Failure Analysis And Risk Pre-warning Of Soft Yoke Mooring System Based On Field Monitoring

Soft yoke mooring system (S YM), as one kind of important equipments for marginal oil fields, is widely used for floating process storage offloading (FPSO) in Bohai Bay. Although the mechanical principle of SYM is simple, the problems are also very prominent:1. The complexity of SYM structure. SYM is a multi-rigid-body mechanism composed by several hinge points, which bear a great stress in the actual process. It is indicated that, the risk of the structure should be increased; 2. The resonance phenomenon of SYM structure. Due to release the several direction constraints, the mooring leg with ballast seems to be a pendulum. When the period of SYM’s Transverse-Resonance is close to the period of tanker motion, it will cause the resonance phenomenon; 3. The problem of shallow water effect. Because the water depth is shallow in Bohai Bay, FPSO has a problem of shallow water effect, which lead the uncertainty of SYM analysis; 4. The complexity of the environment. Bohai Bay has a long term ice age in winter. Meanwhile, during the monsoon season, a sudden change of the environment is also significant. These problems above have become the difficulties of the design and safety management in operation.The traditional design do not specify how to consider the dynamic problem. In theoretical analysis, numerical simulation and model experiment, the environmental loads acted on structures are only in several directions, the complex dynamic characteristics of actual environment are ignored. Meanwhile, traditional design methods do not only exists the scaling effect, but also can not simulate resonance phenomenon and Stern-Spin induced by FPSO’s weathervane.Based on the shortcoming for traditional design methods, supported on the subject "National Science Group of China (grant number 51221961) and the National Key Basic Research (grant number 2011CBO13705) and Development Program (grant number 2011ZX05026-002-02)", the SYM field monitoring, which aimed to improve design and evaluation, are developed. The main problems of failure for the S YMs in process are analyzed and evaluated in this dissertation. Risk pre-warning of SYM is also proposed. Related studies of this dissertation are listed as follows:(1) Design, Implementation and Verification of Monitoring SystemAccording to the characteristics of soft yoke mooring system in shallow water, the monitoring information was determined and designed. The monitoring system was composed of the following four sub-monitoring systems, i.e., environmental condition monitoring system, FPSO movement monitoring system, mooring force and mooring structure attitude monitoring system and hinge Points of SYM Monitoring System. Furthermore, computer software with graphical user interface was developed by C++ and installed in central control unit, and the long-term continuous data were obtained. Then the validity of monitoring system has been verified by the comparisons of several group of redundancy. Based on the monitoring information of SYM, the analysis of the FPSO with SYM motion are carried out based on the measured data.(2) Methods Analysis of SYM Overload FailureIn order to reduce the losses caused by SYM Overload, a novel multi-body dynamic analytical method of the horizontal restoring force was established and investigated under the action of wind, wave and current. By compared with the fibre bragg grating (FBG) strain sensors and the static method in measurement, the correctness of dynamic method was verified, and the redundant measurement of mooring force is achieved.(3) Analysis of SYM Structural Fatigue Failure Caused by ResonanceForced vibration between tank’s roll and SYM’s transverse-oscillation is found, especially in particular draft, load factor and wave direction. The roll of tank is consistent with the natural frequency of transverse-oscillation, which indicates the resonance phenomenon. Based on the long-term statistical analysis of rain flow method for transverse-oscillation, the statistical value and design value are evaluated. In order to reduce the risk of resonance, by the study on the vibration experiment of tune liquid damper (TLD) model, the strategy of transverse-oscillation absorption are proposed.(4) Analysis of SYM Functional Domain FailureBased on the prototype measurements, the following two aspects are studied:The study on the phenomenon of FPSO’s Stern-Spin. The characteristics of current in CNOOC 102 area are discussed, and the number of occurrences of stern-spin during one year is obtained. Finite element numerical model of SYM is established and preliminary discussion is carried out for the influence of failure mode and mechanical properties of SYM; Analysis and evaluation of the upper hinge point of mooring leg. Through the monitoring data of this hinge point in Youyi FPSO, the life consumption of mooring leg is analyzed and determined, the extent of hinge point damage on the mooring performance is also discussed.(5) Establishment of Risk Management MethodAccording to the long-term statistical analysis of actual measurements, failure modes of SYMs are studied and the hierarchy structure of risk factors of SYM is built. Taking the overload destruction of SYM as an example, probability distribution model of restoring force is established to analyze and evaluate the probability distribution during the FPSOs in service. The occurrence probability and consequence degree of this failure mode is determined. A neural network namely RBF method to make the prediction of the restoring force in shallow water is presented, the findings have successfully applied to the SYM forecast in Bohai Bay.Finally, a summary of this dissertation and the work in future are listed.