基于层间摩擦磨损的深水动态柔性立管疲劳机理与预测模型研究

Dynamic flexible riser is one of the most important equipments for producing oil and gas and mining in deep water. A flexible riser comprises couples of unbonded metallic layers, in which many spiral helically wound wires could be found. Due to periodic offshore environment and motions of floating platform, fatigue failure tends to happen for the flexible riser next to floater. During alternative bending of flexible risers, inter-layered friction and wear lead to the complicated slick-slip effect that makes the mechanical responses of helical elements appear strongly nonlinear. That will bring difficulties and challenges into the explanation of fatigue principle and the prediction of fatigue life of flexible risers. In this project, the behavior and principle of friction and wear is investigated for helical elements based on the experimental method. By quasi-static loading test of tension and bending and numerical simulation, slippage and stress are investigated and analyzed accurately during the helical element bending, and then the validity of existing theoretical models is studied. According to the fatigue test loaded by both tension and alternative bending, the fatigue failure mode and principle of flexible risers are investigated, and the quantitative analysis is performed for the influence of inter-layered friction and wear on the fatigue life. The full three-dimensional numerical model of the flexible riser is established and the wear imperfection, which could be obtained from fatigue tests, and local fatigue stress coupling model is developed. Those help improve the model to predict the fatigue life, and then the model is verified comprehensively by existing results from published fatigue tests. The achievement of this project could provide reliable theoretical basis and useful tools for fatigue-resistant design and analysis of the fatigue life of dynamic flexible risers in practical engineering.

动态柔性立管是深水油气开发与深海采矿过程中的重要装备。它采用多层金属螺旋缠绕形式非粘结复合而成，在周期性海洋环境与浮式平台运动的共同影响下容易发生疲劳失效。由于层间接触摩擦磨损，柔性立管在交变弯曲过程中产生复杂的粘-滑摩擦效应，使得螺旋构件应力响应呈现强非线性，为柔性立管疲劳失效机理解释与疲劳寿命预测带来困难。本申请项目采用试验方法对构件摩擦与磨损机理进行研究。基于拉弯准静态加载试验和数值模拟，对螺旋构件弯曲过程滑移与应力响应进行准确分析和规律研究，并考察当前理论模型的适用性。通过拉弯组合动态加载疲劳强度试验，探究柔性立管疲劳失效模式与机理，定量分析层间摩擦磨损对疲劳寿命的影响。建立柔性立管全三维数值模型，并基于试验结果提出磨损缺陷和局部应力变化的耦合模型，进一步完善疲劳寿命预测方法。本申请项目的研究成果可以为深水动态柔性立管疲劳设计分析技术提供坚实可靠的理论基础。

具有多层非粘结复合结构的动态柔性立管是深水油气开发和深海采矿过程中的重要装备，其层间相互摩擦磨损效应为准确预测柔性立管疲劳寿命带来挑战。本项目综合采用试验手段、有限元数值方法、解析方法等探索柔性立管层间摩擦磨损引起的螺旋结构粘-滑发展机理和疲劳失效机理，同时对螺旋结构非线性力学行为和柔性立管疲劳寿命等开展模拟研究，取得的主要成果描述如下：.（1）抗拉铠装钢丝间摩擦系数随磨损程度的增加而增大，但是增加速率逐渐变缓；钢丝表面发生以粘着磨损为主、多种磨损机制并存的磨损，磨损宏观表现为钢丝厚度的下降；通过拟合定量得到钢丝磨损相关系数约为10e-12 mm2/N2量级。.（2）建立了一套柔性立管四点拉弯结构试验装置和基于DIC技术的钢丝相对滑移测量方法，同时建立了柔性立管三维有限模型，研究结果显示：在拉弯作用下，立管弯矩和曲率呈现典型的非线性滞回关系；钢丝滑移总是在中性轴位置开始启动而后向两边扩散，中性轴处钢丝滑移量最大而内外曲率处钢丝滑移量趋于0，钢丝滑移不仅仅沿着钢丝轴线，也包含垂直于钢丝轴线的分量；钢丝应力响应与粘滑形态对应，在粘滞阶段非常大，进入全滑阶段则很小，钢丝角点应力随相位呈现周期性正弦分布特点。. （3）基于四点拉弯结构试验装置开展了柔性立管拉弯组合疲劳试验，结果显示出螺旋钢丝发生了典型的应力疲劳失效，螺旋钢丝厚度由于层间摩擦磨损发生了不同程度的下降，并与材料试验预测结果基本一致；结合数值模型分析，发现考虑层间摩擦磨损后的钢丝疲劳寿命相对于完整钢丝的疲劳寿命低。.（4）建立了计及钢丝层间摩擦磨损的管道疲劳寿命预测模型并通过试验结果验证，分析发现层间磨损通常会导致钢丝在 40°至 80°相位角范围内最先发生疲劳失效，同时随着循环次数的增加，钢丝疲劳损伤、交变应力最大值基本呈现线性增加趋势，进而导致疲劳寿命下降约30%。.本项目研究方法和结论将为深水柔性立管疲劳寿命预测提供坚实的技术支撑。