Numerical Study On Multi-body Hydrodynamic Performance Of FPSO Tandem Offloading Operation

The 21 st century is the century of oceans.Owing to the continuous discovery and exploitation of offshore oil and gas fields,the floating production and storage platform(FPSO)with many advantages such as adapting to a wide range of water depth and strong resistance to environmental forces has been rapidly developed.However,how to safely offload the crude oil on the FPSO to the shuttle tanker has been a research hotspot.Different from the side-by-side offloading operation,the tandem offloading operation can be applied to two ships with different displacement and size and can adapt to more complicated environmental conditions,which is an important kind way for offloading.In the actual tandem offloading operation process,the motion response between the FPSO and the shuttle tanker are not only affected by external forces such as wind,waves,current,and connecting cables,but also affected by the hydrodynamic interference caused by the two ships.If excessive surging or fishtailing motion occurs between the FPSO and the shuttle tanker,then the two vessels will most likely have a collision accident,resulting in disastrous consequences.This requires a tug at the stern of the shuttle tank to keep the shuttle tanker at a safe distance and reasonable angle from the FPSO.So how to quickly calculate the required drag force of the tug is necessary.Using AQWA as a tool,the hydrodynamic analysis of FPSO tandem offloading operation system is carried out.The dynamic coupling model and full coupling model are established respectively.The towing forces required under different models are calculated and compared.The main contents of this paper are as follows:(1)FPSO and shuttle tankers operating in a certain sea area are taken as research objects.The wind and current loads are calculated by numerical method,OCIMF method and experimental method under different water depth draft ratio.The results are compared and the characteristics and application scope of the three methods are summarized.(2)The dynamic coupling model of FPSO-shuttle tanker-tug is established.And the hydrodynamic interference between FPSO and shuttle tanker is studied.The coupling between the two cables and the different environments in different sea areas are considered.The towing drag force required under the parameters is compared with the results of the static model and the quasi-dynamic model.(3)The dynamic full-coupling model of soft yoke mooring system-FPSO-shuttle tanker-tug is established,and the influence of soft yoke mooring system on the tandem offloading system is considered.The towing force required for shuttle tanker under different loads(full load and ballast)in different sea areas is calculated and compared with the results of static model,quasi-dynamic model and dynamic coupling model.(4)Reliability analysis of the fully coupled model is carried out.Firstly,the influence of the distance between two vessels,different loading conditions and different water depth on the required towing force is studied.Then,based on the extreme value theory,the collision and bottom contact probability of FPSO and shuttle tanker are studied.The research results and conclusions of this paper can provide theoretical basis for the selection of tug for FPSO tandem offloading operation and can also provide field guidance for tug's operators.