WO2003002848A1

WO2003002848A1 - Method of laying an underwater flowline

Info

Publication number: WO2003002848A1
Application number: PCT/GB2002/002934
Authority: WO
Inventors: Timothy Geoffrey Ley
Original assignee: Smit Land & Marine Engineering Limited
Priority date: 2001-06-29
Filing date: 2002-06-26
Publication date: 2003-01-09
Also published as: EP1402148A1

Abstract

Method of delivering a flowline (1) to an underwater location, comprising the steps of: releasably attaching a carrier pipe (2) of substantially continuous elongate cross section to the flowline; charging the carrier pipe with a fluid having a specific gravity lower than that of seawater effective to provide buoyancy to the flowline; towing the pipe and the attached flowline to an underwater location; deploying the flowline to the sea bed at the desired underwater location by at least partially flooding the carrier pipe and/or the flowline with seawater; at least partially flooding any unflooded region of the carrier pipe with the sea water; etaching the carrier pipe from the flowline; and recovering the carrier pipe.

Description

DESCRIPTION

METHOD OF LAYING AN UNDERWATER FLOWLINE

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 present invention there is provided a method of

delivering a flowline to an underwater location, comprising the steps of:

releasably attaching a carrier pipe of substantially continuous elongate

cross section to the flowline; charging the carrier pipe with a fluid having a specific gravity lower than

that of seawater effective to provide buoyancy to the flowline;

towing the pipe and the attached flowline to an underwater location;

deploying the flowline to the sea bed at the desired underwater location by

at least partially flooding the carrier pipe and/or the flowline with seawater;

at least partially flooding any unflooded region of the carrier pipe with sea

water;

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 carrier pipe is of substantially continuous elongate cross section which

means that the pipe is of substantially continuous elongate cross section along at

least a major portion of its length, for example along a length of at least 100m,

preferably at least 500m, more preferably at least 1km and most preferably >5km.

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 sealable 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 method 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 method, 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 method 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 a method, 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 method 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 method of the invention.

Figure 4 is a schematic view of a sequence of steps according to a third

preferred method 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.

Figure 9 is a schematic view of a sequence of steps according to a fourth specific

method of the invention.

In a specific method 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 method shown in Fig. 1, the carrier pipe

and flowline assembly is towed by the off-bottom method. Alternative methods

such as controlled-depth 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 method 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 _s x g

= 1500 x 1030 x (9.81 x lO^"5)

= 151 bar

Pressure at C = hx _kx g

= 1500 x 800 x (9.81 x lO"⁵)

= 117 bar

Driving pressure = 151 - 117

= 34 bar

[where h = depth below sea level in metres; _s = specific gravity of seawater; _k =

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. Ih), for example to the shore for reuse, leaving the flowline 1

installed on the seabed.

In a further method 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. 3 c, 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 method 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 method 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 method 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 method, 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 method 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 methods, 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 a method of the invention. In the method 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 methods 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 method 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. Fig. 9 shows a further method 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 method (not shown) seawater is used to displace the kerosene

by controlled 'Free-flooding'. In this method, 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 methods are intended to illustrate preferred

methods of the invention, other methods of the invention are envisaged without

departing from the scope of the present invention.

Claims

1. A method of delivering a flowline to an underwater location,

comprising the steps of:

releasably attaching a carrier pipe of substantially continuous elongate

cross section to the flowline;

charging the carrier pipe with a fluid having a specific gravity lower than

that of seawater effective to provide buoyancy to the flowline;

towing the pipe and the attached flowline to an underwater location;

deploying the flowline to the sea bed at the desired underwater location by

at least partially flooding the carrier pipe and/or the flowline with seawater;

at least partially flooding any unflooded region of the carrier pipe with sea

water;

detaching the carrier pipe from the flowline; and

recovering the carrier pipe.

2. A method according to claim 1 wherein the at least partial flooding of

the carrier pipe is effected at a controlled rate.

3. A method according the claim 2 wherein the rate of at least partial

flooding of the carrier pipe is controlled by at least one pig deployed within the

carrier pipe.

4. A method according to any one of claims 1 to 3, wherein the carrier

pipe and the attached flowline are towed such that the whole flowline (including

both ends thereof) is moved from its starting location to the desired underwater al location.

5. A method according to any one of claims 1 to 4 wherein the carrier pipe

has a density lower than that of seawater.

6. A method according to any one of claims 1 to 5 wherein the carrier pipe

is made of plastics.

7. A method according to any one of claims 1 to 6 wherein the carrier pipe

is of substantially the same length as the flowline.

8. A method according to any one of claims 1 to 7 wherein the fluid is a

gas.

9. A method according to any one of claims 1 to 7 wherein the fluid is

kerosene.

10. A method according to any one of claims 1 to 9 wherein the flowline is

releasably suspended from the carrier pipe by a plurality of chains.

11. A method according to any one of claims 1 to 10 wherein the flowline

is attached to the carrier pipe by a longitudinal beam connected substantially along

the length of the carrier pipe.

12. A method according to claim 11 wherein the beam includes a

protruding surface for attachment of releasable clamps for connecting the

flowline.

13. A method according to any one of claims 1 to 12 wherein the flowline

is releasably attached to the carrier pipe via a series of straps circumscribing the

flowline.

14. A method according to any one of claims 1 to 13 wherein there is

provided a plurality of carrier pipes.

15. A method according to any one of claims 1 to 14 wherein on flooding

of the carrier pipe 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 claim 16 wherein the fluid is driven to sea level by ambient seawater pressure.

18. A method according to any one of claims 15 to 17 wherein the fluid is driven out of the carrier pipe to sea level via a conduit connected to the carrier pipe by a sealable valve.

19. A method according to any one of claims 1 to 14 wherein on flooding of the carrier pipe 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 claims 1 to 19 wherein the carrier pipe includes means for balancing internal fluid pressure with ambient seawater pressure.

21. A method according to any one of claims 1 to 20 wherein one or both of the carrier pipe and the flowline includes a cavity which can be charged with a fluid.

22. A method according to claim 21 further comprising the steps of: charging the flowline cavity and/or the carrier pipe cavity with a fluid having a specifϊc gravity lower than that of seawater;

towing the carrier pipe and flowline to a desired offshore position; and

flooding the flowline and/or the carrier pipe with seawater to deploy the carrier

pipe and flowline to the seabed at the underwater location prior to detaching the

carrier pipe from the flowline and recovering the carrier pipe.

23. A method according to claim 22 wherein the fluid in the flowline

cavity and/or the carrier pipe cavity 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.

24. A method according to any one of claims 1 to 23 wherein the carrier

pipe and flowline are towed to an offshore site prior to charging of the carrier pipe

with fluid.

25. A method according to any one of claims 1 to 24, wherein the carrier

pipe is released from the flowline by a rip-cord which is recovered separately from

the carrier pipe.

26. A method according to any one of claims 1 to 25 comprising the steps

of:

attaching a carrier pipe and a ballast line to the flowline, the carrier pipe

being in fluid communication with the ballast line;

towing 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;

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.