WO1995027101A1

WO1995027101A1 - Vortex induced vibration suppression

Info

Publication number: WO1995027101A1
Application number: PCT/EP1995/001255
Authority: WO
Inventors: Donald Wayne Allen; David Wayne Mcmillan
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 1994-04-04
Filing date: 1995-04-03
Publication date: 1995-10-12
Also published as: NO964172D0; GB2303898A; NO964172L; GB2303898B; NO310370B1; GB9620517D0

Abstract

A method is provided to suppress vortex induced vibrations of a portion of a subsea tubular that is subject to vortex induced vibrations, the method comprising the steps of: providing a plurality of shrouds wherein each shroud is capable of reducing forces caused by shedding of vortexes over a segment of the tubular, wherein each shroud can slide along the tubular and wherein each shroud can be individually secured around the tubular; hingeably connecting the shrouds from end to end; individually securing the shrouds around the subsea tubular at a location on the tubular above the portion of the subsea tubular that is subject to vortex induced vibrations; and allowing the shrouds to slide down the tubular until the shrouds surround the portion of the tubular subject to vortex induced vibrations.

Description

VORTEX INDUCED VIBRATION SUPPRESSION
The invention relates to a method to provide vortex induced vibration suppression for a subsea tubular. Exploration and production of oil and gas from below increasing depths of oceans has given rise to many new challenges. One of these challenges is the control of vortex induced vibrations in long unsupported segments of tubulars. Currents can cause vibrations by alternate shedding of vortexes from the sides of the tubular. These vibrations can be of a great enough magnitude to result in mechanical failure of some tubulars. Long and unbraced segments of tubulars such as pipelines and cable conduits crossing ocean floor valleys, production risers, drilling risers, export risers, mooring cables, and tendons for tension leg platforms are examples of tubulars that can be subject to such vibrations.

Vortex induced vibrations can be suppressed by shrouds, fairings, or strakes protruding from the tubular.

M.M. Zdravkovich, Review and Classification of Various
Aerodynamic and Hydrodynamic Means for Suppressing Vortex Shedding,
Journal of Wind Engineering and Industrial Aerodynamics, 7 (1981) 145-189 discloses an analysis of many different apparatuses to suppress vortex induced vibrations of tubulars in water. Methods to install these apparatuses are not discussed. Installation of such apparatus to suppress vortex induced vibrations is difficult because of the great depths of water under which the tubular may be. A method to install such an apparatus would also desirably be suitable for installation over a tubular that is already in place.

Prediction of the magnitude of vortex induced vibrations is difficult and not precise. A need for an apparatus to suppress vortex induced vibrations may not become apparent until after installation of the tubular.

Although this publication discusses the effectiveness of various shrouds to suppress vortex induced vibrations, installation of the shrouds on existing subsea tubulars remains a problem.

Additionally, construction and placement of the tubular with such a shroud in place would be difficult because the shrouds are generally much more delicate than the tubulars, and handling of the tubulars with the tubulars surrounded by the shrouds, without damagiiig the tubulars, is difficult.

U.S. Patent (Serial No. 08/155,394, filed November 19, 1993) discloses a method to install a device to suppress vortex induced vibrations. This method includes providing a shroud that can be vertically lowered and secured to a subsea pipeline and then lowered onto the pipeline with the shroud optionally being provided in hingably connected segments. The method of this disclosure is applicable to horizonal pipes that extend unsupported over sea floor valleys.

It is an object of the present invention to provide an improved method for suppression of vortex induced vibration of a subsea tubular wherein a shroud to provide such protection is provided.

It is another object of the present invention to provide such a method wherein the shroud can be placed around the tubular after the tubular has been installed.

In accordance with the invention there is provided a method to suppress vortex induced vibrations of a portion of a subsea tubular that is subject to vortex induced vibrations, the method comprising the steps of:
providing a plurality of shrouds wherein each shroud is capable of reducing forces caused by shedding of vortexes over a segment of the tubular, wherein each shroud can slide along the tubular and wherein each shroud can be individually secured around the tubular;
hingably connecting the shrouds from end to end;
individually securing the shrouds around the subsea tubular at a location on the tubular above the portion of the subsea tubular that is subject to vortex induced vibrations; and
allowing the shrouds to slide down the tubular until the shrouds surround the portion of the tubular subject to vortex induced vibrations.

With the method of the invention, a shroud is provided wherein the shroud is secured around the tubular at a convenient location, and slid down the tubular to a location where suppression of vortex induced vibration is desired. Because the shroud can be provided after the tubular is fully installed; the construction of the tubular is not hampered by the presence of the shroud.

The invention will now be described in more detail and by way of example with reference to the drawings, in which
Figs. 1A, 2A, and 3A show profile views of shrouds that can be installed in the method of the present invention;
Figs. 1B, 2B, and 3B show end-views of shrouds that can be installed in the method of the present invention;
Figs. 4A and 4B show details of a hinge mechanism to connect the shrouds of the present invention;
Figs. 5A and 5B show details of an alternative hinge mechanism to connect the shrouds of the present invention.

Referring now to Figs. 1A and 1B, a profile view and an end view, respectively, are shown of a shroud that can be installed to suppress vibrations of subsea tubulars caused by vortex shedding in the method of the present invention. A shroud, 101, is shown as a short fairing shroud having a tail section, 102, comprising a straight fin, 103, and a flow smoothing section, 104. The short fin of the present invention is effective to significantly reduce vibrations do to vortex shedding with total lengths of about 1.25 to 1.5 times the outside diameter of the tubular. Longer fairings are even more effective in reduction of vibrations, but are not preferred because the longer fairings cause increased vibrations and drag forces when the currents are not within about 30 degrees of the centerline of the fairing.

Alternatively, the longer fairings could be mounted so they swivel around the tubular, but this is not preferred because a mechanism that allows such swiveling motion will be relatively expensive to provide and would be likely to eventually fail to swivel when exposed to the subsea environment.

The preferred short fairings can be locked into place so long as the prevailing currents are within about 60 degrees of the centerline of the short fairings because the short fairings do not cause the vibrations to significantly exceed the vibrations of the bare pipe at any relative current angle, and do not significantly increase drag compared to the bare pipe at any relative current angle.

Hinges, 1"5, are provided to permit straps, 106, to be closed around the tubular and result in the shroud being secured around the tubular. Locking pin brackets, 107, and locking pins, 108, are provided to lock the straps around a tubular, 112. The straps may be manually closed by divers, or above the surface of the sea if the tubular is accessible above the surface of the sea. Locking pins may be manually installed after the straps are closed.

Flexible centralizers, 109, may be provided to prevent vibrations of the shroud against the tubular. The flexible centralizers must not create so much friction that the shroud does not readily slide down the tubular to allow placement of the shroud where it is needed on the tubular.

The shroud must be provided with a mechanism to hingably connect the shroud to subsequent similar shrouds. A eye, 110, is shown on each end of the shroud, supported by a brackets, 111, to accomplish this. The eyes of adjacent shrouds can be connected by a locking chain link.

The volume inside the smoothing section may be partially or fully filled with a foam material, 112, to provide support and buoyancy. The foam material may be, for example, a closed cell polyurethane. The foam preferably provides enough buoyancy to reduce the weight to be supported by the chain of shrouds, but not so much that the shrouds do not readily slide down the tubular.

Referring now to Figs. 2A and 2B, another embodiment of the shroud that can be utilized in the practice of the present invention. The shroud of these Figs. is a helical strake shroud comprising four strakes, 201, fixed to two hinged half shells, 202.

The half shells are connected by hinges, 203, so that they can be opened and closed around a tubular, 206, shown as an Avon clad pipe.

Locking pins, 204, and locking pin brackets, 205, provide for a means to secure the shroud around the tubular. The means to secure the shroud around the tubular could be other means known in the art, such as, for example, a J-hook or a latch. Three strakes are shown in Figs. 2A and 2B, but two or more could be effective.

Referring now to Figs. 3A and 3B, another embodiment of the present invention is shown. This embodiment includes a perforaced shroud, 301, supported on rings, 302, to hold the shrouds displaced from the tubular. The perforated shroud is provided in two halves, connected by hinges, 303, and hinge pins, 304. Locking pins, 305, and locking pin brackets, 306, provide a means to secure the shroud around the tubular, 308. Perforations of the shroud are preferably essentially rectangular with a width and a height of about 0.05 to about 0.35 diameters, based on the tubular outside diameter. The shroud is preferably supported about 0.03 to about 0.12 diameters from the outside surface of the tubular. The shroud preferably has a porosity of between about thirty and about eighty percent, where the porosity is defined as the open area in relation to the total shroud surface area plus open area.

Referring now to Figs. 4A and 4B, a front view and a profile view respectively are shown of a hingable connection between adjacent shrouds. Shroud half shells, 401 and 402 of an upper shroud are connected to shroud half shells, 403 and 404 of a lower shroud by wire rope links, 405 and 406. The wire rope links could alternatively be, for example, chain links. The ends of the wire rope links are connected to the shrouds by fittings, 407, with the wire rope ends shown as anchored by a set screw or bolt, 408.

Referring now to Figs. 5A and 5B, a front view and a profile view respectively are shown of an alternative hangable connection between adjacent shrouds. Shroud half shells, 501 and 502 of an upper shroud are connected to shroud half shells, 503 and 504 of a lower shroud by wire rope links, 505 and 506. The ends of the wire rope links are, in this embodiment, connected to the shrouds by swage fittings or thimbles to fittings, 507. The fittings are connected to lugs, 509. The lugs, 509, may be bolted or welded to the half shells.

There are many types of shrouds known to be useful for the purpose of suppressing vibrations caused by vortex shedding, and each of them could be useful in the practice of the present invention. The amount of the tubular that is covered by shrouds can be from as little as ten percent and as much as 100 percent of the portion of the tubular that is affected by vortex induced vibrations. When less than the total portion is covered, the portions covered preferably include the antinodes of the vibration resonant mode shape. Gaps between segments of the tubular covered by shrouds may be provided, for example, by suspending lower shrouds by cables from the upper shrouds.

The shrouds of the present invention may be of any length that is convenient for handling and fabrication, but lengths of between about four and about twelve feet are preferred because they can be handled manually, and are sufficiently long that installation will not require an excessive amount of time.

The shrouds of the present invention may be hingably connected to adjacent shrouds on the deck of a platform or vessel, and suspended to a point where the shrouds may be conveniently connected to the tubular. If the tubular extends to above the surface of water, such as some production risers, the shrouds may be connected to the tubulars above the surface of the water. If connection of the shrouds to the tubular must be done below the surface of the water, the shrouds can be installed by divers at a location near the water surface, and preferably no more than 100 feet below the surface of the water.

The last shroud may be suspended from an easily accessible fixed point such as a oil or gas production platform, or may be fixed directly to the tubular. Subsequent lower shrouds remain suspended from the upper shroud by the hingable connections between the shroud segments.

Claims

1. A method to suppress vortex induced vibrations of a portion of a subsea tubular that is'subject to vortex induced vibrations, the method comprising the steps of: providing a plurality of shrouds wherein each shroud is capable of reducing forces caused by shedding of vortexes over a segment of the tubular, wherein each shroud can slide along the tubular and wherein each shroud can be individually secured around the tubular; hingably connecting the shrouds from end to end; individually securing the shrouds around the subsea tubular at a location on the tubular above the portion of the subsea tubular that is subject to vortex induced vibrations; and allowing the shrouds to slide down the tubular until the shrouds surround the portion of the tubular subject to vortex induced vibrations.

2. The method of claim 1 wherein the shrouds are individually secured around the tubular at a location above a point that is 100 feet below the surface of sea.

3. The method of claim 1 wherein the shroud is a fairing shroud.

4. The method of claim 3 wherein the fairing shroud has a total length of between about 1.25 and about 1.5 of the outside diameter of the tubular.

5. The method of claim 1 wherein the shroud comprises helical strakes.

6. The method of claim 1 wherein the shroud is a perforated shroud.

7. The method of claim 2 wherein the shrouds are secured around the tubular by divers below the surface of the sea.

8. The method of claim 7 wherein the shrouds are hingably connected above the surface of the sea.