The installations of sub-sea equipments are very complex operations, requiring previous analysis in order to define the correct procedure and the environmental “window” for a safe operation. This paper addresses the installations of a Mid Water Arch (MWA) that consists of a structure to provide risers support. Connecting the risers to the MWA largely eliminates the dynamic forces that would otherwise cause friction and fatigue. Such structure is composed by the riser guides and several buoyancy tanks. It is kept in the water by means of tethers connected to an anchor. The MWA is to be installed 42m from the seabed. The installation procedure has several steps evolving the launching of each component of the MWA (anchor, main structure and the tethers). A tug boat with an A-frame is used during the whole launching, and an assisting vessel is required to keep the buoy away from the tether and the launching cable. The presence of exciting waves induces oscillatory motions in the whole system, and may cause large dynamic forces in the cables and tethers. Due to the complexity of the multi-body system, a comprehensive numerical and experimental analysis was then carried out in order to dimensioning the launching cables and to define the limit environmental condition. The numerical analysis was carried out in the Numerical Offshore Tank (TPN), a multi-processor offshore system simulator that considers the 6 DOF of each body and all environmental forces acting in them. The lines are modeled by finite-element method. Furthermore, a full set of small-scale experiments were carried out at the State of Sa˜o Paulo Technological Research Institute (IPT) towing tank, considering the system excited by a sinusoidal motion at the top, emulating the wave excitation. Comparisons between numerical and experimental results were performed, with good adherence between them. The validated numerical simulator was then used to make predictions of the behavior of the systems during the installation, considering several environmental conditions and configurations.
Brazilian offshore scenario requires innovative solutions in order to increase the efficiency of operations, while still keeping the safety and the economical constraints. The global market and naval industry cannot meet the demand of the growing exploration activities, related to the recent discovery of new oil fields. Furthermore, they are located at a large distance from the shore (approximately 300km) and the environmental conditions are severe. These facts impose more logistics problems for materials and persons transportations. The retrofitting of vessels is an alternative for increasing their operability and this option has been studied by Transpetro, subsidiary of Petrobras responsible for the Ducts, Terminals, Maritime Transport and Natural Gas segments. This paper presents two real cases of retrofitting related to the application of new technologies for positioning, including Dynamic Positioning (DP) Systems or manually operated thrusters. In the first case, a conventional tanker will be converted to a DP2 tanker, in order to operate close to drill ships and to transfer fluids during the drilling operation. The second case is related to an offshore delivering vessel used for bunkering operation. This vessel is moored at a monobuoy, and up to 2 receiving vessels may be connected to it. An aft-azimuth thruster will be installed in the vessel, increasing its capacity to change heading in harsh environmental conditions and to keep a safe distance from the monobuoy during fast variations of environmental conditions. Both cases require the application of a comprehensive design methodology for thruster dimensioning. The retrofitting imposes tight constraints to the layout and positioning of the thrusters. Also, the proximity to others vessels and the harsh environmental conditions in the offshore fields requires a combination of static and dynamic calculation in order to evaluate power consumption and drift motions of the vessels.
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