Transient Response Evaluation Of FPSO With Different Failure Scenarios Of Mooring Lines

Floating platforms usually adopt mooring positioning method to ensure that the platform moves within a fixed range.However,the mooring line may fail due to its own fatigue,overload or performance deterioration,etc.,causing the platform to drift significantly and collide with neighboring ships and platforms,causing huge economic losses.A series of time-domain numerical simulations for evaluating the performance changes of a 4 × 3 combined distribution of internal turret Floating Production Storage and Offloading(FPSO)with different broken modes of mooring lines was carried out in this paper and explore the mechanism of platform motion response changes in different broken modes.Firstly,according to the ship’s lines,the geometric model of platform based on the ANSYS-AQWA software is established.The wave loads,RAO,QTF,etc.of FPSO under regular waves are derived through calculations using potential flow theory in frequency domain.Design the platform mooring and conduct a time-domain coupling analysis of the FPSO mooring system.The static and dynamic results are compared with the MARIN tank experiment to verify the accuracy of the mooring system layout and the reliability of the analysis results.Secondly,taking the 100-year hurricane load in the Gulf of Mexico as example,three different collinear and non-collinear incident angles of wind,wave,and current were set as environmental conditions.Taking the #2 mooring line with the highest tension when the mooring is complete as a reference,4 different platform mooring failure modes of single mooring line or two mooring lines failure under ipsilateral,opposite,and adjacent sides are set respectively.Analysis the stiffness changes of the mooring system under different failure modes,it provides a basis for subsequent mechanism analysis.It can be seen that the stiffness of the mooring system under a certain direction is determined by the number of broken lines in that direction and the failure location.The greater the number of mooring lines failures,the smaller the stiffness,and the failure on the same side has a smaller stiffness than the failure on the opposite side.Thirdly,a nonlinear time-domain coupling analysis is performed on the FPSO mooring system under different broken modes,and the results of the motion response of the platform and the change of the mooring line tension is obtained,and the corresponding influence mechanism was investigated.The results indicated that the failure of mooring lines mainly influences the responses of sway,surge,and yaw,and the change rule is closely related to the stiffness and symmetry of the mooring system.Under the same degree of freedom,the greater the stiffness,the smaller the motion response,the smaller the symmetry damage,and the smaller the platform motion response.When one or two moorings on the same side fail,the remaining moorings will bear the instantaneous environmental load impact,causing the tension of the remaining moorings to increase sharply in a short time.The maximum instantaneous increase can reach 2.6 times that when the mooring is complete.Finally,in view of the two uncertain factors of non-extreme sea conditions and different wave seeds,the extreme response of the FPSO mooring system under failure conditions is studied.The results show that the average peak tension of the mooring line should consider about conservative 8% variable uncertainty,and the average peak motion of the platform should consider about conservative 15% variable uncertainty if applying only one certain irregular wave condition in the random sea state analysis,based on the simulation results in this paper.