WO2014092622A1

WO2014092622A1 - Mooring buoy

Info

Publication number: WO2014092622A1
Application number: PCT/SE2013/000190
Authority: WO
Inventors: Bertil Moritz
Original assignee: Flowocean Limited
Priority date: 2012-12-13
Filing date: 2013-12-10
Publication date: 2014-06-19
Also published as: EP2932567A1; SE1200769A1; JP2016500464A; EP2932567A4; SE537490C2

Abstract

A buoy anchored at sea for mooring a floating vessel having an first electric cable (9) comprises a hollow cylindrical body (1) with a mechanical mooring device (5) and a slip ring arrangement (4) having a ring part (15) and a brush part (16) for high voltage power transmission. The mechanical mooring device comprises a girdle (6) journalled around the buoy, and that the slip ring arrangement comprises a ring-shaped diving bell construction (17).

Description

Mooring buoy

TECHNICAL FIELD

The present invention concerns mooring of a floating industrial plant at sea. More precisely the invention concerns a buoy for mooring a floating industrial plant at sea. Especially the invention concerns a mooring buoy for a floating windmill plant system. In particular the invention concerns a mooring buoy with a slip ring arrangement for transporting high voltage electric power from a floating vessel to a stationary structure or vice versa.

A slip ring arrangement comprises a stationary part and a moveable part. One of the parts comprises one or a plurality of electrically conducting rings and the other part comprises a set of an equivalent number of brushes in contact with the rings. The brush arrangement may comprise a plurality of trailing sets of brushes. Thus either the ring part or the brush part may constitute the stationary part or the moveable part respectively. By high voltage in this context should be understood 12 to 36 kilovolt. Normally a power plant generates about 6 MW which indicate that the slip ring arrangement should be designed to transfer 100 to 300 A or more. The power rating of such an electrical swivel must be high as the generation of three phase power at 35 KV and 600 amps per phase may occasionally be required.

BACKGROUND OF THE INVENTION

Floating vessels with power generating capacity are used as a power source for offshore equipment. Such vessels also comprise floating wind mill plants used for electric power generation. The vessel may be moored to a buoy. A cable is then extended from the buoy to the place where the power is needed. Winds and tides cause the floating vessel to weathervane about the buoy which is fixed by anchoring means at the sea bottom. For this reason an electrical swivel is required to transmit the power generated on the vessel to a nonrotating cable.

A slip ring assembly may be mounted in an anchored buoy and used to transmit power from a floating power generating vessel to an offshore installation. A subsea cable is used to connect the slip ring to the installation. The slip ring acts as an electric swivel to allow the floating vessel to weathervane around the buoy as may be required. The slip ring conducts a plurality of phases of high voltage power through an open stack of rings to spaced brushes which are connected to the subsea cable. Since the weather condition is harsh and the swivel may be washed over by water the ring and brush arrangement needs to be protected from penetrating water.

A known slip ring arrangement is to put the ring part and the brush part in a container. Insulating oil then completely fills the container in which the ring stack is located. The oil has a breakdown voltage which is more than four times greater than the breakdown voltage of air. This allows the assembly to have smaller dimensions than would otherwise be required. The insulating oil is able to be sampled, filtered, and changed without emptying the chamber. Access ports and viewing windows may enable additional maintenance to be performed on the ring without removing the outer surrounding housing. Besides the bigger slip ring arrangement the larger oil volume is needed.

A known design of a high voltage swivel comprises an annular outer element defining a cylindrical chamber around a longitudinal axis and an inner cylindrical element coaxial with the outer element and rotatable relative to the outer element around said longitudinal axis. The inner and outer elements each comprise two axially spaced electrical conductors which conductors are rotatable with the inner and outer elements. The conductors form two pairs placed with contact surfaces in mutual electrical contact. One conductor in each pair is provided at the inner element. The other conductor is provided at the outer element and being connected to a respective voltage line. The voltage lines extend to an input terminal and to an output terminal respectively. The conductors are surrounded by an insulating material. From US 4252388 a high power slip ring assembly is previously known for transmitting high voltage three phase power at sea. The slip ring assembly comprises a slip ring mounted in a buoy for power transmission from a generating vessel to an offshore installation. A vessel moored at the buoy can weathervane around the buoy in response to wind and current conditions. The conductors of the inner element, a rotor, are annular plates mounted on a central frame of dielectric support brackets, alternately with washer-like dielectric barriers. The conductors of the outer element, a stator, are provided by carbon brushes mounted in brush holders and contact the conductive copper rings of the contacts of the rotor. The open ring stack allows high dielectric insulating oil to circulate through the stack, allowing smaller dimensions compared to using air as a dielectric.

From US 7137822 a high voltage swivel is previously known, the object of which is to provide a high voltage swivel of compact and reliable design which can be operated at relatively high voltages. Hereto the high voltage swivel includes that the electrical contacts of the outer element are provided in a recess in an annular solid outer insulating ring. By the use of solid insulating material in the form of insulating rings, a high dielectric strength can be achieved such that high voltages on the electrical contacts are possible, such as voltages up to 33 kV at for instance a nominal current of 395 A. Furthermore, the use of the solid insulating material allows the size and weight of the swivel to be reduced, and hence helps to limit bending moments on the turret swivel stack. Reduced contamination of the insulating material by particles resulting from wear of the electrical contacts occurs, such that the insulating properties are maintained during the lifetime of the swivel.

In one embodiment, the conductors at the outer and at the inner element comprise annular contact surfaces. By the use of ring-shaped contact surfaces, instead of known carbon brushes, contamination of dielectric oil in the swivel, which can be used inside for instance 11 kV, can be avoided. Dielectric oil is protected from overpressure, over temperature and leakage via a Buchholz Relay unit comprising a compensation bladder for accommodating thermally induced expansion and contraction of the dielectric oil. The present swivel is suitable for high voltages, such as 33. ^V-at utteDts.ot39£ -A .orjnaore. From US 6294844 is previously known an artificial wind turbine island comprising a framework on a plurality of platforms. Each platform is connected to a rotatable float by means of a bearing. A connecting piece is connected, via a bearing, to an anchored float. The anchored float is anchored to the sea or lake bottom by means of a number of anchoring cables. The connecting piece can freely rotate, via bearing, with respect to anchored float.

The wind generator installation relates to an installation for converting wind energy into electric energy. The installation comprises a plurality of windmills provided with vanes which rotate about a substantially horizontal shaft. The shaft is supported in bearings in a gondola. The gondola also accommodates an electric generator. The horizontal shafts extend substantially parallel to each other, in such a manner that said vanes rotate in a substantially common plane. Float bodies are attached to the framework, which function to keep the installation afloat.

SUMMARY OF THE INVENTION A primary object of the present invention is to seek ways to improve a mooring buoy providing an underwater connection for a floating vessel.

This object is achieved according to the invention by a buoy characterized by the features in the independent claim 1 or by a method characterized by the steps in the independent claim 8. Preferred embodiments are described in the dependent claims.

According to the invention the mooring buoy contains a slip ring arrangement housed in an air pocket created by a diving bell construction. The diving bell construction comprises an air tight chamber open only at the bottom. When sank into water an air volume will be captured in the chamber thus preventing water to enter the chamber. The slip ring part is held by insulators in the upper part of the air pocket and the brush part is protruding into the air pocket from the bottom opening. Thus the electric components inside the chamber are insulated by air. In an embodiment of the invention the chamber holding the air pocket is designed as a ring construction enveloping the mooring buoy. The ring-shaped chamber comprises an inner wall, an outer wall and a ceiling. The inner wall comprises part of the outer side of the buoy. The outer wall comprises a stabile skirt tightly fixed to the buoy by the ring-shaped ceiling. Thus the inner wall and the outer wall and the roof form a circular air pocket around the buoy. In an embodiment a slip ring for each phase of the electric power is held by insulators at the upper part of the air pocket. The slip rings are connected to an electric cable which is led to the inner part of the buoy by an air tight passage. The brush part is fixed to a girdle construction beneath the ring-shaped chamber. The girdle is journalled around the buoy and comprises arrangement for mooring the vessel.

In an embodiment the mooring buoy is fixed at the sea bottom by anchoring means. At least three anchor cables are attached to anchorage points around the buoy just below the girdle. In an embodiment the buoy has air injecting means by which the buoy can be raised or lowered in the sea. In a mooring situation the buoy may be raised in the water such that the girdle is positioned above the sea level. Then the floating vessel may easily be mechanically and electrically moored to the buoy. Having mechanically moored the floating vessel to the buoy the electric cable from the vessel may be connected to the brush part on the girdle by a water tight connecting device. In an embodiment the brush part is part of the vessel^'s mooring equipment and raised into contact with the slip rings in the chamber. In a first aspect of the invention the object is achieved by a buoy anchored at sea for mooring a floating vessel having an electric cable. The buoy comprises a hollow cylindrical body with a mechanical mooring device and a slip ring arrangement having a ring part and a brush part for high voltage power transmission. The mechanical mooring device comprises a girdle journalled around the buoy and the slip ring arrangement comprises a ring-shaped diving bell construction. In an embodiment of the invention the ring part is fixed inside the diving bell construction and the brush part is fixed to the girdle and protruding into the diving bell construction from beneath. In an embodiment the ring part comprises a plurality of slip rings and the brush part comprises an equal number of brushes. In an embodiment the ring-shaped diving bell construction comprises an inner wall, an outer wall and a ceiling part.

In an embodiment the inner wall comprises the wall of the cylindrical buoy, the ceiling comprises a flat ring attached to the outer surface of the buoy and the outer wall comprises a stable skirt hanging down from the ceiling part. In an embodiment an electric cable from the vessel is connected by a watertight contact device on the girdle and the electric cable connecting the ring part is passing the inner wall of the diving bell construction by a watertight passage. In an

embodiment the brush part comprises a plurality of trailing parallel brushes.

In a second aspect of the invention the object is achieved by a method for mooring a floating vessel having an electric cable for high voltage power transmission to a buoy with a hollow cylindrical body with a mechanical mooring device and a slip ring arrangement having a ring part and a brush part. The method include providing around the buoy a ring-shaped diving bell construction to envelope the slip ring arrangement, mooring the vessel to the buoy by a girdle journalled around the buoy, and connecting the cable to the slip ring

arrangement. In an embodiment providing the ring-shaped diving bell construction comprises forming a watertight chamber by an inner wall, an outer wall and a ceiling part, enveloping the slip ring arrangement by fixing the ring part inside the diving bell construction and fixing the brush part to the girdle, and making the brush part protrude into the diving bell construction from beneath to make contact with the slip rings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which:

Fig 1 is a mooring buoy with a mechanical mooring device and a slip ring arrangement according the invention, Fig 2 is a cross section of a slip ring arrangement according to the invention, and

Fig 3 is a three dimensional embodiment part of the mooring buoy.

DESCRIPTION OF PREFERRED EMBODIMENTS

A buoy according to the invention is shown in Fig 1. The buoy comprises a hollow body 1 having a cylindrical cross section. The buoy is anchored to the sea bottom with a plurality of anchoring cables 2. In the embodiment shown there are three anchoring cables connected to an equal number of anchoring lugs 3. The buoy further comprises a slip ring arrangement 4 and a mooring device 5. The mooring device comprises a girdle 6 journalled on the buoy. The girdle provides an endless rotation of a moored vessel around the buoy. In the embodiment shown the girdle comprises two pins 7 for connecting a mooring arm 8 from the vessel. A first high power cable 9 from the vessel is connected to the slip ring arrangement by a not shown contact device. From the slip ring arrangement the power is transmitted by a second high power cable 10 which other end is connected to a shore installation. In the embodiment shown the buoy comprises a platform 11 for work facilities or helicopter landing. In a normal position the water level W lies above the mooring device and the slip ring arrangement. Thus in a normal position both the mooring device and the slip ring arrangement is positioned under water. The buoy arrangement makes use of the fact that an underwater mooring of a floating vessel results in a more steady behaviour of the vessel. The sea waves otherwise raise and lower the stem of the vessel in a manner not wanted. By an underwater mooring the waves have a less influence on the movement of the vessel. When mooring a vessel the buoy is first raised by pumping air into the hollow body such that the mooring device is positioned above the sea level. The mooring arm and the first cable are connected by work being done on the water surface. Then the buoy is lowered into its working position by letting air out of the hollow body. The slip ring arrangement is shown as a cross section in Fig 2. The slip ring arrangement comprises a ring part 15 and a brush part 16 housed in a ring- shaped diving bell construction 17. In the embodiment shown the diving bell construction forms a watertight chamber 18 comprising an inner wall 19, an outer wall 20 and a ring-shaped ceiling 21. The inner wall and the outer wall form a ring-shaped opening 22 facing downwards. The brush part comprises three brushes fixed by a solid insulator to a brush holder ring 23. The brush holder ring is journalled around the cylindrical surface of the buoy and connected to the mooring device by a follower. The brushes are connected to the first high voltage cable 9. The first cable comprises in the embodiment shown three phases.

However any number of phases may be included in the slip ring arrangement. The ring part comprises three slip rings attached by busbars to an insulator 24 attached to the wall of the buoy. The insulator is making an insulated passageway through the wall to the inside of the buoy and by which the slip rings are connected to the second high voltage cable 9.

When lowered into the sea the water will try to enter through the opening 22 to the watertight chamber 18 of the slip ring arrangement. An air pocket 25 will be framed by the watertight chamber and prevent the water to intrude. Due to gas compression the water 26 will raise to a certain level 27 inside the chamber but most of the chamber will be air filled. Even though the slip ring arrangement is lowered in the sea also at large depths the diving bell construction will prevent water to enter the watertight chamber. Thus the electric components in the chamber will be air insulated from each other. In harsh conditions at sea the buoy may be tilted and water will enter the chamber to a certain extent. By designing the outer and inner wall or the chamber to become deeper such water entering will be hindered. As a precaution an air inlet 28 may be introduced by which fresh air is pressed into the chamber. In the embodiment shown in Fig 3 the first high voltage cable is attached to the girdle 6 of the mooring device 5. In this embodiment there are no contact devices. The first cable carries at its end a plurality of brushes. These brushes are inserted into the chamber at mooring. The brush holder ring 23 is in this embodiment connected to the girdle by a follower arm 29. In an embodiment the dimension of the buoy is around four meter in diameter. The anchoring cables are made of reinforced plastic or coal fibres. The anchoring cables may be in the range of 500 meters. In this example the ring-formed watertight chamber of the slip ring arrangement is like a corridor of around 12 meters length.

Although favourable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art. For instance the brush part may comprise any number of trailing brushes to ensure a sufficient non resistance contact path through the slip ring arrangement. The brush holder ring may comprise a plurality of bulkheads to minimize the water intrusion into the chamber when the buoy is tilted. The outer wall of the chamber may comprise a rubber skirt instead of a metal sheet. The diving bell construction may be made of any formable material such as metal, rubber or plastic.

Claims

1. Buoy anchored at sea for mooring a floating vessel having a first electric cable (9) comprising a hollow cylindrical body (1) with a mechanical mooring device (5) and a slip ring arrangement (4) having a ring part (15) and a brush part (16) for high voltage power transmission, c h a r a c t e r i z e d i n that the mechanical mooring device comprises a girdle (6) journalled around the buoy, and that the slip ring arrangement comprises a ring- shaped diving bell construction (17).

2. Buoy according to claim 1 , wherein the ring part is fixed inside the diving bell construction and that the brush part is fixed to the girdle and protruding into the diving bell construction from beneath.

3. Buoy according to claim 1 or 2, wherein the ring part comprises a plurality of slip rings and the brush part comprises an equal number of brushes.

4. Buoy according to any of the preceding claims, wherein the ring-shaped

diving bell construction comprises an inner wall (19), an outer wall (20) and a ceiling part (21).

5. Buoy according to claim 4, wherein the inner wall comprises the wall of the cylindrical buoy (1), the ceiling comprises a flat ring attached to the outer surface of the buoy and wherein the outer wall comprises a stable skirt (20) hanging down from the ceiling part.

6. Buoy according to any of the preceding claims, wherein the first electric

cable (9) from the vessel is connected by a watertight contact device on the girdle and that a second electric cable (10) connecting the ring part is passing the inner wall of the diving bell construction by a watertight insulator (24).

7. Buoy according to any of the preceding claims, wherein the brush part

comprises a plurality of trailing parallel brushes.

8. Method for mooring a floating vessel having a first electric cable (9) for high voltage power transmission to a buoy with a hollow cylindrical body (1) with a mechanical mooring device (5) and a slip ring arrangement (4) having a ring part (15) and a brush part (16), c h a r a c t e r i z e d b y providing around the buoy a ring-shaped diving bell construction (17) to envelope the slip ring arrangement, mooring the vessel to the buoy by a girdle (6) journalled around the buoy, and connecting the cable to the slip ring arrangement.

9. Method according to claim 8, wherein providing the ring-shaped diving bell construction comprises forming a watertight chamber (18) by an inner wall (19), an outer wall (20) and a ceiling part (21), enveloping the slip ring arrangement by fixing the ring part inside the watertight chamber and fixing the brush part to the girdle, and making the brush part protrude into the chamber from beneath to make contact with the slip rings.

10. Use of a buoy according to any of claims 1 to 7 or a method according to claim 7 or 8 for mooring a floating windmill plant at shallow waters.