Composite Structures for Offshore and Harsh Environment Applications

石油和天然气工业日益增长的需求要求先进的技术适用于恶劣环境下的深水钻井。复合材料被认为是在这些条件下运行的海洋结构物的“使能技术”。通过用复合材料代替钢支架，显著减轻了重量，将张力腿平台的功能从1500米扩展到3000米以上。此外，复合材料隔水管的上级隔热性能优于钢隔水管，可在低温深水中保持流动性。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局限性。扩展有限元方法XFEM能够独立于有限元网格预测裂纹路径。与此同时，XFEM在其当前的实施遭受了许多挑战，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。这种方法在解决位移场和位移梯度不连续性组合时特别有用。因此，更适合于分析复合材料的疲劳损伤和裂纹扩展。此外，这种方法是基于引入鬼节点，而不是丰富的节点的情况下，XFEM。虚节点可以引入到现有单元中，而不会对裂纹区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这种方法，以加强对复合材料老化损伤机制的预测。这一分析工作是由一个测试程序验证分析预测。该测试计划致力于混合复合材料，包括两种类型的纤维，因此与当前的复合材料制造技术相一致。在拟议的测试计划中考虑了复合材料的其他制造方面，如固化方法、纤维类型和树脂技术。因此，所提出的计划旨在通过提出适合预测海上复合材料老化机制的准确技术，满足恶劣环境下的深水钻井要求，该技术由与当前海上技术一致的材料测试计划支持。

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.