Risk Assessment For FPSO Tandem Offloading System

The tandem offloading operation with tanker is the main way to transport oil from FPSO to land at present.Once the fault occurs in tandem offloading system,it will cause the interruption of crude oil production and even lead to the leakage of crude oil and then pollute the environment.At present,the research on the offloading system is mainly focused on the hydrodynamic analysis of structure and the risk prediction in offloading operation is relatively weak.The research of risk source,risk evolution process,occurrence probability and loss prevention measures of offloading system has some guiding significance for the establishment of operating procedures,staff training,and daily maintenance and design of the system,.In this paper,the corresponding risk assessment and hydrodynamic analysis of the tandem offloading system are carried out.The equipment composition of offloading system is studied and the process of operation is divided.The Risk identification and evaluation of key operating units are made by combining the JSA method and the risk matrix method and the corresponding mitigation and control measures are made.The study on key risk source and risk evolution process of tandem offloading system is carried out by FTA method.The four kinds of probability models are used to simulate the occurrence probability of different basic events.Then,the occurrence probability of accident,the maintenance cycle and the maintenance scope of offloading system are analyzed.The hydrodynamic analysis software AQWA is used for studying the dynamic response of tandem offloading system composed of a soft yoke mooring FPSO and a tanker.The occurrence probability of cable fault,collision and excessive relative heading motion between two ships under different working conditions is calculated.Then,the above three risks are analyzed with ALARP criterion.The analysis results show that the risk source of tandem offloading system is from FPSO offloading system,oil tankers,tugs,human factors and environmental factors.The maintenance cycle of the tandem offloading system can be set to 3-4 years,and the partial maintenance strategy can achieve the similar maintenance effect with all the equipment maintenance and it can also save the maintenance cost effectively.The influence law of wind,wave and current on dynamic response of system is different.Under the specific conditions of this paper,the cable length setting to be 60 m can not only ensure the safety of the system,but also reduce the cost by shortening the length of the oil pipeline as much as possible.The movement of FPSO and tanker in Sway and Yaw direction shows obvious low frequency characteristics.With the increase of pulling force of the tug,the response amplitude in surge direction of FPSO and tanker reduces and its response frequency increases.The response frequency of cable tension also increases gradually.The motion of two ships in horizontal direction is mainly composed of the motion in sway direction.The critical sea state may not be the most dangerous sea state,and the maximum response value of relative heading motion between two ships may occur at the time when the value of the wind,wave and current is lower than its critical value.