VORTEX INDUCED VIBRATION SUPPRESSION SYSTEMS AND METHODS

Field of the Invention

This invention is related to vortex induced vibration suppression devices that can be attached to offshore structures to reduce drag and/or vortex induced vibration (VIV).

Background of the Invention

Whenever a bluff body in a fluid environment, such as a cylinder, is subjected to a current in the fluid, it is possible for the body to experience vortex-induced vibrations (VIV). These vibrations may be caused by oscillating hydrodynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency.

Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV. Equipment exposed to VIV may include structures ranging from the smaller tubes of a riser system, anchoring tendons, or lateral pipelines to the larger underwater cylinders of the hull of a minispar or spar floating production system (a "spar"). Risers as used herein are defined to be a non-exclusive example of a marine element subject to VIV. Generally a riser system is used for establishing fluid communication between the surface and the bottom of a water body. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore. A typical riser system may include one or more fluid-conducting conduits that extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, there may be provided auxiliary conduits such as, for example, choke and kill lines, pressurized fluid lines, hard pipes, and electrical lines, which extend relatively parallel to the main conduit. These auxiliary conduits and lines are commonly referred to as umbilical elements and/or umbilicals.

There are generally two kinds of water current induced stresses to which elements of a riser system may be exposed. The first kind of stress as mentioned above is caused by vortex-induced alternating forces that vibrate the underwater structure in a direction perpendicular to the direction of the current. These are referred to as vortex-induced vibrations (VIV). When water flows past the structure, vortices are alternately shed from each side of the structure. This produces a fluctuating force on the structure transverse to the current. These vibrations can, depending on the stiffness and the strength of the structure and any welds, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions have been known to cause structures such as risers to break apart and fall to the ocean floor. The second type of stress is caused by drag forces which push the structure in the direction of the current due to the structure's resistance to fluid flow. The drag forces may be amplified by vortex induced vibrations of the structure. For instance, a riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more so than a stationary riser. This results in greater energy transfer from the current to the riser, and hence more drag.

Many methods have been developed to reduce vibrations of sub sea structures. Some of these methods to reduce vibrations caused by vortex shedding from subsea structures operate by stabilization of the wake. These methods include streamlined fairings, wake splitters and flags. Streamlined or teardrop shaped, fairings that swivel around a structure have been developed that almost eliminate the shedding or vortexes. Other conventional methods to reduce vibrations caused by vortex shedding from sub sea structures operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure. Examples of such methods include the use of helical strakes around a structure, or axial rod shrouds and perforated shrouds.

Copending U.S. Patent Application 60/805,136, filed June 19, 2006, having attorney docket number TH 1500, later published as WO 2007/149770, discloses a system for producing oil and/or gas, including a subsea structure defining an interior of the system, the structure subject to a water current; a covering exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure; a vortex induced vibration suppression device exterior to the covering; the covering including a first mechanism and the vortex induced vibration suppression device including a second mechanism, wherein the first mechanism and the second

mechanism are adapted to maintain the vortex induced vibration suppression device in a location along a length of the subsea structure. U.S. Patent Application 60/805,136 is herein incorporated by reference in its entirety.

Subsea structures often have umbilical lines, pipes, electrical lines, cables, ropes, lift lines, and/or other smaller lines that run along the length of the structure. When the structure is initially installed these lines are sometimes fitted into insulation or buoyancy modules. However, when other lines are desired and are to be added, for example to run a line on the outside of the structure or after the structure has been installed, the lines need to held outside of the insulation or buoyancy modules. These additional lines then may interfere with the VIV and/or drag suppression devices that are installed exterior to the insulation or buoyancy modules.

There is a need in the art for improved apparatus and methods for suppressing VIV.

There is a need in the art for apparatus and methods for suppressing VIV that do not suffer from the disadvantages of the prior art.

There is a need in the art for apparatus and methods for providing VIV suppression to a subsea structure comprising additionally installed lines and/or for providing protection to the additionally installed lines.

There is a need for systems and methods of installing VIV suppression devices to a subsea structure with additionally installed lines.

These and other needs will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.

Summary of the Invention In one aspect, the invention provides a system including a subsea structure defining an interior of the system, the structure subject to a water current; a covering exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure, the covering selected from insulation and buoyancy modules; at least one additional line exterior to the covering; a collar exterior to the covering adapted to maintain the at least one line adjacent to the covering; and a vortex induced vibration suppression device exterior to the at least one line, the collar, and the covering.

In another aspect, the invention provides a method of reducing drag and/or vortex induced vibration of a subsea structure, including installing the subsea structure in a body of water, wherein the subsea structure is subject to one or more water currents; installing a covering exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure; installing at least one line exterior to the covering; installing a collar exterior to the covering, the collar maintaining the at least one line adjacent to the covering; and installing a vortex induced vibration suppression device exterior to the covering, the at least one line, and the collar. Advantages of the invention may include one or more of the following: improved apparatus and methods for suppressing VIV; apparatus and methods for suppressing VIV that do not suffer from the disadvantages of the prior art; apparatus and methods for providing VIV suppression to a subsea structure comprising additionally installed lines and/or for providing protection to the additionally installed lines; and/or systems and methods of installing VIV suppression devices to a subsea structure with additionally installed lines.

Brief Description of the Figures

Figure 1 illustrates a subsea structure system.

Figure 2a illustrates a subsea structure system.

Figure 2b illustrates a side view of a riser with foam.

Figure 2c illustrates a cross-sectional view of a riser with foam. Figure 2d illustrates a side view of a riser with foam.

Figure 2e illustrates a cross-sectional view of a riser with foam.

Figure 2f illustrates a cross-sectional view of a riser with foam and a fairing.

Figure 3a illustrates a cross-sectional view of a riser with foam.

Figure 3b illustrates a cross-sectional view of a riser with foam and a fairing.

Detailed Description

Referring now to Figure 1 there is illustrated offshore system 100. System 100 includes surface structure 102 near the water surface, which is connected to riser 104, which riser 104 is connected to subsurface structure 106, which is

adjacent to seafloor 108. The water has current 1 10, which may cause vortex- induced vibration of riser 104. To counter VIV, fairings 1 14 may be installed along the length of riser 104. Collars 1 12 are used to keep fairings from moving along the length of riser 104. Referring now to Figure 2a, system 200 is illustrated. System 200 includes surface structure 202 near the surface of the water, which is connected to riser 204. Riser 204 is also connected to subsurface structure 203 near the seafloor 208. Exterior to riser 204 is foam 206, which may serve to insulate and/or provide buoyancy to riser 204. Current 210 is in the water, which may cause VIV on riser 204 and foam 206.

Foam 206 may contain one or more lines between surface structure 202 and subsurface structure 203. Additional lines 212 have been added after the installation of foam 206, so they are exterior to foam 206. Fairing holders 216 have been installed to hold additional lines 212 adjacent to foam 206. Fairings 214 are installed exterior to fairing holders 216. Fairing holders 216 are shown inside fairings 214, and serve to keep fairings 214 at a desired location along subsurface structure 203 and/or allow fairings 214 to rotate about fairing holders 216 without interfering with or being tangled with additional lines 212. One or more fairing holders 216 may be used for each fairing 214. Referring now to Figure 2b, riser 204 is shown with foam 206 exterior to the riser, one or more additional lines 212 have been run along the length of riser 204 exterior to foam 206. Fairing holders 216 have been installed to hold additional lines 212 adjacent to foam 206.

Referring now to Figure 2c, there is a cross-sectional view of riser 204 and foam 206. Riser 204 is in the interior. Exterior to riser 204 is foam 206, which is illustrated as two sections, 206a and 206b, which maybe connected around riser 204. Internal pockets 222 house lines 220, for example hydraulic lines, electrical lines, choke and kill lines, or other lines as are needed. Split-half pockets 226, defined between sections 206a and 206b, also house lines 220, and external pockets 224 also house lines 220.

Referring now to Figure 2d, riser 204 and foam 206 are illustrated with grooves 21 1 spaced along the length of riser 204. Fairing holders 216 have been placed in grooves 21 1 . Grooves 21 1 serve to maintain fairing holders 216 at a desired location along the length of riser 204 and foam 206.

Referring now to Figure 2e, a cross-sectional view of riser 204 and foam 206 is shown. Fairing holder 216 may have portion 216a with openings 216b and 216c, adapted to receive additional lines 212a and 212b, respectively. Fairing holder 216 may be fixed in place so that additional lines 212a and 212b can be held in place adjacent foam 206 by fairing holder 216.

Referring now to Figure 2f, fairing 214 has been attached exterior to fairing holder 216. To attach fairing 214 about foam 206 and fairing holder 216, fairing 214 may be held open and pulled around foam 206 and fairing holder 216. Fairing is then closed around foam 206 and fairing holder 216. When fairing halves are aligned, screws 232 may be attached to insert 230 to hold the two halves of fairing 214 together.

Referring now to Figure 3a, a cross-sectional view of riser 304 and foam 306 is shown, with grooves 312, and with various lines 320 provided in internal pockets 322, external pockets 324, and split-half pockets 326. Fairing holder 316 is attached exterior to riser 304 and foam 306. Fairing holder 316 may have a portion with openings 316a, 316b, and 316c, adapted to receive additional lines 312a, 312b, and 312c, respectively. Fairing holder 316 may be fixed in place so that additional lines 312a, 312b, and 312c can be held in place adjacent foam 306 by fairing holder 316. Fairing holder 316 may have a substantially constant thickness about riser 304 and foam 306 to allow a fairing to rotate about fairing holder 316.

Referring now to Figure 3b, fairing 314 has been attached exterior to fairing holder 316. To attach fairing 314 about foam 306 and fairing holder 316, fairing 314 may be held open and pulled around foam 306 and fairing holder 316. Fairing is then closed around foam 306 and fairing holder 316. When fairing halves are aligned, screws 332 may be attached to insert 330 to hold the two halves of fairing 314 together.

Fairing holder 316 may also have axial protrusions (not shown) extending outwardly from its outer surface. Such protrusions may serve to keep a space between fairing holder 316 and fairing 314, and facilitate the weather vaning of fairing 314 about fairing holder 316. In addition, such protrusions may serve to keep a space between fairing holder 316 and lines 312a-312c and/or to prevent damage to the lines from the weather vaning of fairing 314 about fairing holder 316

Fairings may be replaced with strakes, shrouds, wake splitters, tail fairings, buoyancy modules, or other devices as are known in the art. Suitable sleeves,

suitable collars, and suitable devices to install exterior to structures, and methods of their installation are disclosed in U.S. Patent Application Number 10/839,781 , having attorney docket number TH1433; U.S. Patent Application Number 1 1 /400,365, having attorney docket number TH0541 ; U.S. Patent Application Number 1 1/419,964, having attorney docket number TH2508; U.S. Patent Application Number 1 1/420,838, having attorney docket number TH2876; U.S. Patent Application Number 60/781 ,846 having attorney docket number TH2969; U.S. Patent Application Number 60/805,136, having attorney docket number TH1500; U.S. Patent Application Number 60/866,968, having attorney docket number TH31 12; U.S. Patent Application Number 60/866,972, having attorney docket number TH3190; U.S. Patent Number 5,410,979; U.S. Patent Number 5,410,979; U.S. Patent Number 5,421 ,413; U.S. Patent Number 6,179,524; U.S. Patent Number 6,223,672; U.S. Patent Number 6,561 ,734; U.S. Patent Number 6,565,287; U.S. Patent Number 6,571 ,878; U.S. Patent Number 6,685,394; U.S. Patent Number 6,702,026; U.S. Patent Number 7,017,666; and U.S. Patent Number 7,070,361 , which are herein incorporated by reference in their entirety.

Suitable methods for installing fairings, collars, and other devices to install exterior to structures, are disclosed in U.S. Patent Application Number 10/784,536, having attorney docket number TH1853.04; U.S. Patent Application Number 10/848,547, having attorney docket number TH2463; U.S. Patent Application Number 1 1/596,437, having attorney docket number TH2900; U.S. Patent Application Number 1 1/468,690, having attorney docket number TH2926; U.S. Patent Application Number 1 1/612,203, having attorney docket number TH2875; U.S. Patent Application Number 60/806,882, having attorney docket number TH2879; U.S. Patent Application Number 60/826,553, having attorney docket number TH2842; U.S. Patent Number 6,695,539; U.S. Patent Number 6,928,709; and U.S. Patent Number 6,994,492; which are herein incorporated by reference in their entirety.

The collars and/or fairings may be installed on the connector member before or after the connector member is placed in a body of water.

The collars, fairings and/or other devices exterior to the structure may have a clamshell configuration, and may be hinged with a closing mechanism opposite the hinge, for example a mechanism that can be operated with an ROV.

The collar may be a copper ring, or have a copper ring that is part of its structure.

Fairings may be provided with copper plates on their ends to allow them to weathervane with adjacent fairings or collars. Fairings may be partially manufactured from copper.

A biodegradable spacer may be placed between adjacent fairings to keep them from binding and allow them to weathervane after the spacer has degraded.

Illustrative Embodiments In one embodiment, there is disclosed a system including a subsea structure defining an interior of the system, the structure subject to a water current; a covering exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure, the covering selected from insulation and buoyancy modules; at least one additional line exterior to the covering; a collar exterior to the covering adapted to maintain the at least one line adjacent to the covering; and a vortex induced vibration suppression device exterior to the at least one line, the collar, and the covering. In some embodiments, the subsea structure is selected from an umbilical, a riser, and a tendon. In some embodiments, the covering comprises foam adapted to provide buoyancy and/or thermal insulation to the subsea structure. In some embodiments, the vortex induced vibration suppression device comprises a fairing or a helical strake. In some embodiments, the system also includes at least one old line exterior to the subsea structure, the at least one old line within the covering.

In one embodiment, there is disclosed a method of reducing drag and/or vortex induced vibration of a subsea structure, including installing the subsea structure in a body of water, wherein the subsea structure is subject to one or more water currents; installing a covering exterior to the subsea structure, covering at least a portion of an outside surface of the subsea structure; installing at least one line exterior to the covering; installing a collar exterior to the covering, the collar maintaining the at least one line adjacent to the covering; and installing a vortex induced vibration suppression device exterior to the covering, the at least one line, and the collar. In some embodiments, the installing the collar comprises installing at least two collars per vortex induced vibration suppression device. In some embodiments, the vortex induced vibration suppression device comprises a fairing.

In some embodiments, the vortex induced vibration suppression device is adapted to rotate about the subsea structure in response to the one or more water currents. In some embodiments, the collar is fixed in relation to the subsea structure, and maintains the vortex induced vibration suppression device at a desired location along a length of the subsea structure

Those of skill in the art will appreciate that many modifications and variations are possible in terms of the disclosed embodiments, configurations, materials and methods without departing from their spirit and scope. Accordingly, the scope of the claims appended hereafter and their functional equivalents should not be limited by particular embodiments described and illustrated herein, as these are merely exemplary in nature.