Pipelaying

1. An in-line accessory structure for a subsea pipeline, the structure comprising:

a subsea foundation arranged to support the structure in an installed orientation;

a frame attached to the foundation;

an elongate pipe section supported by the frame, the pip section being arranged to be integrated into the subsea pipeline and hence defining a flow axis through the structure; and

at least one branch having a lower end in fluid communication with and facing forward the pipe section and an upper end in fluid communication with and facing forward a connector hub;

wherein, in the installed orientation, the branch inclines inwardly and upwardly in a transverse direction between its lower end and its upper end, toward an upright longitudinal plane containing the flow axis and extending substantially orthogonally with respect to a supporting seabed.

2. The structure of claim 1 , wherein the branch intersects the upright longitudinal plane.

3. The structure of claim 1 , wherein the upright longitudinal plane is substantially vertical when the structure is in the installed orientation.

4. The structure of claim 1 , wherein the pipe section is substantially straight and the flow axis coincides with a central longitudinal axis of the pipe section.

5. The structure of claim 1 , wherein the branch comprises at least one elbow defining:

an upper branch portion that inclines upwardly toward the connector hub and the upright longitudinal plane; and

a lower branch portion that inclines downwardly toward the pipe section and the upright longitudinal plane.

6. The structure of claim 5 , wherein the lower branch portion extends laterally outwardly from the pipe section in the transverse direction.

7. The structure of claim 5 , wherein the elbow is, at least partially, laterally outboard of the connector hub in top plan view.

8. The structure of claim 7 , wherein at least part of the connector hub is laterally inboard of the pipe section in top plan view.

9. The structure of claim 5 , wherein the lower branch portion intersects an upper quadrant of a cross-section of the pipe section.

10. The structure of claim 9 , wherein at least a majority of a circumferential line of intersection between the lower branch portion and the pipe section lies above a central longitudinal axis of the pipe section.

11. The structure of claim 10 , wherein the circumferential line of intersection between the lower branch portion and the pipe section, is laterally outboard of the upright longitudinal plane.

12. The structure of claim 1 , further comprising a valve in the branch between the pipe section and the connector hub.

13. The structure of claim 5 , further comprising a valve disposed in the upper branch portion between the elbow and the connector hub.

14. The structure of claim 1 , wherein the transverse direction is substantially orthogonal to the upright longitudinal plane.

15. The structure of claim 14 , wherein the transverse direction lies in a transverse plane that extends substantially orthogonally with respect to a supporting seabed when the structure is in the installed orientation.

16. The structure of claim 15 , wherein the transverse plane is substantially vertical when the structure is in the installed orientation.

17. The structure of claim 1 , wherein the upright longitudinal plane in the connector hub.

18. The structure of claim 17 , wherein the upright longitudinal plane substantially bisects the connector hub.

19. The structure of claim 1 , wherein a central longitudinal axis of the connector hub is substantially parallel to the upright longitudinal plane.

20. The structure of claim 19 , wherein the central longitudinal axis of the connector hub is substantially vertical in the installed orientation.

21. The structure of claim 1 , wherein the at least one branch comprises first and second branches each having a lower end in fluid communication with the pipe section and an upper end in fluid communication with a respective connector hub, the first and second branches being spaced longitudinally from each other along the pipe section and being disposed on respective mutually-opposed sides of the upright longitudinal plane.

22. The structure of claim 1 , wherein the foundation comprises at least one mudmat that is movable relative to the frame from a compact stowed state into an extended deployed state, in which latter state the mudmat extends in a plane substantially parallel to a supporting seabed when the structure is in the installed orientation.

23. The structure of 22 , wherein the lane of the mudmat in the deployed states, is substantially orthogonal to the upright longitudinal plane.

24. The structure of claim 22 , wherein the mudmat is angularly movable around a pivot axis between the stowed and deployed states.

25. The structure of claim 24 , wherein, in the stowed state, the mudmat is inclined inwardly toward the upright longitudinal plane, moving upwardly from the pivot axis.

26. The structure of claim 25 , wherein the inclination of the stowed mudmat converges with opposite inclination of the branch about the upright longitudinal plane, which branch extends from an opposite side of the pipe section.

27. The structure of claim 26 , wherein the stowed mudmat converges with the opposed branch above the pipe section.

28. The structure of claim 27 , wherein a gap between the stowed mudmat and the opposed branch is narrower than the pipe section.

29. The structure of claim 22 , wherein at least one of the mudmats and at least one of the branches are disposed in longitudinal succession along the pipe section on at least one side of the pipe section.

30. The structure of claim 29 , wherein the at least one of the mudmats and the at least one of the branches are disposed in longitudinal succession along the pipe section on each side of the pipe section, the at least one of the mudmats on opposite sides of the pipe section being offset longitudinally from each other to define longitudinally extending recesses with opposite longitudinal offset that accommodate the at least one of the branches on opposite sides of the pipe section.

31. The structure of claim 22 , wherein at least one of the branches originating from one side of the pip section is opposed by one of the mudmats supported by the frame on the other side of the pipe section.

32. The structure of claim 22 , wherein the or each mudmat is expandable from the stowed state to increase its load-bearing area in the deployed state.

33. The structure of claim 32 , wherein the or each mudmat comprises leaves that are pivotable relative to each other from substantially parallel relation in the stowed state to substantially co-planar relation in the deployed state.

34. A subsea pipeline comprising at least one in-line structure as defined in claim 1 .

35. An in-line accessory structure for a subsea pipeline, the structure comprising:

at least two mudmats that are attached to and movable relative to a frame from a compact stowed state into an extended deployed state;

an elongate pipe section supported by the frame, the pipe section being arranged to be integrated into the subsea pipeline; and

at least two branches in fluid communication with the pipe section; wherein at least one of the mudmats and at least one of the branches are disposed in longitudinal succession along the pipe section on each side of the pipe section, the at least one mudmat on opposite sides of the pipe section being offset longitudinally from each other to define longitudinally-extending recesses with opposite longitudinal offset that accommodate the at least one branch on opposite sides of the pipe section.

36. The structure of claim 35 , wherein the at least one of the branches originating from one side of the pipe section is opposed by the at least one of the mudmats supported by the frame on the other side of the pipe section.

37. A subsea pipeline comprising at least one in-line structure as defined in claim 35 .