Preliminary design of composite riser stress joints

The objective of this paper is to investigate the technical feasibility of using hybrid carbon and glass-fibre reinforced epoxy composite tubes as production risers for a tension leg oil platform tethered in 1000 m of water. The axial forces and bending moment distributions applied to the riser were calculated by hydrodynamic finite element analysis, taking into consideration the extreme environmental conditions, large displacements, waves, currents and platform motions, which could occur in a 100 year recurrent storm. Loads induced by pre-tensioning the riser, the weight of the riser, external water pressure and the internal pressure that could arise in the event of a blow-out in the well were also considered. The riser was of 220 mm internal diameter and the walls were reinforced with layers of carbon-fibre wound at ±20° to the tubes axis, sandwiched between circumferentially wound glass-fibre reinforced epoxy layers on the inner and outer surfaces. The thickness of the carbon-fibre reinforced layers tapered along the 24 m lengths of the top and bottom sections of the riser, which are described as tapered joints. The strengths at various sections of the composite tubes were calculated using orthotropic, laminated, thick cylinder theory and progressive failure analysis. The wall thicknesses were chosen by comparing the predicted first failure load and 1/3rd final failure load envelopes with the various combinations of axial tensile and bending loads and internal and external pressures that the tubes could encounter. The possibility of delamination occurring at the ply drop-offs in the walls of the tapered joints was investigated using finite element methods and fracture mechanics. The composite riser was shown to satisfy all the design requirements and to weigh less than half the weight of an equivalent steel component.