Design of Riser Turret Mooring Systems

The riser turret mooring system, or RTM, is an FPSO configuration suitable for shallow depths typical of developments to date in Australian waters. It provides some significant advantages, in particular: decoupling of the motions of the FPSO from the risers and mooring lines; and the capability for the vessel to disconnect for avoidance of tropical cyclones. It also provides some peculiar engineering challenges when it comes to design: the modelling of the turret requires special care since the RTM is very different to a ship-shaped floating body, hence the presently available engineering packages should be used with caution in order to provide a sound and robust design. The correct modelling of the system is crucial for addressing limiting operational conditions early in the design process under the various system configurations: during FPSO disconnection and reconnection, during tandem-moored tanker offloading and behaviour of the RTM when disconnected. The accurate modelling of these configurations has been found to be extremely useful in the analysis and drafting of the vessel management procedures associated with all these operations. Challenges still remain for some of the physical response attributes of this type of RTM installation - in particular there is uncertainty in the prediction of conditions under which the disconnected RTM may experience Vortex Induced Motions (VIM) under the combined action of current and waves. Approaches for the prediction of VIM are presented in this work. Since all RTMs installed globally have been in this region, it is important to have a proper understanding of the key issues discussed both for developing new designs and for maintenance and life extension of existing installations. Based on recent experience with the design of these mooring systems for new development and in the reassessment of existing facilities, this paper presents and discusses the state of the art design methodologies with their advantages and limitations as well as addressing the areas in which more understanding is still needed.