Composite Structures for Offshore and Harsh Environment Applications

石油和天然气工业日益增长的需求需要先进的技术，适合恶劣环境下的深水钻井。复合材料被认为是海上结构在这些条件下运行的“使能技术”。将钢立管替换为复合立管，可以显著减轻重量，将张力腿平台的功能从1500米扩展到3000米以上。此外，与钢制立管相比，复合立管的绝热性能更好，可以在低温下保持深水中的流动保障。有限元分析是分析复合材料结构的有效方法，但在预测老化损伤机理方面存在一定的局限性。扩展有限元法XFEM能够独立于有限元网格预测裂纹路径。同时，目前实现的XFEM还面临着许多挑战，例如无法预测裂纹分支。因此，本研究提出应用Nitsche方法增强有限元分析的失效预测能力。这种方法在处理位移场和位移梯度不连续的组合时特别有用。因此更适合分析复合材料的疲劳损伤和裂纹扩展。此外，在XFEM的情况下，这种方法基于引入虚节点而不是充实节点。幽灵节点可以被引入到当前存在的元素中，而不会对裂缝区域的相邻元素产生任何影响。因此，提出在有限元商用软件中实现该方法，以增强对复合材料时效损伤机理的预测。这种分析工作与验证分析预测的测试程序并行。该测试项目主要针对混合复合材料，包括两种类型的纤维，因此符合当前复合材料立管的制造技术。复合立管的其他制造方面，如固化方法、纤维类型和树脂技术，都在拟议的测试程序中考虑。因此，该方案旨在通过提供准确的技术来预测海上复合材料的老化机制，并支持与当前海上技术相一致的材料测试方案，以满足恶劣环境下的深水钻井要求。

The increasing needs of oil and gas industry require advanced technologies suitable for deep water drilling in harsh environment. Composite materials are identified as the "enabling technology" for offshore structures operating in these conditions. Significant weight saving attained by replacing steel risers with composite ones extends the functionality of tension-leg platform from 1500 m to beyond 3000 m. Moreover the superior thermal insulation of composite risers over their steel counterparts maintains flow assurance in deep water at low temperatures. Finite element analysis is efficiently used to analyze composite structures however it suffers from few limitations in predicting aging damage mechanism. The eXtended Finite Element Method XFEM enabled predicting crack path independent of the finite element mesh. Meanwhile the XFEM in its current implementation suffers a number of challenges such as inability to predict crack branching. Therefore this research proposes enhancing failure prediction capabilities of finite element analysis by applying Nitsche's approach. This approach is particularly useful in addressing displacement field and displacement gradient discontinuities combined. Hence more suited for analyzing fatigue damage and crack propagation in composite materials. Moreover this approach is based on introducing ghost nodes rather than enrichment nodes in the case of XFEM. Ghost nodes can be introduced in currently existing elements without any effect on neighboring elements in the crack zone. Therefore it is proposed to implement this approach in finite element commercial software to enhance prediction of aging damage mechanism in composites. This analytical effort is paralleled by a testing program for validation of analytical predictions. The testing program is dedicated towards hybrid composites incorporating two types of fibres and hence consistent with current manufacturing techniques of composite risers. Other manufacturing aspects of composite risers such as curing methods, fibre types and resin technology are accounted for in the proposed testing program. Therefore the proposed program is designed to fulfil deep water drilling in harsh environment requirements by presenting accurate technique suitable for predicting aging mechanism in offshore composites supported by material testing program consistent with current offshore technologies.