although available an extensive literature describing its fundamental issues, these vortex-induced phenomena still deserve investigation, particularly in the offshore platforms installed in regions with high current speed. Recently, the Vortex-Induced Motions - VIM, a particular case of vortex-induced vibration with high magnitude of response amplitude, have been observed in SPAR platforms installed in Gulf of Mexico - GoM, opening a new investigation field. For those motions, aspects such as asymmetric mooring restoring stiffness and the probable three-dimensionality of the flow turn the problem even more complex. Since 2003, in partnership with University of Sao Paulo and consultant companies, PETROBRAS has been studying the use of monocolumn floaters for oil production in Campos Basin and GoM. Considering the environmental conditions in these areas and assuming that monocolumn floaters can exhibit similar VIM behavior of SPAR platforms, it was started an experimental investigation focusing on VIM responses of small-scale monocolumn floaters in towing tank. Although based on the state of art procedures used for SPAR platforms, the monocolumn experiments considered the different geometry of this concept and the larger susceptibility to the three-dimensional effects, due to the smaller relation draft/beam. Special attention was given to the ratio roughness/beam in order to guarantee similarity between the experiments and its respective real cases. Additionally, different heading conditions were also tested. Thus, the present work presents a set of preliminary results and discussions concerning VIM of monocolumn floaters and its impact on the mooring line design and riser specification.Copyright

A Phenomenological Model for Vortex-Induced Motions of the Monocolumn Platform and Comparison With Experiments

Vortex-Induced Motions (VIM) of floating structures is a very relevant subject for the design of mooring and riser systems. In the design phase, Spar VIM behavior as well as Semi Submersible and Tension Leg Platform (TLP) flow-induced motions are studied and evaluated. This paper discusses flow-induced behavior on the Monocolumn concept by presenting a phenomenological model and comparing its results with a set of experiments that took place in the IPT Towing Tank - Brazil (September 2008). The experimental results have shown some fundamental differences from previous VIM tests on other units such as Spars. This numerical model attempts to identify these disparities in order to better understand the mechanics of this phenomenon. The model is based on a time-domain, two degree-of-freedom structural model coupled with a van der Pol type wake oscillator. The comparison was performed in order to calibrate the model, to study and better understand the tests results, and finally to identify important aspects to investigate in further experiments.Copyright

VORTEX-INDUCED MOTION OF A MONOCOLUMN PLATFORM: NEW ANALYSIS AND COMPARATIVE STUDY

This paper presents a new analysis and a comparison of results obtained from Vortex-Induced Motion (VIM) model tests of the MonoGoM platform, a floating unit designed for the Gulf of Mexico. The choice of scale between the model and the platform in which the tests took place was a very important issue that took into account the basin dimensions and mooring design. The tests were performed in three different basins: the IPT Towing Tank in Brazil (September 2005), the NMRI Model Ship Experimental Towing Tank in Japan (March 2007) and the NMRI Experimental Tank in Japan (June 2008. The objective of this work is to discuss the most relevant issues regarding the concept, execution and procedures to analyze comparatively the results obtained from model tests. The approach employed in the tests was designed to build a reliable data set for comparison with theoretical and numerical models for VIM prediction, especially that of Monocolumn platforms.

Parametric analysis of a phenomenological model for vortex-induced motions of monocolumn platforms

Phenomenological models are an important branch in VIV (Vortex-Induced Vibrations) and in VIM (Vortex-Induced Motions) studies to complement the results achieved via CFD (Computational Fluid Dynamics), as the latter tool is not presently a suitable tool for intense use in engineering analysis, due to high computer power requirements. A phenomenological model for evaluating the VIM on monocolumn platforms is presented and its results are compared with experimental ones. The main objective is to present a parametric analysis, focusing on the physical significance of the modifications in parameter values. The following parameters are varied: aspect ratio (L/D), structural damping (ξ), fluid damping (γ) and Strouhal number (S). The results are presented in terms of: non-dimensional amplitudes of motion (AX/D and AY/D), added mass coefficient (Ca) and periods of motion (TX and TY). The phenomenological model is based on a time-domain, two degree-of-freedom structural model coupled with van der Pol wake oscillators. The governing equations are solved through fourth-order Runge-Kutta schemes.

Damping Coefficient Analyses for Floating Offshore Structures

The damping evaluation of floating offshore systems is based on the viscous effects that are not considered in numerical models using the potential theory. Usually, different techniques for different systems are used to evaluate these hydrodynamic coefficients. The total damping is separated by potential and viscous damping, the first one is evaluated numerically and the second through experiments at reduced scale model. Common techniques considering linear motion equations cannot be applied to all degrees of freedom. Some methods were compared for results of decay test, such as: exponential and quadratic fit. Fourier transform (FT) spectral analysis and Hilbert Huang transform (HHT) can be used to evaluate the signal natural frequency and with HHT this can be done during the time domain. Also, analysis through the Random Decrement Technique (RDT) is presented to demonstrate the damping evaluation for irregular waves. The method to obtain external damping was presented for the different techniques in an ITTC semi-submersible model. The linear method is not sufficient to predict the damping coefficient for all the cases, because in most of them, the viscous damping was better represented by a quadratic fit. The HHT showed to be a good alternative to evaluate damping in non-linear systems.Copyright