Low-Frequency Motions of Tankers on Tandem Offloading Condition

ABSTRACT In this paper a number of hydrodynamic aspects of tandem mooring operations of floating VLCCS will be discussed. The effect of shielding of the wind and current on the export tanker are established by means of model test experiments. The interaction of two floating vessels on the wave drift forces exerted on each other is treated by means of potential theory calculation methods. One of the findings of this study is that the current loads and wave drift forces on the export tanker are reduced with respect to the undisturbed case, provided that the F(P)SO heading angle is close to 180 deg. However, for larger heading angles even an increase of the longitudinal forces on the export tanker can be observed. INTRODUCTION Tandem Mooring Nowadays, oil off loading from storage tankers is common practice due to the presence of several dozens of tanker-based FpSOS and FSOS moored at sea. With the in Creasing number of marginal fields in development it may be expected that more (easily transferable)tanker-based F(P)SOS will follow. For the economy of the oil production system it is beneficial if the 'weather window? for oil offloading is large: an export or off take (OT) tanker should be able to stay connected and keep loading in relatively high sea states. Model tests and experience have learned that a tandem mooring arrangement can be maintained in higher sea states than a side-by-side mooring, so that all present F(P)SO designs assume that offloading takes place by an export tanker which is hawser moored to the F(P)SO stern. Operability The limitations for the operability of tandem off loading are determined by two aspects:The low frequency drift motions of the export tanker with respect to the F(P)SO, which may be due to wave drift forces as well as galloping (mooring instability).The wave induced (high frequency)relative motion between the two tankers in way of the bow hawser, at the time that the low frequency excursion is at a maximum. Improvement of the operational limits of tandem offloading has been focused on optimization of bow hawser characteristics and bow hawser length. Properly chosen elasticity will help reduce the peak loads due to the second aspect mentioned above, while a properly chosen bow hawser length will avoid galloping, ref. [1] and [2]. Still, low frequency 'fishtailing? motions of the export tanker form the basis for the limitations in the operability of tandem moored offloading. Modern methods to improve on this are in development: thruster assisted bow hawser mooring and dynamically positioned(DP) offloading. In thruster assisted mooring the bow (tunnel) thrusters of the export tanker are used to maintain a good alignment of the bow hawser, whilst the main propeller provides some astern thrust to avoid slack. In DP offloading the bow hawser is not necessary: an automatic thruster control system takes care that the tanker keeps position behind the F(P)SO, [3].