IMPROVEMENTS IN COATING PIPES

The present invention relates to improvements in coating pipes, and in particular to a method for coating pipeline field joints and a coated pipeline field joint.

Pipelines in the oil and gas industry are typically formed from many lengths of steel pipe that are welded together end-to-end as they are being laid. To prevent corrosion of the pipes, they are coated with one or more protective or insulative layers. The pipes are usually coated at a factory remote from the location in which they are to be laid. This is generally more cost effective than coating them on site. At the factory, the coating is applied to the outside of the pipes whereupon a short length is left uncoated at either end of the pipe. The uncoated ends are necessary to enable the pipes to be welded together to form the pipeline.

Before the pipeline can be laid the welded ends, known as field joints, must be coated in the region of the joint to prevent corrosion of the pipes. The coating in these regions is referred to as the field joint coating.

Polypropylene is widely used as a coating for pipes used to form pipelines. The pipe coating can take several different forms depending on the particular application and will normally consist of more than one layer. A conventional pipe coating will typically comprise a thin layer of a primer, such as an epoxy-based material, that is applied in either liquid or powdered form to the outer surface of the steel pipe. To ensure a good bond between the pipe and the primer, the pipe is typically blast cleaned and etched with an appropriate anchor pattern. In many applications, the primer is in powdered form and may be a Fusion Bonded Epoxy

(FBE).

A second layer will then usually be applied over the primer during the curing time (i.e. time taken to harden or set) of the primer, so as to allow the two layers to bond. This second layer is typically a thin layer of polypropylene. A third layer, usually a thicker layer of extruded polypropylene, may then be applied on top of the second layer. This layer is typically 3 mm in thickness depending on the

particular application. If a high degree of thermal insulation is not important, the third layer can serve as the outermost or final layer of the coating, in which case the coating is commonly referred to as a 3 layer polypropylene (3 LPP) type coating. Such coatings are widely used in pipelines where thermal insulation is not needed.

In pipeline applications where thermal insulation is important, a fourth layer may be applied comprising an extruded or foamed polypropylene-based material. Alternatively, the fourth layer may be a polypropylene layer that has been further modified to enhance its thermal insulation properties, e.g. such as syntactic polypropylene incorporating embedded hollow glass microspheres. Usually, such coatings will also include a fifth or final layer of a solid protective layer of polypropylene to improve the mechanical strength of the coating and protect the underlying layers from environmental damage.

A pipe coating of the above form is commonly referred to as a 5 layer polypropylene (5LPP) type coating. Such coatings are widely used in pipelines where a high degree of thermal insulation is required, and many other variations incorporating multiple additional layers can be employed to obtain the required combination of mechanical and thermal properties.

Two common processes for coating field joints of pipelines formed from polypropylene coated pipes are the Injection Moulded Polypropylene (IMPP) and Injection Moulded Polyurethane (IMPU) techniques.

An IMPP coating is typically applied by first blast cleaning and then heating the pipe using induction heating, for instance. A layer of powdered FBE primer is then applied to the heated pipe, together with a thin layer of polypropylene - which is added during the curing time of the FBE. Exposed chamfers of factory applied coating on the pipe are then heated. The field joint is then completely enclosed by a heavy duty mould that defines a cavity around the uncoated ends of the pipes, which is subsequently filled with molten polypropylene. Once the

polypropylene has cooled and solidified, the mould is removed leaving the field joint coating in place.

Field joint coatings of the IMPP type provide similar or identical mechanical and thermal properties to the pipe coatings, as the factory applied coating and field joint coating materials are compatible thermosetting plastics. The compatibility of the pipe coating and field joint coating permits fusion to occur between the pipe and the field joint coatings, thereby imparting great integrity to the coatings.

However, a disadvantage of IMPP coating techniques is that they generally involve complex moulding processes which may be time consuming and expensive when laying pipelines off-shore, for example. In order for the polypropylene to solidify it must cool down and set, which consequently increases the amount of time required to complete the pipelaying operation, as the thickness of the coating increases, so too does the cooling time of the polypropylene. The moulds can be water cooled to accelerate the cooling, but nonetheless, considerable time is typically required before the moulds can be removed and the joint is able to withstand any mechanical loading.

Another disadvantage is caused by the need to heat the ends of the factory applied coating prior to fitting of the mould over the field joint and injecting molten polypropylene. Control of the heating step, so the factory applied coating is sufficiently heated to allow fusion to occur when the molten polypropylene is introduced into the mould, is difficult.

Yet another disadvantage of this technique is the relative inflexibility of the required equipment when applied to confined working spaces. The mould must be of heavy duty construction, often incorporating hydraulic opening and closing mechanisms in order to withstand high moulding pressures. The extruding machine which dispenses polypropylene into the mould must be closely coupled to the mould as it is not possible to pump molten polypropylene at the high rates required for rapid mould filling over long distances. Moreover this equipment must be capable of moving onto and off the field joint area in order to permit other

processes to be carried out. Accommodation of the equipment therefore presents very serious problems in many pipeline construction environments, particularly those on an offshore pipelaying vessel.

By contrast, an IMPU coating uses a chemically curable material instead of injecting polypropylene as the infill material in the IMPP field joint. Typically, the initial step in the IMPU technique is to apply a liquid polyurethane primer onto the exposed blast cleaned surface of the pipe. Once the primer has been applied, a mould is positioned to enclose the field joint in a cavity and the chemically curable material is injected into the cavity defined by the mould. The infill material is typically a two component urethane chemical. When the curing process is sufficiently advanced, the mould can be removed and the field joint coating can be left in place.

Field joint coatings of the IMPU type are useful in that the infill material solidifies by way of a chemical reaction that is largely independent of any external heating. The component chemicals can be specially formulated to cure in less time than is required to cool typical polypropylene coatings. As a result, the interval between filling the mould and the instant at which the mould can be removed and the joint subjected to mechanical loading, is significantly shorter than for a similar joint coated by IMPP techniques. The speed of the chemical reaction is largely independent of the size of the field joint or volume of infill material, unlike the cooling time in the IMPP coatings, and therefore considerable time can be saved during the pipelaying operation.

Moreover, as the component chemicals exhibit low viscosity prior to mixing, they can be pumped over considerable distances to the site of the pipelaying operation. This enables some of the bulky equipment to be located remotely from the coating site, where space is often limited. As the component chemicals are mixed at the time they are injected into the mould, the mixed infill material largely retains the low viscosity of the chemicals, and therefore the mould does not need to withstand high pressures. As a consequence, moulds of lightweight design and simplicity may instead be used with this type of coating technique.

However, a disadvantage of IMPU coating techniques is that due to the chemically dissimilar nature of the pipe and field joint coatings, the maximum bond strength that can be achieved between polyurethane and polypropylene is lower than the maximum bond strength that can be achieved between polypropylene and polypropylene. Because of this there is a perceived risk that fractures may occur between the pipe and field joint coatings, which is undesirable as it would allow water to penetrate the pipe coating. This could cause corrosion of the pipe, and were water to come into contact with polyurethane close to a pipe operating at an elevated temperature the polyurethane could become degraded through a process known as hydrolysis.

The present invention seeks to overcome, or at least mitigate, some of the above mentioned problems.

According to a first aspect of the present invention there is provided a method of coating a pipeline field joint between two joined lengths of pipe, each length being coated along part of its length with a pipe coating, the method comprising the steps of: applying a layer of a first coating material to the field joint such that it contacts and extends between the pipe coating of each of the two lengths of pipe; and subsequently applying a layer of a second coating material to the field joint, wherein the first coating material is capable of fusing with at least a component of the pipe coating and the second coating material is a curable material.

According to a second aspect of the present invention there is provided a coated pipeline field joint formed by two joined lengths of pipe, each length being coated along part of its length with a pipe coating, the field joint having a coating comprising: a layer of a first coating material that contacts and extends between the pipe coating of each of the two lengths of pipe; and

a layer of a second coating material applied subsequently to that of the first coating material, wherein the first coating material is capable of fusing with at least a component of the pipe coating and the second coating material is a curable material.

Provision of a layer of coating material which can fuse with at least a component of the pipe coating enables a layer of coating to be applied over the field joint which is fused with the pipe coating, forming a highly resistant barrier to moisture and other contaminants which can withstand flexing or other loading of the pipeline. Provision of a layer of curable material enables the field joint coating to be relatively easily built up to a required thickness and cured relatively quickly.

Preferably the layer of the first coating material is fused with the pipe coating of each length of pipe. The second coating material may be applied directly over the first coating material, or one or more other layers of material may be applied between the two layers.

The first coating material may comprise any suitable material which will fuse with the pipe coating and may comprise a thermoplastic material, such as a polyolefin and in one embodiment is polypropylene. The first coating material may be the same or substantially the same as at least a component of the pipe coating.

The second coating material may cure chemically. In some embodiments the curable material may comprise a urethane based chemical, which upon curing forms polyurethane. Although polyurethane may be used as the second coating material, other chemically curable materials may alternatively be used in accordance with the present invention. For instance, the second coating material may be formed from an epoxy based compound or other chemically similar derivative.

A filler could be incorporated into the second coating material, or into the coating layer formed by the second coating material. In particular, two or more part shells,

typically two half shells, may be provided in the layer of second coating material to reduce the amount of material required.

There are many advantages of using a chemically curable material as the second coating material, as since the material cures by chemical reaction the field joint coating solidifies in a shorter time than a conventional IMPP coating. In fact, a high degree of control can be exercised over the curing time of the present field joint coating, as the coating material can be specially formulated to cure rapidly, thereby saving valuable time during the pipelaying operation.

The second coating material may comprise two or more components that are combined prior to applying the coating material. The components may be specially formulated so that the coating material quickly cures to minimise the time required before the field joint coating can withstand flexing and other loading of the pipeline. The components may be in a liquid or semi-liquid state prior to mixing, such that the curable material subsequently solidifies during curing in order to facilitate setting of the coating.

The liquid or semi-liquid components may possess relatively low viscosity, which allows for pumping of these materials over relatively large distances from the pipelaying site, typically of the order of 15 meters but potentially much further. In this way, the coating materials and pumping equipment may be situated at a site remote from the field joint working station.

Application of the second coating material may include arranging a mould around the field joint and injecting the curable material into the mould. The second coating material may be formed by combining two or more components. Any components may be combined prior to injection into the mould, or during injection into the mould, or in the mould itself. The injected mixture typically retains the relatively low viscosity of the components which thereby reduces the pressure during injection and allows lightweight moulds to be used compared to the heavy duty moulds associated with IMPP coating techniques.

Typically, the layer of the first coating material has a thickness in the range of about 1.0 mm to about 5.0 mm and the layer of the second coating material independently has a thickness of at least 5.0 mm, or at least 20mm. Preferably the layer of second coating material is of sufficient thickness to extend slightly beyond the factory coating. As such it could have a thickness of the order of 150mm. However, it is to be appreciated that any relative thicknesses may be used depending upon the particular application and desired degree of thermal insulation. It is preferred that the layer of the first coating material is of less thickness than the layer of the second coating material.

The method of the invention may further comprise the step of applying a layer of a primer material between the field joint and the first coating material. This primer material may be a powdered or liquid primer, and in one embodiment is a Fusion Bonded Epoxy. Heat may be applied to one or more of the pipe, the primer material and the first coating material prior to and/or during applying the first coating material. Heating the one or more of the pipe, primer material and first coating material may fuse the first coating material to the pipe, increasing the integrity of the seal provided by the first coating material.

Where a primer is used, the first coating material is typically applied while the primer layer cures, as known in the art. An adhesive layer may also be applied that is associated with the primer to further improve the bonding between the pipe, primer and first coating material.

The first coating material may be in the form of a tape and applying the tape may include the step of wrapping the tape around the field joint, preferably in a helical pattern although other patterns may be used. Heat may be applied to the tape before and/or during and/or after wrapping the tape around the field joint. Heating the tape and/or field joint promotes the wrapped layers of the tape to fuse together more efficiently, thereby creating a more secure protective layer around the field joint.

An arrangement suitable for applying a polypropylene tape to a field joint is discussed in EP 1318904.

The first coating material may alternatively be applied in powdered form or by flame spraying in order to build up the first layer. Alternatively, a continuous sleeve of the first coating material may be positioned around the field joint and fastened to the coating materials by conventional techniques, which in one embodiment involves a plastic welding process. In another embodiment, the first coating material may instead be in the form of a heat-shrinkable sleeve that is heat-shrunk to coat the area of the field joint.

Of course it is to be appreciated that any suitable technique of applying the first coating material may be used in accordance with the present invention.

In some embodiments, one or more of the pipe coatings may each be modified to include a component that protrudes beyond the body of the coating at the uncoated end of the pipe. The protruding component is preferably a region of reduced thickness compared to the remainder of the pipe coating and preferably extends from a layer close to the base of the pipe coating and may define a tail section that extends along least part of the length of the uncoated end of the pipe. The tail section may be circumferential and concentric with the longitudinal axis of the pipe. The protruding component or tail section is preferably formed form a material which will fuse with the first coating material.

In some embodiments, the tail section may have a length in the range of about 10 mm to about 150 mm, and is most preferably in the range of about 25 mm to about 100 mm. However, it is to be appreciated that the length of the tail section may vary depending on the particular application and configuration of the pipes and/or field joint.

In the method of the invention, the first coating material may be applied to cover at least some of the respective tail sections of the pipe coatings, to allow the coating material to fuse with the tail sections to form a highly resistant barrier to

moisture and other contaminants. Where the first coating material is in the form of a tape, the tape is wrapped around the field joint such that it overlaps and covers at least part or all of the respective tail sections.

The fused layer formed by the first coating material and the protruding components provides a high level of corrosion protection for the underlying pipes, as the layer forms a highly effective seal. The provision of a fused layer underneath the curable material reduces the reliance on the bond between the curable material and the pipe coatings. In the unlikely event that any disbondment where to occur between the curable material and the pipe coatings, the pipes would not be expected to be exposed to moisture or contaminants as the integrity of the fused layer will not be significantly affected.

The tail section may be selected to have a thickness that provides sufficient thermal insulation to prevent excessive temperatures from arising at any location where moisture and the curable material may be in contact. Hence, the field joint coating of the present invention reduces the risk of degradation of the curable material.

Of course it is to be appreciated that the first coating material may be applied over the protruding components in some other form to that of a tape, e.g. as powdered or molten polypropylene or as a polypropylene sleeve. However, the advantages of having a fused layer are still achieved with these alternative arrangements, as whatever form the first coating material takes, and however it may be applied, it can be made to overlap with and/or cover the respective protruding components of the pipe coatings.

The field joint coating of the invention may further comprise an outer sleeve that encases the field joint. This sleeve may be a thermoplastic sleeve that is fastened in place via plastic welding or heat-shrinking etc. Such a sleeve further enhances the integrity of the field joint coating and consequently reduces and/or eliminates the risk of an undesirable failure of the coating during pipeline operations.

Although the present invention is ideally suited for coating field joints of oil and gas pipelines, it will be recognised that one or more of the principles of the invention could also be used in other applications where a relatively rapid and reliable coating is required.

Embodiments of the invention will now be described in detail by way of example and with reference to the accompanying drawings in which:

Figure 1 shows a side cross-sectional view of a pipeline field joint coating according the invention; Figure 2 shows part of an end portion of a conventionally coated 5LPP pipe as known in the art; and

Figure 3 shows part of an end portion of 5LPP pipe according to a preferred arrangement of the invention.

With reference to Figure 1, there is shown an arrangement of a field joint coating according to and applied using the method of the present invention. The field joint in Figure 1 has been formed by welding two factory coated pipes Ia, Ib end-to- end as typically performed at the pipelaying site. As shown, each pipe Ia, Ib has a respective originally uncoated end portion 2a, 2b that facilitates the joining of the pipes at the location in which they are to be laid.

In the example of Figure 1, the pipes Ia, Ib have been factory coated according to a 5LPP type arrangement, but it is to be understood that the field joint coating of the present invention may be used with any suitable factory coating.

Subsequent to welding of the pipes together, the uncoated region of the joined pipes, the field joint, has been coated.

Initially the pipes Ia, Ib were pre-heated using any suitable conventional technique, such as induction heating. A primer layer 3 was then applied to the field joint. In this embodiment the primer layer 3 is powdered FBE, but liquid primers may alternatively be used. The primer layer 3 forms a surface or substrate

to which a subsequent polypropylene layer 5 can more securely bond. In this example, the polypropylene layer was applied while the primer layer 3 cured.

The polypropylene layer 5 is applied to the field joint formed by the welded pipes so that it overlaps the factory coating either side of the joint, and extends continuously from the factory coating of one pipe Ia to the other Ib. The polypropylene layer 5 may be applied using any conventional technique, but in this arrangement, the polypropylene is in the form of a tape that has been repeatedly wrapped around the field joint in a helical pattern to form a layer of thickness of about 3mm. Heat was applied before, during and/or after wrapping the tape around the field joint, and the tape itself can be pre-heated before application.

The polypropylene layer 5 fuses with the one or more polypropylene layers of the pipe coating of each pipe. The fusion of the layers arises in part to the compatibility of these materials and thereby leads to a much stronger and secure coating than would otherwise be achieved by using coatings of different materials.

During application of the polypropylene layer residual heat in the pipe and/or heat applied during the coating process causes at least the surface of the tape and/or the surface of the factory coating to soften or melt so that the material of the pipe coating and applied coating combine to a degree. Subsequently the materials set and become fused together.

Subsequently a polyurethane layer 6 is applied over the polypropylene layer 5 to provide additional mechanical strength and thermal insulation. The polyurethane layer 6 is formed from a polyurethane based chemical that cures to form polyurethane by way of chemical reaction. The polyurethane is applied by way of conventional injection moulding techniques, such that the field joint is enclosed by a mould that creates an empty cavity around the field joint into which curable components of the coating are injected. However, it is to be appreciated that other techniques may alternatively be used to apply the polyurethane. The chemicals selected to produce the polyurethane can be in the form of two or more components, e.g. liquid or semi-liquid chemicals, which when combined together

react to form solid polyurethane. Any other suitable curable coating could be used arranged to harden after application without the need for significant heating or cooling.

The polyurethane layer is of sufficient thickness so that it protrudes beyond the factory applied pipe coating by about 5mm, enabling it to extend over the pipe coating adjacent the field joint.

In the example of Figure 1, the polypropylene layer 5 is relatively thin compared to the polyurethane. This minimises the time required for the polypropylene to cool and set.

As the polypropylene coating is fused to the factory coating of each pipe it forms an effective barrier to moisture reaching the surface of the pipe. However, as the greater volume of the field joint coating is formed from a cured polyurethane material the time saving advantages of a polyurethane coating are obtained together with the better moisture resistant qualities of a polypropylene coating.

The integrity of the field joint, and in particular its resistance to moisture ingress is further enhanced by the preparation of the cut back portions of the factory applied pipe coating.

Figure 2 shows part of an end portion 2c of a conventional coated pipe for use in an insulated pipeline. The pipe has a 5LPP type coating that has been left uncoated at its end portion 2c. The end portion 2c of the pipe coating is commonly referred to as a "cut-back", as some machining of the edge or end face of the pipe coating has taken place after coating the pipe.

The pipe coating comprises conventional layers, starting with a primer layer of FBE 3 which is overlaid with a relatively thin layer 9 of polypropylene as the primer cures. On top of the thin polypropylene layer 9 there is usually a thicker layer 10 of extruded polypropylene, which in turn provides a surface to which a further insulative layer 1 1 of foamed polypropylene material, for example, can

bond. To improve the mechanical strength of the factory coating, a final layer 12 of solid polypropylene may be applied.

Figure 3 shows a modified form of cut back of a 5LPP pipe coating, as employed in the joint shown in figure 1. The cut back is modified as compared to a conventional cut back to leave a tail of polypropylene coating 7 that protrudes beyond the body of the remainder of the coating. The tail of coating provides a surface onto which the polypropylene layer 5 of the field joint coating can overlap.

The tail section 7 is circumferential and concentric with the longitudinal axis of the end portion 2a of the pipe Ia (see Figure 1). However, it is to be appreciated that the tail section may be configured to conform to other desired geometries depending on the particular application.

Referring to inset A in Figure 1 , there is shown an enlarged view of the tail section 7 of the polypropylene layer 9 that has been coated with a field joint coating according to the method of the present invention. In this example, the tail section is relatively thin compared to the thickness of the polypropylene layer 9. The tail section advantageously provides a relatively thin coating of polypropylene with which the polypropylene 5 of the field joint coating can substantially overlap and fuse, as shown as overlap 8 in inset A. Thus, in preferred embodiments, the polypropylene layer 5 of the field joint coating can be wrapped repeatedly around the field joint so that the respective tail sections of the pipes Ia, Ib are partly or fully covered by the polypropylene. In this way, a fused layer is formed between the polypropylene of the field joint coating and the polypropylene of the pipe coatings.

As the tail section follows the shape of the underlying pipe Ia, it provides an increased area and a concentric surface to which the polypropylene layer 5 can readily conform, enabling a well fused seal to be established between the tape and pipecoating.

In the event that the polyurethane layer of the field joint coating separated from the factory coating of the pipes water could only penetrate as far as the fused polypropylene coating overlying the polypropylene tail 7. The coating and tail are sufficiently thick to ensure that where the pipe carries material at a typical elevated temperature the surface of the polypropylene coating underlying the polyurethane layer doe not reach a temperature at which hydrolysis is likely to occur.

To further improve the strength and integrity of the field joint an outer sleeve such as a thermoplastic sleeve (not shown) can be fastened, such as by plastic welding, heat- shrinking or securing with straps, around the field joint to encase the coatings and form an additional impermeable barrier to moisture and contaminants.

The above embodiments are described by way of example only. Many variations are possible without departing from the invention.