Risers and umbilicals are typically used in offshore hydrocarbon production to transport fluids (such as produced hydrocarbons or hydraulic fluids) between a well head at the sea bed and a surface vessel, for example a rig or a drill ship. Motion of the surface vessel caused by wind, waves, ocean currents and so on, leads to bending of the risers and umbilicals. This places periodic mechanical stresses on the risers and/or umbilicals, which can lead to mechanical failure.
The weakest point of each riser or umbilical is the point immediately adjacent to the connection to the surface vessel. This is the region subject to the highest mechanical stress, and therefore the region most likely to fail.
In order to strengthen the riser at this point, it is known to attach a bend stiffener 1 to the riser (the word riser is used herein to describe any pipe or umbilical extending from the surface vessel towards the well head or other subsea installation), as illustrated in FIG. 1. The bend stiffener 1 is disposed around a riser 2 that is connected at a riser connection point 3 to a surface vessel. This point is known as the “hang-off”.
Existing bend stiffeners in use tend to be polyurethane stiffeners moulded into a truncated conical shape. Other types of material that have similar elastic properties to polyurethane may be used.
Conical bend stiffeners have an insulating effect around the riser, as they do not allow free movement of water. If the riser is carrying hot fluids, such as high temperature hydrocarbons, this insulating effect can give rise to high temperatures between the bend stiffener and the riser. Similarly, if an umbilical contains a high voltage power cable, this can give rise to heat. If the heat cannot dissipate away from the riser in the hang-off region, if may damage the riser or the bend stiffener. Heat is known to degrade polyurethane, so over time the bend stiffener and any polymeric outer sheath on the riser will become more prone to mechanical failure. This leads to a reduction in the lifetime of the outer sheath and the bend stiffener.