GB2394522A - Method and apparatus for paying out pipelines - Google Patents

Abstract

There is disclosed method and apparatus for paying out pipelines (200, 205) from a vessel, in particular relating to the offshore oil and gas industry. Relative movement between the vessel (14) and the seabed can be an obstacle, as it can induce unacceptable pipeline stress. Active tension regulation systems (230) exist, but they are unsuitable for applications involving multiple pipelines. Passive systems (260), however, are suitable. A method is disclosed whereby, prior to attachment to a main pipeline (205) subject to motion, a piggyback pipeline (200) is bent temporarily by plastic deformation to form a curved portion (340) after it leaves a supply reel (250), the curved portion by elastic deformation isolating the pipe as it leaves the supply from longitudinal oscillatory stress and motion. Suitable apparatus comprises pipeline feeder (420), bender (440) and straightener (460), arranged so that the curved portion is free to expand and contract elastically, thereby isolating the pipeline upstream of the apparatus from longitudinal oscillatory stress and motion.

Description

METHOD AND APPARATUS FOR PAYING OUT PIPELINES

This invention relates to methods and apparatus for paying-out a pipeline or multiple 5 pipelines from a supply of pipe carried on a floating vessel. The invention finds particular application in the offshore oil and gas industry, but is not limited to such applications. 10 Different types of pipe laying systems are known. In an "S-lay" system, such as that described in US 5,893,682 (Oliver)), a pipeline is paid out at a shallow angle, describing a rather stretched S-shape to the seabed. Substantial horizontal tension is maintained in the pipe through use of thrusters on the vessel, to maintain this shape. In US '682 the pipeline is fabricated by welding together short pipe sections on the deck of 15 a vessel, but it is also known that pipeline can be prc-fabricated and stored on a large reel on the vessel.

During the course of laying offshore pipeline, a pipe laying vessel may be subject to various forces of motion, such as those caused by sea swell, current and wind. The 20 induced motion can have a detrimental effect upon the pipeline being laid, inducing strains upon it beyond its design capabilities. Some of these forces can be compensated for by use of the vessel's thrusters. However, more random oscillatory movements, such as sea-swel1, are much harder to eliminate. This relative movement between the vessel and the seabed can be an obstacle to laying, as it can induce unacceptable 25 stresses/strains in the pipeline, especially where it meets the seabed. Vessel-mounted systems have therefore been developed to compensate for this motion to maintain constant tension and so reduce the stress upon the pipeline, allowing laying of pipe to continue in more challenging environments. In these systems, the pipeline exhibits motion back and forth relative to the vessel, within limits, and the work stations where 30 the pipe is fabricated and tested are designed to cope with this.

Another known pipe laying arrangement is "J-lay", where the pipeline leaves the vessel from an inclined tower or ramp, so as to hang in a natural catcnary curve to the seabed.

Less horizontal tension is required, and J-lay is therefore more suitable for use at greater depths. Due to the greater distance between surface and seabed, motion 5 compensation is not so often required, although it may be fitted in some J-lay systems.

In many installations, it is desired to lay two or more pipes simultaneously. US 5,975,802 (Willis/Stolt) shows a vessel having an inclined tower in which a main pipe is fabricated from sections on deck, and a smaller pipe, stored on a reel, is laid in 10 parallel with the main pipe and strapped to it as a "piggy-back" line. Both pipes are bent plastically upwards from the deck and over the top of the tower, and then straightened before being strapped together and entering the sea in the manner of J-lay.

In US '802, the weight of the suspended pipeline is taken by tensioners on the ramp, 15 and the pipeline therefore does not move relative to the vessel, except when being paid out under direct control. In other arrangements, however, it may be desired to couple a piggy-back line with a main pipeline having constant tension regulation, where the main pipe is moving relative to the vessel. This presents substantial engineering challenges, as both independent pipeline fabrication supply systems (main fabrication 20 line and piggy-back reel) would have to match their motion relative to the vessel to allow them to be attached to each other. Failure to do so could cause serious damage, considering the forces and inertia involved. As a consequence, where use of a tension regulation system results in relative motion of the coupled pipeline with respect to the vessel, it has, until now, remained unfeasible to perform tensioncompensated coupled 25 Reel/S-laying pipe laying.

Similar problems arise in other circumstances, beside the particular example above. For example, it is not generally practicable to combine laying of a large pipeline from a reel with constant tension regulation, because the inertia of the loaded reel does not permit 3() the dynamic control necessary to accommodate oscillatory motion of the pipeline.

Accordingly, it is an object of the invention to enable the use of constant tension regulation in a wider range of situations during the course of laying offshore pipelines, while keeping the stresses imposed upon the pipeline (and any piggy-back line) within acceptable limits. The inventors have recognised in particular that it is possible to 5 provide a region of elasticity in the normally rigid pipeline, and so isolate the supply of a pipeline from the oscillatory motion downstream.

The invention in a first aspect provides a method of paying-out a pipeline from a supply of pipe carried on a floating vessel, wherein the pipeline is bent temporarily by plastic 10 deformation to form a curved portion after it leaves the supply, the curved portion by elastic deformation serving to isolate the pipe as it leaves said supply from longitudinal oscillatory motion of the pipeline caused by forces applied downstream (in the direction of feeding the pipeline) of the curved portion.

15 The supply may comprise a reel, although the invention is not limited to such a case.

The pipeline may be a piggy-back line being fed in parallel with a larger pipeline to which it is attached, downstream of the curved portion, although the invention is not limited to such a case.

In such a case, the larger pipeline may be controlled by a constant tension regulation system, resulting in said oscillatory motion. The elasticity of the curved portion avoids the need to synchronisc the feeding of the piggyback line with the oscillations of the .. main ppohne.

The curved portion may be fixed at a location on the vessel, rather than travelling with the pipeline as it is fed.

The curved portion may comprise at least one substantially complete loop of pipe, the 30 pipeline upstream and downstream of the loop being substantially parallel. This permits the method to be applied or not, without changing substantially the layout of the vessel and the path followed by the pipeline.

The curved portion may be oriented normal to the deck of the vessel, thereby minimising deck space used.

5 The bending may be performed by feeding the pipeline by a track-type tensioner between inner and outer radius controllers.

The straightening may be performed by a straightener mounted so as to be able to move relative to the vessel in accordance with said oscillatory motion of the pipeline.

1() The method may include fixing the straightener relative to the vessel, prior to straightening a portion of pipeline.

The method may include repeating cycles of distinct feeding and holding phases, said 15 feeding phase comprising drawing new pipe from said supply while (a) bending the pipeline as it enters said curved portion and (b) straightening the pipeline as it leaves said curved portion, said holding phase comprising clamping the pipeline upstream of the curved portion, fixing it relative to the vessel.

20 In a case where the pipeline is a piggy-back pipeline being attached to a larger pipeline fabricated from shorter pipe sections on-board the vessel, the feeding phases and holding phases may be synchronised with feeding phases and welding phases of the pipeline fabrication process.

25 The bending and straightening may alternatively be performed on a more irregular basis. The invention further provides an apparatus for regulating the feeding of a pipeline during offshore pipe laying from a vessel, the regulating apparatus comprising: 30 - a feeding device for receiving said pipeline from a supply of pipe and for holding said pipeline in fixed relation to the vessel while also permitting the pipeline to be fed on demand in a direction downstream from said supply;

- bending means downstream of said feeding device for plastically bending the pipeline to form at least one curved portion; - a straightener for receiving said curved portion of pipeline and for straightening said pipeline prior to laying, 5 the apparatus being arranged such that in operation said curved portion between the feeding device and straightener is free to expand and contract elastically, thereby to isolate the pipeline upstream of said regulating apparatus from longitudinal oscillatory motion of the pipeline caused by forces applied downstream of the regulating apparatus. It is noted that the process of US '802, for example, involves bending and straightening the pipeline as it passes from fabrication to laying. In that known case, however, any curved portions of the pipeline are constrained by radius controllers and do not provide the longitudinal elasticity characteristic of the present invention.

The apparatus may be arranged to receive said pipeline from a reel on the vessel.

The apparatus may be arranged to receive a piggy-back line, alongside apparatus for feeding a larger pipeline to which the piggy-back line will be attached, downstream of 20 the regulating apparatus. The apparatus for feeding the larger pipeline may comprise stations for fabricating said larger pipeline from shorter pipe sections.

The apparatus may be arranged to maintain said curved portion at a fixed location on the vessel, rather than travelling with the pipeline as it is fed.

The apparatus may be arranged such that in operation the curved portion comprises at least one substantially complete loop of pipe, the pipeline upstream and downstream of the loop being substantially parallel.

30 The apparatus may be arranged such that in operation the curved portion is oriented normal to the deck of the vessel.

The feeding device may be track type tensioner. It may be mounted at a fixed location on the deck of the vessel. Such tensioners are well-known.

The bending means may comprise opposed inner and outer radius controllers. These 5 may also be of track type, but typically passive rather than driven.

The straightener may be mounted so as to be movable relative to the vessel in accordance with said oscillatory motion of the pipeline.

10 Means may be provided operable to fix the straightener relative to the vessel, prior to straightening a portion of pipeline.

These and other features and advantages of the invention will be apparent from the description of certain preferred embodiments which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure l a is a side view of a vessel laying a combined main and piggy-back pipeline, using conventional equipment (J-lay example); Figure l b is a plan view of the vessel and equipment of Figure 1; Figure 2 is a schematic diagram of an improved apparatus of laying a combined piggy back and main pipeline; Figure 3a is a side view of a practical embodiment of the apparatus shown in Figure 2; Figure 3b is a plan view of the equipment described with reference to Figure 3a; Figure 4a is a more detailed side view of a regulation loop which forms a part of the apparatus shown in Figures 3a and 3b; Figure 4b is a plan view of the regulation loop of Figure 4a; and Figure 5 is a plan view of the equipment mounted upon a pipe laying vessel (S-lay example).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figures la and lb show a combined piggy-back line 10 and main pipeline 12 being laid by a pipe laying vessel 14 in a known manner, where the piggyback line is attached by 5 steel straps and rubber blocks (not shown) to the main pipeline at 20, just before both enter the sea. This type of arrangement is described in more detail in US 5,975,802 (Willis/Stolt), mentioned above. A main pipeline production line 22 provides a source of pipeline, which is conveyed into the sea by rising it up a tower 24 and passing it around a radius controller 26 until it is presented to the surface of the sea at the desired 10 angle. The pipeline in this example is fabricated on-board by welding shorter sections together. In the example shown the pipeline is passed vertically into the sea, whereas the dashed illustration of the tower 30 in the same diagram shows the position for passing the pipeline into the sea at a lower angle. A tensioner 40 supports the full weight of the pipeline, to minimise tension on the pipeline as it is being fabricated and l 5 as it passes around the radius controller 26. The source of piggy-back pipeline 10 is a deck-mounted reel 42.

The piggy-back pipeline passes around its own radius controller 44 following substantially the same route as the main pipeline. Each pipeline 12, 10 also has an 20 associated deck radius controller, 50, 52, to prevent over-bending of the pipes as they bend up to the top of the tower.

In the above know example, it is assumed that the main pipe 12 is held and paid out by the tensioner on a purely progressive basis, and both pipes can be paid out under direct 25 control.

A vessel from which a pipeline is being laid is subject to various forces of motion such as those caused by sea swell, current and wind. The pilot of the vessel will seek to compensate for the constant forces, such as current, by use of engines and thrusters. As 30 these forces change slowly over time the pilot is able to monitor the vessel's position using usual positioning equipment, such as sonar buoys, GPS or radar, and compensate to accommodate. I lowever, a certain amount of unpredictable "higher frequency"

motion cannot be compensated for by the pilot, hence in some situations the need for vessel-mounted compensation, or 'regulation'.

l his unpredictable motion will tend to be "back and forth" or oscillatory motion, such 5 as those caused by sea swell or violent winds. In the case of sea swell, for example, as the vessel rides the peaks and troughs of the surface of the sea, the distance between the end of the pipe on the seabed and the vessel will vary, the amount of which depends on the sea state at the time. When the vessel is at the bottom of a trough the pipeline will be "long", seeking to push into the vessel (or at least become relatively slack) and at the 10 top of a peak will be "short", seeking to pull out of the vessel. Without any form of regulation the forces on the vessel and its pipe laying equipment caused by this periodic motion would either be damaging, or too great to allow normal pipe laying operations to be performed.

15 Accordingly, and in an S-lay configuration in particular, it may be desired to employ a heave compensation or other strain-relief system, which allows the pipelines to move, relative to the vessel. In this case, however it is very difficult to attach the piggy-back pipeline alongside the main pipeline, in particular because of the need to control the source of further pipeline, such as a continuous reel (due to its mass, and corresponding 20 moment of inertia). The novel apparatus and methods described below aim to overcome these problems by matching the relative motion between the two pipelines, allowing them to be attached to one another and for other operations such as field joint

coating to be performed.

25 Figure 2 is a schematic diagram of different stages required to perform "S-lay" pipeline laying with a piggy-back line. In this set-up, a small "piggy-back" line 200 is being laid with a main pipeline 205, and attached to it at 210.

The main line 205 is laid using conventional S-lay equipment, using a deck-mounted 30 constant tension regulation system 230 to provide strainrelief to ensure that the stress imposed upon the main pipeline remains within acceptable limits. The majority of the tension upon the main pipeline is accommodated by the tension regulation system 230,

thereby minimising the stress placed upon the pipeline itself, in particular at the interface with the seabed. Further sections 240 of pipeline are attached to the pipeline in a conventional manner.

5 The source 250 of the small pipeline 200 being attached to the main pipeline 205 is a reel in this example. Therefore no further fabrication steps are required prior to attaching it to the main pipeline. However, the skilled person will appreciate that the small pipeline could be fabricated in the same way as the main pipeline, by attaching small sections of pipeline to the one being laid.

However, to allow attachment of the pipelines to each other, the system must match the movement of the piggy-back pipeline with that of the main pipeline. To allow for unpredictable oscillation of the main pipeline, the novel apparatus effectively forms an elastic portion in the piggy- back pipeline. Specifically, in this embodiment, a 15 Regulator 260 is provided between the portion of piggy-back pipeline being attached to the main pipeline, and the reel 250 other source, as described later. This provides an essentially passive supply for the piggy-back line 200 avoiding the need to control two active supplies in perfect synchrony. For safe handling, the pipe on the reel must be kept taught by a certain back-tension, which can now be performed without regard to 20 the oscillatory motion downstream of the regulation loop 260.

Figures 3a and 3b are side and plan views respectively of a practical embodiment of the pipeline laying system. The deck-mounted piggy-back pipeline equipment comprises, listed in order of source to sea, a) a reel 250 of piggy-back pipeline, mounted upon a 25 reel-seat 310, b) a spooler seat 320 and reel tensioner 325 to manage the unreeling of the pipe, c) a roller box 330 (also known as a "crocodile") to guide the pipe into the Regulator 260 itself, which includes a loop 340 of pipe, and various support equipment, described later.

30 Figures 4a and 4b are side and plan views respectively showing the Regulator 26() in more detail. The term "regulation,' is applied to describe when the loop is operating to provide strain-relief of the pipeline piggy-back line during oscillatory motion of the

main pipeline. The system also has another mode of operation for passing the pipeline further into the sea, which is described later.

Regulator 260 is formed by three pieces of equipment in conjunction with the pipeline 5 200. A tensioner 420 is provided for holding the pipeline to isolate portions upstream of the regulator from longitudinal motion downstream, and for feeding the pipeline into the regulator according to demand. A deck radius controller 440 comprising inner and outer radius controller tracks is provided for bending the pipe into a set degree of curvature. A straightener 460 is for bending the pipe from the curve back to a straight 10 configuration while it is being fed out of the regulation loop, or for supporting the pipeline while regulation is taking place. The pipeline is fed through all three pieces of equipment as it progresses from the deck-side 'A' to sea-side 'B', in-between forming a loop 340 of pipeline of a size determined by the bending curvature.

15 In a practical embodiment, strain-relief is provided by plastically deforming the pipeline to a predetermined radius of curvature to form a "regulation loop" (comprising one or possibly more loops, the number depending upon the nature of compensation to be provided). The pipeline is then straightened back as it exits the loop, before passing into the sea. The nominal configuration of the loop is the loop relaxed in its 20 unconstrained shape after plastic deformation ("spring-back effect"). When the section of pipeline entering the sea moves with respect to the section on the deck (which is held stationary to facilitate fabrication of the pipeline), then the loop will accommodate this movement by increasing or decreasing its diameter. The loop is designed so that the pipeline stresses stay within defined stress tolerances during these movements.

The regulator 260 in this example is designed to operate in two modes: feeding or "move-up" mode, where the pipeline is further transported into the sea, and holding or "regulation" mode, where the relative Notion of the vessel with respect to the pipeline is accommodated at side "B" but isolated from side "A", allowing further sections of 30 pipeline to be fitted while the vessel is moving with respect to the main pipeline.

The loop of coiled pipeline 340 is held up by the clamping actions of the Tensioner 420 and Deck Radius Controller 440. If necessary, there can be additional simple structures of any type (not shown), such as vertical pipes either side of the pipeline to ensure that the regulation loop cannot fall over, should it become unclamped.

Tensioner 420 is of conventional design. In "regulation" mode the 'l'ensioner is firmly clamped to the pipeline passing through it, thus clamping the pipeline at side "A" directly to the vessel. In "move-up" mode, the Tensioner also feeds the pipeline through the regulation loop and support equipment, through the Straightener and I () thereafter into the sea.

Deck radius controller (DRC) 440 is used to bend the pipeline feeding into the regulation loop from a straight configuration into a curve. The degree of curvature is set so that the pipeline forms a complete loop for the end of the pipe to enter the l 5 Straightener 460. The diameter of the loop is selected to accommodate, for example, i2 metres of longitudinal pipeline movement by elastic deformation, while ensuring the plastic deformation does not occur throughout the anticipated range of operation.

Factors such as pipeline diameter and material will influence the amplitude of motion that can be accommodated in this way. If it is considered that the regulator 260 cannot 20 provide the desired motion compensation by using just one loop, it is possible to arrange it to use multiple turns of the loop by continuing the curve to produce further loops, before feeding the end of the pipeline into the Straightener. For example, six loops of the same type would provide a range of longitudinal movement of approximately +12 metros.

DRC 440 uses two pipe handling tracks, an inner track 442 on the inside of the loop, and an outer track 444 on the outside of the loop. Unlike with conventional pipe handling tracks which are typically in a straight configuration, the DRC tracks are set on a curve, so that a pipeline being guided between them is shaped into the required 30 degree of curvature. The force required to push the pipe through the DRC is provided by the Tensioner 420.

The DRC only operates to bend the pipe during "move-up" mode. It need not contribute to "regulation" mode but may be arranged to support the lower part of the loop in the desired vertical configuration.

5 The Straightener 460 is used in "move-up" mode to re-shape the pipe back into a straight configuration, before it enters the sea. However, it only straightens the pipeline when the pipeline is being pushed through the whole regulation loop by the Tensioner.

The Straightener operates in a conventional manner, comprising a series of tracks arranged on opposite sides of the pipe to apply forces at points spaced along the pipe.

As described before, the regulation loop operates like a giant spring, with upstream end A' clamped, by tensioner 420, the downstream end 'B' moving with respect to the vessel, according to the level of tension the regulation loop is accommodating at the time. Straightener 460 is not enabled to perform any straightening while the regulation 15 loop is regulating the tension on the pipeline ("regulation" mode), because constant straightening and bending of the pipe as it passed through the straightener would soon induce metal fatigue of the pipe. Instead, during regulation the straightener 460 is arranged to clamp itself to the pipe, and the whole straightener assembly is mounted upon a carriage 462 which is able to move freely upon runners 464 in the longitudinal 20 direction of the pipeline. A separate clamp mounted on the carriage could also be provided for the purpose. The straightener is enabled to move in equal measure upon its runners, as the sea-side part of pipeline (section "B") moves with respect to the vessel due to relative motion of the vessel with respect to the sea. The skilled person will appreciate that the length of the runners 464 will be chosen to accommodate the 25 greatest range of longitudinal motion of the pipeline in "regulation mode".

After move-up and bending of a new portion of pipe, the tension in the loop 340 will be released and straightener 460 will have been positioned upon the runner 464 so that loop 340 and carriage 462 are capable of accommodating the anticipated positive and 30 negative motion of the vessel, relative to the steady state.

Figure 3a also shows the range (depicted by the dashed lines 482, 484) of operation of the regulation loop 260 during "regulation". The loop in its nominal configuration 340 is in a relaxed (steady-state) condition, with no residual stresses within the loop, but with a residual strain due to the plastic deformation made. However, forces acting upon 5 outboard end of the pipeline place stress upon the loop, the other end of which is fixed relative to the deck by the tensioner 420. A compressive force will cause the loop to expand as indicated at 482. If the force is tensile, the loop will contract, as indicated at 484. As long as the expansion or contraction is restricted to the elastic range of deformation of the loop the counterforce provided by the loop will follow a predictable 10 law, increasing as the tensile force upon it increases. Of course, however, if too much compressive or tensile force is applied to the loop the material from which the pipe is constructed will become over-stressed, subjecting it to plastic deformation.

Detailed operating sequences will be given later. Briefly, during "regulation" mode the 15 regulation loop is subject to varying compressive and tensile forces, contracting and extending respectively as the pipeline is pulled out of and pushed into the vessel by the aforementioned forces, the straightener following the longitudinal movement by rolling on its support. The periodic motion of the vessel is thus compensated for without placing undue stress on the pipeline, vessel or its deck-mounted equipment, and without 20 the need for active control of the reel or tensioner.

During feeding or "move-up" mode, longitudinal movement of straightener 460 is arrested by locking its carriage in the steady-state position, straightener is enabled to straighten the pipeline as it passes through it, and tensioner 420 is driven to feed the 25 pipeline through the regulation loop, pulling more pipeline out of the reel. Tensioner 420 forces the pipe through the Deck Radius Controller 440, the action of which shapes the pipe into a loop as the pipeline passes through it.

Figure 5 is a plan view of a practical embodiment on a pipe laying vessel of a "piggy 30 back" pipeline being laid in conjunction with a main pipeline being laid by "S-lay". A basic S-lay arrangement is shown in US 5,893,682, mentioned above. The unreeled pipeline passes through the spooler seat 320, reel tensioner 325 and roller box 330,

through the regulator 260, before being joined to the main pipeline, before both pipelines arc transported into the sea in a conventional manner. One can observe that the "piggy-back" line mounted upon its reelseat 310 and regulator 260 are offset on the deck of the ship, with respect to the main pipeline. This is possible due to the flexibility 5 of the "piggy-back" line, which may be only 2"-4" (approximately 50mm to l00mm) in diameter, for example. The "piggy-back" line is physically attached to the main pipeline at point 400, in a conventional manner. The skilled reader will appreciate that the "piggy-back" fabrication line and regulation loop could be placed in alternative positions, depending upon the vessel deck configuration. For example, the regulation 10 loop and source of"piggy-back" pipeline could be mounted upon a raised deck above the deck on which the main pipeline is being fabricated. The loop 340 could be horizontal, and so forth. Because the "piggy-back" pipeline regulation loop 340 is a passive device,

not 15 requiring active control, the two compensation systems are easily matched in their ability to compensate for relative motion in equal measure. Use of an active control system controlling, for example, rotation of the piggy-back reel to match the relative motion of the main pipeline would involve complex monitoring and control systems for both pipelines to ensure that both compensation systems were operating exactly in 2() unison (not "fighting" each other). Failure to do so could cause serious damage, considering the forces and inertia involved.

In "regulation" mode the piggy-back reel 250, 310, spooler seat 320, reel tensioner 325, crocodile 330 and Deck Radius Controller 440 play no part in compensating for relative 25 motion. The Tensioners 325, 420 firmly grip the pipe to arrest its motion and provide stability to the regulation loop of the pipeline while it is in use. Likewise, the Straightener is disabled from straightening the pipeline, but is enabled to grip the pipe and move freely upon runners.

30 "Move-up", for example when a newly-fabricated section of main pipeline is fed into the sea to make room on deck for one or more sections to be added, is achieved in the following steps:

À disable "piggy-back" line regulation by enabling the Deck Radius Controller 440, enabling the Straightener, and locking the Straightener carriage 462 to the vessel; À release the gripping action of the Tensioners 325, 420; 5 À move up the main fabrication line (not shown in Figs. 2-5) pipeline by using vessel repositioning to draw the main pipeline through the main pipeline tension regulation system; À whilst doing so, match the reel speed of the "piggy-back" pipeline approximately to the feed rate of the main pipeline by manual control of the 10 feed rates of Tensioners 325, 420. The regulation loop will accommodate an amount of mismatch between the feed rates of the main and "piggy-back "pipelines; À a human operator can use the diameter of the regulation loop to gauge whether the feed rates of the Tensioners should be increased or decreased, seeking to 15 return its diameter to its steady state position; À lock Tensioners 325, 420 and free the carriage 462 to return to regulating mode.

In this case there is some residual tension from the bending process, the position at which the carriage is locked for move-up should be selected so that it will return by the springback effect to an appropriate mid- travel position, rather than 20 starting the regulation mode with the carriage too near one limit; À during fabrication of a new section of pipeline to the free end of the main pipeline, and while the main pipeline constant tension regulation system (not shown) is causing the pipeline to move with respect to the vessel, operate the regulation loop in "regulation" mode, to compensate for this relative motion; 25 À when the fabrication on the new section is complete, operate the regulation loot, in "move-up" mode to feed the "piggy-back" pipeline through the regulation loop, and simultaneously draw the main pipeline through its constant tension regulation system and out of the vessel, further into the sea. This also involves repositioning the vessel relative to the seabed; 30 À at the same time, during "Move-up" mode, further attach the "piggy-back" pipeline to the main pipeline, to extend the combined pipeline, and perform further finishing steps, such as coating;

À to continue the process of fabricating a "piggy-backed" pipeline, repeat the above steps as necessary.

Further uses of the regulation loop are hereafter described. It is possible to isolate the 5 move-up steps of both pipelines, so that while one pipeline is being extended the other is being moved-up, and vice versa. This provides the advantage of allowing work to continue on one of the pipelines while the other is being moved up, for example if the fabrication process for the former is more complicated.

10 It is also not necessary to operate the regulation loop in distinct feeding and holding cycles. The feed tensioner 420 and straightener 460 (which is free to move on its carriage 462) could be operated, either synchronously or independently in response to demand for the pipeline, to achieve a more or less continuous flow of pipe. Each unit can be controlled either manually or automatically, so as to maintain the loop 340 15 within limits, and to maintain the average position of the carriage 462 near the mid-

point of its permitted travel.

It is also envisaged that a regulator of this type may be used to provide motion compensation in the main or sole pipeline. For a reel-lay operation with large diameter 20 pipe, for example, if its hard to control the reel itself dynamically. In this case, as in the piggy-back example, the carriage 462 with straightener 460 may be free to travel in regulation mode. Alternatively, the carriage itself may carry a clamp and be driven by rams to form the motion compensation. Conversely, the spring- back regulation loop could be used as a buffer betwocn a weld-on-board pipeline fabrication process and a 25 more continuous move-up process. In this case, of course, the elasticity in the buffer would have to be at least equal to the length of pipe section between welds.

Those skilled in the art will appreciate that the embodiments described above are presented by way of example only, and that many further modifications and variations 30 are possible within the spirit and scope of the invention.

Claims (1)

1. A method of paying-out a pipeline from a supply of pipe carried on a floating vessel, wherein the pipeline is bent temporarily by plastic deformation to form a curved 5 portion after it leaves the supply, the curved portion by elastic deformation serving to isolate the pipe as it leaves said supply from longitudinal oscillatory motion of the pipeline caused by forces applied downstream of the curved portion.

2. A method as claimed in claim 1, wherein the supply of pipe comprises a reel.

3. A method as claimed in claims 1 or 2, wherein the pipeline is a piggyback line being fed in parallel with a larger pipeline to which it is attached, downstream of the curved portion. 15 4. A method as claimed in claim 3, wherein the larger pipeline is controlled by a constant tension regulation system, resulting in said oscillatory motion.

5. A method as claimed in any of the preceding claims, wherein said curved portion is fixed at a location on the vessel, rather than travelling with the pipeline as it is fed.

6. A method as claimed in any of the preceding claims, wherein said curved portion comprises at least one substantially complete loop of pipe, the pipeline upstream and downstream of the loop being substantially parallel.

25 7. A method as claimed in any of the preceding claims, wherein said curved portion is oriented substantially normal to the deck of the vessel, thereby minimising deck space used. 8. A method as claimed in any of the preceding claims, wherein said bending is 30 performed by feeding the pipeline forcibly between opposed inner and outer radius controllers.

9. A method as claimed in any of the preceding claims, wherein said straightening is performed by a straightener mounted so as to be able to move relative to the vessel in accordance with said oscillatory motion of the pipeline.

5 10. A method as claimed in any of the preceding claims, further comprising the step of fixing the straightener relative to the vessel, prior to straightening a portion of pipeline.

11. A method as claimed in any of the preceding claims, further comprising repeating cycles of distinct feeding and holding phases, said feeding phase comprising drawing 10 new pipe from said supply while (a) bending the pipeline as it enters said curved portion and (b) straightening the pipeline as it leaves said curved portion, said holding phase comprising clamping the pipeline upstream of the curved portion, fixing it relative to the vessel.

15 12. A method as claimed in any of the preceding claims, wherein, in a case where the pipeline is a piggy-back pipeline being attached to a larger pipeline fabricated from shorter pipe sections on-board the vessel, said feeding phases and holding phases are synchronized with feeding phases and welding phases of the pipeline fabrication process. 2() 13. A method as claimed in any of the preceding claims, wherein the bending and straightening is performed on an irregular basis.

14. Apparatus for regulating the feeding of a pipeline during offshore pipe laying from 25 a vessel, the regulating apparatus comprising: - a feeding device for receiving said pipeline from a supply of pipe and for holding said pipeline in fixed relation to the vessel while also permitting the pipeline to be fed on demand in a direction downstream from said supply; - bending means downstream of said feeding device for plastically bending the 30 pipeline to form at least one curved portion; a straightener for receiving said curved portion of pipeline and for straightening said pipeline prior to laying;

the apparatus being arranged such that in operation said curved portion between the feeding device and straightener is free to expand and contract elastically, thereby to isolate the pipeline upstream of said regulating apparatus from longitudinal oscillatory motion of the pipeline caused by forces applied downstream of the regulating 5 apparatus.

15. Apparatus as claimed in claim 14, arranged to receive said pipeline from a reel on the vessel.

1() 16. Apparatus as claimed in claims 14 or 15, arranged to receive a piggy-back line, alongside apparatus for feeding a larger pipeline to which the piggy-back line will be attached, downstream of the regulating apparatus.

17. Apparatus as claimed in claim 16, wherein said apparatus for feeding said larger 15 pipeline comprises stations for fabricating said larger pipeline from shorter pipe sections. 18. Apparatus as claimed in any of claims 14 to 17, arranged to maintain said curved portion at a fixed location on the vessel, rather than travelling with the pipeline as it is 20 fed.

19. Apparatus as claimed in any of claims 14 to 18, arranged such that in operation said curved portion comprises at least one substantially complete loop of pipe, the pipeline upstream and downstream of the loop being substantially parallel.

20. Apparatus as claimed in any of claims 14 to 19, arranged such that in operation said curved portion is oriented normal to the deck of the vessel.

21. Apparatus as claimed in any of claims 14 to 20, wherein said feeding device is a 30 track type tensioner.

22. Apparatus as claimed in claim 21, wherein said tensioner is mounted at a fixed location on the deck of the vessel.

23. Apparatus as claimed in any of claims 14 to 22, wherein said bending means 5 comprises opposed inner and outer radius controllers.

24. Apparatus as claimed in claim 23, wherein said radius controllers are of a substantially passive track type.

10 25. Apparatus as claimed in any of claims 14 to 24, wherein said straightener is mounted so as to be movable relative to the vessel in accordance with said oscillatory motion of the pipeline.

26. Apparatus as claimed in any of claims 14 to 24, wherein means are provided 15 operable to fix said straightener relative to the vessel, prior to straightening a portion of . ppehne. 27. A method of paying-out a pipeline from a supply of pipe carried on a floating vessel substantially as hereinbefore described with reference to Figure I a, lb, 2, 3a, 3b, 4a, 4b 20 or 5 of the drawings.

28. Apparatus for regulating the feeding of a pipeline during offshore pipe laying from a vessel substantially as hereinbefore described with reference to Figure la, lb, 2, 3a, 3b, 4a, 4b or 5 of the drawings.