Polyurethane bend stiffener

1. A bend stiffener comprising (i) a flexible polymeric sleeve having a mass of at least 500 kg, an axial bore for receiving an elongated flexible member and (ii) a flange, end plate, locating cylinder-and-sleeve arrangement or guide connector and latch assembly for attaching the sleeve to a support, wherein all or a portion of the flexible polymeric sleeve is a polyurethane elastomer produced in a reaction of an isocyanate-terminated prepolymer and a curing agent at an isocyanate index of 90 to 120, wherein the isocyanate-terminated prepolymer has an isocyanate content of 7 to 15% by weight and is a reaction product of at least one aromatic polyisocyanate and at least one polyether polyol having a hydroxyl equivalent weight of 350 to 2000 and a nominal functionality of 2 to 2.3, and wherein the curing agent is one or more diol chain extenders having an equivalent weight per isocyanate-reactive group of up to 125 or a mixture containing one or more such diol chain extenders and one or more polyether polyols having a hydroxyl equivalent weight of 350 to 2000 and a nominal functionality of 2 to 2.3, and the polyurethane elastomer has a hard segment content from 40 to 60% by weight.

2. The bend stiffener of claim 1 , wherein the isocyanate-terminated prepolymer has an isocyanate content of 9 to 12% by weight.

3. The bend stiffener of claim 2 , wherein the aromatic polyisocyanate is diphenylmethanediisocyanate (MDI).

4. The bend stiffener of claim 2 , wherein the polyether polyol used to make the prepolymer has a hydroxyl equivalent weight of 350 to 750.

5. The bend stiffener of claim 2 , wherein the curing agent contains no more than 25% by weight of a polyether polyol having a hydroxyl equivalent weight of 350 to 2000.

6. The bend stiffener of claim 1 , wherein the prepolymer is a reaction product of diphenylmethanediisocyanate (MDI) with one or more poly(tetramethylene oxide) diol(s) having a hydroxyl equivalent weight of 350 to 750, and the curing agent is (i) one or more chain extenders or (ii) a mixture of one or more chain extenders, 0 to 10% by weight of one or more poly(tetramethylene oxide) diol(s) and 0 to 10% by weight of one or more crosslinkers, and the prepolymer has an isocyanate content of 9 to 12% by weight.

7. The bend stiffener of claim 2 , wherein the chain extender is the only isocyanate-reactive material in the curing agent.

8. The bend stiffener of claim 2 , wherein the chain extender is 1,3-propane diol.

9. A method of making a flexible bend stiffener sleeve, comprising mixing an isocyanate-terminated prepolymer and a curing agent at an isocyanate index of 90 to 120, introducing the resulting mixture into a mold and curing the mixture in the mold to form a polyurethane elastomer constituting all or a portion of the sleeve, wherein the isocyanate-terminated prepolymer has an isocyanate content of 7 to 15% by weight and is a reaction product of at least one aromatic polyisocyanate and at least one polyether polyol having a hydroxyl equivalent weight of 350 to 2000 and a nominal functionality of 2 to 2.3 and wherein the curing agent is one or more diol chain extenders or a mixture containing one or more such diol chain extenders and one or more polyether polyols having a hydroxyl equivalent weight of 350 to 2000 and an average nominal functionality of 2 to 2.3, and the polyurethane elastomer has a hard segment content from 40 to 60% by weight, wherein the prepolymer and curing agent are cured in the mold until they have cured sufficiently that the sleeve can be demolded without damage, the sleeve is demolded, and the sleeve is then postcured by heating it outside of the mold for a period of 10 minutes to 24 hours at a temperature of 40 to 120° C. and the sleeve has a mass of at least 500 kg.

10. The method of claim 9 , wherein the isocyanate-terminated prepolymer has an isocyanate content of 9 to 12% by weight.

11. The method of claim 10 , wherein the aromatic polyisocyanate is diphenylmethanediisocyanate (MDI).

12. The method of claim 10 , wherein the polyether polyol used to make the prepolymer has a hydroxyl equivalent weight of 350 to 750.

13. The method of claim 9 , wherein the prepolymer is a reaction product of diphenylmethanediisocyanate (MDI) with one or more poly(tetramethylene oxide) diol(s) having a hydroxyl equivalent weight of 350 to 750, and the curing agent is (i) one or more chain extenders or (ii) a mixture of one or more chain extenders, 0 to 10% by weight of one or more poly(tetramethylene oxide) diol(s) and 0 to 10% by weight of one or more crosslinkers, and the prepolymer has an isocyanate content of 9 to 12% by weight.

14. The method of claim 9 , wherein the chain extender is the only isocyanate-reactive material in the curing agent.

15. The method of claim 9 , wherein the chain extender is 1,3-propane diol.

16. A method for attaching a flexible pipe that extends from an undersea wellhead to a supporting apparatus, comprising inserting the flexible pipe into or through the axial bore of the sleeve of the bend stiffener of claim 1 , and affixing the bend stiffener to the supporting apparatus via the flange, end plate, locating cylinder-and-sleeve arrangement or guide connector and latch assembly of the bend stiffener.

17. The method of claim 16 , wherein the supporting apparatus is a buoy, a floating or fixed production platform, a ship or other floating structure.