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.
A great deal of works has been developed on the Spar 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, the present paper presents a meticulous study on VIM for this type of platform concept. 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 inline and cross-flow motion amplitudes, 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.
The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to protect drilling equipment from environmental forces, and its design aims the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies have been carried out to predict the water dynamics inside the moonpool. At most, analytical tools have been used with experimental results, to obtain a good evaluation of viscous effects. Another line of development uses the moonpools as a device to reduce motions of ships or oil platforms. In his context, the use of moonpools in monocolumn type platforms was studied during the development of the concept, through the partnership between PETROBRAS and University of Sa˜o Paulo–USP. An alternative that became viable in the last years is the use of numerical methods to evaluate potencial parameters, being only necessary simple experiments to obtains viscous data to complete the model. This work, that is a continuation of articles about the issue written before, intends to consolidate the calculation method of moonpool to monocolumn units.
The steel catenary risers (SCR) have been studied to be used in ultra-deep oil fields. In Brazil, the oil field is going to deeper water. The geographical characteristics impose this type of production risers, and the necessity of these risers is real. However, in order to avoid risers fatigue problems, some premises related to low vertical motions have to be respected. To reach the necessary good seakeeping responses, new type of hull shape has been proposed, the monocolumn hull, to which some motion minimization devices have been developed. There are several conceptions of monocolumn type hull, but the concept presented in this paper is different from the existing monocolumn hull. To have good seakeeping capability, three main minimization devices were proposed: the structural skirt, the inclined wall, and the moonpool. The inclined wall, a variation of a section area near the water-plane, works together with the skirt, an enlarged bilge keel at the bottom of the platform, modifying the effect of damping, added mass and inertia. The moonpool is the old tank usually found in drilling units, properly modified to work as a motion minimization system. It works as a passive heave motion reduction system, shifting the natural period far from the wave spectrum. Some numerical analyses were made to present the effects obtained from these devices. These numerical results from WAMIT® were compared to experimental showing to be according to them.
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.
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