WO2018001426A1

WO2018001426A1 - Vessel having a mooring arrangement and method of mooring a vessel to an offshore structure

Info

Publication number: WO2018001426A1
Application number: PCT/DK2017/050203
Authority: WO
Inventors: John Friis LØNDAL
Original assignee: MHI Vestas Offshore Wind AS
Current assignee: Vestas Offshore Wind AS
Priority date: 2016-06-29
Filing date: 2017-06-22
Publication date: 2018-01-04
Anticipated expiration: 2018-12-29

Abstract

Avessel (2) having a mooring arrangement (4) for mooring the vessel (2) to an offshore structure (6), the mooring arrangement (4) comprising; -a starboard side and a port side mooring device (8', 8''), wherein each mooring device (8', 8'') comprises, -amounting structure (14), for attaching the mooring device (8', 8'') to the vessel (2), -an adhering means (16) comprising a suction cup (18), for releasable adhering of each mooring device (8', 8'') to the offshore structure (6), and -an arm (20) being disposed between the mounting structure (14) and the adhering means (16), wherein the arms (20) are configured for maintaining separation of the vessel (2) and the offshore structure (6) when the adhering means (16) are adhering to the offshore structure (6).

Description

VESSEL HAVING A MOORING ARRANGEMENT AND METHOD OF MOORING A VESSEL TO AN OFFSHORE STRUCTURE

FIELD OF THE INVENTION The present invention relates to a vessel having a mooring arrangement for mooring the vessel to an offshore structure.

BACKGROUND OF THE INVENTION

An increasing number of wind farms are established off-shore, as an off-shore location enables the wind turbines of the wind farm to produce relatively more energy compared to an onshore location.

The wind turbine generators can be erected on a foundation that is fixed in relation to the sea bed. Various foundation types are available depending upon criteria such as the depth and condition of the seabed. A non-limiting list of foundation types include gravity-, monopile-, tripod- and j acket foundations.

The foundation types mentioned has an upper part that extends more or less above the sea level. The sea level can vary quite considerably dependent on tidal conditions at the installation site. The height of the upper part is such that the upper end of the upper part never becomes submerged. A tower interface is arranged at the upper end of the foundation such that a wind turbine tower can be mounted on top of the foundation without becoming submerged under water.

From time to time it is necessary for personnel to access the wind turbine. One option for accessing an offshore wind turbine is to hoist personnel from the helicopter to the wind turbine.

Another option is to access the wind turbine from the sea on a seagoing vessel. Personnel safety is a major concern as the personnel have to change between the non-stationary vessel that is subject to heave-, yaw- and rolling motions due to waves and the stationary foundation. This problem is pronounced in rough sea conditions. The wind turbine therefore becomes accessible only when the waves are below a certain threshold. The lower this threshold the fewer days the wind turbine will be accessible. As there is a desire to maximise the number of days where the wind turbine is accessible the threshold wave height much be as high as possible. One way of achieving this is by restricting the movements of the ship relative to the foundation as personnel are changing between the vessel and the foundation as much as possible.

EP 2 298 641 disclose a mooring system to be used for accessing foundations that are equipped with a boat landing/access installation comprising two beams extending vertically and a ladder located between the beams.

The vessel is equipped with two grippers that are attached side-by-side to the bow of said vessel. Each gripper has two jaws that are configured to close around one of the beams to keep the bow of the ship immobilised in relation to the ladder. The vertical beams of the boat landing shall be structurally capable of carrying the forces applied by the vessel.

DESCRIPTION OF THE INVENTION

An object of the invention is to indicate an alternative mooring system and a method of mooring a vessel.

According to a first aspect, the invention provides a vessel having a mooring arrangement for mooring the vessel to an offshore structure, the mooring arrangement comprising;

- a starboard side and a port side mooring device, wherein each mooring device comprises,

- a mounting structure, for attaching the mooring device to the vessel,

- an adhering means comprising a suction cup and/or an electromagnet, for releasable adhering of each mooring device to the offshore structure, and

- an arm being disposed between the mounting structure and the adhering means,

wherein the arms are configured for maintaining separation of the vessel and the offshore structure when the adhering means are adhering to the offshore structure.

In the present context the term "vessel" should be interpreted to mean "watercraft or other artificial contrivance used or capable of being used, as a means of transportation on water".

In the present context the term "adhering" should be interpreted to mean "to hold fast or stick by as if by gluing, suction, grasping or fusing", and by the term "releasable adhering" to mean selectively sticking to a surface followed by release of the sticking to the surface. The method may comprise mooring a vessel having a mooring arrangement with an adhering means to an offshore structure, wherein the method comprises the step of bringing the adhering means into abutment with the surface of the offshore structure and establishing a vacuum between the adhering means and the surface of the offshore structure.

With the invention a mooring system and a method of mooring a vessel to an offshore structure is provided that is an alternative to existing systems. An advantage of the proposed system is that it does not require any external beams mounted to the offshore structure.

The mooring system provides two ways of establishing adherence to the surface of the offshore structure - either by suction or a magnetic field. Adherence by suction requires the surface to be smooth but does not require the offshore structure to be made of a particular material. Adherence by magnetic field does not require the surface of the offshore structure to be smooth but requires the offshore structure to be made of a magnetisable material.

In the present context the term "smooth" should be interpreted to mean "having a flat, even surface, not rough not having any bumps, ridges or uneven parts at least where the suction cup abuts the surface of the offshore structure".

Moreover, the adhering means when engaged with the offshore structure is stationary and thus, causes less wear on the offshore structure and the mooring arrangement itself.

In embodiments where the mooring device comprises a shock-absorber, a damper arrangement and/or a rotation limiter the forces applied onto the offshore structure by the vessel during engagement and mooring are comparably less. Thus, foundation can be dimensioned accordingly, for example with a thinner gauge of the wall material.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which fig. 1 shows a side view of a vessel according to the invention, where the vessel is approaching an offshore structure,

fig. 2 shows a plan view of the vessel of fig. 1,

fig. 3 shows a plan view of the vessel of fig. 1 where the vessel is moored to the offshore structure,

fig. 4 shows a side view of an embodiment of a mooring device according to the invention fig. 5a shows an aft view of the mooring device of fig. 4, and

fig. 5b shows a front view of the mooring device of fig. 4.

In the explanation of the figures, identical or corresponding elements will be provided with the same designations in different figures. Therefore, no explanation of all details may be given in connection with each single figure/embodiment.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a side view of a vessel 2 with a mooring arrangement 4 and fig. 2 and 3 show plan views of the vessel 2. In fig. 1 and 2 the vessel 2 is approaching an offshore structure 6 and in fig. 3 the vessel 2 is moored to the offshore structure 6.

The offshore structure 6 can for example be a monopile foundation for an offshore wind turbine. In the embodiment shown in fig. 1-3, the monopile foundation is cylindrical with a constant diameter. This can be clearly seen in fig. 2 and 3. The offshore structure 6 is made of a smooth and magnetisable material.

In alternative embodiments the offshore structure 6 may have a polygonal shape with straight side faces.

The vessel 2 comprises a starboard side and a port side mooring device 8', 8".

When the vessel 2 is moored to the offshore structure 6, service personnel can disembark or embark via a gangway 10, see fig. 2-3 on the vessel 2 and a ladder 12 on the offshore structure 6. Fig. 5 shows a side view of an embodiment of a mooring device 8', 8" according to the invention and fig. 6a and 6b shows an aft view and a front view respectively of a mooring device 8', 8". Each mooring device 8', 8" comprises a mounting structure 14 for attaching the respective mooring device 8', 8" to the vessel 2. Moreover, the mounting structure 14 of the embodiment shown is configured for supporting various components of the mooring device 8', 8". An adhering means 16 is provided for releasable adhering of each mooring device 8', 8" to the offshore structure 6, such that the vessel 2 can be moored to the offshore structure 6. The adhering means 16 comprises a suction cup 18 and an electromagnet 19. An arm 20 is attached to the mounting structure 14 at one end and the adhering means 16 is attached to the other end, such that separation between the vessel 2 and the offshore structure 6 is ensured when the adhering means 16 is adhering to the surface of the offshore structure 6. The surface of the suction cup 18 may be flexible in order to be able to conform to the surface of the offshore structure 6 to establish an air tight connection. The suction cup 18 may also be angled.

In an alternative embodiment the adhering means consist of a suction cup 18.

In an alternative embodiment the adhering means consist of an electromagnet 19.

A suction pipe 22 is provided and connected to the suction cup 18 at one end and to a suction device (not shown) at its other end. When the suction cup 18 is brought into contact with the surface of the offshore structure 6 and the suction device is turned on, a vacuum is established in the suction cup 18 that will make the adhering means 16 adhere to the surface of the offshore structure 6 and together with the arm 20 maintain separation between the offshore structure 6 and the vessel 2. In the embodiment shown the adhering means comprises an electromagnet 19 to complement the suction cup 18. The electromagnet 19 is connected to a power source (not shown) through a power chord 42. The electromagnet 19 increases the adhering force of the adhering means 16. The power source may advantageously be independent of the suction device such that it provides redundancy should the suction device fail. In the embodiment shown the mounting structure 14 comprises a base 22 to which an upright member 24 on each side of the arm 20 is mounted. A first pivot 26 connects the arm 20 to the upright member 24, such that the arm can rotate about the first pivot 26.

In the embodiment shown the arm 20 comprises a second pivot 28. The second pivot connects the adhering means 16 to the arm 20 such that the adhering means 16 can rotate about the second pivot 28.

The first and second pivots 28, 30 have rotation axes that are substantially parallel.

This arrangement allows the vessel 2 to move up and down with the heave motions caused by waves. This embodiment lowers the loading that the mooring arrangement 4 is subjected to by allowing heave motions of the vessel 2.

In the embodiment shown the mooring device comprises a damper arrangement 32. The damper arrangement 32 comprises means for restricting the rotation of the arm 20 about the first pivot 28.

An example embodiment of the damper arrangement 32 is shown in fig. 4 to 5. This arrangement comprises means for restricting the rotation of the arm 20 about the first pivot 28. The means for restricting the rotation of the arm 20 comes in the form of a plurality of spring elements 34 arranged four-by-four on each side of the arm 20, wherein the spring elements 34 are arranged with their longitudinal axis tangential to the first pivot at a distance from the first pivot 28.

The damper arrangement 32 allows for some rotation of the arm 20 about the first pivot 28. The vessel 2 may therefore move up and down in response to the heave motions caused by the waves. How much movement that is allowed is a trade-off between trying to limit the loads that the mooring devise get subjected to and the safety of the personnel. If no movement is allowed the personnel safety is at its highest because the movement of the gangway 10 in relation to the stationary ladder 12 on the offshore structure is limited.

However, in this situation the mooring device 8', 8" and the vessel 2 are potentially subjected to high loads as the waves change from crest to through. Therefore there is an optimum that ensures acceptable safety without having to provide a too strong and therefore heavy structure. The spring element 34 may comprise an adjustment mechanism for adjusting the dampening.

In the embodiment shown the mooring device 8', 8" comprises a clutch 36 with an integrated lock. The clutch 36 is arranged between the damper arrangement 32 and the arm 20. The clutch 36 can selectively engage and disengage the damper arrangement 32. The lock is arranged to lock the arm 20 in a desired position. This may be used during transportation between offshore structures. Furthermore, engagement of the lock may be maintained when the wave height is low and therefore the loading on the mooring device 8', 8" is low. When the wave height increase the loading increase thus, the lock may be released and the clutch 36 may be engaged to couple in the damper arrangement 32 to maintain the loads on the mooring device 8', 8" below acceptable levels.

In the embodiment shown the second pivot 30 comprises a rotation limiter 38. The rotation limiter 38 includes a stop that maintains the adhering means 16 in an upright position such that it will readily engage the surface of the offshore structure 6 when the vessel 2 approaches said offshore structure 6.

In an embodiment, the rotation limiter comprises a coupling means that is arranged to selectively couple/decouple the rotation limiter or arranged to adjust the position of said at least one stop, so that the rotation of the adhering means about the second pivot can be changed according to the operational mode of the mooring system.

In the embodiment shown the arm 20 comprises an extension means 44, here in the form of a hydraulically driven linear actuator. Other suitable actuation means include pneumatic and electrical actuators. The extension means 44 allows adjustment of the separation distance between the vessel 2 and the offshore structure 6. Moreover, the arm 20 in its most retracted position may decrease the risk of the most forward end of the mooring device 8', 8" to interfere with other vessels or structures when the mooring arrangement 4 is not in operation. In the embodiment shown the mooring device 8', 8" comprises a shock-absorber 45. The shock-absorber is integrated with the arm 20 and dampens forces applied in the longitudinal direction of the arm 20. The shock-absorber 45 comprises a switching means 46 for selectively engaging/disengaging the shock-absorber 45.

In an embodiment the adhering means 16 comprises a friction lining 48 in the interface between the adhering means 16 and the surface of the offshore structure 6. The friction lining 48 further increases the ability of the adhering means 16 to maintain its position on the surface of the offshore structure 6 once engaged.

Turning to fig. 1 and 2 the vessel 2 is approaching the offshore structure 6. The approach may be at an angle such that either the starboard side mooring device 8' or the port side mooring device 8" engages the surface of the offshore structure 6. The arm 20 may also be extended or retracted to adjust the distance between the gangway 10 and the ladder 12.

The shock-absorber 45 may advantageously be engaged such that the impact forces applied by the vessel onto the offshore structure, when the starboard side mooring device 8' come into contact with the offshore structure 6 is decreased. The suction device is turned on to establish a vacuum between the suction cup 18 of the engaged mooring device 8', 8" and the surface of the offshore structure 6 and/or power is applied to the electromagnet 19 of engaged mooring device 8', 8" to create a magnetic field between the electromagnet 19 and the surface of the offshore structure 6. Then, the propulsion system of the vessel 2 is operated to yaw the vessel 2 until the other mooring device 8', 8" becomes engaged with the surface of the offshore structure 6. Then suction is applied to the suction cup 18 of the just engaged mooring device 8', 8" and/or power is applied to the electromagnet 19 of the just engaged mooring device 8', 8". The shock-absorber 45 may be disengaged to lock the length of the arm 20 to maintain sufficient separation between the vessel 2 and the offshore structure 6.

Operation of the propulsion system of the vessel may be maintained to push the vessel 2 towards the offshore structure. With the vessel 2 safely moored to the offshore structure, as shown in fig. 3, personnel may embark/disembark via the gangway 10 and ladder 12.

When the service operation is completed the suction device is turned off and if applicable electrical power to the electromagnet 40.

Propulsion system of the vessel 2 may be operated to increase the separation between the vessel 2 and the offshore structure.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The invention can be implemented by any suitable means; and the scope of the present invention is to be interpreted in the light of the accompanying claim set. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A vessel having a mooring arrangement for mooring the vessel to an offshore structure, the mooring arrangement comprising;

- a mounting structure, for attaching the mooring device to the vessel,

- an arm being disposed between the mounting structure and the adhering means,

wherein the arms are configured for maintaining separation of the vessel and the offshore structure when the adhering means are adhering to the offshore structure, and wherein the arm comprises an extension means, wherein the extension means is configured for selectively extending or retracting the arm.

2. The vessel according to claim 1, wherein the extension means is a linear actuator selected from a group consisting of hydraulic, pneumatic and electrical actuators.

3. The vessel according to any of the previous claims, wherein the mooring device comprises a shock-absorber, wherein the shock-absorber is operably attached to the arm, wherein the shock-absorber is configured for dampening in the longitudinal direction of the arm.

4. The vessel according to claim 3, wherein the shock-absorber comprises a switching means, for selectively engaging/disengaging the shock-absorber.

5. The vessel according to any of the previous claims wherein the arm comprises a first pivot, wherein said first pivot is arranged between the arm and the mounting structure, such that the arm may rotate about the first pivot.

6. The vessel according to any of the previous claims wherein the arm comprises a second pivot, wherein said second pivot is arranged between the arm and the adhering device, such that the adhering device may rotate about the second pivot.

7. The vessel according to claim 5, wherein each mooring device comprises a damper arrangement, wherein the damper arrangement comprises means for restricting the rotation of the arm about the first pivot.

8. The vessel according to claim 7, wherein the damper arrangement comprise at least one spring element.

9. The vessel according to claim 8, wherein said at least one spring element is connected to the arm at a connection point with a radial distance to the first pivot and wherein said at least one spring element is oriented with its longitudinal axis tangential to the first pivot.

10. The vessel according to any of claims 7 to 9, wherein the mooring device comprises a clutch and a lock, wherein the clutch is arranged between the damper arrangement and the arm for selectively engaging the damper arrangement, and wherein the lock is arranged to selectively lock the arm against rotation about the first pivot.

11. The vessel according to claim 6, wherein the second pivot comprises a rotation limiter, wherein the rotation limiter comprises at least one stop, wherein said at least one stop is arranged to limit the rotation of the adhering means about the second pivot.

12. The vessel according to claim 11, wherein said at least one stop is arranged to maintain the adhering means in a substantially upright position, when the rotation limiter is in engagement with said at least one stop.

13. The vessel according to any of the previous claims wherein the adhering means comprise a friction lining located in the interface between the adhering means and the offshore structure.

14. Method of mooring a vessel having a mooring arrangement with an adhering means to an offshore structure, wherein the method comprises the steps of bringing the adhering means into abutment with the surface of the offshore structure and establishing a vacuum and/or a establishing a magnetic field between the adhering means and the surface of the offshore structure and with the extension means adjusting the separation distance between the vessel and the offshore structure.