Closely-Moored Floating Bodies in a Production and Offloading Facility - Requirement for and Application of a Coupled Analysis Capability
Authors: Connaire A.D. , Lang D.W. , Galvin C.D. 
Published: October 2008

Abstract
In remote deepwater fields, it is efficient to exploit reservoirs via numerous wells from a dedicated dry-tree wellhead facility. Reservoir fluids are passed to a closely located floating production, storage and offloading (FPSO) vessel, from which oil is then intermittently offloaded to a shuttle tanker via offloading lines and an anchor leg mooring buoy. While accepted as being commercially effective, the complexity of the behaviour of such integrated systems imposes constraints upon the designer unlike those seen for simpler field architectures. Specifically, the response of each component in such systems is influenced by the mechanical and hydrodynamic coupling effects and proximity of the other components. Consequently, if the feasibility of such systems is to be proven or demonstrated, coupled analysis techniques, which facilitate the accurate prediction of all responses without imposing conservatism associated with commonly used analysis techniques, must be employed by the designer.
A coupled analysis capability has been developed and extensively verified, which integrates radiation/diffraction theory with a beam finite element (FE) time-dependent structural analysis technique for slender offshore structures. This paper describes the coupled analysis capability and its application to a West of Africa type offloading system. The underpinning theoretical background and the inclusion of coupled frequency dependent excitation, inertia, damping and stiffness characteristics of floating components together with similar characteristics for attached mooring and transfer lines in a non-linear time-domain finite element solution scheme, is outlined. Numerical examples are described to illustrate the operation of the technique for design of deepwater systems.

Introduction
As the emphasis on development shifts to increasingly remote, deepwater fields, novel field architectures have to be employed to economically exploit these reserves. The lack of available export pipeline infrastructure in remote locations necessitates the use of systems that rely on intermittently offloading oil to a shuttle tanker by means of an anchor leg mooring buoy.
Such a system typically consists of a dedicated dry-tree wellhead platform, which may for example, be a tension-leg platform (TLP) or Spar, that facilitates both drilling and production for closely located wells. Reservoir fluids are transferred via transfer lines from the wellhead platform to a (FPSO) unit. Oil is then offloaded from the FPSO by means of either a catenary anchor leg mooring (CALM) or taut anchor leg mooring (TALM) offloading buoy, which is connected to the FPSO by steel or flexible offloading lines. Safety considerations necessitate that a horizontal separation of up to 2km be maintained between the FPSO and the offloading buoy/shuttle tanker. During the offloading procedure, the shuttle tanker connects to the offloading buoy using a hawser, which serves to maintain the shuttle tanker in position.