Composite Structures for Offshore and Harsh Environment Applications

石油和天然气行业日益增长的需求要求适应恶劣环境下的深水钻井的先进技术。复合材料被认为是在这些条件下运行的海上结构物的“使能技术”。用复合立管取代钢质立管可显著减轻重量，将张力腿平台的功能从1500米扩展到3000米以上。此外，复合立管的隔热性能优于钢制立管，可在低温下保证深水中的流动。有限元分析是分析复合材料结构的有效方法，但其在预测老化损伤机理方面的局限性较小。扩展有限元方法XFE使裂纹路径的预测不依赖于有限元网格。同时，XFE在目前的实现中遇到了许多挑战，例如无法预测裂纹分支。因此，本研究建议应用Nitsche的方法来提高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问题时特别有用。因此更适合于分析复合材料的疲劳损伤和裂纹扩展。此外，这种方法基于引入重影节点，而不是在Xfem的情况下引入丰富节点。可以在现有单元中引入鬼节点，而不会对裂纹区中的相邻单元产生任何影响。因此，建议在有限元商业软件中实现该方法，以增强对复合材料老化损伤机制的预测。这一分析工作与验证分析预测的测试程序相平行。该测试计划专门针对含有两种纤维的混杂复合材料，因此与当前复合材料立管的制造技术相一致。复合材料立管的其他制造方面，如固化方法、纤维类型和树脂技术，也在拟议的测试计划中考虑在内。因此，提出的方案旨在满足恶劣环境下的深水钻井要求，提供适合于预测海洋复合材料老化机理的准确技术，该方案支持与当前海洋技术相一致的材料测试方案。

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.