Development and application of a DP float-over analysis program
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Copyright 2015 by the International Society of Offshore and Polar Engineers (ISOPE). Offshore float-over installations using dynamically positioned (DP) vessels, compared to conventional float-over method, have the advantage of better maneuverability, minimal vessel and field preparation, quick set-up and operation, and better safety and economy due to simplicity. China National Offshore Oil Corporation (CNOOC), collaborating with OffshoreTech LLC (OTL) and Texas A&M University (TAMU), has developed an in-house DP float-over timedomain analysis tool (DPTIMED) to predict motions and forces during the float-over process. The software simulates the vessel motions under environmental loads from wind, wave and current. It has the ability to account for nonlinearities including second order wave forces, slow drift motions of the vessel, as well as nonlinear fender stiffness. In addition, the thruster forces required to minimize the motion is predicted using Kalman Filter and Proportional Derivative (PD) conrol theory in the program. The thruster forces are used as feedback force in the simulation to investigate the effectiveness of the DP system to control/minimize the vessel motions during the float-over operation. The software tool was successfully applied to help design CNOOC's first DP float-over installation of HZ 25-8 DPP topside performed on heavy transport vessel HYSY278. The vessel and the deck were modeled as one 6-DOF rigid body. The sway fenders were modeled as non-linear compression-only springs. Thruster and propeller forces were calculated interactively based on Kalman Filter and PD theory. The simulation was carried out for various stages of entry phase (when the vessel is entering the jacket slot) and exit phase (when the vessel withdraws from the jacket slot after the mating). Limiting environmental conditions for the entry and exist phase are defined in the analysis, based on the actual thruster capacity of vessel HYSY278. In addition, the interactive loads on the surge and sway fenders between the jacket and the vessel were calculated for selecting the size and capacity of the fenders. This paper presents DP float-over analysis methodology, procedure, and results of HZ 25-8 DPP topside installation. The results show that the vessel position can be well controlled by the DP system under the design environmental conditions. The capability of the analytical tool is endorsed by the successful execution of the project.
