Offshore Platform Prototype Monitoring And Modal Analysis Of The Mooring Structure

The offshore platform is the important place for offshore oil drilling and production and the staff living. It plays a key role in the operations and the safety of the staff. Offshore platform is a large ocean engineering structure and the environment is extremely complex. The studies on marine platform were developed through theoretical analysis, laboratory model test or numerical calculation. In these studies, the external loads of structure are controllable, easier to implement, but more or less different with the reality. The results weren’t completely consistent with the reality. Based on the offshore platform prototype monitoring, obtaining the first-hand monitoring data is significant to the research and design evaluation of offshore platform.In order to adapt to the marine environment, a variety of different types of offshore platforms have been designed. Based on two types of platform structures, different prototype measurement technologies were studied for them, as well as different prototype monitoring schemes. And real-time monitoring systems were developed. The long-term prototype monitoring data were obtained including responses of offshore platform structures, platform motions, and environmental loads.Some statistics of the motion and environment load time series were extracted by segments based on the prototype monitoring data of a type of FPSO in Bohai Bay. The statistics characteristics and distributions of different measurement data were given in time domain or frequency domain. Through further analysis of statistical results, FPSO periodic motions, some distribution characteristics of the environmental load were found, and periodic and symmetric motions of FPSO soft yoke mooring system were analyzed. The relationship between the relative wind direction and FPSO heading motion was preliminarily revealed.Mooring system is the positioning device for offshore platform to limit its movement range, ensuring safety operations of offshore platform. In connection with the nonstationarity of the huge offshore structure monitoring data, a method of constructing structure free response based on the non-stationary response data is proposed. This method uses a unilateral moving average to stabilize loads and the response data in the dynamic model of the multi-degree-of-freedom (mdof) structure, analyzes spectral characteristics of moving averages in some orders, researches filtering effects when the order q=1 (difference) and q> 1 and constructs the free vibration equation of the structure by the random decrement technique (RDT). Through the identification of the second-order linear system simulation model, a comparison of modal parameters with the theoretical values is given, which are identified by different methods, and it shows good effects on the analysis of non-stationary response data using this method. Finally, the method is applied to the mode identification of the Floating Production Storage and Offloading (FPSO) single-point mooring structure, the distribution of the modal parameters of single-point mooring structure is analyzed and modal parameters identified by different values of q are compared.