Rodolfo T. Gonçalves

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 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.

A MODEL SCALE EXPERIMENTAL INVESTIGATION ON VORTEX-SELF INDUCED VIBRATIONS (VSIV) OF CATENARY RISERS

Vortex Self-Induced Vibrations (VSIV) of a reduced scale model of a catenary riser are experimentally investigated. The riser model dynamics was assessed with a submerged optical motion capture system and significant VSIV were revealed as result of oscillatory vertical motion imposed to the top. Such a behavior recovers similar ones reported in the technical literature by other authors and resembles previous fundamental studies, by Sumer and Fredsoe, with rigid cylinders forced to oscillate in a plane and elastically mounted in the transversal direction. The present experiments are preliminary and pertain to a much more comprehensive experimental set, within a research project aimed at studying the nonlinear dynamic behavior of risers, through experimentally validated analytical and numerical, nonlinear reduced-order models.Copyright

Conceptual Design of Monocolumn Production and Storage With Dry Tree Capability

A new concept and a preliminary study for a monocolumn floating unit are introduced, aimed at exploring and producing oil in ultradeep waters. This platform, which combines two relevant features—great oil storage capacity and dry tree production capability— comprises two bodies with relatively independent heave motions between them. A parametric model is used to define the main design characteristics of the floating units. A set of design alternatives is generated using this procedure. These solutions are evaluated in terms of stability requirements and dynamic response. A mathematical model is developed to estimate the first order heave and pitch motions of the platform. Experimental tests are carried out in order to calibrate this model. The response of each body alone is estimated numerically using the WAMIT ® code. This paper also includes a preliminary study on the platform mooring system and appendages. The study of the heave plates presents the gain, in terms of decreasing the motions, achieved by the introduction of the appropriate appendages to the platform.

The Influence at Vertical First Order Motions Using Appendages in a Monocolumn Platform

Oil discovery in ultra-deep-waters is carrying out the ocean engineering to develop new conceptions of offshore exploration and production systems. A promising alternative is a monocolumn platform with moonpool. Concept design has shown its good stability make feasible the use as offshore solution. In order to solve the problem brings by increase on water deep, the use of steel catenary risers and wellhead dry completion has been shown a good solution because it decreases the maintenance cost. However, this solution needs a very low vertical motion. In general, platform like the monocolumn present resonance on vertical motion (15–20 sec to heave motion, 20–25 sec to roll/pitch motion) close to typical wave energy. This fact is so that designers introduce mechanisms to leave the natural periods out of wave energy. The moonpool as passive absorber motion was a solution found by designers. The water inside moonpool acts out-of-phase with platform motion. This difference on phases can attenuate the platform dynamic response. Other solution is water-line-area (AWL) reduction that reduced the restoring forces and consequently increased the value of vertical motions natural periods. These two alternatives showed very efficient to improve the monocolumn performance. This paper presents the gain, in terms of motions, achieved as the appendages introduction on the platform. These appendages consist of two plates, one in form of a disk and other around it. It can be located on the bottom at platform and inserted inside at the moonpool. Model test conduced on the scaled platform shows that heave and pitch natural period increased approximately 4 seconds when this appendages is allocated on the bottom. Practically any change is noted when it is allocated only inside of the moonpool.Copyright

CFD Calculations for Free-Surface-Piercing Low Aspect Ratio Circular Cylinder With Solution Verification and Comparison With Experiments

The flow around free-surface piercing, low aspect-ratio circular cylinder is investigated by means of unsteady Reynolds averaged Navier-Stokes (URANS) calculations together with verification procedures and comparison with small-scale experimental and Particle Image Velocimetry results. A two-phase interface capturing model is used to handle the free-surface flow, together with k-ω SST turbulence model. We investigate physical and modeling aspects of this problem in order to gain more knowledge about the interaction of free-surface and free-end effects so that this mechanism is better understood and taken into account when modeling the problem in engineering-applied situations, such as the vortex induced motion of spars, tension-leg platforms and semi-submersibles.The case herein presented is a captive, low aspect-ratio cylinder (L/D = 2.0) with flow velocity corresponding to Reynolds and Froude numbers (both based on diameter) of Re = 4.3 × 104 and FnD = 0.31, respectively. We will show that appreciable free-surface effects are perceived on the flow, but with dominance of free-end effects, at least in terms of forces. Furthermore, we investigate different boundary conditions that would represent this free-surface problem to show that the separation of viscous and free-surface effects is not valid in this instance. Therefore, the interaction between viscous and free-surface effects is also tangentially investigated. In order to support our conclusions, we will show forces with uncertainty estimation and field variables obtained with different modeling strategies, unveiling physical and numerical aspects of this problem.Copyright