The State Estimation Of TTR Riser Under VIV Loading

Energy resources are the key point for the sustainable and steady development of national economy. While petroleum stands as the core source for current energy consumption, deepwater crude oil represents the leading opportunity for petroleum industry in future. Deepwater risers, the only essential components which connect the offshore platform and subsea drilling system, are the most fundamental issues for the safety of offshore oil production. However, seriously affected by the severe environmental conditions and unclear loading patterns, risers are vulnerable to be destroyed by all these factors. Vortex-induced vibration (VIV) is the major concern for risers since it may lead to strength failure as well as fatigue damage, and it is of great importance to conduct real-time monitoring and estimation of riser under VIV loadings. Thus, this dissertation would investigate in the referred issues from the following aspects:First, the basic mechanism, influencing parameters, analytical method and research advance of VIV is briefly summarized. Then, by analyzing the statistics from real VIV experiment with proper data processing strategies, the characteristics of risers with large aspect ratio under VIV loadings have been qualitatively and quantitatively revealed, which provides theoretical basis for the lift force model and dynamic equation of risers conceived in the following chapter.Second, the finite element model of TTR riser which considers the coupling effects of in-line and cross-flow motion, fluid-structure interaction, inner fluid transmission and the real velocity distribution of South China Sea and Gulf of Mexico has been built in Matlab. The VIV response of TTR riser has been analyzed in time domain with Newmark-β method and its modal analysis has been conducted thereafter.Third, the strain information has been derived from FEM model and time domain displacement response data. With Miner criterion commonly used in fatigue cumulative damage theories, S-N curve for risers and Rain flow counting (RFC) method, the distribution of fatigue damage of important nodes for the riser has been estimated. In addition, the effects of elasticity modulus, top tension and inner fluid on the fatigue damage of risers have been further investigated.Finally, a preliminary study on the optimized state estimation strategy of TTR riser with limited sensors has been put forward. The H2norm is introduced first for an initial selection of possible sensor arrays which may provide optimal strategy for the reconstruction of riser under VIV excitation. The strategy has fully taken practical installment as well as engineering economic issues into consideration. Then the Kalman filter is introduced to give state estimation of the whole riser based on the selected sensor arrays, and its accuracy has been further illustrated in detail.