Research On The Applicability Of Tug Under Environmental Loads In FPSO-tanker Tandem Offloading Operation

At present,the offloading in tandem between FPSO and shuttle tanker,which needs little mooring force and can well adapt to different ship tonnages,loading conditions and complexity of the marine environment,has been widely used.The operating system composed of FPSO and shuttle tanker is moving under the complex environmental loads.The different sizes of FPSO and shuttle tanker cause different environmental loads and relative motion.The excessive relative motion of FPSO and shuttle tanker could cause collision.During the offloading operation in tandem,the maximum available towing force,which provided by the stern tug,has immediate impact on operation safety.In order to study tug dragging force in offloading operation in tandem and put forward reasonable theoretical basis for tug selection,it is necessary to calculate the towing force.Based on the different assumptions of the environmental load and the operation condition,three models of drag force calculation are proposed,which are static model,quasi dynamic model and coupling model.In the static model,first step is setting up the equilibrium equation of the system mechanics model.Secondly,the geometric modeling of the oil tanker and FPSO should be made.Calculating the wave loads by AQWA and getting the wind loads and current loads by empirical formulas.Finally,the distribution curve of the drag force could be made in the extreme permissible operating environment condition,and the drag force required for the tug could be solved.Static model is easy to operate and use,and the results of drag force could be got speedily by this model.In the quasi dynamic model,environmental loads,such as winds,waves and flow,are comprehensively considered,while FPSO and shuttle tanker are circling around the single point.Setting up the quasi dynamic equilibrium model and getting the value of environmental loads by empirical formulas and AQWA-Workbench tools.Finally,fitting the environmental load curves and plugged into force balance equations,the required minimum towing force for operation safety could be calculated.Both operational safety and economical efficiency can be aligned by this towing force to select tug.In the coupling model,first step is putting the three-dimensional geometric models of FPSO and shuttle tanker into the AQWA platform,then the wind and current loads coefficients,which calculated by empirical formulas,should be put in.Secondly,the single point mooring and cable connection of the offloading operation in tandem system should be finished.At the same time,the parameters of the sea area are input to calculation platform.Finally,after the frequency domain and time domain calculation the result of the tug drag force could be obtained.Compared with above models,coupling model is most close to the actual working situation,and has great calculation precision.According to the calculation results of three models,we complete a comparative analysis,and summarize the advantages and disadvantages of the three models.Finally,some advice is given for tug choice.The results are practical for FPSO operating management to avoid unnecessary bigger bollard pull force and at the same time ensure safety for offloading operation.