EP2623413B1

EP2623413B1 - A method and system of providing access between a floating vessel and a marine structure

Info

Publication number: EP2623413B1
Application number: EP13153045.3A
Authority: EP
Inventors: Arnoldus Martinus Josephus Van Poppel; Rudolf Schreijer; Jan De Vries
Original assignee: Sea Maester Bv
Current assignee: Sea Maester BV
Priority date: 2012-01-31
Filing date: 2013-01-29
Publication date: 2016-08-31
Anticipated expiration: 2033-01-29
Also published as: EP2623413A1; DK2623413T3; NL2008207C2; PT2623413T; ES2605460T3

Description

The invention is directed to a method of providing access between a floating vessel and a landing position of a marine structure. The invention is also directed to a mooring system for providing access between a floating vessel and a landing position of a marine structure.
When at sea, or on similar large body of water such as larger lakes, it is often necessary for personnel, equipment or fuel to transfer from a vessel to a marine structure and back. In the context of this application, the term "marine structure" refers to floating structures and vessels which may be fixed to the sea bed. Examples are offshore structures such as an oil or gas rig, wind turbine or the like where maintenance workers need access from time to time or land based structures which are sometimes only accessible from sea, like for example light houses. The term fixed to the sea bed can mean directly fixed to the sea bed via a base structure or via anchors and the like. The term vessel can mean any vessel which is employed to carry personnel, equipment and/or fuel to and from the marine structure.
Conventionally, a worker is required to step from a transfer vessel onto a flexible or rigid ladder or onto a rigid gangway at the side of the marine structure. This is inherently hazardous, particularly for those who are not accustomed to working in a marine environment. In all but the calmest of seas, relative motion between the vessel and the marine structure is substantial. Stepping from a moving vessel onto a stationary ladder is difficult and it is easy to slip and, potentially, to fall into the water. This carries the danger of being crushed between the vessel and the marine structure.
For these reasons, safety regulations limit the transfer of personnel from a vessel to a marine structure to times when the sea conditions are within certain parameters, typically to circumstances where the wave height is less than about 0.7m to 1m. The consequence of this is that many working days are lost when wave conditions are such that access to the fixed structure is not possible. This can represent a major expense for those involved in the construction and maintenance of offshore facilities.
Various methods and vessels have been developed to provide access between a floating vessel and a landing position of a marine structure in a greater range of sea conditions. For example, WO 2002/020343 describes a system is known that is used for achieving a flexible walkway connection between a vessel and an offshore construction. For this purpose, the vessel is provided with a telescopically extendable walkway which at one end is movably mounted on the vessel around two shafts. At the free end of the walkway a coupling device is provided, which is made in such a way that it can be coupled to a substantially vertically pointing grip bar connected to the offshore construction. While mooring, the vessel is manoeuvred to a suitable starting position in relation to the grip bar. Next, the walkway is aimed towards the middle part of the grip bar by means of a suitable swivel movement and the walkway is extended until the coupling device encompasses the grip bar. Then, the coupling device's two hydraulically controlled coupling jaws - which can move towards each other from an open position - are closed.
A disadvantage of the system of WO 2002/020343 is that the coupling procedure can sometimes be somewhat cumbersome, especially during stormy weather. The forces that are generated at the moment that the arm hits the grip rod, are very hard to control under these circumstances.
WO 2009/048323 describes a similar system as in WO 2002/020343 in that the coupling is now achieved by swivelling an arm from the floating vessel and grabbing a vertically positioned mooring cable running from an upper position on the offshore structure and a lower point of the offshore structure at some distance above the sea level. The walkway is coupled by means of a coupling device which rest on a support beam which is part of the offshore structure.
A disadvantage of this system is that the swivelling operation is cumbersome and requires some precision when performed in order to avoid that the swivelling arm collides with other parts of the offshore structure.
The methods of WO2002/020343 and WO2009/048323 require trained operators.
Closest prior art document WO 2006/013342 describes a method wherein an extendable gangway is extended from a vessel to a marine structure using a guide wire. The guide wire connects the marine structure and the vessel and is held at a constant tension. When the gangway is in place it is connected to the marine structure by means of a mounting means which allow the gangway to move relative to the marine structure.
A disadvantage of using a guide wire as in WO 2006/013342 is that the weight of the gangway rests on said wire thereby risking that this wire breaks. For this reason the gangway of WO 2006/013342 is provided with inflatable members such that in case of failure of the guide wire the gangway will float thereby providing safe refuge for personnel. Another disadvantage is that when extending the gangway towards the marine structure the weight of the gangway will cause the wire to bend. This will result in a less efficient guiding of the gangway. In a worst case the gangway cannot move forward due to a resulting deep dip in the wire caused by the weight of the gangway.
WO 2010/147478 describes a method of providing access between a floating vessel and a landing position of a marine structure where a mooring cable is fished up from a buoy and attached to the vessel. By moving the vessel away from the marine structure and extending the gangway using the taut mooring cable as a guide wire the gangway is manoeuvred towards a landing platform of the marine structure. The free end of the gangway is arranged in an opening in the landing platform. By a continuous press force of the gangway into the opening of the landing platform the free end of the gangway remains in contact with the landing platform enabling transfer of personnel between the vessel and the marine structure. The continuous press force is achieved by insertion and withdrawal of the telescopic parts of the gangway.
A disadvantage of the method of WO 2010/147478 is the use of a guide wire as explained above. The continuous press force which is required in this technology on the bridge parts will require more robust hydraulics. The robust hydraulics will in turn result in a heavier gangway construction. This heavier gangway will only increase the problem of the guide wire to bend under the weight of the gangway. The continuous push force of the bridge will also require that the thrusters of the vessel will have to continuous work against this force in order to keep the ship in position. This may result in unacceptable high fuel consumption.
The object of the present invention is to provide a method of providing access between a floating vessel and a landing position of a marine structure which is simpler to perform, which can be performed under stormy conditions and is easily disconnected in case of an emergency.
This is achieved by a method of providing access between a floating vessel and a landing position of a marine structure by performing the following steps:

(a) positioning the vessel near the marine structure,
(b) fishing up two parts of a mooring cable, which parts of the mooring cable extends from the marine structure and wherein the intermediate part of the mooring cable connecting the two parts can run freely via a position at the landing position of the marine structure,
(c) attaching one part of the mooring cable to the extendable end of an telescopically extendable bridge, which bridge is attached at or near its other end to the floating vessel and attaching the second part of the mooring cable to a winch fixed to the floating vessel,
(d) hauling the winch such that the extendable end of the bridge is pulled towards the landing position until the extendable end of the bridge reaches the landing position and
(e) maintaining a pull force on the mooring cable such that the extendable end of the bridge remains positioned at the landing position enabling safe access between the floating vessel and the marine structure.

In the method according to the invention the mooring cable hoists the extendable bridge to the landing position. This is advantageous over using a guide wire because the weight of the bridge does not press the wire downwards. Another advantage is that no specially trained operators are required to moor the bridge to the landing position.
The telescopically extendable bridge may comprise a number of bridge parts which can move relative to each other and preferably at least two bridge parts. The telescopic movement of the bridge parts is suitably achieved by using hydraulics as is well known to the skilled person. These hydraulics will be referred to as bridge part hydraulics. The bridge is fixed at its fixed end to the vessel, either directly or via a platform or the like. A suitable position is the flat work deck of an offshore support vessel. The bridge suitably has a pivotal attachment near or at its fixed end for pivotal movement along a horizontal by means of one or more hydraulic luffing cylinders and the bridge has a pivotal attachment near or at its fixed end for pivotal movement along a vertical by means of slewing hydraulics. The bridge may suitably extend somewhat in a direction opposite the extendable part to provide some balancing weight. The balancing will suitably not exceed the weight of the opposite positioned bridge parts, more preferably not exceed the weight of the bridge parts in their extended position.
In step (d) the bridge part hydraulics, the hydraulic luffing cylinders and the slewing hydraulics are suitably relieved of pressure in a gradual manner. In step (e) the hydraulic luffing cylinders and the slewing hydraulics are preferably relieved of pressure while the bridge part hydraulics are preferably kept under a low pressure to damp the relative movement of the vessel relative to the marine structure. Relieving the pressure of the hydraulic luffing cylinders and the slewing hydraulics enables the hydraulics to move more freely and compensate for the movement of the vessel relative to the marine structure. Thus the bridge may move pivotal along the horizontal axis and vertical axis. In the direction of the telescopically bridge the movement is suitably somewhat dampened. More preferably the pressure in these hydraulics is, suitably gradually, relieved when the bridge and the vessel are in position and when the force on the mooring cable exceeds a certain predetermined pull force in step (d).
Preferably the telescopically extendable bridge comprises of two bridge parts wherein the extendable bridge part can extend to a maximum extension. In step (e) and suitably also in step (d) the actual extension of the two bridge parts is kept between an inner arid outer limit of the maximum possible extension. This provides a buffer along which the bridge parts can move to compensate for the relative movement of the vessel with respect to the marine structure.
In step (e) a constant force is suitably maintained on the mooring cable by the winch. This force is suitably reduced to a minimum such to maintain a connection. This force will hold the bridge in position at the landing position. The mooring cable is suitably designed such that in case of emergency the cable will break when the vessel moves away from the marine structure. Preferably the winch and mooring cable is provided with a gross overload protection (GOP). In case of an emergency the mooring cable will reel of the winch and finally disconnect at the designed force of the GOP.
The end of the bridge is kept at the landing position by means of the pull force of the mooring cable. For security reasons it may be suited to have a safety connection, which may be a steel cable or the like, which in case the mooring cable fails will keep the bridge at the landing position. Such a safety coupling may also be by means of a magnetic force between bridge and landing position.
The floating vessel is suitably kept in position in steps (a)-(e) by means of dynamic positioning or by means of a two point mooring. Dynamic positioning is well known and may for example be achieved by using thrusters in combination with satellite navigation. Two point mooring may be achieved by means of an anchor mooring at the bow of the vessel while the bridge is deployed from the stern of the vessel.
In step (b) the mooring cable may be a cable having two loose ends or a cable in the form of a closed loop. In case the mooring cable has two loose ends it is preferred that the parts being picked up in step (b) are the two loose ends. In case the mooring cable is a closed loop one part is attached to the bridge in step (c) and another part is fixed to the winch. The mooring cable may be lowered from the marine structure, for example by means of a crane or may be fixed to a buoy to be fished up in step (b). Preferably the mooring cable is detached from a buoy. The buoy is suitably anchored to the sea bed and has fixing means to fixe the two loose ends of the mooring cable. Detaching a mooring cable from a buoy and fishing up the parts of the mooring cable is a well known operation which can be performed under a wide range of sea conditions.
In case a mooring cable is used having two loose ends, it is preferred that in step (c) the loose end of the mooring cable is attached to the extendable end of the telescopically extendable bridge via a first cable part, also referred to as pendant. This first cable part is fixed at one end to the extendable end and which other end is connected to the loose end of the mooring cable. The use this extra cable part which is initially fixed to the free end of the bridge is advantageous because it enables one to connect the mooring cable to the free end of the bridge without having to attach the loose end of the mooring cable to the free end of the bridge. This latter operation might be cumbersome, especially at high wave conditions.
In step (c) the second loose end of the mooring cable is suitably fixed to the winch via a second cable part, which second cable part is fixed to the winch at one end and which other end is connected to the second loose end of the mooring cable.
To facilitate easy transport of the mooring cable at the landing position it is preferred that the mooring cable runs via a pulley at the landing position. Non-moving means to guide the mooring cable at the landing position are also suited. Examples are bollard type structures, preferably having some sort of security to avoid the mooring cable to disconnect.
The invention is also directed to a mooring system for providing access between a floating vessel and a landing position of a marine structure comprising

(i) a marine structure having a landing position at an elevation above the water level, a mooring cable freely running via a point at the landing position and wherein the mooring cable has two parts positioned at some distance from the marine structure and
(ii) a floating vessel comprising a winch and a telescopically extendable bridge attached to the vessel at one end of the bridge and provided with a connection means for a cable at the extendable end of the bridge.

In case the vessel and the marine structure are connected one part of the mooring cable is attached to the extendable end of the bridge and one part of the mooring cable is attached to the winch. See also above.
The preferred telescopically extendable bridge of the mooring system comprises at least two bridge parts which can move telescopically relative to each other by means of bridge part hydraulics. The bridge has a pivotal attachment near or at its fixed end for pivotal movement along a horizontal by means of one or more hydraulic luffing cylinders. The bridge has a pivotal attachment near or at its fixed end for pivotal movement along a vertical by means of slewing hydraulics. More preferably the telescopically extendable bridge comprises of two bridge parts.
The preferred winch of the mooring system has a control to maintain a constant force on the mooring cable. Preferably the mooring cable is provided with a gross overload protection (GOP) such that in case of emergency the cable will disconnect from the winch when the vessel moves away from the marine structure.
The preferred floating vessel of the mooring system is provided with a dynamic positioning system.
The mooring cable may be a cable having two loose ends or a cable in the form of a closed loop. The mooring cable may be lowered from the marine structure, for example by means of a crane or the parts of the mooring cable may be fixed to a buoy to be fished up.
In case the mooring cable is fixed to a buoy it may be picked up in step (b) by detaching the cable from the buoy.
The mooring cable may be a cable having two loose ends or a cable in the form of a closed loop. A mooring cable in the form of a closed loop suitably has connections operable for connecting with the winch and the end of the bridge. In case of two loose ends, one loose end of the mooring cable is suitably attached to the extendable end of the telescopically extendable bridge via a first cable part which first cable part is fixed at one end to the extendable end and which other end is connected to the loose end of the mooring cable.Suitably the second loose end of the mooring cable is fixed to the winch via a second cable part, which second cable part is fixed to the winch at one end and which other end is connected to the second loose end of the mooring cable.
Suitably the mooring cable runs via a pulley or a bollard at the landing position.
The above mooring system is preferably used in the above method according to the invention.
The invention will be illustrated by making use of Figures 1-6. These figures illustrate the various steps (a)-(e) of the method of providing access between a floating vessel and a landing position of a marine structure according to the present invention.

Figure 1 shows a mooring system (1) for providing access between a floating vessel (2) and a landing position (3) of a marine structure (4). The marine structure (4) has a landing position (3) at an elevation above the water level (5). In Figure 1 the landing position (3) is at the so-called spider deck. A mooring cable (6) runs freely via a pulley or a bollard (3a) at the landing position (3). The mooring cable (6) has two loose ends (7, 8) positioned at a buoy (9) at some distance from the marine structure (4). The buoy (9) is attached to the sea bed (10) by means of an anchor (11).
The vessel (2) has a winch (12) and a telescopically extendable bridge (13). In figure 1 the bridge (13) is in its resting position.
In Figure 2 the bridge (13) has turned towards the marine structure (4) and is already partly extended. The vessel (2) reverses to a position where it can fish up the loose ends (7,8) of the mooring cable (6). Figure 2 also shows a first cable part (14) that is fixed at one end to the extendable end (15) of bridge (13) and is free at its other end. Also a second cable part (16) is shown that is fixed to the winch (12) at one end and which other end is free.
Figure 3 shows vessel (2) which is suitably kept in position by means of dynamic positioning. As shows, the loose ends (7,8) of the mooring cable (6) have been fished up and disconnected from buoy (9). Loose end (7) of the mooring cable (6) is attached to the extendable end (15) of the telescopically extendable bridge (13) via first cable part (14). Second loose end (8) of the mooring cable is (6) fixed to the winch (12) via second cable part (16). In this situation the hydraulics of the bridge (13) are all under pressure and active.
In Figure 4 shows the next situation wherein the mooring cable (6) is put under tension by winch (12). The mooring cable (6) which runs via a pulley or a bollard (3a) will exert a pulling force on the end (15) of the bridge. When the force is sufficiently strong the hydraulics of the bridge (13) will be gradually relieved of pressure such that the bridge can move more freely as a response to the movement of the vessel relative to the marine structure. The extendable bridge (13) will extend as a result of the pulling force of the mooring cable (6) and by the fact that the vessel (2) remains positioned.
Figure 5 shows a next situation wherein the bridge has come to its extended position. If the end (15) of the bridge (13) has not reached the landing position (3) the vessel (2) may move towards the marine structure (4). In this situation the hydraulics are relieved of pressure and thus not active, apart from the bridge part hydraulics which suitably will exert a small pull force on said bridge parts.
Figure 6 shows the situation of step (e). The bridge parts (17) and (18) are allowed to move relative to each other. The end (15) of bridge (13) will be in contact with a landing pier (19). The pull force exerted by the mooring cable (6) keeps the end (15) of the bridge at this landing pier (19). The place of contact at the landing pier (19) is preferably made of a plastic or wood. Preferably some sort of security connection is present to maintain a connection in case the pull force exerted on the mooring cable fails.
Personnel, supply, equipment and the like may now move safely exchanged via the bridge between the vessel and the marine structure. Via the bridge also supply lines for fuel for the marine structure may be positioned. These fuel supply lines can be suitably connected at the supply lines of the marine structure extending downwards towards the landing position. In prior art situations these fuel lines would extend into the sea and be connected to a buoy. This involved fishing up the fuel lines when the marine structure required refuelling. Furthermore additional inspection of these lines are required because of the harsher conditions the submerged parts of the fuel lines have to endure. By not having to extend these lines into the sea but only to the landing position above sea level a more robust system is obtained.
Figure 7 illustrates a prior art method of extending a bridge (20) towards the marine structure (4) via a guide wire (21). The guide wire (21) runs through a guide opening (25) as present on the bridge (20). The guide wire (21) is attached to the marine structure 4 at the landing position (24) at one end and attached to a vessel (23) at its other end. The guide wire is kept under tension by thrusters (26) of vessel (23). The luffing cylinders (22) will have to be relieved of pressure in such a mooring operation and the full weight of the bridge (20) will be exerted on the guide wire (21). Because of this a dip in the guide wire (21) will result as shown and in a worst case the bridge (20) cannot be moved towards landing point (24).

Claims

A method of providing access between a floating vessel (2) and a landing position (3) of a marine structure (4) by performing the following steps:
(a) positioning the vessel (2) near the marine structure (4),

(b) picking up two parts of a mooring cable (6), which parts of the mooring cable (6) extends from the marine structure (4) and wherein the intermediate part of the mooring cable (6) connecting the two parts can run freely via a position at the landing position (3) of the marine structure (4),

(c) attaching one part of the mooring cable to the extendable end (15) of an telescopically extendable bridge (13), which bridge (13) is attached at or near its other end to the floating vessel (2) and attaching the second part of the mooring cable (6) to a winch (12) fixed to the floating vessel (2),

(d) hauling the winch (12) such that the extendable end (15) of the bridge (13) is pulled towards the landing position (3) until the extendable end (15) of the bridge (13) reaches the landing position (3) and

(e) maintaining a pull force on the mooring cable (6) such that the extendable end (15) of the bridge (13) remains positioned at the landing position (3) enabling safe access between the floating vessel (2) and the marine structure (4).
A method according to claim 1, wherein the telescopically extendable bridge (13) comprises at least two bridge parts (17, 18) which can move telescopically relative to each other by means of bridge part hydraulics, wherein the bridge (13) has a pivotal attachment near or at its fixed end for pivotal movement along a horizontal by means of one or more hydraulic luffing cylinders and wherein the bridge (13) has a pivotal attachment near or at its fixed end for pivotal movement along a vertical by means of slewing hydraulics and wherein the bridge part hydraulics and the hydraulic luffing cylinders are relieved of pressure in step (e).
Method according to claim 2, wherein the bridge part hydraulics exerts a small pull force on the bridge parts (17, 18) in step (e).
Method according to any one of claims 1-3, wherein in step (e) a constant force is maintained on the mooring cable (6) by the winch (12).
Method according to any one of claims 1-4, wherein the mooring cable (6) is provided with a gross overload protection (GOP).
Method according to any one of claims 1-5, wherein the floating vessel (2) is kept in position in steps (a)-(e) by means of dynamic positioning or by means of a two point mooring.
Method according to any one of claims 1-6, wherein in step (b) the mooring cable (6) is picked up by detaching the mooring cable (6) from a buoy (9).
Method according to any one of claims 1-7, wherein the parts of the mooring cable (6) are two loose ends (7, 8) of the mooring cable (6) and wherein in step (c) the loose end (7) of the mooring cable (6) is attached to the extendable end (15) of the telescopically extendable bridge (13) via a first cable part (14) which first cable part (14) is fixed at one end to the extendable end (15) and which other end is connected to the loose end (7) of the mooring cable (6).
Method according to any one of claims 1-8, wherein the parts of the mooring cable (6) are two loose ends (7, 8) of the mooring cable (6) and wherein in step (c) the second loose end (8) of the mooring cable (6) is fixed to the winch (12) via a second cable part (16), which second cable part (16) is fixed to the winch (12) at one end and which other end is connected to the second loose end (8) of the mooring cable (6).
Method according to any one of claims 1-9, wherein the mooring cable (6) runs via a pulley or a bollard (3a) at the landing position (3).
Mooring system (1) for providing access between a floating vessel (2) and a landing position (3) of a marine structure (4) comprising
(i) a marine structure (4) having a landing position (3) at an elevation above the water level (5), a mooring cable (6) extending from the marine structure (4) wherein the mooring cable (6) has two parts positioned at some distance from the marine structure (4) and wherein an intermediate part of the mooring cable (6) connecting the two parts is freely running via a point at the landing position (3) and

(ii) a floating vessel (2) comprising a winch (12) and a telescopically extendable bridge (13) attached to the vessel (2) at one end of the bridge (13) and provided with a connection means for a cable at the extendable end (15) of the bridge (13), characterized in that
the parts of the mooring cable (6) are two loose ends (7, 8) of the mooring cable (6) and wherein one loose end (7) of the mooring cable (6) is attached to the extendable end (15) of the telescopically extendable bridge (13) via a first cable part (14) which first cable part (14) is fixed at one end to the extendable end (15) and which other end is connected to the loose end (7) of the mooring cable (6) and wherein the second loose (8) end of the mooring cable (6) is fixed to the winch (12) via a second cable part (16), which second cable part (16) is fixed to the winch (12) at one end and which other end is connected to the second loose end (8) of the mooring cable (6).
Mooring system according to claim 11, wherein the telescopically extendable bridge (13) comprises at least two bridge parts (17, 18) which can move telescopically relative to each other by means of bridge part hydraulics, wherein the bridge (13) has a pivotal attachment near or at its fixed end for pivotal movement along a horizontal by means of one or more hydraulic luffing cylinders and wherein the bridge (13) has a pivotal attachment near or at its fixed end for pivotal movement along a vertical by means of slewing hydraulics.
Mooring system according to any one of claims 11-12, wherein part of the mooring cable (6) is attached to the extendable end (15) of the bridge (13) and another part of the mooring cable 6 is attached to the winch (12) and wherein the mooring cable (6) consists of a closed loop.