Multiple floating structures are being considered for a variety of offshore oil and gas projects. Examples include (1) FPSO and a shuttle tanker during loading; (2) LNG carriers and floating (or fixed) offshore unloading terminals; and floating mobile offshore drilling rigs (MODUs) and moored floating production systems (FPSs); and tender assisted drilling operations. The responses of each structure in the system can be influenced by interactions with the other structure(s), and the relative motion between the structures during operations is an important design and operational consideration. An analysis tool that can accurately and reliably predict the relative motions of multi-structure systems is needed. The analyses must consider the hulls of the floating structures, the mooring lines and any risers attached to each structure, and any interconnections between the structures (e.g., lines, umbilicals, yokes, and fenders). Few existing numerical models fully account for the complete hydrodynamic and line interactions between the coupled floating units. In this study, WINPOST was extended to be able to analyze the motions and interactions of an FPSO (or LNG Carrier) and a shuttle tanker during both side-by-side and tandem offloading scenarios.
The hydrodynamic interaction and mechanical coupling effects of two floating platforms connected by elastic lines are investigated by using a time-domain multi-hull/mooring/riser coupled dynamics analysis program. Particular attention is paid to the contribution of off-diagonal hydrodynamic interaction terms on the relative motions during side-by-side offloading operation. An exact method termed the Combined Matrix Method (CMM) was developed that includes all the vessel and line dynamics and the 12×12 hydro-dynamic coefficients in a combined matrix. The CMM method was compared with two typical approximation methods: the No Hydrodynamic Interaction (NHI) method which is an iteration method that does not consider the hydrodynamic interaction between two vessels; and the Separated Matrix Method (SMM) iteration method that partially considers the hydrodynamic interaction between two vessels that ignores the off-diagonal cross-coupling terms in 12×12 hydrodynamic coefficient matrix. Comparisons were made for a side-by-side offloading operation in two different environmental conditions. The numerical examples show that there is a significant discrepancy at sway and roll modes between the exact and the approximation methods, which means that the cross-coupling (off-diagonal block) terms of the full hydrodynamic coefficient matrix play an important role in the case of side-by-side offloading operation. Therefore, such approximation methods should be used with care. The fender reaction forces, which exhibit large force with contact but no force without contact, were also included in the time-domain simulation studies.