Research On Dynamic Analysis And Structural Lectotype Of Ice-resistant Offshore Platforms

At present, the design of ice-resistant platforms is still in the state of static design. But itis indicated that the ice-induced vibration of ice-resistant platforms is quite intensive, anddirectly endangers the structural safety according to the field data monitored on severalplatforms in the Liaodong Bay of Bohai Sea in past years. A systematic research on thedynamic characteristic, ice-induced fatigue and structural lectotype of ice-resistant offshoreplatforms is presented based on in situ results of ice-induced vibration and the currentprogress Of ice load.Firstly, the full-scale field tests and the current achievements of ice load are summarized.It is found that the ice-induced steady vibration occurs on vertical structures, and theice-induced sympathetic vibration also happens on conical structures. The ice load model,which was developed based on the field synchronous results of ice load and structuralresponses, provides a probability to perform the dynamic analysis of ice-resistant platforms.The main works in this thesis are obtained with adopting the current achievements of ice load.The finite-element models of four ice-resistant structures are established, on whichfull-scale tests are carried on. The damping ratio of structures is determined with logarithmicreduced method using free damped curve of acceleration response obtained from field data,which can improve the precision of dynamic analysis. The modal analysis is conducted, and iscompared with the field data to prove its rationality.The dynamic characteristics of ice-resistant jacket platforms in the Bohai Sea areanalyzed, and it is pointed out that the platforms, as typical compliant structures, haverelatively large static rigidity according to dynamic response analysis of the four platforms.For the dynamic characteristics, both of the basic frequencies of vertical and conicalstructures are close or consistent with the frequency of ice load. The phenomenon of dynamicmagnifying, as a typical compliant characteristic, is obvious on the ice-resistant jacketplatforms. For the static characteristics, the static rigidity is relatively large with smalldisplacement, and the static safety stock of the structure is enough under the maximum staticice load. The characteristics above can be applied for dynamic optimization and vibrationcontrol of the structure.A general analysis about fatigue for ice-resistant jacket platform in the Bohai Sea isprocessed, and the method of computing ice-induced fatigue life of tube node is studied indetails. Based on the field data of ice load history and ice-induced response, the probability offatigue failure for ice-resistant jacket platforms is discussed. Through the safety fatigueanalysis, the flowcharts of fatigue analysis are given, and the further perfect estimationmethod of ice-reduced fatigue life is improved for ice-resistant platforms. With the analysisabove, it can be concluded that the ice load and the ice fatigue environmental parameters arethe key problems for estimating the ice-induced fatigue life. Moreover, the ice fatigueenvironmental model of JZ20-2 area in the Bohai Sea is established based on the field dataand simulated numerical results in the last decade. The ice-reduced fatigue lives ofJZ20-2NW platform that was built recently, and of JZ20-2MSW platform that is in service areanalyzed with the spectral method in details.Moreover, the influences of ice-indueed acceleration on the stuff and the pipeline systemon the platform are discussed. It is pointed out that the influence of ice-induced accelerationshould be also taken into consideration, and it is necessary to analyze the deck acceleration inthe structural design. The maximal acceleration amplitudes under ice combinationalconditions of various ice thickness and ice velocity are computed with spectrum method. Andthe accuracy of numeric results is proved with the field data. Meanwhile, the application ofnumeric results in practical engineering is discussed.At last, the ice-resistant structural types and their structural characteristics are discussed.There are two kinds of ice-resistant structures as man-made island and jacket platform. Thephylogeny and structural characteristics of the two kinds of ice-resistant structures aresummarized, and the ice-resistant performances of single-legged, three-legged andfour-legged platforms are compared. The results reveal that the monopod jacket platform withice-breaking cone is feasible for satellite platform, and the four-legged platform withice-breaking cones can be adapted as centre platform aiming at the exploitation of margin oilfield in the ice-covered zone of Bohal Sea. The feasibility of simple ice-resistant platform inthe Bohai Sea and the relative key problems are discussed briefly also.