GB2377001A

GB2377001A - Flowline delivery

Info

Publication number: GB2377001A
Application number: GB0205461A
Authority: GB
Inventors: Timothy Geoffrey Ley
Original assignee: Smit Land & Marine Engineering
Current assignee: Smit Land & Marine Engineering
Priority date: 2001-06-29
Filing date: 2002-03-08
Publication date: 2002-12-31
Also published as: GB0115918D0; GB0205461D0

Abstract

A method of delivering a flowline 1 to an underwater location, includes the steps of: ```releasably attaching a carrier pipe 2 to the flowline 1, ```charging the carrier pipe 2 with a fluid 31 having a specific gravity lower than that of seawater, ```conveying the pipe 2 and the attached flowline 1 to an underwater location, ```preferably neutralising the buoyancy within the carrier pipe 2 by replacing the fluid 31 with seawater, and ```detaching the carrier pipe 2 from the flowline 1 and recovering the carrier pipe 2. The flowline may include a cavity which can be charged with a fluid. In such a case, the method of the invention may further include the steps of: ```charging the flowline 1 cavity with a fluid having a specific gravity lower than that of water, ```towing the carrier pipe 2 and flowline 1 to a desired offshore position, and ```flooding the flowline 1 with seawater to convey the carrier pipe 2 and flowline 1 to the underwater location, prior to detaching the carrier pipe 2 from the flowline 1 and recovering the carrier pipe 2. In each case the fluid may be a gas such as air or nitrogen, or a liquid such a kerosene or other hydrocarbon.

Description

DESCRIPTION FLOWLINE DELIVERY The present invention relates to offshore flowlines, particularly to the delivery of flowlines by towing.

Flowlines are used for handling fluids to or from an offshore well or cluster of wells. In the present invention, the term"flowline"is intended to refer to all types of flowline, pipeline or riser, including catenary risers. Risers are a particular form of flowline which are installed in a vertical orientation to enable delivery of a fluid, across a height differential, for example from the seabed to a surface vessel or installation. Typically, flowlines are deployed as "bundles" which comprise a plurality of rigid or flexible pipelines, flowlines or risers which are grouped together and may be enclosed within a carrier pipe. Power or umbilical cables may be deployed with the flowlines and may be included as part of the bundle.

Flowline bundles are conveyed offshore to the required site of installation by towing in long lengths either along the seabed, through the water column or on the surface.

Typically, the bundle comprises a steel carrier pipe which provides buoyancy for the flowlines at the desired depth.

Lengths of some 3-7 km of flowline within a carrier pipe bundle may be towed offshore.

The carrier pipe, which is typically about 1 metre in diameter, has walls of steel and must be internally pressurized, typically by charging with gas such as nitrogen, to withstand hydrostatic pressure in deep water.

Once installed, the carrier remains on or just above the seabed, serving as a protective casing for the bundle of flowlines and/or cable contained within.

The continuing search for new sources of oil and gas has led to a desire to exploit wells in deeper waters and thus to the need for new technology able to withstand higher pressure conditions. As hydrostatic pressure increases, the use of steel carrier pipes becomes expensive and impractical owing to the increased wall thickness required to withstand the high gaseous pressures required within the pipes to resist buckling from seawater pressure.

At depths of beyond about 800-1000 metres, the use of steel carrier pipes is impractical. The abovementioned system of delivering flowlines is therefore limited to relatively shallow conditions. Even in relatively shallow conditions, bundle methods which employ the use of a steel carrier pipe, may suffer additional costs when compared to other pipeline/flowline installation costs due to relative complexity of the method.

An object of the present invention is to provide a flowline delivery system which addresses at least some of

the abovementioned problems. In the present application, the term"flowline"is to be understood to refer to a pipeline or other means for handling fluids, and may refer to a single flowline or a bundle.

According to the invention there is provided a method of delivering a flowline to an underwater location, including the steps of: releasably attaching a carrier pipe to the flowline, charging the carrier pipe with a fluid having a specific gravity lower than that of seawater, conveying the pipe and the attached flowline to an underwater location, detaching the carrier pipe from the flowline and recovering the carrier pipe.

The carrier pipe may be made of any suitable material.

Preferably it is of a material or combination of materials having a density lower than that of seawater, to provide buoyancy. Preferably, it is made of plastics, for example polyethylene. The carrier pipe may be of substantially the same length as the flowline, or it may be divided into smaller lengths.

The fluid may be liquid or gaseous. Preferably, the fluid is a hydrocarbon. More preferably it is kerosene.

Alternatively the fluid may be a gas such as air or nitrogen.

The carrier pipe may be attached to the flowline via any suitable means. The flowline may be suspended from the carrier pipe by a plurality of chains along the length of the carrier pipe. Preferably the chains include a release bar or wire.

Alternatively or additionally, the flowline may be attached to the carrier pipe by a longitudinal beam connected substantially along the length of the carrier pipe. Preferably the beam includes a protruding surface for attachment of releasable clamps for connecting the flowline.

Alternatively or additionally, the flowline is releasably attached to the carrier pipe via a strap circumscribing the flowline. Preferably a plurality of straps are located along the flowline. A plurality of carrier pipes may be employed in the method.

Preferably the fluid is expelled from the carrier pipe at the underwater location. Preferably it is expelled prior to detachment of the carrier pipe from the flowline.

The fluid expelled from the carrier pipe is preferably displaced by seawater. Preferably the fluid from the carrier pipe is recovered. More preferably, the fluid is recovered to sea level, for example to a tanker or other storage device from where it may be reused or disposed of.

Yet more preferably, the fluid is driven to sea level by

ambient seawater pressure.

The fluid may be driven out of the carrier pipe to sea level via a conduit. Preferably the conduit is connected to the carrier pipe by a salable valve.

Alternatively, the fluid may be expelled from the carrier pipe into the surrounding seawater. In such case the expelled fluid is preferably a gas. Preferably it is air or nitrogen. Alternatively, the expelled gas may be released via a conduit to the deck of a tug or other vessel, where its rate of release can if required, be controlled.

According to the method of the invention the carrier pipe preferably includes means for balancing internal fluid pressure with ambient seawater pressure. Said means may comprise a movable member such as a piston, for providing a movable barrier between the fluid and the ambient seawater.

Alternatively or additionally, said means may include a membrane interface between ambient seawater and the fluid, enabling pressure compensation between the seawater and the fluid. Said means may be located at an end region of the carrier pipe and/or at an intermediate position along the carrier pipe.

The step of recovering the fluid may be effected by opening the valve to allow fluid communication between the carrier pipe and the conduit, whereby the fluid is driven

out of the carrier pipe and along the conduit by pressure of ambient seawater.

In a preferred embodiment of the invention, the flowline may include a cavity which can be charged with a fluid. In such a case, the method of the invention may further include the steps of: charging the flowline cavity with a fluid having a specific gravity lower than that of water, towing the carrier pipe and flowline to a desired offshore position, and flooding the flowline with seawater to convey the carrier pipe and flowline to the underwater location, prior to detaching the carrier pipe from the flowline and recovering the carrier pipe.

In the abovementioned preferred embodiment, the fluid in the flowline cavity is preferably a gas. Preferably it is released into the seawater upon flooding.

Preferably, the above method further includes the step of flooding the carrier pipe with seawater prior to detaching it from the flowline. In such case, the fluid may be released from the carrier pipe into a tanker or other storage device or into the surrounding seawater.

Flooding of the carrier pipe prior to detaching it serves to neutralise buoyancy and to prevent a sharp deviation in the path of the carrier pipe upon detaching it.

Alternatively, the flowline may itself have sufficient ballast to remain at or near the seabed without the need for flooding. The flowline may, for example, be sufficiently dense to remain at or near the seabed without the need for flooding. Alternatively it may have a cavity which is preflooded prior to towing. In such case, the method of the invention may further include the steps of: towing the carrier pipe and flowline to the desired location, and flooding the carrier pipe with seawater to convey the assembly to the underwater location, prior to detaching the carrier pipe from the flowline and recovering the carrier pipe.

During flooding of the carrier pipe with seawater, the fluid therein may be released into a tanker or other storage device or into the surrounding seawater.

In an alternative embodiment of the invention, preferably the carrier pipe and flowline are towed to an offshore site prior to charging the carrier pipe with a fluid having a specific gravity lower than that of seawater, the carrier pipe and flowline then being conveyed to the underwater location by displacing the fluid in the carrier pipe with seawater. The carrier pipe is then detached from the flowline and recovered. Preferably the fluid is also recovered.

In such an embodiment, preferably there are a plurality of carrier pipes. More preferably there are two carrier pipes, one of which acts as a ballast line. In such a case, the method of the invention comprises the steps of: attaching a carrier pipe and ballast line to the flowline, conveying the carrier pipe, flowline and ballast line to an offshore site, successively charging the carrier pipe and the ballast line with fluid from a tanker positioned at the offshore site, the fluid having a specific gravity lower than that of seawater, whereby the ballast line is charged with fluid via the carrier pipe, the carrier pipe being in fluid communication with the ballast line, flooding the ballast line and the carrier pipe successively with seawater, the carrier pipe being flooded with seawater via the ballast line, and detaching and recovering the carrier pipe and ballast line from the flowline.

Preferably, the carrier pipe includes means for balancing internal fluid pressure with ambient seawater pressure, for example a pig or other means as described above. Preferably the ballast line also includes such means.

The use of the fluid in the above described methods serves to neutralise buoyancy. Buoyancy is further neutralised upon replacement of the fluid with seawater.

An advantage of the invention is that concrete-coated flowlines may be used. Concrete has the advantage of providing physical protection and stability.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of a sequence of steps according to a preferred embodiment of the invention.

Figure 2 is a cross-sectional view of a method of recovering the fluid from the carrier pipe which may be used in conjunction with a method according to the invention.

Figure 3 is a schematic view of a sequence of steps according to a second preferred embodiment of the invention.

Figure 4 is a schematic view of a sequence of steps according to a third preferred embodiment of the invention.

Figure 5 is a cross-sectional view of a possible sequence of steps for releasing the flowline from the carrier pipe according to the invention.

Figure 6 is a perspective view of a releasable attachment means according to the invention.

Figure 7 is a cross-sectional view of a further releasable attachment means according to the invention.

Figure 8 is a side view of the releasable attachment means depicted in Figure 7.

In a specific embodiment of the invention as shown in Fig. 1, a flowline 1 is suspended from a polyethylene carrier pipe 2 by connecting chains 3 (not shown in Fig.

1). Chains 4 are suspended from the flowline along its length to provide stability and to act as a guide when the flowline is towed above the seabed. The carrier pipe and bundle are laid on the seabed 8 from a shallow water laybarge in a configuration as shown in Fig. la. The carrier pipe at this stage contains seawater.

A tanker 7 is then connected to the carrier pipe 2 via a hose 5. Kerosene is pumped via the hose 5 into the carrier pipe, displacing the seawater out of the carrier pipe. The pressure of the kerosene in the carrier pipe is equalized with that of the ambient seawater pressure by a pig 20 which acts as a movable barrier between the ambient seawater and the kerosene within the carrier. The position of the pig is shown in Fig. 2. Any variations in seawater pressure will be compensated by movement of the pig along the carrier pipe. For example, at increasing depths, the increase in seawater pressure will force the pig to slide along the carrier pipe, compressing the kerosene until the

pressure of the kerosene is equal to that of the seawater.

The pressure within the carrier pipe will thus always be equal to that of the seawater, the carrier pipe thus providing the correct level of buoyancy. Fig. lb shows the step of displacing seawater by kerosene and the resultant increase in buoyancy of the carrier pipe. In Fig. lc, the entire carrier pipe 2 is flooded with kerosene, providing sufficient buoyancy for the pipe and flowline to be located just above the seabed, the chains 4 remaining in contact with the seabed 8.

Once the carrier pipe is filled with kerosene and the correct level of buoyancy for towing is achieved, as shown in Fig lc, the carrier is connected to a tug 6 and is towed to the desired deep-sea offshore location, as illustrated in Fig Id. The trailing end of the carrier pipe is connected to trail tug 9 to facilitate movement to the desired location. In the embodiment shown in Fig. 1, the carrier pipe and flowline assembly is towed by the offbottom method. Alternative methods such as controlleddepth tow or bottom tow might however be employed within the scope of the present invention.

Once positioned in the desired deep-sea location as shown in Fig. Ie, a tanker 7 is connected via a hose 5 to the carrier pipe 2 for recovery of the kerosene. This process is shown in Fig. If, and in greater detail in Fig.

2. Upon opening of valve 22, kerosene from the carrier pipe is forced into and along the hose 5 by the comparatively greater pressure of the ambient seawater which drives pig 20 along the carrier pipe. In an alternative embodiment not shown in the drawings, the carrier pipe may be provided with two pigs located near each end of the carrier pipe. The contents of the carrier pipe in such case will be exposed to the ambient seawater pressure at both ends. This will be of particular advantage when the pipe is towed at a gradient and is exposed to different pressures at either end.

Referring to Fig. 2, the driving pressure is equal to the pressure difference"x"across the valve 22, which is the difference between the ambient seawater pressure at A and the pressure of kerosene within the hose at C.

At a depth of 1500 metres:

Pressure of seawater at A = Pressure of kerosene at B = h x p x g = 1500 x 1030 x (9. 81 x 10-5) = 151 bar Pressure at C = h x pk X g = 1500 x 800 x (9. 81 X 10-5) = 117 bar Driving pressure = 151-117 = 34 bar

[where h = depth below sea level in metres; Ps = specific gravity of seawater; Pk = specific gravity of kerosene; g = gravitational force constant Once the kerosene has been recovered and the hose 5 disconnected from the carrier, the carrier pipe is released from the flowline as shown in Fig. Ig, and is towed away (Fig. lh), for example to the shore for reuse, leaving the flowline 1 installed on the seabed.

In a further embodiment of the invention shown in Fig.

3, a flowline 1 is suspended from a polyethylene carrier pipe 2 by connecting chains 3 (not shown in Fig. 3).

Chains 4 are suspended from the flowline along its length to provide stability and to act as a guide when the flowline is towed above the seabed 8.

Initially the carrier pipe 2 contains air and is sufficiently buoyant to float at the seawater surface as shown in Fig. 3a. Kerosene is then delivered from a tanker 7 via connecting hose 5 into the carrier pipe, displacing the air out of the pipe. Flooding of the pipe with kerosene reduces its buoyancy such that the pipe is displaced downwards to a position just above the seabed, as shown in Fig. 3b. As the carrier pipe 2 moves downwards, the pressure of the kerosene within the pipe is equalized with that of the ambient seawater pressure by pig 20 as

shown in Fig. 2, hereinbefore discussed.

Once the carrier pipe is filled with kerosene and the correct level of buoyancy for off-bottom towing is achieved, as shown in Fig. 3c, the carrier is connected to a tug 6 and is towed to the desired deep-sea location, as illustrated in Fig. 3d. The trailing end of the carrier pipe is connected to trail tug 9 to facilitate movement to the desired location.

Once positioned in the desired deep-sea location as shown in Fig. 3e, a tanker 7 is connected via a hose 5 to the carrier pipe 2 for recovery of the kerosene. The carrier pipe is gradually flooded with seawater as the kerosene is recovered to the tanker. Fig 3f shows the gradual shift of the carrier pipe towards the seabed 8 as it fills with seawater and becomes less buoyant. This displacement of kerosene by seawater is shown in greater detail in Fig. 2, as described earlier.

The carrier pipe, now flooded with seawater is then released from the flowline as shown in Fig. 3g, and is towed away (Fig. 3h), for example to the shore for reuse, leaving the flowline 1 installed on the seabed. Since the carrier pipe is made of polyethylene, this provides sufficient buoyancy for the pipe to lift off the seabed once detached from the flowline, thereby enabling it to travel through the water column.

Fig. 4 shows schematically yet a further embodiment of the invention. A flowline 1 is suspended from a polyethylene carrier pipe 2 by straps 53. The method of attachment of the straps is described in more detail in the description accompanying Figs. 7 and 8. Whilst the present embodiment employs straps 53, other means of attachment such as those described hereinafter may additionally be employed without departing from the scope of the invention.

In Fig. 4a the carrier pipe and flowline assembly is connected to vessel 400 and is launched from a land-based site or from a shallow-water lay-barge.

In Fig. 4b the carrier pipe 2 and the flowline 1 are charged with a gas, preferably air or nitrogen to suit ambient seabed pressure at the site of installation. The carrier pipe and flowline assembly is then towed to the desired offshore site by vessel 400, assisted by trail tug 401, as shown in Fig. 4c.

On arrival at the site, a'pull-down'cord 402 is attached to the assembly and to the vessel 400 via anchoring means such as a clump weight 403 on the seabed 8, to assist with locating the end of the flowline in the required target area of the seabed. The cord 402 may be made of any suitable material, and may for example be a wire or rope.

Once positioned correctly at the above location, a

flooding line 404 is connected to the flowline and the flowline is flooded with seawater, for example by pumping, as illustrated in Fig. 4e. Alternatively, the flowline may be allowed to free-flood via a controllable valve at the end of the pipeline, in which case the line 404 represents a control umbilical without a flooding element. Seawater 407 enters via an opening at an end region of the flowline, the seawater exerting pressure on a movable pig 405 within the flowline cavity. The pressure of the seawater causes the pig to be forced along the flowline cavity pushing against the gas and causing it to be expelled via an exit point at a distal end region of the flowline. The gas inside the flowline is thus displaced progressively and in a controlled manner by seawater, and the assembly moves downwards towards the seabed 8 as the buoyancy of the flowline decreases.

The flowline is then laid on the seabed 8 in the desired location, monitored by survey vessel 406, as shown in Fig. 4f.

Once positioned on the seabed, the carrier pipe 2 is also flooded with seawater, the gaseous contents being released into the surrounding sea or via a conduit to the surface. The carrier pipe is then released from the flowline and is towed away from the site for re-use or otherwise. The abovementioned method which employs a gas

such as air or nitrogen offers economic advantages since the flowline assembly is towed at or near the surface of the sea. This allows the use of vessels which are less costly, not necessarily being vessels specifically for pipe-laying, this being of particular advantage in more remote areas.

While Fig. 4 shows a method of a preferred embodiment of the invention, the following modification to the described method is envisaged: Referring to the step of Fig. 4e, the clump weight 403 may be dispensed with. In this embodiment, the flowline is flooded over a length sufficient to create a'belly'in the pipe profile which touches down onto the seabed while tension is initially maintained by the tug (400). The assembly is guided to the desired underwater position by means of transponders; when sufficient pipe has been laid on the seabed, tension on the tug (400) can be relaxed and the end of the pipe laid down on the seabed.

In a yet further embodiment of the invention, the above described method as illustrated in Fig. 4 may be modified by use of a flowline which need not be flooded according to Figs. 4e-4h. In such a method, the flowline must have sufficient ballast to remain at or near the seabed upon detachment of the carrier pipe. The flowline may, for example, itself be sufficiently dense to remain at

or near the seabed without the need for flooding, or it may be preflooded with seawater prior to towing of the assembly offshore. After towing the assembly out to the desired offshore location, only the carrier pipe is flooded with seawater to lower the assembly to the seabed as shown in Fig. 4g.

In the above described embodiments, a possible mechanism of release of the carrier pipe 2 from the flowline 1 is shown in Fig. 5. Fig. 5a shows the flowline 1 suspended by carrier pipe 2 by connecting chain 3. The flowline 1 is shown here as a bundle having two separate lines 38 and 39, but other variants including a single line or more than two lines plus cables may also be employed within the scope of the invention. The carrier pipe is filled with kerosene 31 and is buoyant at a level just above the seabed 8 as is desired for off-bottom towing. In Fig. 5b, the carrier pipe 2 is flooded with seawater 32 thereby reducing its buoyancy level. Detachment of the carrier pipe 2 and chain 3 is effected by withdrawal of release wire 34 from eye 36. The carrier pipe 2 and connecting chain 3 can then be towed away as described above, leaving the flowline 1 and suspended chain 4 on the seabed 8, as shown in Fig. 5c.

Fig. 6 shows an alternative system for releasably attaching a carrier pipe to a flowline according to an

embodiment of the invention. In the embodiment shown, a pair of carrier pipes 2 are connected together. Each carrier pipe 2 bears an I-shaped beam 61 along its length.

A flowline 1 is attached to each of the beams 61 via a plurality of releasable clamps 42 which slide along projecting edge 66 of the beam. Further embodiments are envisaged having a single carrier attachable to one or more flowlines.

Figs. 7 and 8 show a further alternative system for releasably attaching a carrier pipe to a flowline. In the accompanying drawings, the release method is illustrated with respect to the embodiment of the invention as illustrated in Fig. 4 but the principles may similarly be applied to other situations within the scope of the invention.

Shown in Fig. 7 in cross-section, a carrier pipe 2 bears a lug 51 provided with eyes 52 through which passes a Kevlar strap 53. The strap circumscribes the flowline 1 and is secured by a release pin 54 inserted in one of the eyes 52 and in turn attached to a release wire 59. The flowline is thus supported below the carrier pipe by the strap. A plurality of straps 53 are provided at intervals along the carrier pipe 2, as shown in the side view of Fig.

8.

The release pin 54 secures the strap 53 by passing

through a terminal loop 57 in the strap. In Fig. 7a, the carrier pipe and flowline are filled with a fluid 101 such as air or nitrogen and the assembly thus remains buoyant at sea level. In Fig. 7b, the flowline 1 is flooded with seawater 100 and the system settles at or near the seabed.

Flooding of the carrier pipe with seawater is then also carried out as shown in Fig. 7c. At the stage shown in Fig. 7d the carrier pipe 2 is detached from the flowline 1.

Detachment is effected by withdrawal of the release pin 54 from the eye 52, by movement of the release wires 59, shown in Fig. 8. The release wires 59 are triggered by a control wire 58, which is connected to each release wire and which follows the length of the carrier pipe. The detached flowline is left on the seabed whilst the detached carrier pipe 2 which contains seawater may be towed away, for example to the shore for reuse.

In an alternative means for releasably attaching the carrier and flowline (not shown in the drawings), the carrier pipe and flowline are released by a rip-cord which is recovered separately from the carrier pipe, thus leaving the carrier pipe free from attachments, the rip-cord being reusable.

While the above described embodiments are intended to illustrate preferred embodiments of the invention, other embodiments of the invention are envisaged without

departing from the scope of the present invention.

Claims

CLAIMS 1. A method of delivering a flowline to an underwater location, including the steps of: releasably attaching a carrier pipe to the flowline, charging the carrier pipe with a fluid having a specific gravity lower than that of seawater, conveying the pipe and the attached flowline to an underwater location, detaching the carrier pipe from the flowline and recovering the carrier pipe.
2. A method according to claim 1 wherein the carrier pipe has a density lower than that of seawater, to provide buoyancy.
3. A method according to claim 1 or 2 wherein the carrier pipe is made of plastics.
4. A method according to any one of the preceding claims wherein the carrier pipe is of substantially the same length as the flowline.
5. A method according to any one of the preceding claims wherein the fluid is a gas.
6. A method according to any one of claims 1-5 wherein the fluid is kerosene.
7. A method according to any one of the preceding claims wherein the flowline is releasably suspended from the carrier pipe by a plurality of chains.

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8. A method according to any one of the preceding claims wherein the flowline is attached to the carrier pipe by a longitudinal beam connected substantially along the length of the carrier pipe.
9. A method according to claim 8 wherein the beam includes a protruding surface for attachment of releasable' clamps for connecting the flowline.
10. A method according to any one of the preceding claims wherein the flowline is releasably attached to the carrier pipe via a series of straps circumscribing the flowline.
11. A method according to any one of the preceding claims wherein there are a plurality of carrier pipes.
12. A method according to any one of the preceding claims wherein the fluid is expelled from the carrier pipe at the underwater location.
13. A method according to claim 12 wherein the fluid is expelled prior to detachment of the carrier pipe from the flowline.
14. A method according to claim 12 or 13 wherein the fluid expelled from the carrier pipe is displaced by seawater.
15. A method according to any one of the preceding claims wherein the fluid from the carrier pipe is recovered.
16. A method according to claim 15 wherein the fluid is recovered to sea level.
17. A method according to any one of claims 12-16 wherein

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the fluid is driven to sea level by ambient seawater pressure.
18. A method according to claims 12-17 wherein the fluid is driven out of the carrier pipe to sea level via a conduit connected to the carrier pipe by a salable valve.
19. A method according to claims 12-14 wherein the fluid is expelled from the carrier pipe into the surrounding seawater or back to the surface where it is released.
20. A method according to any one of the preceding claims wherein the carrier pipe includes means for balancing internal fluid pressure with ambient seawater pressure.
21. A method according to any one of claims 14-20 wherein the fluid is recovered by allowing ambient seawater to drive the fluid out of the carrier pipe.
22. A method according to any one of claims 1-11, wherein the flowline includes a cavity which can be charged with a fluid.
23. A method according to claim 22 further comprising the steps of: charging the flowline cavity with a fluid having a specific gravity lower than that of seawater, towing the carrier pipe and flowline to a desired offshore position, and flooding the flowline with seawater to convey the carrier pipe and flowline to the underwater location prior

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to detaching the carrier pipe from the flowline and recovering the carrier pipe.
24. A method according to claim 23 wherein the fluid in the flowline is a gas which is expelled into the surrounding seawater or back to the surface where it is released upon flooding the flowline with seawater.
25. A method according to claim 23 or 24 further comprising the step of flooding the carrier pipe with seawater prior to detaching it from the flowline.
26. A method according to claim 25 wherein upon flooding the carrier pipe with seawater, the fluid is released from the carrier pipe into a tanker or other storage device.
27. A method according to claim 25 wherein the fluid is released from the carrier pipe into the surrounding seawater or back to the surface where it is released.
28. A method according to any one of the preceding claims, wherein the carrier pipe and flowline are towed to an offshore site prior to charging the carrier pipe with fluid, the carrier pipe and flowline then being conveyed to the underwater location by displacing the fluid in the carrier pipe with seawater.
29. A method of delivering a flowline to an underwater location, including the steps of: attaching a carrier pipe and ballast line to the

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flowline, conveying the carrier pipe, flowline and ballast line to an offshore site, successively charging the carrier pipe and the ballast line with fluid from a tanker positioned at the offshore site, the fluid having a specific gravity lower than that of seawater, whereby the ballast line is charged with fluid via the carrier pipe, the carrier pipe being in fluid communication with the ballast line, flooding the ballast line and the carrier pipe successively with seawater, the carrier pipe being flooded with seawater via the ballast line, and detaching and recovering the carrier pipe and ballast line from the flowline.
30. A method according to any one of the preceding claims, wherein the carrier pipe is released from the flow line by a rip-cord which is recovered separately from the carrier pipe.