This paper outlines the main benefits of composite and hybrid flexible risers in deep water applications and describes the main challenges in qualifying such riser solutions for safety and reliability.
The most prominent new riser concepts that utilize modern composite materials for strength, weight reduction and corrosion resistance are briefly introduced. They use pultruded carbon fibre composite rods as tensile strength elements or thermoplastic composite pipes (TCPs) as the complete riser section or as a layer in an unbonded flexible riser wall. These concepts provide the bending flexibility required for efficient installation and accommodation of movement of the floating offshore unit. The bending flexibility is achieved by accommodating relative movements between elements of the unbonded layers in the pipe wall and by the inherent flexibility of the TCP.
The shortcomings of current design practices for application to hybrid and composite risers are identified. Industry addresses these challenges by using modern design formats and structural reliability methods as well as best industry practice for qualification of new technology. The main qualification challenges are summarized, and recommendations are provided for how to address them. Work is in progress to capture the current state of the art in new tailor-made standards to simplify the safe and responsible exploitation of the new hybrid and composite riser technologies in offshore oil and gas field developments without needing to resort to structural reliability analysis and technology qualification.
A riser is a pipe that connects a reservoir below the seabed, where the hydrocarbons have been trapped for 100's of millions of years, with our society which today needs the hydrocarbons for energy supply. As easy hydrocarbon supplies get depleted, exploration has moved to deeper waters where the reservoirs also contain aggressive components such as CO2 and H2S. Current fields are developed in down to 3000 m water depth and the reservoir flow can contain more CO2 and other aggressive chemical compounds than hydrocarbons. Conventional riser concepts that use steel as the main loadbearing elements approach their limits in these environments because the high density (weight) of steel makes the riser very heavy and because of the sensitivity of the steel materials to corrosion.