深水油气柔性立管多场热质耦合迁移及冷凝机理研究

The marine flexible risers are quite suitable for the developments of deepwater oil and gas reservoirs due to the excellent characteristics of the design abilities, economics and dynamic behaviors. At present, the theories and methods of the gas permeations in the composite walls have not yet considered the influences of the multiphase flow in the inner pipes and the gas-liquid convection in the annuluses between the internal pressure sheaths and the outer sheaths. Therefore, the migrating mechanisms may be further clarified, the prediction accuracies of the penetration rates are expected to improved for the ultra-deep water applications, and the specific forms of the annulus condensates need to be confirmed. The project aims to study the mechanisms of the mass and energy transferring of the typical flexible risers. Firstly, the unsteady migrating and condensing model of the multi-field heat and mass couplings in the "flow-solid-mass-seawater" system is development, in which the effects of the multiphase flow in pipes and the natural convection in annuluses on the time-varying behaviors of the migration are considered. Secondly, the optimal numerical algorithm for solving the strong nonlinear model is proposed for the good convergence and stability. Thirdly, the formation process and the spatial and scale distributions of the local condensates are investigated based on the model prediction results, the experimental data and the stochastic fractal theories. Fourthly, the migration characteristics of the gas - liquid phase in the closed annulus cavity are studied under the action of non-equilibrium internal and external fields, including the inner concentration, temperature, gravity fields and the outer venting, shutdown cooling and vortex-induced vibrating effects. Lastly, the distributions of the temperatures, pressures and condensates as well as their dynamic responses are revealed with the different flexible riser configurations. The research results can provide the theoretical basis for the flow assurance solutions of the deepwater flexible risers, such as the corrosions and hydrates in the annulus.

海洋柔性立管具有可设计性强、经济性好、动力特性优良等特点，尤其适用于深水油气藏的开发。目前针对管壁气体渗透的理论及方法尚未考虑管内多相流和环空气液对流的影响，存在迁移机理有待澄清、应用于超深水环境时渗透速率预测精度不高、环空冷凝液形态尚不明确等问题。本项目拟针对典型柔性立管物质和能量互作传递机理开展研究。建立“流-固-质-海水”多场热质耦合迁移冷凝非稳态模型，考虑多相管流和环空自然对流对迁移时变性的影响；开发求解强非线性模型收敛性好、稳定性强的最优数值算法；研究基于模型预测结果、实验数据以及随机分形理论，环空局部冷凝液的形成过程、空间分布与尺度分布；查明在浓度场、温度场、重力场与排气泄压、停输冷却、涡激振动等非平衡态内外场联动作用下，环空多孔连通密闭腔体内气液相整体的迁移特性；揭示不同悬挂构型立管环空的温压、冷凝液分布及其动态响应规律，为环空的腐蚀、水合物等流动保障问题提供理论依据。

应对深水油气田开发所用柔性立管的环空内浸没、腐蚀、鼓胀等安全挑战，重点开展了小分子气体在多场热质耦合机理下的渗透及冷凝理论研究，主要完成了：（1）通过机理分析构建了柔性立管渗透、冷凝及迁移数理模型，考虑了内管气液多相流动与传热的影响，采用有限差分法与控制体积法相结合的数值计算方法对模型进行了求解；（2）搭建了一套全透明蓝宝石反应釜的冷凝实验装置，模拟研究了环空局部水蒸气滴状冷凝的成核、生长和滑落过程以及分形几何特征；（3）结合实验和数值模拟方法，较为系统地查明了柔性立管的渗透、冷凝及迁移规律，建议适当调整屏蔽因子和放空压力，将放空频率控制在合理的范围内；（4）开展了环空安全检测理论方法研究，提出了由低压向高压的阶梯步进检测方法，并将研究成果应用于南海某在役柔性立管的现场检测及安全评价。上述研究成果可为海上油气柔性立管的环空安全保障提供科学依据和技术指导。