Dynamic Characteristics And Response Analysis For Mooring Posioning System Of Deepwater Platform

At present, the exploitation of offshore oil and gas resources is concentrated in the Bohai sea area of China territorial waters. While South China Sea as three fourths of our territorial water, the gas and oil resources development is nearly empty. According to preliminary estimate, the oil reserves in the South China Sea approaches 23 billion to 30 billion tons, accounting for one third of China's total amount of resources. As the center of the world's four major offshore oil and gas accumulation, this area gets high reputation of"The Second Persian Gulf". In China, gas and oil resources exploitation is primarily in the 200m water depth. The development of maximum drilling depth comes near 500 meters and maximum operating depth comes up to 300 meters. Compared with foreign ocean engineering technology, there is still a gap between theirs and our deepwater oil and gas development. Unfortunately, Vietnam, Philippines and other neighboring countries have begun their exploitation in advance in the disputed areas of South China Sea. Therefore, it's momentous to accelerate the pace of oil exploitation in South China Sea.Deepwater oil platforms are generally required by mooring positioning system to maintain it at a relatively fixed position, so the design of mooring positioning system plays an overwhelming role in the overall design process of floating structure. Mooring positioning system performance depends mainly on the ocean environmental conditions, the environmental loads acting on floating structure and mooring lines, the material and dynamic performance of mooring line, the arrangement of mooring lines, water depth and other factors. Establishing an appropriate model and obtaining corresponding numerical analysis techniques can help study from the mechanism of the dynamic characteristics of mooring positioning system, predict mooring positioning performance, and further guide the design and construction.The research on mooring positioning system analysis mainly consists of two parts: performance investigation of mooring system and influence to floating structure. Performance investigation of mooring system mainly includes: static and dynamic characteristic, fatigue and anchor system. Influence to floating structure mainly includes: influence of mooring damping and coupled effect with floating structure. The two methods of solving these problems are still based on numerical simulation and model test, but there are many other new problems in the procedure. The main works of this dissertation are as follows:1. The piecewise extrapolating method is employed to the static characteristic analysis of mooring system, and the influence of vertical displacement to restoring force of hybrid mooring line is also estimated. The dynamic response analysis of mooring line is conducted through nonlinear finite element method. Then the analysis of natural frequency and stiffness of hybrid mooring line are compared, and the material characteristics of polyester mooring line are summarized2. The damping contribution of mooring line to floating platform is calculated by the nonlinear finite element method in time domain. The influences of various parameters to mooring damping are studied and the results show some regularity. The regularity is respectively fitted by uniform formula. The mooring damping characteristic of hybrid mooring line is compared.3. The model tests of HYSY-981 semi-submersible platform are conducted in the Joint Laboratory of Wind Tunnel & Wave Flume at Harbin Institute of Technology. A truncated mooring system is designed based on the static equivalent principle. Then the motion responses of semi-submersible platform and its mooring system are obtained from experimental results. Base on the comparative results of model test and numerical simulation, the viscous damper compensated system in truncated mooring lines is proposed in the paper. The numerical simulations for truncated model tests of semi-submersible platform with viscous damper compensated system in mooring lines are conducted and the motion responses of semi-submersible and its mooring line are compared with the primary results.4. The motion responses of DDMS platform and its mooring system are calculated under each short-term sea state, and the results are based two mooring systems which have the same static restoring force characteristic. The two mooring systems respectively consist of chain-wire-chain and chain-polyester-chain. The Rain Flow counting method is employed to obtain the fatigue load spectrum in each short-term sea state based on the time series of mooring line tension. The Palsren-Miner linear cumulative law model is used to compare the fatigue damage of the hybrid mooring lines in long-term sea state.5. The numerical model and specified calculation flow for ultimate pullout load of drag embedment anchor are established by ABAQUS. The influence of depth, fluke/shank angle and mooring angle to static ultimate pullout load of drag embedment anchor are studied. The influence of depth and load cycle times to dynamic ultimate pullout load of drag embedment anchor are studied and the results are compared with static ultimate pullout load.