Unbonded Flexible Pipe: Composite Reinforcement for Optimized Hybrid Design

Abstract

Abstract The introduction of composite materials technology to unbonded flexible pipe systems is enabling solutions for the increasingly severe service conditions and logistical challenges of deepwater hydrocarbon recovery. Conventional unbonded flexible pipe is a composite structure, combining the chemical resistance of polymers with the high stiffness and strength of functionalised layers of metallic reinforcement. The pursuit of reducing pipe weight for deepwater applications is driving the adoption of fibre reinforced composite materials as structural elements. However the design of unbonded flexible pipes also demands a systems engineering approach to balance the pipe properties with the demands of the installation logistics and long-term dynamic environment; across the whole process, the introduction of composite materials can provide both enabling and optimisation tools. The global dynamics of the flexible pipe structures are conventionally managed by using combinations of ancillary items such as tethering systems, buoyancy and/or weights. A global analysis is carried out on the whole system to ensure stability and manage interactions for all anticipated load conditions. As deeper water systems are considered, the self-weight of metallic reinforced structures becomes critical and considerable buoyancy may be required, adding to the cost and complexity of installation and system analysis. Alternatively a fully composite pipe itself may be buoyant, requiring weight to be added to achieve stability. The introduction of high specific modulus and strength composite materials enables the designer to optimise both the pipe internal load response and the system dynamics by carefully balancing the materials selection and composite design of the interacting functional layers in a hybrid metallic/composite design; neither too heavy nor too light, but optimised for strength and dynamic response, for simple, minimal ancillary requirement installation schemes. This paper details both thermoplastic and thermoset composite materials technologies introduced into the unbonded flexible pipe designer's toolbox, discusses qualification processes and prototype testing results, and presents case study data for optimised flexible pipe system designs for deepwater applications.