Fibre reinforced polymer tube

1. A fibre reinforced polymer (FRP) tube having a longitudinal axis and a circular cross section, the FRP tube comprising a plurality of concentric layers of an FRP material forming a shell of the tube,

wherein at least one blind threaded longitudinal bolt hole is provided from a transverse end surface of the shell;

wherein the bolt hole extends in at least three of the plurality of layers, a middle layer encompassing a plane passing through a center of the bolt hole, an inner layer encompassing a plane of an innermost extent of the bolt hole, and an outer layer encompassing a plane of an outermost extent of the bolt hole;

wherein the FRP material for each of the plurality of layers is formed from at least one wound continuous fibre filament in a solid polymer resin;

wherein the at least one filament of each of the inner and outer layers has been wound at a first angle to the longitudinal axis corresponding to 90° minus a lead angle of the threading of the bolt hole ±5° or ±2°; and

wherein the at least one filament of the middle layer has been wound at a second angle to the longitudinal axis corresponding to 90° minus half of a thread angle of the threading of the bolt hole ±5° or ±2°.

2. The FRP tube according to claim 1 , wherein the at least one filament of the inner layer has been wound only at the positive first angle and the at least one filament of the outer layer has been wound only at the negative first angle.

3. The FRP tube according to claim 1 , wherein the first angle is within a range of 82-92°.

4. The FRP tube according to claim 1 , wherein the second angle is within a range of 55-65°.

5. The FRP tube according to claim 1 , wherein the plurality of layers comprises at least one intermediate layer positioned between the middle layer and the inner layer or the outer layer, wherein the filament of each of said at least one intermediate layer has been wound at an angle to the longitudinal axis which is within a range of between the second angle and the first angle.

6. The FRP tube according to claim 1 , wherein the polymer resin is a thermosetting or thermoplastic resin, e.g. comprising epoxy, polyester, polyurethane and/or polyvinyl ester.

7. The FRP tube according to claim 1 , wherein the fibre filament comprises glass fibre, carbon fibre, aramid fibre, polyethylene terephthalate fibre and/or natural fibre.

8. A fibre reinforced polymer (FRP) tube assembly comprising:

a first FRP tube in accordance with claim 1 ,

a connecting part such as an end fitting or a second FRP tube in accordance with any preceding claim, and

a joint joining the transverse end surface of the first FRP tube to the connecting part by means of threaded bolts screwed into the respective at least one blind threaded longitudinal bolt hole of the first FRP tube and the connecting part, whereby the first FRP tube and the connecting part are fastened to each other and longitudinally arranged next to each other.

9. The FRP tube according to claim 1 , wherein the first angle is 87°, and wherein the lead angle is 3°.

10. The FRP tube according to claim 1 , wherein the second angle is within a range of 58-62°.

11. The FRP tube according to claim 1 , wherein the second angle is 60°.

12. The FRP tube according to claim 1 , wherein the plurality of layers comprises at least one intermediate layer positioned between the middle layer and the inner layer or the outer layer, wherein the filament of each of said at least one intermediate layer has been wound at an angle to the longitudinal axis which is between the second angle +2° and the first angle −2°.

13. The FRP tube according to claim 1 , wherein the polymer resin comprises epoxy, polyester, polyurethane and/or polyvinyl ester.

14. The FRP tube according to claim 1 , wherein the polymer resin comprises epoxy.

15. The FRP tube according to claim 1 , wherein the fibre filament comprises glass fibre.

16. A method of forming a shell comprising a plurality of concentric layers of a fibre reinforced polymer (FRP) material, of an FRP tube having a longitudinal axis and a circular cross section, wherein at least one blind threaded longitudinal bolt hole comprising a thread is provided from a transverse end surface of the shell, the method comprising:

forming each of the plurality of layers of the shell by, around a support tube, applying a polymer resin and winding at least one continuous fibre filament at an angle to the longitudinal axis, followed by curing of the polymer resin to form the layer having the wound filament in a solid resin matrix, wherein:

the at least one filament of an inner layer of the plurality of layers is wound at a first angle to the longitudinal axis corresponding to 90° minus a lead angle of the thread of the bolt hole ±5° or ±2°,

the at least one filament of a middle layer, concentrically outside of the inner layer, is wound at a second angle to the longitudinal axis corresponding to 90° minus half of a thread angle of the thread of the bolt hole ±5° or ±2°,

the at least one filament of an outer layer, concentrically outside of the middle layer, is wound at 180° minus the first angle to the longitudinal axis;

drilling the at least one bolt hole from a transverse end surface of the shell of the formed plurality of layers; and

threading the at least one drilled bolt hole with said thread.

17. The method of claim 16 , wherein the winding of the filament is performed by a filament winding machine with a winding head adjacent to the shell being formed and moving in a longitudinal direction along the shell at different speeds depending on the angle to the longitudinal axis at which the filament is being wound in the different layers.

18. The method of claim 16 , wherein the at least one filament of the inner layer is wound only at the positive first angle and the at least one filament of the outer layer has been wound only at the negative first angle.

19. The method of claim 16 , wherein the first angle is within a range of 82-92°.

20. The method of claim 16 , wherein the second angle is within a range of 55-65°.