CA2054217A1 - Vertical helix fluid transfer system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A rotating fluid transfer system for offshore oil fields is disclosed. The system uses flexible pipe and allows at least five or six rotations before the system reaches its limit. It uses flexible pipe in a vertical column arranged around a central rigid pipe which acts as a bend restrictor. The bottom of the pipes are attached to the mooring device and the top of the pipes are attached to a tower that stands on the vessel deck. As the vessel rotates around the mooring device the flexible pipes are twisted around the central rigid pipe forming a helix. An axial swivel is provided in each flexible pipe to relieve torsion in the flexible pipe.

Description

2 .~ 7 VERTICAL HELIX FLUID TRANSFER SYSTEM

Field of the Invention The present invention relates to hydrocarbon production from offshore oil fields to a ship-shaped floating production facility. In particular, it relates to the transfer of fluids between the mooring assembly which does not rotate relative to the seafloor and the floating vessel which can rotate around the mooring assembly.

Backqround of the Invention When ship-shaped floating production facilities are used to produce offshore oil and gas fields ~hat are located in areas with severe environmental conditions they are usually moored so that the floating vessel can weathervane around the mooring system. This significantly reduces the mooring loads. There are many methods of mooring a weathervaning floating facility but they all have a mooring device at the sea surface that is restrained from any appreciable rotation by the mooring system. Fluids are transferred from the seabed to this mooring device at the surface and then to the weathervaning floating facility. Because of the relative rotation between the weathervaning floating facility and the mooring device the fluid transfer lines must be provided with some form of swivel.

2 ~ 7 Conventionally, the majority of floating production facilities have used multi-path, multi-pass swivels which are mechanical devices that use elastomeric seals. These swivels have all been for use with low fluid pressures S and a relatively low number of lines and amount of fluids. For higher pressures and larger volumes of fluids, multi-pass swivels will become very large. This raises concern about reliability of the sealing system and the a~ility of maintaining and repairing the swivel.
As a result there have been many proposed fluid transfer schemes that use flexible pipe. Most of these are based on flexible pipe wound on a drum that is mounted on the axis of rotation of the vessel. The significant limitation of these schemes is the limited number of rotations that the vessel can make before the system runs out of pipe on the drum. Most schemes only allow between one and two rotations. The reason for this limit is usually because the size of the system becomes too large, especially in deck area required. When the limit is reached the system must either be disconnected which, if an alternative transfer system is not provided, results in the shut down of production, or the vessel can be rotated back to its original position using thrusters.
It is therefore desirable to have a system that can make a high number of turns so the number of rewinding operations can be kept to a minimum or so that there is sufficient time available to wait for a suitable weather window to rotate the vessel.

Summary of the Xnvention The present invention is a rotating fluid transfer system that uses fle~ible pipe and allows at least five or six rotations before the system reaches its limit. It - 2~ 2~

uses ~lexible pipe in a vertical column arranged around a central rigid pipe which acts as a bend restrictor. The bottom of the pipes are attached ~o the mooring device and the top of the pipes are attached to a tower that stands on the vessel deck. As the vessel rotates around the mooring device the flexible pipes are twisted around the central rigid pipe forming a helix. An axial swivel is provided in each flexible pipe to relieve torsion in the flexible pipe.
The use of flexible pipe requires that attention be paid to the radius of bend when ~he pipe is flexed. There is a minimum bend radius below which it must not be bent or permanent material deformation will occur. If the pipe is to be bent a large number of times under high axial loads or high internal pressures then the minimum bend radius must be increased. One objective of the present invention is to use the flexible pipe so that it does not bend more than the minimum static and dynamic bend radii and that the tension and torsional forces are controlled.
Over most of the length of the flexible pipe the central rigid pipe acts as a bend restrictor. The radius of the central rigid pipe can be less than the minimum bend radius required for the flexible pipe because the flexible pipe lays at an angle across the rigid pipe. At any given helix angle a cross-section of the central core will locally be an ellipse. The flexible pipe touches the ellipse at the location of the maximum radius of the ellipse. This is the bend radius of the flexible pipe.
Because the flexible pipe goes around the central rigid pipe in a helix the plane of bending will continuously change and the flexible pipe bend radius will always be at the maximum radius of the ellipse. Thus the minimum bend radius is a function of the radius of the central pipe and the helix angle. The number of rotations is 2 ~ 2 ~ ~1 dependant on the helix angle, the radius o~ the central rigid pipe, and the vertical length of the flexible pipe.

One feature of the helix forming around the central rigid pipe is that the bend radius changes in ~roportion to the number of rotations. The bend radius of the flexible pipe is infinite when the system is in the unrotated condition and gradually reduces until it is the minimum bend radius at the maximum number of rotations.
When there is no rotation the flexible pipes hang vertically so there is no bending. As rotation starts a helix begins to form and the local cross-section of the central rigid pipe will be an extremely elongated ellipse with the major radius approaching infinity. Thus the bend radius of the flexible pipe is almost infinite. As the amount of rotation increases the angle of the helix changes such that the geometry of the ellipse becomes less elongated and the major radius, and thus the flexible pipe bend radius, decreases. When the number of rotations reaches its limit the major radius of the ellipse will have decreased to the extent that it should be at the minimum bend radius allowed for the flexible pipe. In operation it is expected that the vessel will reach the maximum number of rotations very infrequently and the number of times the vessel rotates a small amount will be very large. As shown above, the bend radius for a low amount of rotation is very large and conversely the bend radius for a high number of rotations is small.
Thus for the majority of the flexing of the flexible pipe the bend radius will be large and bending to the minimum bend radius will be infrequent. These are the characteristics required for long life of the flexible plpe.
When the vessel rotates around the mooring device and the flexible pipe is wound around the central rigid 2 ~ 7 pipe the flexible pipe, if held rigidly at either end, would also be rotated about its own axis the same angular amount as the vessel rotation. To avoid this twisting and therefore avoid any torque on the flexible pipe an axial swivel is placed at the end of the pipe. With the use of a swivel there will be a relative rotation between the flexible pipe and the central rigid pipe. The relative motion between the two sur~aces could be handled by employing low friction materials. But the preferred method is to use a series of sleeves one above the other on the central rigid pipe. Each length of sleeve is able to rotate independently of its neighbours to accommodate the different amount of relative rotations from top to bottom.
The sleeves on the central rigid pipe also serve a second function. When the flexible pipes form a helix the vertical height decreases producing a vertical movement of the flexible pipes relative to the central rigid pipe. Again this motion could be accommodated by the use of low friction materials but the preferred method is to ~mploy the same sleeves used for the relative rotations. Gaps are required between the sleeves to allow for vertical movement.
For simplicity of design and operation, the flexible pipes are suspended from their top end. Because the vertical length of the flexible pipes change as they form a helix, a semi circular loop is used at the lower end so that each flexible pipe doubles back on itself.
An extra amount of flexible pipe in the loop accommodates the change in the vertical length of the helix. The flexible pipe must be constrained sideways at the bottom of the helix before going into the loop to prevent the helix formation from going into the loop where it will not be properly controlled. The simplest method is to have the flexible pipes pass through holes in a guide ~0~2~7 structure so that the sides provide constraint and the pipe is ree to slide vertically. The holes must be oversized or elliptical to accommodate the pipes at an angle when they form the helix. I sliding of the pipes in the holes is not acceptable because of the possibility of wear then rollers can be used.
When the helix is at its maximum number of rotations the loops at the bottom will be at their smallest. For this the loops are arranged to have a radius greater than the minimum bend radius. This is controlled by the depth of the loop and the distance between the flexible pipe end and guide holes where the pipe enters the helix. When there is no helix there will be more flexible pipe available for the loop so the loop will increase in size and increase the radius of bend.
Thus the minimum bend radius will only be reached when the maximum number of rotations occurs.
At the top of the flexible pipes when there is no rotation the flexible pipes will hang straight down.
When the helix is formed the flexible pipes will bend towards the helix angle and also to either side of the csntral rigid pipe depending on the rotation. This bending at the ends of the flexible pipe as they enter the top of the helix can be controlled in several ways.
A flexible bend restrictor can be placed over the flexible pipe or a bellmouth can be used. If a bellmouth is used it would be attached to the flexible pipe so that it would rotate with the pipe. The axial swivel for each pipe would also be located at the top end of the flexible pipes so the support for the rotating bellmouth would work in conjunction with the swivel support. The ends of the flexible pipes can be oriented vertically or at an angle. If they are vertical then when there is no helix the is no bending. I they are placed at an angle such as at the angle that the helix forms at maximum rotation 2 :~ 7 then the total amount of bending in this condition is reduced. Setting the angle between these two extremes will reduce the amount of bending to a minimum.
From the above it can be seen that the ~lexible pipe is controlled over its total length so that it is not bent beyond its minimum bend radius, is not put in torsion and the minimum bend radius only occurs at the maximum number of rotations of the transfer system.
The number of rotations depends on the vertical height of the helix. For a system about the heiyht of a drilling derrick at least five turns are possible. If the production vessel is equipped with thrusters then at some time during the time period taken to produce these five rotations a calmer weather period should occur when the vessel can be rotated with the thrusters back to its original position. If thrusters are not available or if the risk of production shut down is to be avoided then a temporary bypass system can be provided to allow the system to be rewound to the neutral position. In a bypass arrangement the flows from the mooring device, instead of entering the helix at the beginning o~ the loops, are directed towards independent exterior loops.
Connectors with flow shut off valves are provided at the end of the helix loops and at the exterior bypass loops.
During normal operations the bypass loops are disconnected. When the bypass loops are to be used the vessel must be in approximately the right orientation relative to the mooring device so the external bypass loops can be connected. With the bypass in place the helix loops are disconnected allowing the bottom end of the helix to be rotated to unwind the helix. The size of the external bypass loops determines how much vessel rotation is possible during the time when the bypass is in operation. If the need to have a specific vessel orientation to connect the bypass loops is too much of a 2 ~

limitation then extra positions for the loops to connect to can be provided.

Brief Description of the Drawin~s The invention is illustrated by way of example in the accompanying drawings in which:

FIGURE lA is a perspective view of a fluid transfer system using flexible pipe in a vertical column. It illustrates a system with no bypass arrangement and only two flexible pipes are shown for simplicity and with the system unrotated;
FIGURE lB is the same as Figure lA but with the system rotated;
FIGURE 2 is a cross-section of a vertical helix fluid transfer system with no bypass;
FIGURE 3 is a diagrammatic drawing of a flexible pipe wound in a helix around a central rigid pipe and shows that the bend radius of the flexible pipe is the major radius of an elliptical cross-section of the central rigid pipe;
FIGURE 4 is a diagrammatic drawing of the flexible pipe entering the helix from a vertically oriented bellmouth and shows how the bellmouth protects the flexible pipe from excessive bending;
FIGURE 5 is a diagrammatic drawing of the flexible pipe entering the helix from an angled bellmouth:
FIGURE 6A is a perspective view of a fluid transfer system using flexible pipe in a vertical and with a bypass arrangement; only two flexible pipes are shown for simplicity and with the system unrotated;
FIGURE 6B is the same as Figure 6A but with the system rotated; and FIGURE 7 is a cross-section of a vertical helix 2~42 ~ ~

fluid transfer system with a b~pass.

Detail Description of the Invention Referring to Figures 1 and 2, a circular ~ooring device 3 of a floating production system is shown. In some applications this could be the turret of a turret moored floating production system where the mooring winches are on the turret and the mooring lines exit from the bottom of the turret and go to the anchors on the seafloor. Surrounding and above the turret is a derrick 2 securely mounted on the deck of a vessel. The vessel can rotate around the turret. The fluid transfer system consists of flexible pipes from the turret to the top of the derrick. The flexible pipes 1 surround a vertical central rigid pipe 4. ~t the top of the fluid transfer system the flexible pipes attach to axial swivels 5 that are attached to rigid pipas 10 which go down through the derrick structure to the vessel deck. At the bottom of the fluid transfer system the flexible pipes pass through holes 6 in guide plate 7, loop back to pass through guide plate 7 again and connect to the rigid piping 9 on the turret.
When there is no rotation of the vessel around the turret the flexible pipes hang straight down as shown in Figure lA. When the vessel rotates around the turret, the derrick 2, which is attached to the vessel, rotates the tops of the flexible pipes which causes them to be twisted into a helical form 15 as shown in Figure lB. An axial swivel 5 is located at the top of each flexible pipe so that as the flexible pipe bends around the central rigid pipe 4 no torque is developed in the flexible pipe. If no swivel were used then the flexible pipe would be rotated about its own axis the same number of rotations as that to produce the helix.

2 ~3 As the flexible pipes form a helix, the central rigid pipe 4 acts as a bend restrictor for the ~lexible pipes 1 to prevent the flexible pipes from bending more than their minimum permitted bend radius. The radius of the central rigid pipe can be less than the minimum bend radius required for the flexible pipe because the flexible pipe lays at an angle across the rigid pipe.
This is shown in figure 3. At any given helix angle 11 a cross-section of the central core will locally be an ellipse 12 (Figure 3B). The flexible pipe touches the ellipse at the location 13 of the maximum radius 14 of the ellipse. This is the bend radius of the flexible pipe as they form a helix around the central rigid pipe 4. Because the flexible pipe goes around the central rigid pipe in a helix the plane of bending will continuously change and the flexible pipe bend radius will always be at the maximum radius of the ellipse.
On the outside of the central rigid pipe are sleeves 16 that are free to slide up and down and rotate relative to the central rigid pipe. The sleeves can also rotate independently of each other but are linked so that they have a limited range of travel axially relative to each other. The sleeves have two purposes. One is to allow the pipes to rotate on the central rigid pipe by different amounts throughout the height of the system.
It is possible to allow the pipes to slide on the central rigid pipe without the use of sleeves, but the use of sleeves will ensure that any secondary forces due to friction are minimized. The second purpose of the sleeves is to accommodate the change in length of the helical arrangement with a change of the helix angle. As with the rotation, it is possible to allow the pipe to slide on the central rigid pipe but the sleeves will minimize problems due to friction. To allow each sleeve to slide independently there must be a gap between ~ 0 ~ 7 sleeves. Because the flexible pipes are held at the top of the assembly the vertical movement of the pipes will be greatest at the bottom of the assembly. Therefore the sleeves must have progressively more movemen~ in proportion to their distance from the top. This can be accomplished by either having stops located on the central rigid pipe for each sleeve or by having all the sleeves joined so that there is a limited movement between sleeves.
Figure 2 shows the top of each flexible pipe terminated in an end fitting 17, a bellmouth 18 and a swivel 5. The end fitting 17 is the normal end fitting used for flexible pipe that is suitable for connecting to other equipment. The swivel 5 is as described previously. The bellmouth 18 is attached to the flexible pipe and the end fitting and is used to restrict the amount of bend of the flexible pipe. Since the bellmouth 18 is attached to the pipe and the endfitting it will rotate with the pipe and thus minimize sliding if the pipe moves from side to side. The bellmouth can be oriented vertically as shown diagrammatically in Figure 4 or at an angle as shown diagrammatically in Figure 5. In Figure 4 the flexible pipe is shown leaving the vertical bellmouth 18 either straight down (position 19) or at the maximum helix angle (position 20). In the plan view (Figure 4B), the flexible pipe is shown to be able to go either side of the central rigid pipe 21 as indicated by positions 21 and 23. With the bellmouth vertical and no rotation on the system the flexible pipe hangs straight down. From this position the angle of bend progresses from zero to the maximum helix angle as the system is rotated. The radius of the bellmouth can be either constant or it can be varied so that at low angles of bend the radius is larger. This increased bend radius would benefit the flexible pipe since the highest number 2 ~ ~ ~ 2 ~ ~

of times that the flexible pipe will be bent will be at low angles. For a vertical bellmouth the flexible pipe will be moved away from the central rigid pipe in proportion to the size of the bellmouth.
An alternative position for the bellmouth is to place it at an angle. Figure 5 shows the bellmouth 50 placed at the maximum angle of the helix. This has the potential for having the least angle of bend of the flexible pipes when helix is at the maximum angle, which is when the flexible pipe will be the most heavily loaded. From the forgoing it can be seen that there is a wide range of options for positioning the bellmouth to suit any specific configuration or si~e of flexible pipes.
A bellmouth has been shown for limitiny the bend radius of the flexible pipe at the top of the flexible pipes. An alternative is to use a local bend restrictor attached to the pipe that provides progressive stiffening of the pipe Still further possibilities exist with the use of static structural shapes.
Loops 8 ~Figure lA) at the bottom end of the flexible pipes provide a means of accommodaking ~he height change of the helix as the helix angle~increases.
Figure 2 shows the loops in more detail. The loop is formed by the flexible pipe being constrained by the end fitting 26 and the hole 6 in the guide plate 7. ~hen the helix has its maximum angle the loop will be as shown in position 25 and the radius of the bend will be at the minimum bend radius. When there is no rotation and therefore no helix, the loop will be as shown in position 24. Because there is more flexible pipe available for the loop in the unrotated condition, loop position 24 will have a larger bend radius. As the helix forms and the angle increases, flexible pipe will be drawn through hole 6. Hole 6 will be larger than the flexible pipe to accommodate the flexible pipe when it is at the maximum helix angle. The hole could also be elliptical in shape.
Whatever the hole shape the edges must be smooth and rounded to allow free sliding of the flexible pipe.
Alternatively, rollers can be provided.
Several other features are shown in Figure 2.
~hut-off valves 27 can be provided at the ends of the flexible pipe to facilitate removal of the flexible pipes or to shut off flow in the event of a fire or other emergency. To protect the flexible pipes from fire, since they are less fire resistant than rigid pipe, a shroud 29 can be used. The loops at the bottom end of the assembly can also be enclosed within the structure 31 and guide plate 7. The flexible pipe end terminations 17 and 26 can also be locally enclosed.
The number of rotations of the system is dependant on the helix height. Figure 2 shows the flexible pipe when it is unrotated (position 1) and when it is in a helix (position 28). The length of helix for one rotation is shown as 30 and is dependant on the helix angle and diameter of the central rigid pipe.
Electrical power and communication si~nals must also be transferred as well as the fluids. These lines could also be part of the helix arrangement with an appropriate electrical swivel at the top end instead of a fluid swivel. But it is more convenient to have the power and control lines go through the middle of the central rigid pipe with the swivel at either the top or the bottom. In Figure 2 the swivel 32 is shown in the lower position. In this way the electrical system is kept segregated from the hydrocarbon system.
The system described so far is for a fluid transfer system that requires the vessel to be rotated whenever the fluid transfer system ie to be brought back to the neutral position. This is acceptable if the system has l 2 ~ ~

been designed for more than two or three turns. If it is considered to be operationally inconvenient to rotate the vessel then a method of unwinding the helix can be provided. Figure 6 shows an addition to the basic system that allows the helix to be unwound without having to shut down the flows. A bypass provides an alternative flow path during the unwinding of the heli~. The helix flowpath is disconnected using connector 34 and a bypass flexible pipe loop 44 is connected using a connector 35.
With the helix flowpath disconnected at connector 34, guide plate 55 can be rotated until the helix is unwound or, if preferred, to any desired position such as winding it past neutral to form a helix in the opposite direction. Figure 6A shows the unwound condition while Figure 6B shows the system with a helix formed. Only two flexible pipes are shown for clarity. The bypass system is shown adjacent to the helix loop but the bypass loops could all be clustered on one side of the system.
The bypass and unwinding arrangement is shown in more detail in Figure 7. Guide plate 55 is free to rotate relative to support 47. It serves the same function as guide plate 7 in Figure 2. At the end of the helix flexible pipe loop 45 is the termination 39, a shut-off valve 37 and a connector 34. This flexible pipe end assembly can be moved up and down relative to guide plate 55. Bypass loop 44 also has a termination 38, a shut-off valve 36 and a connector 35 that can move up and down relative to carriage 43. The other end of bypass loop 44 has an end termination 40 that is connected to rigid piping 41 that leads to the vessel deck. Flow from the turret goes up the structural support 47 via rigid piping 48 to a tee divider 49 where it passes through double shut-off valves at 33 and 42 and then to connectors 34 and 35.
During normal operation the bypass is shut-off and 2 ~ 7 disconnected; the double shut-off valves 33 are closed, shut-off valve 36 is closed and connector 35 is disconnected and lowered. In this condition the vessel 54 is free to rotate about the turret 3 and the fluids flow through the double shut off valves 42, connector 34 and shut-off valve 37 into loop 45. To unwind the helix and still maintain fluid flows the bypass loop can be connected when the vessel is approximately oriented with the turret so that the bypass connector is near its associated piping on the turret. Carriage 43 can then be moved circumferentially until the connector is correctly aligned. With connector 34 connected and locked valves 36 and 37 can be opened to allow flow into the bypass.
Flow to the helix can now be stopped by ~losing valves 37 and 42. When connector 6 is released and lowered guide plate 55 can be rotated to unwind the helix. This procedure is then reversed after the helix has been unwound.
When the bypass is in operation full rotation of the turret is not possible. But bypass loop 44 will allow some move~ent of the turret and the amount is governed by how much length of flexible pipe there is in the loop.

Claims (8)

1. In a floating hydrocarbon production facility, a rotatable fluid transfer system for effecting transfer of fluids between a moving assembly, which does not rotate relative to the seafloor, and a floating vessel which can rotate around the mooring assembly; said fluid transfer system comprising:
a vessel having a deck;
a turret of said mooring assembly mounted through the deck of said vessel so that the latter may rotate around said turret;
a derrick securely mounted on said deck and extending upwardly to terminate above said turret;
a substantially vertical, central rigid pipe extending upwardly from said turret and secured adjacent its upper end to the upper end of said derrick;
a plurality of flexible pipes extending from said turret to the top of said derrick;
axial swivel devices secured to the upper ends of said flexible pipes and rigid pipe members securing said axial swivel devices to the top of said derrick whereby fluid in said flexible pipes may travel upwardly through said swivel devices and rigid pipe members and down through the derrick structure to said vessel deck;
a guide plate at the lower end of said system, and a plurality of oversized apertures therein for passage of said flexible pipes through said guide plate;
rigid piping stations on said turret; the lower ends of said flexible pipes looping below said guide plate and passing upwardly therethrough radially outwardly of said central rigid pipe and connecting to said piping stations;
said vessel being rotatable around said turret whereby said derrick rotates the tops of said flexible pipes to cause them to twist into a helical form around said central rigid pipe.

2. A system according to claim 1 wherein said central rigid pipe provides a bend restrictor for said flexible pipes to prevent said flexible pipes from bending more than their minimum permitted bend radius.

3. A system according to claim 1 wherein said central rigid pipe is provided with a plurality of outer sleeves which, in total, extend substantially the length of said central pipe and are free to slide up and down and rotate relative to said central pipe, said sleeves being linked to one another so as to have a limited range of travel axially relative to one another.

4. A system according to claim 1 including a bellmouth attached to the upper end of each flexible pipe for rotation therewith, said bellmouth limiting the bend radius of said flexible pipe, each said bellmouth having an end fitting located in one of said axial swivel devices.

5. A system according to claim 1 wherein the flexible pipe loops below said guide plate provides a means of accommodating height change of the helix as the angle thereof increases.

6. A system according to claim 1 including shut-off valves at the lower terminal ends of said flexible pipes to facilitate removal therefrom or to shut off fluid flow.

7. A system according to claim 1 including a shroud enveloping said central rigid pipe and said flexible pipes.

8. A system according to claim 1 including a bypass arrangement to provide an alternate flow path during unwinding of said helix, including a bypass flexible loop connected at one end to double shut-off valves which connect with said looping below said guide plate, the other end of said bypass loop having an end termination connected to rigid piping leading to said deck.