Offshore drilling or production vessel with single length mooring line of high strength polyolefin fibers

1. An offshore drilling or production vessel comprising:

a platform; and

a mooring system attached to the platform, wherein the mooring system comprises:

(i) a support frame with a winch-drum mounted thereon;

(ii) a single length mooring line comprising high strength polyolefin fibers for mooring the platform to the ocean floor, the mooring line comprising a first portion which is hauled on and paid off by the winch-drum and a second portion which is anchored to the ocean floor, wherein

the first portion extends continuously into the second portion through a tapered portion of the mooring line; and wherein

the first portion of the mooring line has a first mass (M1) of polyolefin fibers per unit length and a specific strength (S1) and the second portion of the mooring line has a second mass (M2) of polyolefin fibers per unit length and a specific strength (S2), wherein the ratio M1/M2 is greater than 1 and the ratio S1/S2 is between 0.50 and 0.98.

2. The vessel of claim 1 , wherein, said first portion substantially extends at most 1 meter from the winch-drum.

3. The vessel of claim 1 , wherein said second portion substantially extends from at least 100 meters below the waterline.

4. The vessel of claim 1 , wherein the ratio M1/M2 is between 1.3 and 3.0.

5. The vessel of claim 1 , wherein the first portion of the mooring line has a specific strength of at least 1.3 kN/(g/m).

6. The vessel of claim 1 , wherein the second portion of the mooring line has a specific strength of at least 1.5 kN/(g/m).

7. The vessel of claim 1 , wherein the polyolefin fibers are fibers manufactured from homopolymers or copolymers of polypropylene or polyethylene.

8. The vessel of claim 1 , wherein the polyolefin fibers are ultrahigh molecular weight polyethylene (UHMWPE) fibers.

9. The vessel of claim 1 , wherein the polyolefin fibers have a tensile strength of at least 0.5 GPa.

10. The vessel of claim 1 wherein the polyolefin fibers have deniers in the range of from 0.5 to 20.

11. The vessel of claim 1 , wherein the polyolefin fibers are creep-optimized UHMWPE fibers obtained by spinning an UHMWPE comprising olefinic branches (OB) and having an elongational stress (ES), and a ratio

(

O

⁢

⁢

B

/

1000

⁢

⁢

C

E

⁢

⁢

S

)

between the number of olefinic branches per thousand carbon atoms (OB/1000C) and the elongational stress (ES) of at least 0.2, wherein said UHMWPE fibers when subjected to a load of 600 MPa at a temperature of 70° C., have a creep lifetime of at least 90 hours, preferably of at least 100 hours.

12. The vessel of claim 1 , wherein the winch-drum has a width to create helical windings, such that in a coiled state the winch-drum comprises several layers of the first portion of the mooring line, wherein the spacing between the windings of the rope is at least 0.5 times a diameter of the first portion of the mooring line.

13. A single length mooring line comprising high strength polyolefin fibers, wherein

the said single length mooring line includes a first portion and a second portion, the first portion extending continuously into the second portion through a tapered portion of the mooring line; and wherein

the first portion of the mooring line has a first mass (M1) of polyolefin fibers per unit length and a specific strength (S1) and the second portion of the mooring line has a second mass (M2) of polyolefin fibers per unit length and a specific strength (S2), wherein the ratio M1/M2 is greater than 1 and the ratio S1/S2 is between 0.5 and 0.98.