The conversion of ageing tankers is a tempting alternative to newbuildings when FPSO units are planned. The low cost of an already classified hull and an existing pumping facility compensate for the repair and updating costs a conversion may demand. Newbuildings on the other hand allow for improved production plant and storage tank lay-out. The trade offs between conversions and newbuildings are analyzed from the point of view of strength and general arrangement. Converted tankers carry a serious strength drawback since the emptied engine space enhances the high buoyancy region at the stern which significantly increases bending moment amidships. In general one or more cargo tanks must be kept at partial loading to compensate for the increased bending moment. A new building lay-out is analyzed in which the tanker engine space is completely eliminated. Utility systems are resized and placed on the main deck aft of the production plant. Accommodation spaces are also resized and rearranged in order to fit into the reduced deck space available. The gains in strength are quantified and the lay-out implications are discussed in detail. A comparative study is presented between a conversion of a hypothetical tanker and an equivalent new building.
A floating structure is proposed to act as a dry wellhead completion support for a mono-column design developed by Petrobras: the MONOBR FPSO. After the discussion of the design issues involved, the MONOBR design is introduced and the peculiarities affecting the insertion of a floating structure into its moonpool are analyzed. In what follows, the main characteristics of the proposed floating structure layout are presented in the context of its purpose of allowing for the effortless vertical motion relatively to the housing mono-column unit, followed by the considerations pertinent to the ballast system devised to allowing for the adequate operation, by the characteristics of the roller mechanisms proposed to ensure the aligned motion desired and by the analysis of the structural issues and the proposed structural layout.
This study aims to analyze the influence of the kinetic energy of the fluid adjacent to the hull of a tanker ship in its vertical vibration frequencies, comparing them with experimental measurements obtained during sea-trials. The one-dimensional modeling of ships allows the construction of simple finite element models from the structural elements of its master section, with structural and added masses, and their frequencies are verified by full-scale measurements, during the sea-trials. The numerical results of these models, with the value of the effective shear area as a fraction of the total area of the strength steel are compared to those obtained in full-scale measurements during sea trials of an oil tanker to be converted to Offshore Construction Vessel. Global vibration measurements were carried out in two of the six ships with the same hull. Accelerometers were installed in eleven strategic points of each hull. Vibration data acquisition was performed simultaneously for these locals in thirteen rotations of the main engine. The amplitude spectra of vibration velocity on the frequency range of measurements were obtained and were plotted graphs of the evolution of the main harmonics, depending on the rotation of the main engine, in order to identify four natural frequencies of the overall vibration of the hull, which were compared to the numerical model. The calculation is performed by the added mass formulations from Burrill, Todd, Kumay and Lewis/Landweber [8] curves, including in all three-dimensional effect by Townsin [17] coefficients, which is checked against the experimental results. The comparison between numerical and experimental results allows assessing the influence of the kinetic energy of the fluid surrounding the hull in the natural frequencies of vibration of the numerical model of the tanker ship and simulating their dynamic behavior after conversion in Offshore Construction Vessel.
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