Simulation of Float-over Topside Installation Using a Dynamic Positioning Vessel

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.  OffshoreTech LLC (OTL) has developed an in-house DP float-over time-domain 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) control theory in the program.  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.  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.