Composite Structures for Offshore and Harsh Environment Applications

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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.