WO2001058749A1

WO2001058749A1 - Method and device for offshore loading of hydrocarbons

Info

Publication number: WO2001058749A1
Application number: PCT/NO2001/000051
Authority: WO
Inventors: Jan Fosso
Original assignee: Ingenium As
Priority date: 2000-02-14
Filing date: 2001-02-14
Publication date: 2001-08-16
Also published as: NO310605B1; NO20000739D0; AU2001236220A1; NO20000739A

Abstract

The invention relates to a method and a means for offshore loading of hydrocarbons from a stationary loading station (1) to a dynamically positioned vessel (2) which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes (51, 52, 53) suspended in separate arcs from the loading station (1) to the vessel (2). Each of the pipes (51, 52, 53) are in one end suspended from a swivel (61, 62, 63) arranged in the loading station (1), and in their other end suspended from a swivel (71, 72, 73) arranged in the vessel (2). The rotation of the vessel (2) according to the prevailing wave direction is carried out as a combination of a rotation of the vessel around its own axis and a movement along an arc (3) around the loading station (1).

Description

Method and device for offshore loading of hydrocarbons

The invention relates to a method for offshore loading of hydrocarbons from a stationary loading station to a dynamically positioned vessel which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes suspended in separate arcs from the loading station to the vessel

The invention also relates to a means for offshore loading of hydrocarbons from a stationary loading station to a dynamically positioned vessel which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes suspended in separate arcs from the loading station to the vessel

When producing hydrocarbons from reservoirs located underneath the sea floor, it is often desirable to load the hydrocarbons to a vessel for storage or transport ashore This loading can be done from a floating storage buoy, a fixed or floating storage tank, or from production equipment located on the sea floor The vessel may be a tanker, but it can also be another structure, e g a floating platform structure

During the loading the vessel can be kept stationary by anchoring or dynamic positioning In dynamic positioning detectors all the time sense the position and movement of the vessel, and based on signals from the sensors, propellers which bring the vessel to the desired position are controlled When loading to a dynamically positioned ship it is required that the ship is lying with the bow against the prevailing wave direction, possibly the prevailing wind direction, and to be able to load in all weather conditions, the ship must therefore be able to rotate 360° during the loading When loading to other types of vessels than ships there may be a corresponding need to rotate the vessel If the weather gets too bad, and the movement of the vessel gets to large, the couplings between the pipes and the vessel are released, in order not to damage the pipes or the couplings

For the loading one or more pipes which have to be flexible are used, since the vessel due to the movement of the sea never will be absolutely quiescent Due to requirements to strength, the pipes are still relatively stiff, and must be treated carefully If nothing is done to prevent it, said rotation of the vessel during a dynamic positioning will result in a twisting of the pipe or the pipes, and, if there are more than one pipe, the pipes may contact each other and rub against each other and attack each other by lateral forces, which may result in damage to the pipes GB 2 168 939 describes loading of hydrocarbons from a piping system on the sea floor to a ship via a riser containing several pipes The riser extends from a foundation on the sea floor to a buoy which via a multiswivel can be connected to an arm of the ship The multiswivel includes several concentric annuh, which on the underside of the swivel are connected to its individual pipe, and on the upper side of the multiswivel are connected to pipes which transport the hydrocarbons to the ship The hydrocarbons in the different pipes can thereby be separately transported through the multiswivel, while at the same time the ship is allowed to rotate The above mentioned problem related to rubbing and lateral forces between the pipes due to mutual contact during a rotation of the vessel is thereby solved The multiswivel with several concentric annuh, is, however, a complicated and expensive device, and it is therefore desirable to solve the problem in a simpler and less expensive way GB 2 173 160 describes loading of hydrocarbons from a piping systems on the sea floor to a ship via a riser which in its lower end is provided with a weight, and m its upper end is provided with a floating body The riser is normally submerged When loading, the riser is hauled up to the ship, where it is connected to an arm via a swivel, which allow rotating the ship without a rotation of the riser The riser includes only one pipe, and thus the problem of rubbing and lateral forces between the pipes during a rotation of the vessel does not exist

NO B 147 868 describes loading of hydrocarbons from a piping system on the sea floor to a ship via risers which are routed to a caisson The caisson carries three coaxial connections, which form separate penetrations for a tool-insertion pipe, a loading pipe and a safety pipe from the caisson to the ship A rotation of the ship is thereby allowed, and the problem of rubbing and lateral forces between the pipes is thereby solved The disclosed pipe connection is, however, a complicated device

NO 150 791 describes loading of hydrocarbons from a piping system on the sea floor to a ship via a riser comprising several pipes, and which ends in a subsea buoy From the buoy flexible pipes are routed slantingly upwards, above a curved surface of the buoy, and then in arcs hanging down, which takes the shape of catenaries, and further to the ship Different pipes have different lengths, and the individual catenaries are thereby separated, which reduces the possibility of rubbing and/or entanglement of the production pipes It is stated that the riser system has an excellent compliance which compensates for the normal pitch, roll and drift of the vessel, and any normal turbulence just below the surface The problem of mutual contact and forces between the pipes during a rotation of the vessel is, however, not mentioned, and the disclosed devices are neither able to solve this problem

NO 154 993 describes loading of hydrocarbons from a piping system on the sea floor to a ship via one or more flexible pipes or houses with a buoy in its upper end During connection before loading the buoy is hauled up to the ship, where it is connected to an arm via a swivel which allows rotating the ship without any rotation of the pipe or hose If more than one flexible pipe or hose are used, a multiswivel is used The problem of rubbing and lateral forces between the pipes during a rotation of the ship is then solved, but as mentioned a multiswivel is a complicated and expensive device

It is further known loading of hydrocarbons from a subsea buoy to a turret on the underside of a ship Such a turret includes inter alia a multiswivel During offshore loading of hydrocarbons from a stationary loading station to a dynamically positioned vessel which during the loading is rotated according to the prevailing wave direction, and in which the hydrocarbons are transported through at least two pipes, according to the above solutions the pipes are connected to the vessel by a multiswivel A multiswivel is as mentioned a pipe connection with one concentric annuh for each pipe, which is complicated and expensive

The object of the invention is to provide a method and a means for offshore loading of hydrocarbons from a stationary loading station to a dynamically positioned vessel which during the loading is rotated according to the prevailing wave direction, which method and means shall not cause twisting of the pipes or contact between the pipes, and be simpler than known methods and devices for the same use A particular object is to enable the vessel to rotate 360°

According to the invention the object is achieved by a method and a means as mentioned in the introductory part, which is characterised by the features of the claims The invention thus consists of a method and a means for offshore loading of hydrocarbons from a stationary loading station to a dynamically positioned vessel which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes suspended in separate arcs from the loading station to the vessel The loading station may be a floating or subsea buoy which is connected to a piping system on the sea floor, or another fixed or floating structure, e g a loading arm of a storage buoy Each of the pipes are in their one end suspended from a swivel arranged in the loading station, and in their other end suspended from a swivel arranged in the vessel The flexible pipes preferably hang in catenaries, since this provides minimum stress both to the pipes and the swivels, but this is no requirement for the invention

According to the invention the rotation of the vessel according to the prevailing wave direction is carried out as a combination of a rotation about the vessel's own axis and a movement of the vessel along an arc around the loading station The vessel can then move between two ends of the arc The vessel's swivels may be located under a projecting arm, or in the underside of the vessel Preferably both the swivels of the loading station and the swivels of the vessel are arranged in a row The row of swivels on the loading station and the row of swivels on the vessel are preferably aligned when the vessel is in a mid position between the ends of the arc

The flexible pipes preferably have different lengths The lengths are preferably adapted in such a way that the pipe which is suspended from a swivel on the loading station which is nearest the vessel is the shortest, and is routed to a swivel on the vessel which is nearest the loading station when the vessel is in its mid position Further, the pipe which is suspended from a swivel on the loading station which is most distant from the vessel is preferably the longest pipe, and is routed to a swivel on the vessel which is most distant from the loading station when the vessel is in its mid position The remaining pipes preferably has intermediate lengths, and thus are suspended in intermediate arcs

The movement of the vessel along the arc preferably take place along an arc having essentially the shape of a circle segment, in order not to subject the pipes to too large bending stresses The vessel preferably moves along an arc of approximately 180°, with the same portion of the vessel facing the loading station The vessel is thereby rotated 180° during its movement along the arc A further rotation of the vessel of 90° around its own axis when it is located in one of the ends of the arc, enables a total rotation of the vessel of 360° without the use of multiswivels As will appear from the special part of the description, the rotation of the vessel does not result in that the pipes are twisted or contact each other

The invention will now be explained in more detail in connection to a description of a specific embodiment, and with reference to the drawings, in which fig 1 illustrates a ship which is loading hydrocarbons from a loading station, which ship is in a mid position, fig 2 illustrates the front part of the ship in fig 1 in a larger scale, fig 3 illustrates the ship and the loading station from above, with the ship in a mid position, a 45° position, a 90° position and a 180° position, fig 4 illustrates the ship in a 45° position, seen in an angle from above, fig 5 illustrates the ship in a 45° position, seen in an angle from below, fig 6 illustrates the ship in a 90° position, seen in an angle from above, fig 7 illustrates the ship in a 90° position, seen in an angle from below, fig 8 illustrates the ship in a 180° position seen in an angle from above, and fig 9 illustrates the ship in a 180° position, seen in an angle from below

Fig 1 illustrates a ship 2 lying in the sea, and which is loading hydrocarbons in the form of oil from a stationary loading station 1 Fig 2 illustrates the front part of the ship 2 and the loading station 1 in a larger scale The loading station 1 is a subsea buoy, and is via a stiff raiser 12 connected to a piping system (not illustrated) on the sea floor, which is connected to oil producing wells (not illustrated) The lower end of the raiser 12 is anchored to the sea floor by a joint, and the raiser 12 with the buoy 1 can thereby yield somewhat for the waves, in order to avoid excessive strain from the waves Three flexible pipes 51, 52, 53 hang in separate arcs from the loading station 1 to the ship 2 Each of the pipes 51, 52, 53 are in one end suspended from a swivel 61, 62, 63 arranged on the underside of a loading arm 10 on the loading station 1, and in the other end suspended from a swivel 71, 72, 73 arranged on the underside of a projecting arm 6 in the bow 7 of the ship 2 The swivels 61, 62, 63 on the underside of the loading arm 10 are not shown in fig 1 or 2, but are illustrated in fig 5 and 7

The swivels 71, 72, 73 are arranged in a coupling buoy 8 (see fig 8), which is connected to the arm 6 with releasable couplings 81, 82, 83 If the weather gets too bad, the pipes 51, 52, 53 must be disconnected from the ship, in order to avoid that they are damaged by strains from the waves and movements of the ship The couplings 81, 82, 83 are then released, causing the coupling buoy 8 with the pipes 51, 52, 53 to fall down in the sea The coupling buoy 8 is anchored to the sea floor by a mooring line 9 with weights 13, and it will therefore fall down at a predictable place, in which it is located when it is not used during loading The coupling buoy 8 includes buoyant bodies, and will therefore seek to the surface, which enable finding it when it again is desirable to connect it to the arm 6

During the loading the oil is transported from the wells, through the piping system on the sea floor, through the raiser 12, the buoy 1, supply pipes 11, the swivels 61, 62, 63, the pipes 51, 52, 53, the swivels 71, 72, 73 and the couplings 81, 82, 83 to not illustrated pipes in the arm 6 From here the oil is transported to process equipment and storage tanks in the ship The loading thus takes place simultaneously with the production, but it should be understood that the invention is equally useable for loading from a storage buoy

The ship is kept stationary by dynamic positioning, which means that sensors all the time detect the position and movement of the ship, and based on signals from the sensors, propellers which bring the ship to the desired position are controlled When loading to a dynamically positioned ship it is required that the ship is lying with the bow against the prevailing wave direction, possibly prevailing wind direction, and in order to be able to load in all weather conditions, the ship therefore must be able to rotate 360° during the loading

Fig 3 illustrates the loading station 1 from above, with the ship 2 in four different positions, which is a result of a rotation of the ship according to the prevailing wave direction The position marked with 0° is designated the mid position, and corresponds to the position of the ship in fig 1 and 2 In the other positions the ship is rotated 45°, 90° and 180°, which is indicated with the corresponding degree figures

It is seen from fig 3 that the rotation of the ship 2 has been carried out as a combination of a rotation around the ship's own axis and a movement along an arc 3 around the loading station 1 The movement of the ship 2 along the arc 3 has taken place along an arc 3 essentially having the shape of a circle segment, which is approximately 180°

During its movement along the arc the ship has moved from the 0° position to the 45° position and further to the 90° position, in which the ship is located in an end 4 of the arc The movement of the ship 2 along the arc 3 has taken place with the same portion of the ship 2, namely the bow with the arm 6, facing the loading station 1, which has resulted in that during its movement along 90° of the arc 3 the ship itself has been rotated 90° In the end 4 of the arc the ship has carried out a 90° rotation around its own axis, from the 90° position to the 180° position.

It is understood that the ship can go through a corresponding movement along that part of the arc 3 which is on the right side in fig 3, and a corresponding rotation around its own axis when it is located in the second end 5 of the arc The ship can thereby be rotated from a 180° position on the one side of the mid position to a 180° position on the other side of the mid position, which amount to a total rotation of 360°

The ship could also have been rotated around its own axis while it was lying in a position along the arc 3, e g the ship could have been rotated 90° while it was located in the mid position, and then have been moved along the arc 3, whereupon it could have been further rotated in another position, possibly during the movement along the arc 3 It is realised from fig 3 that a number of different combinations of a rotation of the ship 2 around its own axis and a movement along the arc 3 enable a total rotation of the ship of 360° The way of rotation and movement which is illustrated in fig 3 is, however, preferred, since it provides a well defined control of the rotation and the movement of the ship The positions of the ship 2 relative to the loading station 1 which are illustrated in fig 4-9 correspond to the positions which are illustrated in fig 3

Fig 4 and 5 illustrate the ship 2 in a 45° position, seen in an angle from above and from below, respectively The pipes 51, 52, 53 are somewhat rotated in their swivels 61, 62, 63 and 71, 72, 73, and no twisting of the pipes has taken place The pipes 51,

52, 53 are separately suspended, and are not subjected to rubbing against each other or lateral forces

Fig 6 and 7 illustrate the ship 2 in a 90° position, seen in an angle from above and from below, respectively The pipes 51 , 52, 53 have been further rotated in their swivels 61, 72, 63 and 71, 72, 73, without being twisted The pipes 51, 52, 53 are still separately suspended, and are not subjected to any lateral strains

Fig 8 and 9 illustrate the ship 2 in a 180° position, seen in an angle from above and from below, respectively The pipes 51 , 52, 53 have been further rotated in their swivels 61, 62, 63 and 71, 72, 73, and still no twisting of the pipes has taken place It is seen that the pipes 51, 52, 53 are still separately suspended

It is seen from the figures, e g fig 5, that the swivels 61, 62, 63 of the loading station are arranged in a row It is further seen that also the swivels 71, 72, 73 of the ship are arranged in a row In the ship's mid position, see fig 2, the two rows of swivels are aligned It is further seen that the flexible pipes have different lengths, the pipe 51 which is suspended from the rear swivel 61 of the loading station 1 to the rear swivel 71 of the arm 6 of the ship hanging at the bottom and being the longest Further the pipe 53 which is suspended from the front swivel 63 on the loading station 1 to the front swivel 73 on the arm 6 of the ship is the shortest, and is hanging at the top The pipe 52 takes an intermediate position, and has a length which is between the length of the pipe 51 and the pipe 53 It is understood that the number of pipes could have been larger, I e more pipes could have been arranged in a manner corresponding to the pipes 51, 52, 53

The configuration of the swivels and the pipes which is illustrated in the figures is preferred, but also other configurations of the swivels and the pipes can enable a rotation of the ship of 360°, as explained above, without a twisting of the pipes or mutual contact between the pipes

The invention has in the above been explained with reference to a preferred embodiment It should, however, be understood that variants of the invention are possible within the scope of the claims, e g associated with the location of the swivels 71, 72, 73 of the ship, which instead of being arranged on the underside of the arm 6 may be arranged e g in the underside of the ship 2

Claims

PATENT CLAIMS

1. A method for offshore loading of hydrocarbons from the stationary loading station (1) to a dynamically positioned vessel (2) which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes (51 , 52, 53) suspended in separate arcs from the loading station (1) to the vessel (2), characterised in that, each pipe (51, 52, 53) in one end being suspended from a swivel (61, 62, 63) arranged in the loading station (1), and in its other end being suspended from a swivel (71, 72, 73) arranged in the vessel (2), the rotation of the vessel (2) according to the prevailing wave direction is carried out as a combination of a rotation of the vessel around its own axis and a movement along an arc (3) around the loading station (1)

2 A method according to claim 1 , characterised in that the movement of the vessel (2) along the arc (3) takes place along an arc (3) essentially having the shape of a circle segment.

3. A method according to claim 2, characterised in that the arc (3) is approximately 180°.

4. A method according to one of the proceeding claims, characterised in that the movement of the vessel (2) along the arc (3) takes place with the same portion of the vessel (2) facing the loading station (1). 5. A method according to one of the proceeding claims, characterised in that the rotation of the vessel (2) around its own axis is carried out when the vessel (2) is located in one of the ends (4,

5) of the arc.

6. A means for offshore loading of hydrocarbons from a stationary loading station (1) to a dynamically positioned vessel (2) which during the loading is rotated according to the prevailing wave direction, in which the hydrocarbons are transported through at least two flexible pipes (51, 52, 53) suspended in separate arcs from the loading station (1) to the vessel (2), characterised in that each of the pipes (51, 52, 53) in one end are suspended from a swivel (61 , 62, 63) arranged in the loading station (1), and in their other end are suspended from a swivel (71, 72, 73) arranged in the vessel (2)

7. A means according to claim 6, characterised in that the swivels (61, 62, 63) of the loading station are arranged in a row.

8 A means according to claim 6 or 7, characterised in that the swivels (71, 72, 73) of the vessel are arranged in a row 9 A means according to one of the claims 6-8, characterised in that the swivels (71, 72, 73) of the vessel are arranged on the underside of a projecting arm (6)

10 A means according to one of the claims 6-8, characterised in that the swivels (71 , 72, 73) of the vessel are arranged in the underside of the vessel (2).