Dry Disconnectable Riser System for Low Keel Clearance Floaters

Abstract

Fixed structures operating in extreme offshore environments (eg: arctic) have to resist and survive the challenging conditions like large ice forces. On the other hand, the floating systems in such environments benefit from their ability to be evacuated and transported in the event of severe condition. However, such an ice-management scheme requires the moorings and risers to be disconnected in the severe conditions and reconnected during more clement conditions. Among the floater designs typically considered for extreme offshore applications is the Spar. The Spar offers low extreme motion responses than other shallower draft floaters and as a result, low fatigue damage on the risers. However, this design has versions with open centerwells that can contain leaked product and cause a potential hazardous condition. Furthermore, most designs that require the risers to be disconnected below the mean water line (wet-disconnection mechanism) not only carry the risk of leakage but have poor access for visual inspection. One solution is to use a continuous flexible riser without in-line connections or terminations in the flexible riser between the seafloor pipe line end manifolds (PLEM) and the production deck manifolds (dry-disconnection mechanism). The risers disconnect at an elevation above the water line and the termination point is lowered to a disconnect buoy supported at the keel. Subsequently the floater is moved away. The main difficulty is that the lowered flexible riser has to be suspended from the disconnect buoy and at the same time avoid contact with the seafloor. This paper describes and discusses a design of a dry-disconnectable flexible riser system comprised of a buoy supporting arches to control the bending in the risers during operation and disconnect. The system is particularly effective when the clearance between the keel and the seafloor is restricted. The rationale is based on strength, control of the minimum allowable bend radius and interference among the risers.