VIBRATION REDUCER FOR REDUCING FLOW INDUCED VIBRATIONS IN A PIPE AND METHOD OF REDUCING SUCH VIBRATIONS

Field of the invention

The present invention relates to damping or hindrance of flow-induced vibrations in coiled tubing and other pipes. More specifically the invention relates to a vibration damper in the form of a band arranged as a spiral spring having a weak spring force, which spring can be extended so as to encompass a substantial part of the pipe exposed for vibration-inducing flows.

Background of the invention and prior art

When water is flowing by a structure of circular cross-section (a pipe, a cable or another elongated element), flow vortices are formed rearward of the structure and accumulated alternately on each side of the structure. The flow vortices affect the structure which thereby is forced crosswise from side to side, back and forth, which results in periodical movements. This is a well-known phenomenon for structures in open waters and is referred to as flow-induced vibrations or vortex-induced vibrations (VIV).

Usually all open waters will have some current representing a flow against a stationary object. Waves will also contribute near the surface. Flow vortices will apply a vibration load to the deployed body and can, inter alia, result in structural damage because of fatigue. This is well known within design of drilling and production risers used in offshore subsea completion. Slender, thin-walled, continuous, circular cross- section elements submerged orthogonally to a flow will in particular be exposed to the phenomenon of flow-induced vibrations.

A coiled tubing as a free-standing riser in open waters will also be exposed to flow-induced vibrations similarly to rigid risers. At present several companies are exploring subsea utilization of coiled tubing. The effect of flow-induced vibrations is by many considered the main element of uncertainty against the feasibility of such utilization.

Many disadvantages are inherent with the prior art devices for vibration damping. The prior art devices require that units be installed/uninstalled as sections on risers, while they are submerged/lifted, which results in a large requirement of labor. Manual work offshore should be limited out of consideration to health, environment and safety. Further such labor results in substantially prolonged operation time spent for bringing risers up or down, which can cause a considerable economical loss, because of longer stop of production and extended period of operation for expensive

means. The equipment that must be used according to the prior art requires provision of substantial storage on-board the rig or vessel, which in some cases is not available. If there is insufficient space for storage on-board the vessel/ship, expensive additional means must be provided for installation of a riser and similarly for taking up the riser. Further, when using the prior art devices due consideration must be taken to buoyancy bodies that may be installed on the riser, to prevent destruction thereof or reducing their effect.

Over time the devices used to reduce flow-induced vibrations will be overgrown by algae, scale, etc. which will diminish the positive effect. With the prior art devices this results in that the riser must be taken up in order to handle the units to be checked/cleaned, which results in stop of production and thereby economical losses.

Patent Publication US 5 421 413 contains a comprehensive description of a general method to reduce flow- induced vibrations, and a more specific description of a device and a method that simplifies installation of the vibration -damping device using a flexible "fairing" or shroud surrounding the riser, coiled tubing, cable, etc. As explained in said publication, in column 3, lines 3-50, a plurality of shrouds or fairings or a continuous shroud is brought over the riser or similar and is fastened on at least one location to the riser preventing the shroud or fearing to float up or down. It would be more preferable with a device without requirement for fastening to the riser or similar.

I patent publications GB 2 335 248 A and US 6 347 911 Bl and patent application WO 2004/020777 Al, devices are described to reduce flow-induced vibrations, and more particularly, cladding segments or strakes with spiral-formed fins that will protect the inner pipe and break the flow vortices. The strake segments are shaped in different ways to save space at storage, as quite many of them are required on a riser, nevertheless they are relatively space-demanding. Common for all of them is that extensive labor is required to install/de-install said segments on a riser and that they will delay the lifting/submerging operations for a riser to a substantial extent. A demand exists for equipment that may reduce some of the disadvantages with equipment and technology mentioned above, in particular equipment that is less space-demanding, simpler in operation and do not affect the time for production in a similar way.

Summary of the invention It is an objective of the present invention to provide a vibration reducer that meets the above-mentioned demand.

The objective is met with a vibration reducer according to the invention, which reduces or removes flow-induced vibrations for a pipe, particularly a coiled tubing, distinguished in that the vibration reducer comprises at least one continuous band

arranged as a spiral spring that elastically can be extended out in order to surround a substantial length of the pipe exposed for vibration-inducing water flows.

That the band is continuous means that the band is continuous in one piece and thereby constitutes one unit. A substantial part of the length of the pipe means a sufficient part of the length of the pipe to reduce vibrations to an acceptable level. It is thereby not obligatory to surround the whole length of pipe submerged into water.

The invention also provides a method to reduce flow-induced vibrations for a pipe, in particular a coiled tubing, by arranging a body around the pipe in axial direction to prevent flow vortices from being generated around the coiled tubing, distinguished by fastening a vibration reducer comprising at least one continuous band at an upper end of the pipe, and in one operation to extend the vibration reducer elastically likewise as a spiral spring to surround a substantial part of the length of the pipe that is exposed to vibration-inducing water flows.

Drawings

The invention will in the following be described in further detail with some embodiment examples and with reference to the drawings, of which

Figure IA is a sketch where the vibration damper is contracted at a vessel/a rig.

Figure IB is a sketch illustrating the vibration damper extended between the vessel/rig and down to a Christmas tree at the seabed,

Figure 2 illustrates the band as extended, and

Figure 3 illustrates several cross-section shapes of the band.

Detailed description The figures IA and IB illustrate a rig 3 or a vessel or another feasible device floating on the surface. This vessel or rig can have/has an opening 4 in the hull to bring a coiled tubing/riser 5 down into the sea and further down to a subsea Christmas tree 6 for connection and optional feeding further into the well bore. The coiled tubing can have an oval or circular cross-section shape. On deck or at the lower part of the opening in the hull, the upper part of the vibration reducer 1 will be fastened by a fastening device 2. The fastening device 2 must carry both the vibration reducer 1 and any optional weight as required to pull the vibration reducer 1 down to the Christmas tree at the seabed.

The vibration reducer 1 will by virtue of its own structural capacity or by use of external means (such as a wire, rope, hose or similar, in the following called a "line") be pulled back to the vessel or rig 3 when it is not in operation or requires cleaning. Further, the vibration reducer 1 will not guide or affect the strength of the interior pipe 5 to any substantial extent. As the vibration reducer 1 and the internal pipe 5 are not fixedly connected to each other, the vibration reducer and the riser can be

lowered or taken up independently of each other, but most conveniently the units are brought down or taken up simultaneously. The vibration reducer 1 is intended to cover the whole riser length exposed for flow-induced vibrations, or a sufficient part thereof to achieve an acceptable low level of vibrations. The vibration reducer will ensure that the conditions for flow-induced loads no longer will be present as the flow conditions of the fluid in the vicinity of the riser is "disturbed" by the vibration reducer.

In connection with the fastening devices 2, there is a deployment/retrieval equipment consisting of at least one winch with at least one line 7. A line can be a wire, rope, hose or similar. The line is fastened to the lower part of the vibration reducer in a fastening means, in order to be able to controllably retrieve or deploy the vibration reducer. The line can, for giving the best possible "stability" for the vibration reducer, be extended through the vibration reducer material in axial direction. The number of through-going lines can be one or several, preferably at least two, diametrically opposed to provide balance. If the vibration reducer comprises several bands, preferably as many through-going lines can be provided as there is bands, optionally multiples thereof. This means that if 2 bands of 180 degrees displacement is used, a number of 2, 4, 6, ... of lines is used, for 3 bands, 3, 6, 9... lines are used.

The vibration reducer 1 will have a shape similar as a flat-coiled spring with internal open diameter, but with only a small spring force to ensure that the range of utilization (operational depth/height) not results in any plastic material flow/deformation of the vibration reducer. Tests have revealed that a vibration reducer functions optimally if there are 3 parallel bands extending with 120 degrees displacement down the pipe. The most preferable lead for the extended spiral or helical will be from 5D to 17D, dependent on utilization, for which D is the diameter of the coiled tubing, but this can be deviated from. Typically the vibration reducer can be made of a band having a cross-section as approximately elliptical/rectangular. The height can generally be in the range 1-10 mm, but for large depths with large bands, or strong flows, the thickness can preferably be increased. The height will preferably be substantially smaller than the width to give the best technical effect, however, it is possible to use a round band, but the vibration reduction and the height as constricted will thereby not be optimal. The band can as such have any cross-sectional shapes providing vibration reduction, from round to flat, of which flat is preferable. The width of the band will be in relation to the diameter of the pipe/coiled tubing 5 the vibration reducer 1 is to remove or reduce the flow-induced vibrations for, typically in the range 0.15D to 0.25D, for which D is the diameter of the coiled tubing. This corresponds to a width, for coiled tubing usually of a diameter D between 2 and 4 inches, of between 7,5 mm and 25 mm. The vibration reducer can be made from metal, such as steel, polymer, composites or any mixture thereof. It will be distinguished by a very small total height as constricted, for example 3/1000 of the length as extended, and with possibility to

cover the whole distance between for example a ship and a wellhead on the bottom, adapted to the requirement. The length or height as retrieved can be further reduced by giving the vibration reducer conical shape as viewed from the side, of which the upper part can have an inner diameter of up to 15 inches (about 38 cm). The vibration reducer in retrieved form will preferably be arranged on a support or a holder that easily can be moved on deck or transported away. The support can be conical, and this can be convenient to reduce the height as retrieved further.

Reference is made to Figure 2 that illustrates a band Ia in extended form. The lead 5 is indicated on the figure. Reference is further made to Figure 3 that illustrates some typical cross-sectional shapes of the band Ia. Further, cross-sectional shapes, from round to even flatter, are possible.

The vibration reducer according to the invention preferably comprises more than one band, preferably 3 bands, which bands are symmetrically displaced or rotated relative to each other. The vibration reducer according to the invention can in one embodiment comprise bands that are rotated/wound in opposite directions.

The lower end of the vibration reducer can be provided with a fastening device. The fastening device can be weighted to pull the vibration reducer down to a Christmas tree or a subsea installation and/or it can be shaped to facilitate optional connection to the Christmas tree or something else, preferably remotely controlled, for example with a ROV (remotely operated vehicle). The vibration reducer can however hang freely in its lower end, provided it will not result in any damage to other equipment.

The coiled tubing can be brought through the vibration reducer having an internal diameter larger than the coiled tubing and equipment fitted thereto, and further down to the Christmas tree or optionally into the well bore. When the coiled tubing be extended down to the Christmas tree the vibration reducer can, if desired, be brought down simultaneously. However, the vibration reducer will not be fastened directly to the coiled tubing in any way. The vibration reducer and coiled tubing can be retrieved and deployed independently of each other.

The vibration reducer has particular advantage at deployment and retrieval, either together with/or independent of the coiled tubing, as this does not increase the requirement for labor substantially on the vessel/rig compared to usual installation. The vibration reducer can be extended even though the coiled tubing is to be retrieved.

Example In one embodiment, where the coiled tubing has a diameter, D = 3 inches (76.2 mm), the vibration reducer comprises a band of thickness of about 3 mm and width of about 10 mm. With a lead in extended form of about 1 000 mm/360 degrees (about 13D) the total height of the vibration reducer in retrieved shape for a depth of 300 m will only be about 900 mm. With 3 parallel bands the height as retrieved will be about

2 100 mm. The vibration reducer can in substance be extended or retrieved similarly as a window blind.