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Abstract
Currently offshore oil and gas industry uses production risers made of high grade steel. The weight of the steel risers has to be supported by tension from the floating platform at the top which limits the capacity of offshore operations. By reducing the weight of the risers, it is possible to exploit natural resources from deeper waters and to increase the production capacity, resulting in significant economic benefits.
Due to the desirable mechanical properties of advanced fibre reinforced polymer (FRP) composites, it has been recognised that offshore risers made of composite materials can lead to considerable weight savings. Previous projects investigating application of composite risers employed fibre reinforcements only in the hoop and axial directions in the design. The prototypes fabricated and tested in these projects confirm that FRP composites can indeed provide significant weight saving over steel risers. The main objective of this thesis is to demonstrate that by tailoring the design employing off-axis reinforcements the weight savings offered by advanced composite materials can be substantially increased.
Two different methodologies for the tailored design have been developed in this thesis to minimise the structural weight of the composite riser: one, using an iterative approach of manual inspection and selection and another employing the optimisation technique of Surrogate Assisted Evolutionary Algorithm. The tailored design approach has been applied to eight different material combinations including high strength and high modulus fibre reinforcements, thermoset and thermoplastic matrices and metallic and thermoplastic liner materials to optimise their laminate configurations for minimum structural weight. The designs are conducted in accordance with the Standards, considering both local load cases and global - functional as well as environmental - loads.
The results show that the tailored design including off-axis reinforcements provide significant weight advantage compared to the conventional approach using only axial and hoop reinforcements. Comparison of the structural weights of the risers with different material combinations shows that the combination of thermoplastic PEEK matrix reinforced with high strength AS4 fibres and PEEK liner offers the highest weight savings.