Catenary Line Dynamic Motion Suppression

Abstract

Dynamic motion decoupling is effected with the use of mass, added mass, buoyancy, submerged weight and drag in areas of relatively low tension. High curvatures of lines on some configurations, together with their low slope may be utilized. The original line configuration may or may not be modified. Known motion suppressing device designs can be used. Because of the low slope on some configurations, said motion suppressing devices can be installed on arbitrarily long line segments to achieve objections required. Novel, drag and added mass enhancing devices effective in all directions can be used to increase the suppression effectiveness and / or in order to reduce the number of devices used. This invention is suitable for use on new designs and it is also suitable for retrofitting on existing, already installed lines.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims the priority of U.S. Provisional Patent Application Ser. No. 60 / 593,269 filed Jan. 3, 2005 and entitled: “Catenary Line Dynamic Motion Suppression Arrangement” the disclosure of which is hereby incorporated by reference in its entirety.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention relates to lines used to connect undersea equipment to related equipment on or near the surface.[0004]2. Description of the Related Art[0005]Petroleum exploration and production is increasingly being conducted off-shore and at ever deeper locations. Typically, a mobile offshore drilling unit (“drilling rig”) is used to create a well. Once the well is completed, a production platform or a buoy is installed at the site to recover the petroleum products which may subsequently be loaded onto a tanker or pumped via pipelines to on-shore facilities.[0006]Exploration and production platforms take many forms. The ...

AI Technical Summary

Benefits of technology

[0058]Undersea dynamic motion of a line, cable, pipe, riser or the like is modified by the attachment of devices which locally change the buoyancy, the submerged weight, enhance the effective mass and modify drag damping of the line at selected locations. Certain mass-enhancing devices according to the present invention effectively add mass without being particularly massive themselves.

[0063]However, the average effective tensions at the top of the line segments between the distributed buoyancy in FIG. 1 (5) or distributed submerged weight in FIG. 2 (5) in these novel designs are significantly lower than those at the line hang-offs.

Problems solved by technology

Currents are seldom uniform along said lines.

Dynamic effects on lines used in offshore engineering can be very complex.

It is often the case that the effective tension near the touch-down becomes periodically negative, making the line susceptible to local buckling, which usually is not desirable and sometimes it is completely unacceptable (example fiber-optic lines).

Bird-caging of umbilical or cable lines can occur, rigid or flexible pipes usually have some built-in resilience, but complex local increases in fatigue damage typically result.

Often, in presently known designs it is difficult to increase the effective tension and to increase the minimum dynamic bending radii to acceptable levels.

Increasing the horizontal tension in the catenaries, which increases also the quasi-static, average effective tension at the touch-down in many known designs is known to often make the dynamic effects described above even worse.

In particular, the said touch down zone line dynamics is in presently known designs both significant and troublesome for simple, free hanging catenary lines attached to floating structures.

In particular, at present, it might be not possible to use larger diameter single pipe or Pipe-in-Pipe (PIP) SCRs on some fields, where smaller diameter freehanging configurations are at present used.

This is because the selection of a simple (freehanging) catenary configuration would have resulted in very high hang-off loads.

These would have become even higher in a case of an accidental flooding of the line with seawater that might inadvertently happen during installation or in operation.

In such cases using a freehanging catenary might be impossible, because the excessive hang-off load resulting might be too high to handle.

Similarly, there might be no installation vessel available anywhere in the world, to handle such a heavy pipe during its installation; or in particular to handle such a large diameter pipe or PIP, in a case of an accidental flooding with seawater.

However, it is noted that:The use of the configurations in question, as implemented in prior art, results in the increase of the suspended lengths used (and in the corresponding increase in costs of the installation that adds to the cost of the associated ‘additional’ hardware used);The selection of one of these configurations in prior art is because of one of the underlying reasons listed above; in the prior art these line configurations are not selected because of the said added advantage.

This method tends to be only partially effective, because this makes the catenary above the clump weight steeper and it can result in the heave motions being transferred more easily down to the location of the clump weight.

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