The offshore industry is often considered very conservative with respect to using new technology. The use of unbonded flexible risers is one of the few technology elements that have been introduced and accepted for use without being fully documented before use. The flexible riser is a great product and has made many field developments possible. When moving into deeper waters with high pressure wells the unbonded flexible riser becomes very heavy and introduces large hangoff forces on the vessel. Experience has indicated that is difficult to carry out proper inspection of these risers. We are therefore investigating riser concepts incorporating new materials and with a simpler cross section that can be used for floating production. Our requirement to a riser is excellent durability, possibility of inspection and installation by reeling. Advanced composite materials have properties such as, low weight, high strength, good durability and very good fatigue performance. These properties make risers manufactured from composite material very interesting. A composite riser can by made by either thermoset or thermoplastic materials. An internal liner must be used, as the composite itself may not be diffusion proof. This liner can be made of metal or plastic and a high quality material may be used, as the amount is modest. The requirement to installation by reeling rules out the metallic inner liner due great mismatch in properties between liner and composite. A very promising concept is the use of a thermoplastic liner together with a thermoplastic composite. This is a riser with excellent mechanical and durability properties and can be installed by the reeling method. An ongoing EU project, AURUM, is investigating this concept in detail. The project has a goal to manufacture an 8″ riser section and includes all the steps in a production process from fibre production to tape winding and analytical modeling.
VIV-induced fatigue damage study of helical wires in catenary unbonded flexible riser in time domain
