WO2013171589A2

WO2013171589A2 - Improvements relating to pipe welding

Info

Publication number: WO2013171589A2
Application number: PCT/IB2013/001771
Authority: WO
Inventors: Olivier Collin; Cesar ATIN
Original assignee: Acergy France Sa
Priority date: 2012-05-17
Filing date: 2013-05-16
Publication date: 2013-11-21
Also published as: GB2502130A; WO2013171589A3; GB201208719D0

Abstract

A method of dissipating heat from a circumferential girth weld when fabricating a pipeline for subsea laying comprises placing an annular or part-annular heat sink in thermal contact with an inner or outer surface of a pipe. The heat sink comprises an annular or part-annular wall and fins for dissipating heat received from the pipe through the wall. The heat sink may be integrated with a line-up clamp or with a cooling module that also comprises a blower for forcing cooling gas flow across the heat sink. Another approach is to encircle the pipe with a blower and to force cooling gas from the blower inwardly against an outer surface of the pipe.

Description

Improvements relating to pipe welding

This invention relates to fabrication by welding of subsea pipelines, as typically performed on a pipelaying vessel such as a lay barge or in other offshore or onshore locations such as spoolbases.

Marine pipelaying techniques involving fabrication of a rigid pipeline on a vessel are generally categorised as either S-lay or J-lay, although variants and hybrids of those techniques have been proposed and used. J-lay operations will be used to exemplify the invention in the description that follows, as the invention has particular benefits in that context. However, the invention may have benefit in any operation in which pipe lengths are girth-welded end-to-end for subsea use, such as in S-lay operations and when spooling pipes for reel-lay operations. The S-lay technique involves welding together successive pipe sections or 'joints' at a series of working stations in a generally horizontal firing line on the deck of a pipelaying vessel, from which the pipeline is launched into the water over a stinger. The pipe adopts a first 'overbend' as it passes over the stinger and a second opposed bend as it nears the seabed. These opposed bends lend a shallow S-shape to the free span of the pipe - hence 'S-lay'.

S-lay techniques designed for shallow-water pipelaying are not suitable for pipelaying in deep and ultradeep water. The J-lay technique is usually preferred when pipelaying in such depths, particularly with larger-diameter pipes. J-lay involves welding single or multiple pipe joints onto the pipe end in an upright orientation in a J-lay tower on a pipelaying vessel. The pipe is launched downwardly into the water as it is formed. The pipe adopts a single bend as it nears the seabed to lend a J-shape to the free span of the pipe - hence 'J-lay'. S-lay benefits from a long production line with several working stations in which welding, inspection and coating operations may be performed simultaneously at different locations as the pipe advances in a launch direction. As this speeds the pipe fabrication process, S-lay is often preferred to J-lay for its inherently greater lay rate. This has led to the development of a variant of S-lay known as 'Steep S-lay', which is adapted for deep and ultradeep water applications where the pipe diameter allows. As the name suggests, Steep S-lay involves setting the lift-off point of the pipe from the stinger as close to vertical as possible. Thus, the pipe experiences a substantial overbend strain in a Steep S-lay operation, undergoing a deflection through substantially 90 degrees as it passes over the stinger. In all cases, the speed of pipelaying depends upon minimising the timescale of operations on the critical path. Given the stepwise, sequential processing steps of welding, weld testing and field joint coating, it is particularly important that welding, testing and coating take no longer than is necessary. Otherwise there will be a

'bottleneck' in the pipeline fabrication and installation process. Any resulting delays can be hugely expensive, tying up marine assets that cost hundreds of thousands of US dollars per day to operate and risking abandonment of the pipelaying operation if sea conditions deteriorate before the pipeline is fully installed.

Where large-diameter pipes must be laid in challenging conditions of depth and current, J-lay remains the favoured pipelaying option. However, unlike the multiple stations typical of S-lay and Steep S-lay, J-lay involves the use of a single welding station and subsequent inspection and coating must be done before the next weld can be made. Welding is therefore inevitably on the critical path, which emphasises the need to achieve high-quality welds consistently and as quickly as possible. In this respect, weld quality cannot be sacrificed for speed: a pipe cannot be launched if it has any critical weld defects and rectifying such defects adds greatly to the cycle time.

Pipe joints used for subsea pipelines are typically of steel. Girth welds between such pipe joints are made by arc-welding processes involving typically four to six passes. The weld region must be cleaned to bare metal between each pass to remove slag.

Each pipe joint end is bevelled, for example with a straight-sided angled cut of about 30° leaving a residual land of typically about 2mm for forming the root pass weld that fuses the inner edges of the opposed pipe walls. Narrower, nearly parallel-sided bevels are possible with a compound angle of cut; these may be advantageous to shorten the subsequent fill passes but they require particularly careful control of heat input. If the root pass is to be made from inside the pipe, the bevel must be adapted accordingly with angled cuts from inside and outside. Before welding, the ends of opposed pipe lengths are bevelled or re-bevelled if necessary and thoroughly cleaned and dressed. The pipe lengths are then pulled together and aligned with a line-up clamp. The pipe lengths must be accurately aligned, typically to a high-low of within 1 .5 mm, to ensure that the internal profile is as even as possible. The weld root gap must be set precisely to ensure full root penetration.

In J-lay operations, an internal line-up clamp or ILUC is usually fitted inside the pipe. The ILUC is suspended on a winch cable extending down from the open end of the upper pipe length held in a J-lay tower of the pipelaying vessel. Its function is to maintain alignment between, and to locate, the adjoining ends of upper and lower pipe lengths during the welding operation. To do so, the ILUC has pneumatically- or hydraulically-operated clamping devices such as shoes, distributed angularly about its longitudinal central axis and acting radially outwardly to bear against the internal surfaces of the adjoining pipe lengths. The shoes may also be operated individually or in opposed pairs to correct ovalisation of the pipe lengths.

The ILUC typically has copper or ceramic backing plate or ring that lies behind the weld root gap to ensure that the root pass weld does not penetrate excessively into the pipe bore. In use, the ILUC is lowered through the interior of the upper pipe length until it bridges the ends of the upper and lower pipe lengths with its backing plate or ring aligned with the weld root gap. Final alignment and adjustment is made and the weld root gap is set before the ILUC locks both pipe lengths against relative movement. As hydrogen can dissolve into molten steel and lead to cold cracking, the weld area is preheated before welding to ensure that the pipe is dry and to prevent the root pass weld from cooling too quickly. In this respect, it will be noted that narrow bevels in thick pipes present a very large heat sink to a relatively small amount of weld metal. For this purpose, pre-heat coils are suitably applied to the pipe ends to heat them by induction. Pre-heating can also be applied by other means, such as by a gas burner.

Pre-heating discourages generation of hydrogen due to disassociation of residual water; it also allows time for dissolved hydrogen to diffuse out before the steel transforms upon cooling. Also, over-rapid cooling may result in the formation of martensite in the weld and in the heat-affected zone (HAZ) adjacent the weld;

martensite gives rise to brittleness and is particularly susceptible to cracking. The root pass, sometimes termed the 'stringer bead', is the initial and most critical weld. It is laid down in a straight line, usually using a single welding head. It is vital that the root pass completely fuses the abutting inner faces of the pipe joints without leaving either unfused areas or excessive protrusion of weld metal into the pipe.

A second 'hot pass' weld is applied over the root pass weld before the temperature of the root pass weld falls below a level at which hydrogen migration is effective. The hot pass weld re-melts the root pass weld slightly to remove any slag inclusions in the root pass weld; it also heat-treats and stress-relieves the HAZ.

One or more fill passes follow the hot pass to fill the remaining bevel groove, between the internal diameter surface and the external diameter surface of the pipe. Multiple welding heads typically weave around the pipe during the fill passes, oscillating axially while moving circumferentially, to ensure complete fusion of the bevel walls. The final weld pass is a cap or cover pass that fills any residual groove atop the fill pass weld, typically leaving the weld standing proud of the pipe by 1 mm or so and overlapping the adjacent outer surfaces of the pipe joints by around 1 to 2 mm. Usually, after completing the hot pass, the I LUC is released and withdrawn from the pipe.

The completed weld is examined by radiography and/or by ultrasound techniques. Any unacceptable defects in the weld must be rectified, by repairing or if necessary by cutting out the welded region, re-bevelling the pipe lengths and repeating the entire welding operation. As rectification of a defective weld takes a long time, everything possible must be done to avoid critical defects arising in the first instance.

After passing inspection, the weld is coated with a field joint coating and is then ready to be launched into the sea as part of a J-lay operation.

Temperature must be controlled carefully throughout the welding process, both in absolute terms and in its rate of change, to ensure correct metallurgical properties in the weld and in the HAZ adjacent to the weld. The size of the HAZ depends on factors including the thickness of the pipe wall, the amount of preheating applied to the pipe and the rate of heat input into the weld. The metallurgical properties of the pipe in the HAZ, such as grain size, vary in accordance with the steep temperature gradient from the weld. The temperature gradient may extend from more than 1400 to 1500 degrees Celsius at the weld to near- ambient temperature in the surrounding parent pipe as little as 300 mm from the weld. After welding, regions of the pipe in the HAZ adjacent the weld will have been heated into the austenitic zone and re-crystallised, leading to equiaxed grains enlarged by the further heat input of repeated weld passes of the pipe. This tempering effect adjacent the weld increases ductility but reduces rigidity. In contrast, regions at the outer edge of the HAZ remote from the weld will still have elongated grains characteristic of the wrought steel of the parent pipe and so will be less ductile but more rigid than regions adjacent the weld.

To maintain correct metallurgical properties and to keep those properties as consistent as possible across the successive welds of the completed pipe, it is necessary to reduce the interpass temperature before starting the next pass. This is to keep the pipe temperature below about 300 degrees Celsius and preferably below about 250 degrees Celsius throughout the welding process. It is also advantageous to shorten the interpass period as this allows the next pass to begin sooner and hence allows the overall welding cycle to be completed in a shorter period of time.

Natural cooling after each weld pass - that is, merely waiting until the weld reaches an acceptable temperature to start the next pass - has a markedly adverse effect on the overall welding cycle time. Accelerated or forced cooling advantageously reduces the overall welding cycle time but cooling too quickly, for example with water quenching, can modify the metallurgical properties in a way that risks a low-quality weld. For example, over-rapid cooling may result in the formation of martensite as noted above and may hinder hydrogen migration from the steel.

EP 1328374 to Saipem describes the problems of excessive temperature when multiple welding heads are used. It addresses those problems by quenching the inside of the pipe with a cooling liquid such as water which may, for example, be sprayed in atomised form from nozzles mounted on a cooling ring within the pipe. Recognising that it is not desirable to introduce a significant volume of liquid such as water into a pipeline, EP 1328374 proposes that less than fifteen litres of cooling liquid is used during each welding cycle. However, the multiple successive welding cycles of J-lay operations will inevitably lead to accumulation of a substantial aggregate volume of liquid in the pipeline over time.

EP 1328374 also acknowledges that quenching with a cooling liquid may itself cause a problem by cooling the pipe too quickly. It addresses that problem by restricting its use to low- to medium-carbon steel, which is less susceptible to deterioration through rapid cooling. However, of course, this does nothing to solve the problem in relation to higher-carbon steels.

As EP 1328374 also notes that actively cooling the root weld may reduce the quality of that critical weld, it proposes to introduce cooling liquid only after at least three weld passes have been completed. Meanwhile - but only after the root pass has been completed - the weld may be cooled with a gas such as air, which may conveniently be blown out of the same nozzles from which a cooling liquid will later be sprayed within the pipe. However, EP 1328374 notes that air is much less effective than water as a cooling medium.

US 4223197 to Hitachi is another example of internal water-cooling applied to pipes during girth welding. It discloses a cooling device comprising a tubular shaft on the central longitudinal axis of the pipe through which water is supplied to water-spray nozzles branching off radially from the shaft. However, US 4223197 is directed to the problem of carbide precipitation when welding stainless steel pipes for use in chemical plants and nuclear equipment. There is no suggestion to do so, but if its teaching was adapted to cool pipes made of the types of steel typically used in subsea applications, their metallurgical properties would suffer. Also, the shape of the cooling device of US 4223197 does not allow its use in an offshore pipeline welding context such as in J-lay operations, where other equipment such as clamps and cables must also be accommodated for movement along the inside of the pipe.

US 2009/0212024 to Muller discloses an internal cooling device. The device contacts the inner surface of the pipe only through thin, flexible washers whose purpose is to centralise the device in the pipe rather than promoting thermal contact between the device and the pipe. The pipe, not the weld, is cooled down by an unspecified coolant and the disclosure describes extraction of coolant rather than blowing, essentially describing a TiG welding method. Coolant is emitted from a heat sink at predefined intervals. This means that cooling is periodic or intermittent and the period of cooling is predefined rather than being reactive in a manner that is apt to control weld temperature as the present invention contemplates.

WO 02/00385 to Kalberg discloses an internal cooling device that blows cooling gas against the interior of the pipe in line with the weld and the HAZ, but again there are no features that promote thermal contact of the cooling device with the pipe. The rationale is not to control the weld temperature just below a limit but instead to quench the weld with a forming gas. The forming gas is stored and transported as a liquid and so must be injected at a very low temperature that could jeopardise or break a girth weld. Also, the use of forming gas introduces a risk of asphyxia or toxicity for nearby workers. Against this background, the present invention resides in methods for dissipating heat from a circumferential girth weld when fabricating a pipeline. One such method of the invention comprises placing an annular or part-annular heat sink in thermal contact with an inner or outer surface of a pipe at a heat sink location. The heat sink may have formations to increase its surface area to dissipate heat conducted from the weld.

In methods of the invention, forced cooling gas flow applied to a heat sink or to the pipe is advantageously controlled in response to sensing temperature in the weld and in the HAZ surrounding the weld. For example, a valve system may be employed for control purposes when taking cooling gas from a high-pressure source. The cooling gas is preferably air or at least predominantly so. This is safe for workers and helps to avoid over-rapid cooling.

Heat may be conducted through the pipe from the weld to a heat sink location that is axially spaced from the weld. It is also possible for a heat sink to be in contact with an inner surface of the pipe at a heat sink location aligned with the weld.

Preferably, the heat sink is placed at the heat sink location before performing a root pass of the weld, conveniently by carrying the heat sink to the heat sink location on an internal line-up clamp. However, the heat sink may be moved and positioned independently of an internal line-up clamp if desired. Advantageously, cooling gas flow is forced across the heat sink, for example between fins of the heat sink that serve to increase the surface area of the heat sink. The cooling gas is suitably piped in from a high-pressure source. Such a forced cooling gas flow is preferably initiated after performing a root pass of the weld and is more preferably continued or repeated during a second pass of the weld and all subsequent passes of the weld.

The invention may involve moving a module comprising the heat sink and an integral forced cooling gas supply along the pipe to the heat sink location. The module may be moved independently of an internal line-up clamp within the pipe.

An internal heat sink may be expanded into a deployed state in thermal contact with an inner surface of the pipe when at the heat sink location; conversely an external heat sink may be clamped or otherwise contracted into thermal contact with an outer surface of the pipe when at the heat sink location.

For compactness and efficient heat transfer, the heat sink location is preferably disposed between a welding bug guide band and the weld or on an opposite side of the weld from the welding bug guide band.

Another method of the invention comprises encircling the pipe with a blower and forcing cooling gas from the blower inwardly against an outer surface of the pipe at a location axially spaced from the weld, to dissipate heat conducted through the pipe from the weld. Again, the blower is preferably disposed between a welding bug guide band and the weld or on an opposite side of the weld from the welding bug guide band. The blower is suitably supplied with cooling gas from a high-pressure source.

The inventive concept also embraces a method of dissipating heat from a

circumferential girth weld when fabricating a pipeline for subsea laying, comprising forcing cooling gas outwardly against an inner surface of the pipe at a location axially aligned with the weld.

The invention allows all welding passes to be completed without applying cooling liquid to the pipe, other than such liquid as may be entrained in a wet cooling gas flow. The inventive concept also embraces related apparatus features. For example, a heat sink of the invention comprises an annular or part-annular wall that is shaped and arranged to be held in thermal contact with an inner or outer surface of the pipe, and formations integral with or attached to the wall to increase the surface area of the heat sink for dissipating heat received from the pipe through the wall. That wall may be divided into portions movable radially into and out of thermal contact with the pipe. The heat sink may be supported on or integrated with an internal line-up clamp, or may be part of an independent structure. A cooling module of the invention comprises a heat sink arranged to be held in thermal contact with an inner or outer surface of the pipe and a blower for forcing cooling gas flow across the heat sink. For example, the heat sink and the blower are suitably annular or part-annular and are spaced from each other along a mutual central longitudinal axis. The blower may comprise angularly-spaced nozzles aimed toward the heat sink, in which case the nozzles of the blower are preferably aimed between angularly-spaced fins of the heat sink.

The inventive concept extends to a pipe for subsea laying in combination with a heat sink or a cooling module of the invention, and to a subsea pipeline made by the methods of the invention. The inventive concept also embraces a pipelaying vessel or other pipe-fabricating facility operating the methods of the invention or equipped with a heat sink or a cooling module of the invention.

The cooling ring module of the invention allows fine control of the cooling cycle, better than water quenching of the weld, with a smooth and controlled reduction of interpass temperature. The cooling ring module includes a gas/air system to cool down a pipe surface. The module is placed against an internal or external pipe surface, near the weld location, to counteract increasing pipe temperature during welding. The cooling ring module is placed ready for use and is activated after the first welding pass is completed. From this point, the cooling ring module dissipates the heat arising from the welding operation, so that the interpass temperature is reduced and limited.

Optional fins are suitably attached to a ring structural part which is in contact with the pipe. The pipe conducts heat from the weld to the fins and the fins then dissipate that heat by thermal exchange with the surrounding air. The size, shape and construction of the fins is chosen to optimise this thermal exchange. As the system of the invention works in parallel to welding activity, in 'hidden time', it does not increase the overall duration of the activity. J-lay involves welding in the '2G' position, with girth welds being made in a generally horizontal plane around a pipe held in an upright orientation. The invention is also apt to be used in S-lay and Steep S-lay techniques that involve welding in the '5G' position, with girth welds being made in a generally vertical plane around a pipe held generally horizontally. The invention is also suitable for use when welding in the '6G' position with the pipe axis held approximately mid-way between the vertical and horizontal, for example at 45° to the horizontal, in which case girth welds are in correspondingly inclined planes orthogonal to the pipe axis. The invention may be used in conjunction with manual, semi-automatic or automatic welding processes. Currently preferred embodiments of the invention are used in J-lay operations and employ an internal cooling ring to inject air. Fins are optional. The heat sink is inserted for contact with the pipe, the root pass of a weld is performed, then cooling is activated and other passes are made. Depending on the target temperature, the cooling temperature or effect may or not be modified; indeed, cooling can be interrupted, stopped or continued. However, the basic principle of preferred embodiments involves continuous blowing.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a perspective view from near sea level of a J-Lay tower on a barge;

Figure 2 is a perspective view of an erector arm loading a double joint into the tower of Figure 1 ;

Figure 3 is a perspective view of a welding operation taking place at a welding station in the tower of Figure 1 ;

Figure 4 is an end view of an external cooling ring in accordance with the invention; Figure 5 is a perspective view of the cooling ring shown in Figure 4;

Figure 6 is a perspective detail view showing the cooling ring of Figures 4 and 5 in use on a welded pipe oriented for J-lay operations;

Figure 7 is a schematic end view of an internal cooling ring in accordance with the invention, shown in a retracted state within a pipe;

Figure 8 is a schematic end view of the internal cooling ring of Figure 7, shown in an extended, deployed state pressed against the internal wall of the pipe;

Figure 9 is a schematic part-sectioned side view of a welded pipe oriented for J-lay operations, the pipe containing an internal line-up clamp and having various cooling features, namely: a pair of internal cooling rings; internal blowers directing cooling air across the internal cooling rings, the weld and the surrounding HAZ; an external cooling ring; and external blowers directing cooling air across the external cooling ring;

Figure 10 corresponds to Figure 9 but shows the external cooling ring positioned below the weld, opposed about the weld with a guide band for a welding bug, positioned above the weld;

Figure 1 1 corresponds to Figure 9 and shows a variant that retains internal blowers directing cooling air across the weld and the surrounding HAZ but omits the internal cooling rings, the external cooling ring and the external blowers;

Figure 12 corresponds to Figure 9 and shows a variant in which an internal cooling ring is axially aligned with a circumferential weld in the pipe;

Figure 13 corresponds to Figure 9 and shows a further variant in which the internal and external cooling rings inject cooling gas against the inner and outer walls of the pipe; Figure 14 is a schematic sectional side view of an internal cooling ring module in accordance with the invention, with integral cooling gas distribution for blowing gas across fins of the cooling ring; Figure 15 is a cross-sectional view on line XV-XV of Figure 14;

Figure 16 is a perspective view of an internal cooling ring for blowing cooling gas directly against an inner skin of a pipe; Figure 17 is a sectional side view of the internal cooling ring of Figure 16 in use within a pipe, in axial alignment with a circumferential weld; and

Figure 18 is a graph that plots temperature curves during a welding cycle for two reference welds and for welding with an internal cooling ring in accordance with the invention.

An example of a J-lay system is found on the Applicant's derrick lay barge Acergy Polaris. To put the invention into context, the operation of Acergy Polaris during J-lay pipelaying will firstly be described with reference to Figures 1 to 3 of the drawings. This J-lay example does not limit the scope of the invention, whose wider applicability has already been explained.

The J-lay tower 10 of the barge 12 is supplied with pipe joints 14 fabricated onshore, which are stored horizontally on the deck 16. In this example, the pipe joints 14 are double joints although triple- or quad-joints could be used if a J-lay tower 10 is tall enough.

As required, the pipe joints 14 are lifted successively in horizontal orientation from the deck 16 to a tower entry level 18 using a pipe elevator system 20 best shown in Figure 2. Here, a pipe joint 14 is loaded into a pivoting erector arm 22 which upends the pipe joint 14 into an upright orientation and passes it over to a tower handling system comprising a tensioner 24. The pipe joint 14 is then lowered down and aligned with the pipeline end 26 held in a support bushing 28 (see Figure 3) at a first work station 30 on the tower 10. The pipe joint 14 is welded to the pipeline end 26 at the first work station 30 before the load of the pipe string is transferred to the tensioner 24 near the top of the J-lay tower 10. The completed pipe string is then lowered down to the support bushing 28 for the addition of the next pipe joint 14. The tensioner 24 and the support bushing 28 alternate to grip the pipeline end, interacting in a so-called 'hand-over-hand' manner.

As the pipe string is lowered, a field joint coating is applied to the welded joint at a second work station 32 suspended from the tower 10 below the first work station 30. The weld can be inspected at either or both of these two work stations 30, 32.

As shown in Figure 3, welding is performed by one or more automatic welding bugs 34 that are driven around the pipe string on a track or guide band 36 fixed on the pipe joint 14 being welded to the pipeline end 26 below. The or each bug 34 moves

circumferentially around the pipe string so that one or more welding heads 38 carried by the bug 34 can run a weld bead within a groove defined between the pipe joint 14 and the pipeline end 26.

Each welding bug 34 requires services including power, data connections, shielding gas and welding wire to be fed continuously to the welding heads 38 during welding. Platforms such as annular turntables at each work station 30, 32 enable pipeline workers 40 such as welders and supervisors to weld, inspect and coat the pipe string in the J-lay tower 10. The turntables provides working platforms for the pipeline workers 40 who control and observe the welding operation and may also support equipment required for the welding operation and for related processes such as weld inspection.

Moving on now to Figures 4 to 6 of the drawings, these show an external cooling ring 42 in accordance with the invention. The external cooling ring 42 may be made of steel or other suitably conductive metal such as aluminium and may be cast and/or machined. A combination of different materials can also be used to allow equipment improvements, such as weight reduction or improved contact with pipe surfaces.

Fins 44 extend radially outwardly from an integral inner circumferential band 46 in planes aligned with the central longitudinal axis of the external cooling ring 42. For compactness where there is limited space in use, the fins 44 taper from the inner circumferential band 46 to a narrower outer edge 48. The inner circumferential band 46 is divided at diametrically-opposed locations into two half-shells 50', 50". The half-shells 50', 50" are joined permanently on one side by a compound hinge 52; on the opposite side, the half-shells 50', 50" are held together temporarily by a pivoting latch pin 54. This quick-release system allows the external cooling ring 42 to be positioned and removed easily and quickly.

By virtue of the hinge 52 and the jaw-like half-shells 50', 50", the external cooling ring 42 is arranged to clamp around an exposed surface of the pipe joint 14 in the manner of a clamshell as shown in Figure 6. The inner circumferential band 46 of the external cooling ring 42 is suitably flexible and resilient to maintain inward clamping pressure on the pipe joint 14.

The external cooling ring 42 is placed on the external skin of the pipe joint 14, close to the welding area, before welding starts. The external cooling ring 42 stays in place until completion of weld filling, and is removed after a desired cooling time has elapsed.

The external cooling ring 42 is preferably part of a module that is independent of any other equipment, although such a module could be connected to other equipment if that is practical. A module comprising the external cooling ring 42 may include a gas supply system to perform cooling by heat convection using a dry or wet gas. For this purpose, electrical and air (or other gas) supplies may be connected to the external cooling ring 42. The module may be activated with a remote control system enabling settings such as gas flow or gas composition to be modified at any time during cooling.

Close contact between the inner circumferential band 46 of the external cooling ring 42 and the pipe joint 14 promotes thermal coupling between them to transmit heat conducted along the pipe joint 14 from the weld to the external cooling ring 42. For this purpose, any insulating or protective coating on the pipe joint 14 is suitably cut back to allow contact between the inner circumferential band 46 and the metal skin of the pipe joint 14. Heat is discharged to the atmosphere by radiation and convection from the external cooling ring 42, aided by the large surface areas of its fins 44.

Figure 6 shows the external cooling ring 42 positioned below the guide band 36 that is fixed to the pipe joint 14. It follows that the external cooling ring 42 is positioned between the guide band 36 and the weld 56 made between the pipe joint 14 and the pipeline end 26 below. This places the external cooling ring 42 as close as possible to the weld 56; however, another approach is to place the external cooling ring 42 below the weld 56 as shown in Figure 10, with the weld 56 therefore being between the guide band 36 and the external cooling ring 42. In practice, the guide band 36 supports welding bugs 34 as shown in Figure 3 but those welding bugs have been omitted from Figure 6 for clarity.

When the weld 56 is complete and the weld 56, the pipe joint 14 and the pipeline end 26 have cooled to a sufficient extent for testing and coating, the guide band 36 and the external cooling ring 42 are removed from the pipe joint 14. The guide band 36 and the external cooling ring 42 may be fixed again to the next pipe joint 14 when the completed pipe string is lowered by the tensioner 24 on the J-lay tower 10.

Figures 7 and 8 of the drawings show an internal cooling ring 58 positioned inside a pipe joint 14. This reflects a preferred design being an internal cooling module, which injects air or other gases on the internal pipe skin or/and the weld root as may be required to follow an optimised cooling cycle during weld passes.

The invention offers two options for internal cooling, namely to blow air or another cooling gas directly against the internal pipe skin, or to blow air or another cooling gas across fins of an internal cooling ring that is in thermal contact with the internal pipe skin.

Figures 7 and 8 show an internal cooling ring 58 that may be made of steel or other suitably conductive metal such as aluminium and may be cast and/or machined. Again, a combination of different materials can also be used to allow equipment

improvements, such as weight reduction or improved contact with pipe surfaces.

An internal cooling ring 58 may be positioned instead, or additionally, inside a pipeline end 26. Also, one or more internal cooling rings 58 may be used instead of, or in addition to, one or more external cooling rings of Figures 4 to 6.

The schematic views of the internal cooling ring 58 in Figures 7 and 8 show that fins 60 extend radially inwardly from an integral outer circumferential band 62. The fins 60 lie in planes aligned with the central longitudinal axis of the internal cooling ring 58. The outer circumferential band 62 is divided by diametrically-opposed gaps 64 into two curved, nearly semi-circular elements 66', 66". These shoes 66', 66" are mounted on double-acting mutually-opposed jacks 68 extending radially outwardly from a central support structure 70. The support structure 70 may, for example, be integrated with a line-up clamp as shown in Figure 9 but it need not be: instead, the elements 66', 66", the jacks 68 and the support structure 70 could together form an internal cooling ring module that is a discrete, independent system.

The gaps 64 between the elements 66', 66" allow the jacks 68 to pull the elements 66', 66" toward each other into a retracted position enabling the elements 66', 66" and the support structure 70 to be inserted into the pipe joint 14. Once inserted to a depth sufficient to place the internal cooling ring 58 at the axial position required relative to the weld 56, the jacks 68 are activated to drive the elements 66', 66" against the inner wall of the pipe joint 14 or of the pipeline end 26, as the case may be.

Close contact between the outer circumferential band 62 and the pipe joint 14 or the pipeline end 26 promotes thermal coupling between them to transmit heat conducted along the pipe joint 14 or the pipeline end 26 from the weld 56 to the internal cooling ring 58. To promote close heat-conducting contact, the outer circumferential band 62 of the internal cooling ring 58 may be flexible and resilient to maintain outward pressure on the inner wall of the pipe joint 14 or of the pipeline end 26. Heat is discharged to the surrounding air by radiation and convection from the internal cooling ring 58, aided by the large surface areas of its fins 60. When the weld 56 is complete and the weld 56, the pipe joint 14 and the pipeline end 26 have cooled to a sufficient extent for testing and coating, the internal cooling ring 58 is withdrawn after the jacks 68 have retracted the elements 66', 66" toward the support structure 70. The internal cooling ring 58 is then ready to be inserted again into the next pipe joint 14 when the completed pipe string is lowered by the tensioner 24 on the J-lay tower 10.

In a variant of the arrangement shown in Figures 7 and 8, the elements 66', 66" of an internal cooling ring 58 may be hinged to each other on one side. In that case, the elements 66', 66" may be latched in an extended position by a dog mechanism on the opposite side from the hinge. Internal cooling has various advantages, which are supported by the temperature curves to be discussed below in relation to Figure 18 of the drawings. Internal cooling provides cooling where assistance with cooling is most needed, bearing in mind that the internal wall of a pipe has a smaller surface area than its external wall and is exposed to a lesser natural flow of cool ambient air. Also, an internal cooling ring 58 may be placed advantageously close to the weld 56. Indeed, two (or more) internal cooling rings 58 may be placed one (or more) each side of the weld 56 if desired; it is even possible for an internal cooling ring 58 to be aligned axially with the weld 56, as will be shown in Figure 12.

To illustrate various cooling options, Figure 9 shows an arrangement in which two internal cooling rings 58 are placed one each side of a weld 56 between a pipe joint 14 and a pipeline end 26. Here, the internal cooling rings 58 are used in addition to an external cooling ring 42 of the type shown in Figures 4 to 6, although this combination is optional.

The pipe joint 14 and the pipeline end 26 shown in Figure 9 have coatings 72 cut back from their adjoining ends. The external cooling ring 42 is clamped to the steel exposed in the resulting gap between the coatings 72, to receive heat from the weld 56 by conduction along the wall of the pipe joint 14. Radiation and convection from the fins of the external cooling ring 42 transfers that heat to the surrounding air, as represented here by wavy lines above the external cooling ring 42.

In the example shown in Figure 9, convection is supplemented by optional external blowers 74 that blow air between the fins of the external cooling ring 42. The blowers 74 can be supported in any convenient way; for example, they may be mounted to an external structure beside the pipe joint 14, or they may be mounted temporarily to the pipe joint 14 itself. More preferably, the blowers 74 are mounted to the external cooling ring 42 to form a discrete self-contained cooling module. The blowers 74 may comprise fan impellers or may be nozzles supplied with compressed air.

Also in the gap between the coatings 72 above the external cooling ring 42, a guide band 36 supports one or more welding bugs 34 carrying one or more welding heads 38, shown here schematically aligned with the weld 56. An internal line-up clamp (ILUC) 76 is suspended on a winch cable 78 extending down from the open end of the pipe joint 14 to bridge the adjoining ends of the pipe joint 14 and the pipeline end 26. The ILUC 76 aligns and locates the adjoining ends of the pipe joint 14 and the pipeline end 26, to lock both of those pipe lengths against relative movement during the welding operation.

The ILUC 76 has shoes 80 spaced angularly about its longitudinal central axis that act radially outwardly to bear against the internal surfaces of the pipe joint 14 and the pipeline end 26. For this purpose, electric, pneumatic or hydraulic actuators (not shown) suitably act between the ILUC 76 and the shoes 80. The shoes 80 centralise the ILUC 76 within the pipe joint 14 and the pipeline end 26; they may also be operated individually or in opposed pairs to correct ovalisation of those pipe lengths.

A copper or ceramic backing plate or ring 82 supported by the ILUC 76 is aligned with the weld 56 in a conventional manner. The ILUC 76 also supports internal cooling rings 58 as illustrated schematically in Figures 7 and 8, one each side of the backing plate or ring 82 and closely adjacent the weld 56. Each internal cooling ring 58 comprises elements 66', 66" mounted on double-acting jacks 68 that act radially to drive the elements 66', 66" against the inner wall of the pipe joint 14 and the pipeline end 26 as appropriate. Again, the jacks 68 acting between the ILUC 76 and the elements 66', 66" are an example of actuators that may be electric, pneumatic or hydraulic.

In this extended or deployed position, the internal cooling rings 58 receive heat from the weld 56 by conduction along the walls of the pipe joint 14 and the pipeline end 26. Radiation and convection from the fins of the internal cooling rings 58 transfers that heat to the air within the pipe joint 14 and the pipeline end 26, as represented in the drawings by wavy lines within the pipe joint 14 shown here around and above the ILUC 76. Warm air will rise within the pipe joint 14 toward its open upper end; in this respect, the pipe joint 14 may contribute an advantageous chimney effect to promote air flow and hence to increase transfer of heat from the internal cooling rings 58.

In this example, convection from the fins of the internal cooling rings 58 is

supplemented by internal blowers 84 that blow air between those fins. Again, the internal blowers 84 may comprise fan impellers or may be nozzles supplied with compressed air or other gas. The internal blowers 84 are shown here supported by the ILUC 76 but they may be supported in any other convenient way. For example, the blowers 84 may be mounted to the internal cooling rings 58 to form discrete self- contained cooling modules. The embodiment of Figures 12 and 13 is an example of such a cooling module, to be described later. It is also possible for upper and lower internal blowers 84 to blow air in the same direction: conveniently, up toward the open upper end of the pipe joint 14. However, the internal blowers 84 are optional and may be omitted entirely, or there may be only one set of them on one level.

The I LUC 76 has various on-board systems shown schematically in Figure 9, namely: a switching system 86 for operation of the various electrical, hydraulic or pneumatic actuators and of the internal blowers 84, taking power and fluid inputs as necessary from an umbilical 88 extending parallel to the winch cable 78; a sensing system 90 for sensing the temperature profile of the weld 56 and the surrounding HAZ, which may, for example, take input from thermocouples on the pipe joint 14 and the pipeline end 26 or from thermal radiation detectors facing the weld 56 and the HAZ (not shown); and a control system 92 for controlling the internal blowers 84 and/or the external blowers 74 in accordance with temperature data received from the sensing system 88, to promote prompt and effective cooling of the weld 56 and the HAZ but without over-cooling. Of course, the sensing system 90 and the control system 92 could be implemented outside the ILUC 76, in which case suitable control signals may be fed to the ILUC 76 via the umbilical 88. It is also possible for control data generated by a sensing system 90 on the ILUC 76 to be fed via the umbilical 88 to an external switching system acting on the external blowers 74.

Figure 10 shows that an external cooling ring 42 may alternatively, or additionally, be mounted close to the weld 56 in a position below the weld 56. In this case, the external cooling ring 42 is therefore opposed to the guide band 36 across the weld 56.

Otherwise, the features of Figure 10 correspond to those of Figure 9 and like numerals are used for like parts. Figure 1 1 shows another variant of the arrangement shown in Figure 9; like numerals are again used for like parts. In this instance, the internal cooling rings 58, the external cooling ring 42 and the external blowers 74 are omitted but the internal blowers 84 remain to direct cooling air across the weld 56 and the HAZ. Again, the control system 90 controls the internal blowers 84 in accordance with temperature data received from the sensing system 88, to cool the weld 56 and the HAZ without over-cooling them.

As in the embodiments of Figures 9 and 10, it is possible for the upper and lower internal blowers 84 of Figure 1 1 to blow air in the same direction; it is also possible to have only one set of internal blowers 84 on one level.

Figure 12 shows a variant of the arrangement shown in Figure 9, in which numerals are used for like parts. Here, the backing plate or ring 82 supported by the I LUC 76 is omitted. The backing plate or ring 82 is replaced in this example by a finned internal cooling ring 58 like that illustrated schematically in Figures 7 and 8, positioned in axial alignment with the weld 56. As before, the internal cooling ring 58 comprises radially- movable elements 66', 66" mounted on double-acting jacks 68.

In an extended or deployed position, the internal cooling ring 58 receives heat directly from the weld 56. Radiation and convection from the fins of the internal cooling rings 58 transfers that heat to the air within the pipe joint 14, with convection being supplemented by internal blowers 84 that blow air between those fins.

Figure 13 shows a further variant of the arrangement shown in Figure 9; again, like numerals are used for like parts and this variant has internal and external cooling rings, either of which could be omitted in favour of the other. In this instance, however, the internal cooling rings 94 and the external cooling ring 96 are adapted to inject cooling gas directly against the inner and outer walls of the pipe. For this purpose, each cooling ring 94, 96 in Figure 13 has a C-section channel shape that ducts a cooling gas such as air supplied from a suitable high-pressure source through nozzles directed against the adjacent pipe wall. The cooling rings 94, 96 suitably bear against the pipe wall and the cooling gas can escape through gaps interspersed around the pipe-contacting edge of the C-section channel shape. The high-speed flow of gas passing over the pipe surface carries away heat radiated from the pipe into the flowing gas. In other respects, the cooling rings 94, 96 of Figure 13 are similar to the counterpart cooling rings 42, 58 of Figure 9, particularly in comprising two or more parts that move relative to each other and the pipe for positioning the cooling rings 94, 96 before use and for removing them after use. For example, actuators in the form of jacks 68 are again shown to drive opposed elements of the internal cooling rings 94 outwardly from the ILUC 76 against the internal wall of the pipe.

The backing plate or ring 82 shown supported by the ILUC 76 in Figure 13 could be omitted like the arrangement shown in Figure 12. This may allow an air-blowing internal cooling ring to be aligned axially with the weld 56 like the arrangement shown in Figure 17, whether that internal cooling ring is supported by the ILUC 76 or otherwise. Fins 44, 60 of a cooling ring 42, 58 may be used on a stand-alone basis, without gas injection. Where gas injection is used, gas may be injected from between the fins 44, 60 or from a module extension inside or outside the ring 42, 58 of fins 44, 60.

Figures 14 and 15 show an internal cooling module 98 in accordance with the invention; again, like numerals are used for like parts. The module 98 comprises an internal cooling ring 58 and a tubular blower ring 100 supported by the cooling ring 58. The module 98 is shown here in use within a pipe joint 14, with the cooling ring 58 in thermally-conductive contact with the inner wall of the pipe joint 14 to dissipate heat. The cooling module 98 shown in Figures 14 and 15 may be suspended inside the pipe joint 14 as a discrete system that can be moved and positioned independently of an ILUC; alternatively it may be supported by an ILUC although it could be movable relative to the supporting ILUC if desired. As before, the cooling ring 58 has fins 60 extending radially inwardly from an integral outer circumferential band 62, in planes aligned with the central longitudinal axis of the internal cooling ring 58. In this instance, convection from the fins 60 is supplemented by air or other cooling gas blown from nozzles 102 distributed around the blower ring 100. The blower ring 100 is supplied with compressed gas through a supply hose that has been omitted from Figures 14 and 15 for clarity. The cooling ring 58 and the blower ring 100 are concentric in plan view on their mutual central longitudinal axis and are separated axially in parallel planes that are orthogonal to that axis. The blower ring 100 is of smaller diameter than the cooling ring 58 and is supported by angularly-spaced, inwardly-inclined struts 104 extending downwardly from the outer circumferential band 62 of the cooling ring 58.

As can be seen in the side view of Figure 14, the nozzles 102 of the blower ring 100 face generally upwardly toward the fins 60 of the cooling ring 58. In plan view as shown in Figure 15, it can be seen that each nozzle 102 is angularly aligned with a gap between adjacent fins 60 of the cooling ring 58.

The internal cooling module 98 of Figures 14 and 15 is presented here in a simplified schematic form to illustrate the principles of the invention. Practical embodiments may divide the cooling ring 58 in a similar manner to that shown in Figures 7 and 8 so that portions of the cooling ring 58 may be pressed against the inner wall of the pipe joint 14 for thermal contact and retracted from the inner wall to move the cooling module 98 after use.

Figures 16 and 17 show another internal cooling module 106 in accordance with the invention, in this case arranged to blow air or other cooling gas directly against the inner wall of a pipe joint 14. The cooling module 106 comprises a tubular blower ring 108 supported by a circular frame 1 10. The frame 1 10 is shown here as a solid-walled tube but it could instead be a fabricated frame of elements such as welded round bars. Air or other cooling gas is blown radially from outwardly-facing nozzles 1 12 distributed around the blower ring 108. The module 106 is shown in Figure 17 in use within a pipe joint 14, with nozzles 1 12 facing against the inner wall of the pipe joint 14 in axial alignment with the weld 56. This positioning is possible where no backing plate or ring 82 is used.

The blower ring 100 is supplied with compressed gas from a compressor or other suitable source through a supply hose 1 14 extending along the interior of the pipe joint 14. A similar supply hose may be used in the preceding embodiment shown in Figures 14 and 15, where the hose was omitted for clarity. Again, the cooling module 106 shown in Figures 16 and 17 may be suspended inside the pipe joint 14 as a discrete system that can be moved and positioned independently of an I LUC. For this purpose, the cooling module 106 is shown here suspended from a wire tackle 1 16. A similar arrangement may be used for suspending the preceding embodiment shown in Figures 14 and 15. Alternatively the cooling module 106 could be supported by an ILUC and could be movable relative to the supporting ILUC if desired.

Turning finally to Figure 18, the graph shown here plots three temperature curves during a welding cycle comprising four passes, namely welding with an internal cooling ring ('ID') and two reference welds without ID cooling, with heat being removed from the weld and HAZ by conduction and forced convection with blown air. Pre-heating takes place to about 100 degrees Celsius in each case shown in Figure 18. It will be seen here that with ID cooling, the temperature reduces quickly - yet smoothly and progressively - after the root pass, reaching just below 150 degrees Celsius after about 60 seconds before the second pass begins. For the reference welds, temperature remains between over 160 degrees Celsius and over 170 degrees Celsius before the second pass begins.

There is considerable divergence between the temperature curves in the second pass: in the case of ID cooling, the second pass temperature peaks at less than 220 degrees Celsius after about 85 seconds whereas the reference weld curves peak at almost 260 degrees Celsius about ten seconds later. ID cooling therefore allows the fill pass to begin correspondingly sooner, whereupon temperature peaks at about 260 degrees Celsius after about 125 seconds before falling to about 230 degrees Celsius after about 160 seconds before the final, cap pass begins. The cap pass can then be completed after about 180 seconds with a peak temperature of about 245 degrees Celsius, with gradual and gentle cooling after that to end at about 175 degrees Celsius after about 270 seconds.

Many variations are possible within the inventive concept. For example, whilst the preferred cooling gas is air, argon or nitrogen are possible alternatives and gases may be used in dry or wet conditions, which may be more practical for the welding process. Mixtures of cooling gases are also possible. The cooling efficiency of a gaseous cooling medium is linked to the supply of ambient air and choice of a pure gas or a gas mixture.

To increase efficiency, various actions can be implemented on cooling fluids: for example, air or other gas may be provided at ambient or below-ambient temperatures. The flow of gas may be discontinuous (e.g. pulsed) or continuous. Air or other gases may be dry or wet. All these configurations can be implemented, varied and controlled in an internal or external cooling ring module in accordance with the invention. A positioning sensor can be used to confirm that the targeted module location is achieved, when necessary. Extra sensors can be used to monitor the cooling flow and the material temperature.

Claims

1 . A method of dissipating heat from a circumferential girth weld when fabricating a pipeline for subsea laying, the method comprising: placing an annular or part-annular heat sink in thermal contact with an inner or outer surface of a pipe at a heat sink location to remove heat conducted from the weld; and forcing cooling gas flow across the heat sink.

2. The method of Claim 1 , wherein heat is conducted through the pipe from the weld to a heat sink location that is axially spaced from the weld.

3. The method of Claim 1 , wherein the heat sink is in contact with an inner surface of the pipe at a heat sink location aligned with the weld.

4. The method of any preceding claim, comprising placing the heat sink at the heat sink location before performing a root pass of the weld.

5. The method of Claim 4, comprising carrying the heat sink to the heat sink location on an internal line-up clamp.

6. The method of any preceding claim, comprising forcing cooling gas flow between fins or around other formations that increase the surface area of the heat sink.

7. The method of any preceding claim, wherein forced cooling gas flow is initiated after performing a root pass of the weld.

8. The method of any preceding claim, wherein forced cooling gas flow is continued or repeated during a second pass of the weld and all subsequent passes of the weld.

9. The method of any preceding claim, comprising moving a module comprising the heat sink and an integral forced cooling gas supply along the pipe to the heat sink location.

10. The method of Claim 9, comprising moving the module independently of movement of an internal line-up clamp within the pipe.

1 1. The method of any preceding claim, comprising expanding the heat sink into a deployed state in thermal contact with an inner surface of the pipe when at the heat sink location.

12. The method of any of Claims 1 to 10, comprising contracting the heat sink into thermal contact with an outer surface of the pipe when at the heat sink location.

13. The method of any preceding claim, wherein the heat sink location is disposed between a welding bug guide band and the weld or on an opposite side of the weld from the welding bug guide band.

14. A method of dissipating heat from a circumferential girth weld when fabricating a pipeline for subsea laying, comprising encircling a pipe with a blower and forcing cooling gas from the blower inwardly against an outer surface of the pipe at a location axially spaced from the weld, to dissipate heat conducted through the pipe from the weld.

15. The method of Claim 14, wherein the blower is disposed between a welding bug guide band and the weld or on an opposite side of the weld from the welding bug guide band.

16. A method of dissipating heat from a circumferential girth weld when fabricating a pipeline for subsea laying, comprising forcing cooling gas outwardly against an inner surface of the pipe at a location axially aligned with the weld.

17. The method of any preceding claim, wherein all welding passes are completed without applying cooling liquid to the pipe other than such liquid as may be entrained in a wet cooling gas flow.

18. The method of any preceding claim, wherein the temperature or flow of forced cooling gas applied to a heat sink or to the pipe is controlled in response to sensing temperature in the weld and in a heat-affected zone surrounding the weld.

19. A heat sink for dissipating welding heat from a pipe for subsea laying, the heat sink comprising an annular or part-annular wall that is shaped and arranged to be held in thermal contact with an inner or outer surface of the pipe, and formations integral with or attached to the wall to increase the surface area of the heat sink for dissipating heat received from the pipe through the wall.

20. The heat sink of Claim 19, wherein the wall is divided into portions movable radially into and out of thermal contact with the pipe.

21. The heat sink of Claim 19 or Claim 20, supported on or integrated with an internal line-up clamp.

22. A cooling module for dissipating welding heat from a pipe for subsea laying, the module comprising: a heat sink arranged to be held in thermal contact with an inner or outer surface of the pipe; and a blower for forcing cooling gas flow across the heat sink.

23. The module of Claim 22, wherein the heat sink is as defined in any of Claims 19 to 21.

24. The module of Claim 22 or Claim 23, wherein the heat sink and the blower are annular or part-annular and are spaced from each other along a mutual central longitudinal axis.

25. The module of Claim 24, wherein the blower comprises angularly-spaced nozzles aimed toward the heat sink.

26. The module of Claim 25, wherein the heat sink comprises angularly-spaced fins and the nozzles of the blower are aimed between fins of the heat sink.

27. In combination, a pipe for subsea laying and a heat sink as defined in any of Claims 19 to 21 or a cooling module as defined in any of Claims 22 to 26.

28. A subsea pipeline made by the method of any of Claims 1 to 18.

29. A pipelaying vessel or other pipe-fabricating facility operating the method of any of Claims 1 to 18 or equipped with a heat sink as defined in any of Claims 19 to 21 or a cooling module as defined in any of Claims 22 to 26.