GB2507252A

GB2507252A - A subsea vibration damping device and clamp with hydrodynamic damping

Info

Publication number: GB2507252A
Application number: GB1215378.9A
Authority: GB
Inventors: Knut Nordheim; Stale Ovretveit
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2012-08-29
Filing date: 2012-08-29
Publication date: 2014-04-30
Anticipated expiration: 2032-08-29
Also published as: GB201215378D0; GB2507252B

Abstract

A subsea vibration damping device 3 comprises a clamp 13 having a plurality of sub elements 14, 15 attachable to each other for jointly defining an inner space 9 which receives and damps vibration of an elongate member or subsea structure, such as a J-tube 4. The vibration-dampening device 3 comprises hydrodynamic dampening means, and fastening means for securing the dampening means to the subsea structure. The hydrodynamic dampening means may comprise a plurality of perforated or slotted bodies 7, 8 attached to, or integrated with, the sub elements 14, 15 of the clamp body, and configured such that when the clamp 13 is secured to the subsea structure 4, the perforated or slotted bodies 7, 8 jointly define a substantially cylindrical body provided with a central through space for receiving the subsea structure 4. Reference is also made to a system and to a method incorporating the subsea vibration damping device and clamp.

Description

Hydrodynanic damper

Technical field

The present invention relates to a hydrodynarnic damper, to a connection means for s securing the damper to a subsea structure, to a system incorporating such a damper, and to a method for installing the damper.

Background

Subsea structures, such as risers or foundations for the offshore industry, etc. typically io extend from the sea surface downwards to various water depths. Sometimes, new structures have to be secured to existing subsea structures below the water surface.

Typically underwater welding or mechanical securing with screw joints is used to secure the new structures to the existing subsea structures.

It can be expensive and risky to send divers down to weld or perform other subsea work during securing of the subsea structures.

Further, iii order to get proper fit between the new and the existing subsea structures, low mechanical tolerances may be needed. This presents still further problems, such as a need of visual inspection and measuring of the existing subsea structure in order to enable colTect dimensioning and design of the new structure. Sometimes, fouling has to be removed from the existing subsea structure before measuring, welding or joining of the new structure to the existing subsea structure, since the fouling may make it impossible to inspect, measure or correctly position the structures relatively each other.

Further, mechanical operations on the existing subsea structure, such as welding and drilling, often result in weakening of the existing subsea structure or damage of anti-corrosive layers adjacent the welded areas.

A special type of subsea structure can be found in offshore wind turbines, which typically comprise a foundation, a tower extending from the foundation upwards, and a turbine provided at a top portion of the tower. Cables extending from the turbine rn downwards to transFer electricity produced are sometimes led through protective tubing called J-tubes. A problem with some wind turbines installed at sea is that the J-tubes may start vibrating, for example due to vortex-induced vibrations caused by fluid passing the wind turbine and vibrations induced by the wind turbine itself. There may be a risk of material fatigue problems in the wind turbine or J-tube caused by the vibrations and therefore it would be advantageous to mitigate such vibrations. Wind turbines with internal J-tubes present even greater problems, since they are difficult to reach for mounting of vibration-dampening means.

One way of mitigating the fatigue problems would be to install supports between the J-tube and inner portions of the tower. However, installing such supports typically requires risk filled, time consuming and expensive mechanical operations, such as welding and screwing. Further, such operations often have to be made under water at s positions where the supports are most effective, but where it is difficult to work due to the water.

Summary

It is an object of the present invention to provide a means for securing a first subsea structure to a second subsea structure. It is a further object of the invention, to secure a dampening means to the J-tube of an offshore wind-turbine. The present invention is defined in the appended independent claims. Embodiments are set forth in the appended dependent claims.

According to an aspect of the invention, there is provided a clamp comprising a clamp body defining a through inner space suitable for receiving an elongate element. The clamp body comprises a plurality of sub dements attachable to each other for jointly defining the inner space.

The provision of said sub elements attachable to each other for jointly defining the inner space, makes it possible to transport the clamp in pieces into spaces where it would otherwise not be possible to fit the clamp, such as into the tower of a wind turbine. Also the provision of said sub elements makes it possible to bring the clamp onto existing structures, such as subsea risers, without having to thread the clamp over an end of the structure to which is to be secured.

The clamp body may be provided with water-swellable means configured to expand within said inner space upon contact with water.

rn The water-swellable means swells upon contaci with waler, wherein it expands such that it at least partly fills the space between a structure positioned in the clamp.

Eventually, the water-swellable means presses against the structure and the clamp such that the clamp is secured to the structure.

Further, the clamp body may be assembled above water onto the structure to which is to be secured, where after the clamp is subsequently lowered into water until it reaches its intended securing position. Thereby, subsea welding or securing with screws will not be needed. Also, the need of divers and subsea robots (Remote Operated Vehicles) is mitigated. This is especially advantageous in confined narrow spaces where divers and robots would not be able to work. Also, the water-swellable means distributes stress well. c

The through inner space may be configured to be suitable for receiving an elongate element having circular cross-section. Such an inner space is of advantage when the device is to be used on the i-tube of an offshore wind turbine, said J-tube having a circular cross-section.

The clamp body may be configured to fonii a closed loop around the inner space when the sub elements are attached to each other. The closed loop around the inner space makes the clamp body withstand large forces created by virtue of the water-swellable means without requiring momentum to be transferred between the sub elements. Rather, expansion of the clamp body is prevented by virtue of tensile strain taken up by the sub elements.

At least two adjacent sub elements may be configured to be hingedly attachable to each other. A hinge connection makes it possible to vary the size of the inner space formed in a convenient manner, such that variations in the size of the At east two adjacent sub elements may be configured to be joined by means of screw joint. The screw joint provides for a strong connection, which may be variably tightened by adjusting the position of the screw, such that the size of the inner space may be varied.

At least two adjacent sub elements may be configured to be joined by means of snap lock connection. The snap lock connection provides for attaching the elements without tools.

According to another aspect of the invention, there is provided a vibration-dampening device suitable for dampening of vibrations of a subsea structure. The vibration-dampening device comprises hydrodynarnic dampening means, and fastening means suitable for securing the dampening means to the subsea structure. The fastening means comprises a clamp according to the above-described aspect of the invention. Further, the hydrodynamic dampening means comprises a plurality of perforated or slotted bodies attached to, or integrated with, the sub elements of the clamp body. The perforated or slotted bodies are configured such that when the clamp is secured to the subsea structure, the perforated or slotted bodies jointly define a substantially cylindrical body provided with a central through space for receiving the subsea structure.

s The substantially cylindncal hydrodynamic dampening body extending around the subsea sftucture, provides dampening properties which are substantially equal in all directions radial to the cylindrical body. This enables easier installation of the dampening device, since special care does not have to be taken as to the axial orientation of the hydrodynamic dampening body relative to the subsea structure. By combinthg the respective slotted or perforated bodies of the hydrodynarnic dampening means with the sub elements of the clamp, the vibration-dampening device of the invention may be transported in pieces into small spaces, where it can be effortlessly assembled and subsequently effortlessly secured to the subsea structure to be vibration dampened.

According to a further aspect of the invention, there is provided a system comprising a subsea structure provided with an elongate member. The elongate member is provided at least partially within a space in said structure. The space is confined from incoming sunlight. Further, the system comprises a vibration-dampening device as described above. The vibration dampening device is secured to said elongate member within said confined space.

Since the vibration-dampening device is provided in a space without incoming light, there is no light to drive growth of fouhng, which in turn leads to reduced fouling problems, and thereby enables use of dampening devices according to the invention even in spaces where access for removal of foufing would be a problem. Mso, the holes or slots provided in the hydrodynamic dampening means can be made in very small sizes, without any substantial risk of malfunction due to fouling, thereby giving engineers greater freedom to design dampers with any hole sizes for improved damping properties.

rn According to yet another aspeci of the invenlion, Ihere is provided a meihod of installing a vibration-dampening device according to the previously described aspect, on a J-tube inside a tower of a wind turbine installed at sea. The method comprises the steps of introducing into the tower of the wind turbine, above sea level, the various separate bodies and elements of the vibration-dampening device. The method further comprises the step of attaching to each other, still above sea level, the sub elements of the clamp body, such that a portion of the J-tube extends through the inner space of the clamp body formed, Also, the method comprises the step of lowering the vibration-dampening device along the J-tube. to a desired position along the J-tube, below sea level. The final step of the method is to suspend the vibration-dampening device until it is secured to the J-tube by the water-swellable means. Such a method provides for low cost post installation of vibration-damper on the J-tube inside the tower of a wind s turbine installed at sea, since it can be performed without disassembling andior moving the wind turbine, and without a need of welding or other risk filled and expensive subsea operations.

The present invention will now be described in more detail, with reference to the io appended drawings showing an embodiment of the invention.

Description of figures

Fig. 1 is a partial side view in cross-section of a wind turbine, shown with a vibration-dampening device according to an embodiment of the invention slidably arranged on the i-tube in a position above sea level, Fig. 2 is a partial side view in cross-section of the wind turbine of Fig. I, shown with the vibration-dampening device in a position below sea level.

Fig. 3 is a top view of the vibration-dampening device also shown in Figs. I and 2 before it has been assembled slidably arranged on the J-tube.

Fig. 4 is a top view of the vibration-dampening device shown in Figs. 1-3 after it has been assembled by attachment of the sub-elements of the clamp, and before the water-swellable means has been activated.

Fig. 5 is a top view of the vibration-dampening device also shown in Figs. 1-4, after the water-swellable means has swoflen to secure the vibration-dampening device to the I-tube.

Fig. 6 is a perspective view showing the contours of a sub-element of the clamp body of the vibration-dampening device also shown in Figs. 1-5.

The figures are not drawn to scale and relative dimensions are merely exemplary, to clearly illustrate the concepts of the invention.

Detailed description

Fig. 1 shows a portion of a wind turbine 1 fastened to a subsea foundation 2. The upper portion of the wind turbine is not shown. A vibration-dampening device 3 according to an embodiment of the invention is slidably arranged on a J-tube 4 in a position above sea level 5. The tower is provided with a service door 6 and an external platform 7. The J-tube 4 extends from the top of the wind turbine downwards and exist the tower close to the bottom of the foundation, as known in the art.

Wind and water flowing around the wind turbine may lead to vortex-induced vibrations in the wind turbine 1, which in turn may cause the J-tube 4 to vibrate. The vibrations of the J-tube 4 may lead to fatigue problems shortening the lifetime of the wind turbine 1.

s Fig. 2 shows the wind turbine 1 also shown in Fig. 1, but with the vibration-dampening device 3 lowered to a position below sea level 5, where it may act to dampen vibrations in the J-tube 4.

Fig. 3 shows the vibration-dampening device 3 in a first stage of installation, before it is assembled onto the J-tube 4. The vibration-dampening device 3 comprises hydrodynamic dampening means and fastening means for securing the dampening means to the 1-tube.

The hydrodynamic dampening means comprises two elongate cage-like bodies 7, 8 of C-shaped cross section (see Fig. 6). The cage-like bodies 7, 8 are made of perforated or slotted sheet material, such as e.g. steel or plastic, and when brought together, they define a substantially cylindrical body (Figs. 4-5) with a central through cylindrical inner space 9 suitable for receiving the J-tube 4. Thus, the cage-likes bodies 7, 8 form a closed ioop around the inner space when they are attached to each other. The diameter of the inner space is larger than the diameter of the J-tube 4, such that the vibration-dampening device 3 may be slid along the 1-tube 4. On each cage-like body 7, 8, there is provided water-swellable means 10, 11 in the form of sheets of a water-swellable material. Water-swellable materials are known. The cage-like bodies 7, 8 are provided with flanges joined by screw joints (not shown).

Fig. 4 shows the vibration-dampening device 3 in a second stage of installation with the cage-like bodies 7, 8 attached around the J-tube 4. The inner portions of the cage-like bodies 7, 8, which define the central through cylindncal inner space 9, form part of the fastening means by together functioning as a clamp. As shown, the water-swellable means 10. II are not yet activated by water in this second stage of installation. Thus, the rn vibralion-dampening device 3 is slidable/movable along Ihe J-Lube 4, such thai ii can be lowered along the J-tube 4 into the water, below sea level. In an embodiment, bearing means (not shown) are provided on the vibration-dampening device 3. The beanng means, protrude into the cylindrical inner space 9 to bear-off against the J-tube 4 dunng lowering. The bearing means thus keeps the water-swellable means lO, ii from damage during lowering.

Fig. 5 shows the vibration-dampening device 3 in a third stage of installation, lowered into the water (See Fig. 2). As shown, the sheet of water-swefiable means 10. 11 is activated by the sea water and has swollen such that it secures the vibration-dampening device 3 by pressing against portions of the J-tube 4. c

For clanty, one of the cage-like bodies 7, 8 of the vibration-dampening device 3 is shown outlined in perspective in Fig. 6.

When installing the vibration-damper, the cage-like bodies are introduced into the tower io of the wind turbine, above sea level (Fig. 1). Inside the tower, the cage-like bodies are attached to each other, still above sea level, such that a portion of the J-tube extends through the inner space of the clamp body formed (Figs. 2-3). The vibration-dampening device is then lowered along the J-tube, until it reaches a desired position along the J-tube, below sea level (Fig. 2). There, the vibration-dampening device is suspended until it is secured to the i-tube by means of the water-swellable means.

It sho&d be understood that instead of securing hydrodynamic dampening means to a J-tube 4. any other structure could be secured to the J-tube under water by means of a clamp according to claim 1. Further, instead of being secured to a cylindrical structure, such as the J-tube, the clamp could be configured such that it could be secured to structures of other shape, such as 1-beams or square beams, by configuring the sub-elements of the clamp body such that they joinfly define another suitable space, i.e. a through space having square or H-shaped cross-section.

Also, the cage-like bodies may have other shapes than C-shaped, as long as they together define a body providing the desired hydrodynarnic dampening effects, and as long they together define an inner space suitable for receiving the structure to which it is to be attached.

rn Subsea structures, such as hydrodynarnic dampers. are prone to glowing of Ibufing. The fouling may gradually decrease the size of fluid passages through slots or holes, which could lead to gradua' reduction of the performance of the dampers. Fouling may be removed, but removal typically requires access by ROV:s or divers, something that is expensive and difficult within confined spaces, such as towers of offshore wind turbines. Therefore, it would seem disadvantageous to use hydrodynamic dampers within towers of wind turbines. However, the inventors have come to the insight that there is a specific advantage to the use of hydrodynamic dampers within confined structures with substantially no incoming light, such as towers of wind turbines. Since the dampers are mounted within the darkness of the towers of the wind turbine, there is no light to drive growth of fouling, which in turn, under normal circumstances, reduces fouling problems. c

The hydrodynarnic dampening means may be attachable/detachable from the fastening means, or it may be at least partly integrated with the fastening means, without departing from the invention.

Claims

C 1 a i m s 1. A clamp (13) for subsea use comprising: a clamp body defining a through inner space (9) suitable for receiving an elongate c]cmcnt (4), charactcrizcd in that the clamp body comprises a plurality of sub elements (14, 15) attachable to each other for jointly defining the inner space (9).
2. A claim (13) according to claim i, wherein the clamp body is provided with water-swellable means (lO, ii) configured to expand within said inner space (9) upon contact with water.
3. A claim (13) according to any one of the preceding claims, wherein the through inner space (9) is suitaNe for receiving an elongate dement (4) having circular cross-section.
4. A clamp (13) according to any one of the preceding claims, wherein the clamp body is configured to form a closed ioop around the inner space when the sub elements (14, 1 5) are attached to each other.
5. A clamp (13) according to any one of the preceding claims, wherein at least two adjacent sub elements (14, 15) are configured to be hingedly attachable to each other.S
6. A clamp (113) according to any one of the preceding claims, wherein at least two adjacent sub elements (14, 15) are configured to be attachable by means of a screw joint.o
7. A clamp (13) according to any one of the preceding claims, wherein at least two adjacent sub elements (14, 15) are configured to be attachable by means of snap lock connection.
8. A vibration-dampening device (3) for dampening of vibrations of a subsea ss structure (4), wherein said vibration-dampening device (3) comprises hydrodynamic dampening means, and fastening means for securing the dampening means to the subsea structure, characterized in that the fastening means comprises a clamp (13) according to daim 1, and in that the hydrodynamic dampening means comprises a plurality of perforated or slotted bodies (7, 8) attached to, or integrated with, the sub elements (14, 15) of the clamp body, and configured such that when the clamp (13) is secured to the subsea structure s (4), the perforated or slotted bodies (7, 8) jointly define a substantially cylindrical body provided with a central through space for receiving the sub sea structure (4).
9. A vibration-dampening device (3) for dampening of vibrations of a subsea structure (4), wherein said vibration-dampening device (3) comprises hydrodynamic io dampening means, and fastening means for secunng the dampening means to the subsea structure, charactenzed in that the fastening means comprises a clamp (13) according to any one of claims 2-7, and in that the hydrodynarnic dampening means comprises a plurality of perforated or slotted bodies (7, 8) attached to, or integrated with, the sub elements (14, 15) of the clamp body, and configured such that when the clamp (13) is secured to the subsea structure (4), the perforated or slotted bodies (7. 8) jointly define a substantially cy'indrical body provided with a central through space for receiving the subsea structure (4).
10. System comprising a subsea structure (1) provided with an elongate member (4).which elongate member (4) is provided at least partially within a space (16) in said structure confined from incoming sunlight, characterized in that the system comprises a vibration-dampening device (3) according to any one of claims 8-9. wherein said vibration-dampening device (3) is secured to said elongate member (4) within said confined space (16). is11. Method of installing a vibration-dampening device (3) according to claim 9 on a J-tube (4) inside a tower of a wind turbine (1) installed at sea, compnsing the steps of: introducing into the tower of the wind turbine (I), above sea level (5), the various bodies, means and elements (7,8, 10, Ii, 13, 14, 15) of the vibration-dampening device (3).attaching to each other, still above sea level (5). the sub elements (14, 15) of the clamp body, such that a portion of the J-tube 4) extends through the inner space (9) of the clamp body, lowering the vibration-dampening device (3) along the J-tube (4), to a desired position along the J-tube (4), below sea level (5), and suspending the vibration-dampening device (3) until it is secured to the J-tube (4) by the water-swellable means (10, 11).