The disconnectable buoy turret mooring (BTM) for an FPSO has been a proven technology for areas where hurricane and seasonal cyclones are predominant, or are subject to ice features which need to be avoided. With the requirements of large buoy payload from deep water moorings and heavy risers, the disconnectable buoy size can be substantial. Recent projects are considering buoy net displacements well above 5000Te. The increased buoy size imposes large inertia loads on the supporting structure and mechanical connectors when connected, and possibly more so on the structural integrity and winching system during disconnect and reconnect operations. In order to provide structure design loads as well as to estimate safe operational windows and procedures, it is desirable to understand the physical properties of 2-body motions when the buoy and ship are close enough to have significant interactions.
This paper focuses on the hydrodynamic and structural (Winch, Mooring/riser, Fender/bumper etc.) interactions with the buoy operating in close proximity to the turret. Since the readily available offshore engineering tools have limitations to deal with close body hydro-interaction, particularly with the presence of moon-pool resonances, rational approximations are derived for hydrodynamic forces during pull-in and disconnection of the buoy. To analyze fully coupled motions and loads in combined environmental conditions, a typical diffraction/convolution based numerical tool (AQWA) is extended by a custom defined external force function. This analysis allows simulating the real time motions during disconnect, reconnect, or at suspended elevations. Various findings from dedicated model tests and analyses are presented and discussed with implications for configuring and operating the disconnectable buoy system.
