GB2377002A

GB2377002A - Flowline delivery

Info

Publication number: GB2377002A
Application number: GB0205462A
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: GB0115919D0; GB0205462D0

Abstract

A method of delivering a flowline 1 to an underwater location, includes the steps of:<BR> ```attaching a carrier pipe 2 to the flowline 1,<BR> ```charging the carrier pipe 2 with a fluid having a specific gravity lower than that of seawater,<BR> ```conveying the pipe 2 and the attached flowline 1 to the desired underwater location, and<BR> ```expelling the fluid from the carrier pipe 2. Suitable fluids include gases such as air or nitrogen, and liquids, for example hydrocarbons such as kerosene. The fluid may be expelled into the surrounding seawater or may be recovered to sea level 7 for reuse. The fluid is preferably displaced with seawater to neutralise buoyancy on the seabed 8. The carrier pipe 2 may be detachable from the flowline 1.

Description

DESCRIPTION FLOWLINE DELIVERY The present invention relates to offshore flowlines, particularly to the delivery of flowlines in deep water.

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.

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: 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 the desired underwater location, and expelling the fluid from the carrier pipe.

The expelled fluid is preferably displaced by seawater in the carrier pipe.

Preferably the fluid is recovered from the carrier pipe. 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. More preferably it is air or nitrogen.

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. Preferably, said means is located at or near both end regions of 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.

According to the invention, the carrier pipe is preferably releasable from the flowline. Preferably the carrier pipe is released from the flowline after recovery of the fluid. Any suitable method of release may be employed, such as, for example, a method as described hereinbelow.

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.

In a preferred embodiment of the invention, the flowline is releasably attachable to the carrier pipe. 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.

According to a preferred embodiment of the invention, the flowline includes a cavity which may be charged with a

fluid. In such preferred embodiment 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 an offshore site, expelling the fluid from the flowline, detaching the carrier pipe from the flowline after the step of expelling the fluid from the carrier pipe, and recovering the carrier pipe.

In the abovementioned preferred embodiment, the fluid is preferably a gas. Preferably it is displaced by seawater upon release. Alternatively the fluid may be a liquid such as kerosene, which is preferably recovered to a tanker or other storage device.

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. Preferably the fluid is 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 recovering the fluid to the tanker, to cause the carrier pipe, ballast line and flowline to settle at the underwater location.

Preferably the carrier pipe and ballast line are then detached from the flowline and are recovered.

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 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 specific embodiment of the invention.

Figure 2 is a cross-sectional view of a preferred method of recovering the fluid from the carrier pipe according to the invention.

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

Figure 4 is a schematic view of a sequence of steps according to a third specific 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, which may be employed in conjunction with a method according to the invention.

Figure 6 is a perspective view of a releasable attachment means which may be used in conjunction with a method according to the invention.

Figure 7 is a cross-sectional view of a further releasable attachment means which may be used in conjunction with a method according to the invention.

Figure 8 is a side view of the releasable attachment

means depicted in Figure 7.

Figure 9 is a schematic view of a sequence of steps according to a fourth specific embodiment of the invention.

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. Ib 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. le, 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 ps 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; pg = 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 is connected to the flowline and the flowline

is flooded with seawater as illustrated in Fig. 4e.

Alternatively, the flowline may be allowed to free-flood via a controllable valve at the end of the pipeline.

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. 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.

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.

Fig. 9 shows a further embodiment of a method according to the invention. Referring to Fig. 9a, flowline 1 is releasably attached to carrier pipe 2 and ballast line 900, shown both in side schematic view and in cross section. The carrier pipe and ballast line are ballasted as necessary with seawater for surface towing. The assembly is towed out to sea by tug 6 and trail tug 9.

The carrier pipe is then linked to a tanker 902 at the offshore site, as shown in Fig. 9b. The carrier pipe is charged with fluid 904 which enters the carrier pipe and progressively fills the pipe against a movable pig 905.

The ballast line 900 is in fluid communication with the carrier pipe 2, so that once the carrier pipe is fully

charged with fluid, the flow of fluid is then diverted into the ballast line 900, as shown in Fig. 9c. The ballast line 900 also includes a pig 906 as an interface between the incoming fluid and the air and water which is expelled from the line.

Once the ballast line and carrier pipe are completely charged with fluid, the assembly attains almost neutral buoyancy in the seawater. The ballast line 900 is then allowed to flood with seawater which displaces the fluid back along the ballast line, as shown in Fig 9d. The displaced fluid 904 is forced by the seawater pressure via the carrier pipe and back in to the tanker 902. In Fig. 9f the ballast line has been fully charged with seawater and the seawater is about to continue its path along the carrier pipe, displacing the fluid. At this stage, to provide additional stability, the flowline 1 is also progressively filled with seawater, and the assembly begins to settle at one end on the seabed 8, guided to the desired position by clump weight 403.

In Fig. 9g, seawater continues to displace the fluid out of the carrier pipe and back to the tanker. Fig. 9 shows the flowline 1, carrier pipe 2 and ballast line 900 charged with seawater, the entire assembly having settled at the seabed 8.

In the final step shown in Fig. 9i, the attachment

means 908 is released from the assembly by a rip-cord to detach the flowline 1 from the carrier pipe 2 and ballast line 900. The attachment means 908 is towed back to shore by tug 909 for disposal or reuse, or is alternatively winched onto the tug, while the carrier pipe and ballast line are also recovered and can be reused.

In a further embodiment (not shown) seawater is used to displace the kerosene by controlled'Free-flooding'. In this embodiment, the tanker (902) can be connected to the same end as tug 9. This enables the dynamic start method to be utilised without the requirement for excessively long hoses.

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: 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 the desired underwater location, and expelling the fluid from the carrier pipe.
2. A method according to claim 1 wherein the expelled fluid is displaced by seawater in the carrier pipe.
3. A method according to claim 1 or 2 wherein the fluid is recovered from the carrier pipe.
4. A method according to any one of the preceding claims wherein the fluid is recovered to a tanker or other storage device.
5. A method according to any one of the preceding claims wherein the expelled fluid is driven to sea level by ambient seawater pressure.
6. A method according to any one of the preceding claims wherein the fluid is expelled from the carrier pipe into the surrounding seawater.
7. A method according to any one of the preceding claims wherein the fluid is a gas.
8. A method according to any one of claims 1-6 wherein

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the fluid is a hydrocarbon liquid.
9. A method according to any one of the preceding claims wherein the carrier pipe preferably includes means for balancing internal fluid pressure with ambient seawater pressure.
10. A method according to any one of the preceding claims wherein the carrier pipe is releasable from the flowline.
11. A method according to any one of the preceding claims wherein the carrier pipe is released from the flowline after expelling the fluid from it.
12. A method according to any one of the preceding claims wherein the carrier pipe is of a material or combination of materials having a density lower than that of seawater.
13. A method according to any one of the preceding claims wherein the flowline includes a cavity which may be charged with a fluid.
14. A method according to claim 13 further including 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 an offshore site above the desired location, expelling the fluid from the flowline, detaching the carrier pipe from the flowline after the step of expelling the fluid from the carrier pipe, and

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recovering the carrier pipe.
15. 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 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.
16. A method according to any one of the preceding claims wherein there are a plurality of carrier pipes.
17. A method of delivering a flowline to an underwater location, including 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

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with seawater via the ballast line, and recovering the fluid to the tanker, to cause the carrier pipe, ballast line and flowline to settle at the underwater location.
18. A method according to claim 17 further comprising the step of detaching and recovering the carrier pipe and ballast line from the flowline.