received by the International Bureau on 25 June 2019 (25.06.2019)

1. An offshore production system, comprising:

a surface vessel; and

a closed-loop cooling fluid circulation system extending subsea from the surface vessel and configured to cool a cooling fluid, wherein the closed-loop cooling fluid circulation system comprises:

a first tendon having an upper end coupled to the surface vessel and a lower end coupled to the seabed, wherein the first tendon is in tension between the upper end and the lower end; and a plurality of circumferentially-spaced conduits disposed about the first tendon, wherein each conduit has an upper end coupled to the surface vessel and a lower end coupled to the seabed, wherein the lower end of each conduit is in fluid communication with the lower end of the first tendon;

wherein the first tendon is configured to flow the cooling fluid from the lower end of the first tendon to the upper end of the first tendon;

wherein each of the conduits is configured to flow the cooling fluid from the upper end of the conduit to the lower end of the conduit.

2. The offshore production system of claim 1 , wherein the closed-loop cooling fluid circulation system further comprises:

a second tendon having an upper end coupled to the surface vessel and a lower end coupled to the seabed, wherein the second tendon is in tension between the upper end of the second tendon and the lower end of the second tendon;

wherein the lower end of the second tendon is in fluid communication with the lower end of the first tendon; and

wherein the second tendon is configured to flow the cooling fluid from the upper end of the second tendon to the lower end of the second tendon. 26

3. The offshore production system of claim 2, wherein the first tendon is coupled to the second tendon with a plurality of axially spaced hubs.

4. The offshore production system of claim 2, wherein the lower end of the second tendon is coupled to the lower end of the first tendon with a jumper.

5. The offshore production system of claim 4, wherein the jumper has a first end coupled to the lower end of the first tendon and a second end coupled to the lower end of the second tendon, wherein the jumper comprises a plurality of loops between the first end of the jumper and the second end of the jumper.

6. The offshore production system of claim 3, wherein the lower end of the first tendon and the lower end of the second tendon are both coupled to the seabed with a lower connection system comprising a terminal block, a flexible joint, and an anchor;

wherein the lower end of each tendon is coupled to the terminal block, wherein the flexible joint extends from the terminal block to the anchor, and wherein the terminal block includes a passage in fluid communication with the lower end of the first tendon and the lower end of the second tendon.

7. The offshore production system of claim 1 , wherein each conduit is coupled to the tendon with a plurality of axially spaced hubs.

8. The offshore production system of claim 1 , wherein the first tendon has an inner diameter that is larger than an inner diameter of each of the conduits.

9. The offshore production system of claim 1 , wherein the plurality of conduits are uniformly circumferentially-spaced about the first tendon.

10. The offshore production system of claim 1 , wherein the first tendon is tensioned with a top tensioner. 27

1 1. The offshore production system of claim 1 , wherein the first tendon comprises a plurality of axially spaced annular fins or a helical fin.

12. The offshore production system of claim 1 , wherein each conduit comprises a plurality of axially spaced annular fins or a helical fin.

13. A system for circulating and cooling a cooling fluid for receiving thermal energy from a process fluid, the system comprising:

a tendon extending subsea from an offshore vessel, the tendon having an upper end coupled to the offshore vessel and a lower end coupled to the seabed; a plurality of circumferentially-spaced conduits disposed about the tendon, wherein each conduit extends subsea from the offshore vessel, and wherein each conduit has an upper end coupled to the offshore vessel and a lower end coupled to the seabed, wherein the lower end of each conduit is in fluid communication with the lower end of the tendon; and

a plurality of vertically spaced hubs coupling the conduits to the tendon, wherein the hubs are configured to maintain a radial spacing between the tendon and the conduits;

wherein the conduits are configured to flow the cooling fluid from the offshore vessel and the tendon is configured to flow the cooling fluid to the offshore vessel.

14. The system of claim 13, further comprising a heat exchanger disposed on the offshore vessel, wherein the heat exchanger has a first inlet in fluid communication with the upper end of the tendon, a first outlet in fluid communication with the upper ends of the conduits, and a first passage extending from the first inlet to the first outlet.

15. The system of claim 13, further comprising a top tensioner coupling the upper end of the tendon to the offshore vessel. 28

16. The system of claim 13, further comprising a plurality of axially spaced annular fins or a helical fin disposed along the outer surface of each conduit.

17. A method for cooling a process fluid on an offshore vessel, the method comprising:

(a) flowing a cooling fluid from a heat exchanger on the offshore vessel down a plurality of conduits extending subsea from the vessel, wherein each conduit has an upper end coupled to the offshore vessel and a lower end proximal the seabed;

(b) flowing the cooling fluid from the lower ends of the conduits into a lower end of a tendon, wherein the tendon has a lower end proximal the seabed and an upper end coupled to the offshore vessel;

(c) applying tension to the tendon during (a) and (b); and

(d) transferring thermal energy from the cooling fluid to the surrounding water during (a) and (b).

18. The method of claim 17, wherein the plurality of conduits are circumferentially- spaced about the tendon.

19. The method of claim 17, further comprising:

(e) flowing a process fluid through a first passage in a heat exchanger on the offshore vessel;

(f) flowing the cooling fluid from the upper end of the tendon through a second passage in the heat exchanger;

(g) transferring thermal energy from the process fluid to the cooling fluid during (e) and (f); and

(h) flowing the cooling fluid from the heat exchanger into the upper ends of the conduits.

20. The method of claim 17, further comprising increasing the transfer of thermal energy from the conduits to the surrounding water during (d) with a plurality of axially spaced fins or a helical fin disposed along an outer surface of each conduit. 29

The method of claim 17, further comprising:

(i) coupling the conduits to the tendon with a plurality of vertically spaced hubs; and

(j) maintaining the spacing of the conduits and the tendon with the hubs.