NO20171484A1 - Umbilical fluid line and umbilical - Google Patents

Abstract

A component of a fluid line for a Subsea power umbilical (2) comprises a metal tube (4) having a semiconducting coating (10). An integrated subsea power umbilical (2) including a fluid line comprises at least one said component. A method of manufacturing a fluid line component for a power umbilical comprises providing a length of metal tube (4) and extruding a coating (10) onto the metal tube, the coating (10) being made to be semiconducting.

Description

UMBILICAL FLUID LINE AND UMBILICAL

The invention relates to mitigation of AC induced corrosion of tubing used in integrated power and service umbilicals, a fluid line for use in an integrated subsea power umbilical and an umbilical comprising the fluid line.

An integrated power umbilical for subsea drilling and exploration etc. may typically include a plurality of power cores, for example three central power cores, and three outer power cores each supplying a three phase high voltage AC supply voltage. There may be a single ring of power cores or the power cores may be distributed other than in a ring, although a ring is convenient for a geometry having maximum distance between the power cores whilst achieving a high packing density (ensuring a small cross section).

A subsea umbilical will also typically include communication (for example optical fibre) and power/signal lines. In addition they may include fluid transport lines which may transport various hydraulic and control fluids. Typically the fluid lines include hydraulic control, and injection of service chemicals and hydrate inhibitors, for example. Typically the fluid transport lines will be formed by steel tubes. Subsea power umbilicals including fluid lines are termed integrated power and service umbilicals or simply integrated umbilicals.

In addition to the various lines carried by the umbilical there will typically be outer and inner sheaths with, for example, a dual layer armour package between the sheaths.

The fluid transport lines are typically made of steel and coated with an insulator for protection against sea water corrosion. In particular, design codes may require the transport lines to be coated with a corrosion protective sheath above 25 ̊C, for example a polyethylene coating may be provided where the operating temperature is in the range 25 to 60°C. Typical coatings include insulating polyethylene and polypropylene coatings.

Factors that can be taken into account when designing a power umbilical include, weight, size (diameter), geometry, power requirements/ system, number and type of lines carried by the umbilical.

US-A-6012495 relates to a subsea line comprising a number of fluid/gas conducting steel tubes and other elongated elements like electrical conductors and cables enclosed, and containing elongated sacrificial elements. At least one of the tubes is made of carbon steel and that at least one sacrificial element which is constituted by one or more tapes or strips made of a material less noble than steel is in substantially continuous contact with the surface of at least one carbon steel tube. The line may include a sea water permeable outer cover.

As explained in US-A-6012495 there are known (GB 2255104 B) subsea lines and an umbilical having corrosion protection satisfying most offshore requirements. The cathodic protection of the stainless steel tubes is obtained by a `built in` sacrificial anode system. The outer surfaces of the small tubes achieve cathodic protection from integrated zinc wires, while the lower surfaces of the tubes are protected by galvanized steel tape. This is, however, also a `dry` design, relying on a non-penetrable outer cover. US-A-6012495 aims to solve the problem when the cover is water permeable.

An example of a power umbilical cross section is illustrated in Figure 1. This power umbilical 1 comprises an assembly of functional elements including steel pipes 4, optical fiber cables 6, reinforcing steel, steel wire ropes or carbon rods 5, electrical power cables 2, and electrical signal cables 3 bundled together with filler material 7 and over sheathed by a polymeric external sheath 8. In this example, the three power cables 2 are bundled together close to the central axis of the umbilical. However, in some cases they may be positioned towards the outside of the umbilical bundle. The steel pipes include a polyethylene insulating coating 9.

When a corrosion protective sheath or coating is applied, the area where the tube is terminated / exposed should also be cathodically protected.

SUMMARY

The present inventors have found that in a subsea umbilical including high voltage AC power cores and steel fluid lines, if the steel tubes carrying the fluids are coated with an insulating material then an electromagnetic field induced on the steel tube by the power cores (or an external source) will set up a potential between the tube surface and the surrounding umbilical-packing, or other elements, across the insulating coating. If the steel tubes are connected to earth potential at both ends then the maximum induced voltage occurs at the midpoint of the tubes. What has been found is that cracks in the insulating coating may cause a high current density (greater than 100 A/m<2>) at the cracks. The consequence of the high current density is an increased corrosion risk.

Typically the steel tubes are coated by extrusion during manufacture and tested using spark testing to ensure there are no cracks. Cracks can of course develop over time. Furthermore, unlike the power and control lines the steel tubes are not manufactured as a single piece but are instead formed from plural lengths (for example, approximately 4km) that are added together. The joins are then manually coated. The coating at the joins may not be tested or the test might not be as thorough as the spark test conducted after the aforementioned extrusion process. In any case it has been found that the joins are particularly susceptible to cracks or crack formation.

A need arises, therefore, to mitigate corrosion of metal, particularly steel, fluid lines in a subsea power umbilical caused by AC induced electromagnetic fields.

There are various possibilities for mitigating the risk of corrosion of the fluid lines in a subsea umbilical.

The first option is to reduce the induced voltage on the tubes. There is some scope for reducing electromagnetic effects through geometry, power core design (including material selection and screening). Possibilities for reducing the induced voltage include providing extra screening on the power cores. For example, metal mesh power screens on an insulating sheath of the power cores. Such screens would increase the losses along the umbilical and would increase the weight and volume of the umbilical which is not desirable. The theory is that using a metal (conductive) screen will reduce the induced voltage on the steel tube. A metal screen may be sufficient for a perfect sinusoidal voltage, although probably not. Unfortunately, the harmonic distortion typically assigned with adjustable speed motor drives may cause significant induced voltage in the steel tube and thereby contribute significantly to AC corrosion. The induced voltage on the tubes with or without an adjustable drive is typically in the order of 10 to 100x. Thus power cores with metallic screen and insulating sheath is not safe with respect to AC corrosion. Normally adjustable speed drives are selected, fix speed motors are normally not selected. In any case, even where the metal screen would be applicable there will still be increased losses along the umbilical.

The distance between the power cores and the steel tubes could be increased. But umbilical designs already tend to provide maximum separation between the cores and the steel tubes and increasing the separation necessarily means increasing the volume of the umbilical.

Finally in terms of reducing the induced voltage there is the option of changing the power system, but the power requirements are generally set by the power requirements of the subsea structure being powered, for example a motor and changing the power system is not necessarily within the purview of the umbilical designer.

The second option is to reduce induced radial current (current density) from the fluid line to the surrounding components of the umbilical. One way is to remove the insulating coating. As noted above a coating is very often required by design codes for subsea umbilical use. Basically, a corrosion protection coating or sheath is normally required when operating temperatures are above 25°C to protect against sea water corrosion. If the tube material is less corrosion resistant then the minimum temperature may be below this value. Such coatings are normally of extruded polyethylene (typically used in the range 25 to 60°C) or polypropylene. In any case, a coating has desirable aspects even when not required since without a protective coating the tubes are at risk of sea water corrosion. Thus even where the coating is not required by regulations, it is often chosen to retain an insulating coating to retain the benefits of the coating.

Another option would be to use, for example, a thermoplastic pipe in place of the steel tube for the fluid line. Whilst a thermoplastic tube might be less expensive, steel tubes have a better life expectancy. The better life expectancy of steel tube makes it the material of choice in most umbilical designs. Thermoplastic tubes may have issues with compatibility with fluid, and aging.

Of course the two different umbilicals could be used (separating the power cores from the fluid lines) but then the advantage of an integrated umbilical is lost.

It is desired, therefore, to mitigate AC induced corrosion for subsea power umbilicals having metal fluid lines.

The invention provides a component of a fluid line of a subsea power umbilical, comprising a metal tube having a semiconducting coating.

The metal tube may be formed from a ferrous metal. The ferrous metal may be steel.

Normally a carbon steel, stainless steel, in particular a duplex or super duplex stainless steel.

The coating material may be an organic semiconducting material. The organic semiconducting material may comprise one or more of, polyethylene or polypropylene, doped with a suitable conducting material. The organic material may be doped with a carbon material.

The coating may be formed over the external surface of the metal tube.

The coating may be formed by extruding onto the metal tube.

The coating material may have a resistivity in the range 0.01 – 100.0 Ω.m. The coating material may have a resistivity in the range 0.1 – 2.0 Ω.m.

The invention also provides a loaded reel for use in manufacturing an integrated power umbilical comprising a component in accordance with the invention.

The invention also provides an integrated subsea power umbilical including a fluid line comprising at least one component according to the invention.

The integrated subsea power umbilical may comprise grounding conductor to provide the semiconducting coating with a grounding path.

The invention further comprises a component of a fluid line for a power umbilical, comprising providing a length of metal tube and extruding a coating onto the metal tube, the coating being semiconducting.

DRAWINGS

Figure 1 shows a cross-section through a typical subsea power umbilical according to the invention;

Figure 2 shows a sectional view and a cross-sectional view through a fluid line for a power subsea umbilical according to the invention;

Figure 3 shows a cross-section through a subsea power umbilical according to an embodiment of the present invention.

DESCRIPTION

At present, it is normal to apply a metallic screen to the power core and to shield the steel tubes and other components of a subsea power umbilical.

In the figures like components are given the same reference numerals.

Figure 2 shows a cross-section of a component of a fluid line for a subsea power umbilical in accordance with an embodiment. The figure is not to scale. The figure shows a steel tube 4 coated with a semiconductor layer 10 on its external surface.

In the present invention for the fluid line of a subsea umbilical, where the chosen tube material is susceptible to corrosion, in order to mitigate corrosion caused by AC induced electromagnetic fields, a semiconducting material is chosen for the coating material of the tubes making up the fluid lines.

The tubes are typically steel, for example an austenitic stainless steel, a duplex stainless steel (e.g.22Cr duplex stainless steel), or a “super-duplex” stainless steel (e.g.25 CR “super-duplex” stainless steel. Of course new improved materials are developed continuously. Any metallic material suitable for use in a fluid line of an integrated power umbilical that is susceptible to AC induced corrosion may be used for the metallic tube of the invention. In an embodiment the tube is a seamless 25Cr super duplex stainless steel tube.

The metallic tubes are typically formed in lengths between 2km and 6km, especially between 3 and 5 km, more particularly 3.5 to 4.5 km on a laying reel. Different vendors may form the lengths for coating in different ways. For example, short 20m lengths may be welded together. Alternatively, a full 4km steel tube may be provided and coated prior to mounting on the laying reel. Consequently the final result is a length of coated tube or pipe on a laying reel ready for laying.

The individual lengths are coated with the semiconducting coating material and prepared on a reel before laying with the other components to form a power umbilical. For a typical umbilical length, say 15km, it is typical to require several lengths of the coated tube which are provided to the laying machine in turn to complete the umbilical. The individual lengths of coated tube are then joined.

Since the coating of the fluid tube line is semiconducting there is no need to perform (indeed inadvisable to perform) a spark test as would be carried out for an insulating coating in accordance with the prior art.

Whilst any suitable coating method can be used and the coating method will often be selected depending on the chosen coating material. In an embodiment the semiconducting material is a polymer material that is extruded onto each length of a steel tube.

If a doping process is required to render the coating material semiconducting then this process may conveniently be done before coating (conveniently the dopant material is added to the polymer granules before extrusion).

Organic materials such as polymers and copolymers of ethylene or propylene are convenient coating materials having excellent engineering properties as coating materials and are susceptible to doping to render them semiconducting. Any suitable semiconducting material may be used as the coating material and the skilled person can determine which coating material will be best for the particular conditions to be faced by the umbilical being designed. In the present embodiment, a steel tube is coated with carbon doped polyethylene. Any suitable material may be used for the coating including non-conductive polymers that may be made semiconducting by doping, for example with carbon. If the dopant is carbon this can be in various forms such as carbon black, graphite, carbon fibres or a mixture of any of one or more forms of carbon. Alternatively, the coating material may be a polymer or copolymer including intrinsically conducting polymers.

Since most polymer materials are not semiconducting doping is necessary to provide the necessary conductivity (by reducing the resistivity of the material). For example, most forms of polyethylene and polypropylene have a resistivity greater than 10<12>Ω.m. (typically 10<13>to 10<18>Ω.m.). On the other hand a conductor may have a resistivity less than about 10<-5>Ω.m (steels are typically 10<-7>to 10<-8>Ω.m).

In the present application, a reference to a semiconducting material or semiconductor means a material having a volume resistivity in the range 10<-4>to 10<10>Ω.m.

In an embodiment of the invention the coating material has a volume resistivity in the range 0.01 – 100.0 Ω.m (1 to 10000 Ω.cm), in a particular embodiment the coating material has a volume resistivity in the range 0.1 – 2.0 Ω.m (10 to 200 Ω.cm).

If the resistivity is too low the coating may act more like a conductor and may not provide protection against sea water corrosion.

If the resistivity is too high no current will flow and there will be a build-up of voltage with a potential high current density where ever there are cracks in the coating.

The amount of dopant that is required will depend on various factors, such as the intrinsic properties of the starting material (e.g HDPE, MDPE, LDPE, Polystyrene, Nylon, Polypropylene or a polypropylene copolymer), and the nature of the dopant. The skilled person is in a position to modify the intrinsic properties of the starting material to provide a final coating have the desired resistivity.

A fluid line comprising one or more components as shown in figure 2 in accordance with the present invention could replace the fluid lines of the prior art umbilical shown in Figure 1. Figure 3 shows an embodiment of an umbilical including fluid lines comprising one or more components forming the fluid lines. Instead of the insulating coating 9 of figure 1, the steel tubes 4 of the umbilical of figure 3 have semiconducting coatings 10 as discussed above.

Not shown is that the semi-conductive coatings are in contact with earth potential and this can be achieved in various ways. It is envisaged that the support and filler components of the umbilical could provide a radial path to earth via the armouring which is typically provided and earthed at both ends. In an embodiment the umbilical includes armouring between inner and outer sheaths and the inner sheath is made semiconducting and the semiconducting protective coatings of the fluid lines are in contact with the inner sheaths. There are various ways this could be achieved that would be evident to the skilled person. Typically the armour wires are earthed at both ends. This means that the semiconducting coating of the metallic tubes needs to be directly or indirectly in electrical contact with the armouring. Furthermore, in this case the inner sheath can be semi-conductive, also any other components between the metallic tube and armouring, if any, may be semi-conductive to allow the current to flow radially from the steel tubes to earth via the armouring (or other outer layer including a conductive path). Although, typical integrated power umbilical designs are “wet”, electrical conductivity (“sea water”) in axial direction may be poor. It is, therefore, well known to provide grounding to the electrical cables and metal screens of the power cores of an integrated power umbilical and these techniques can be extended to the protective sheaths of the fluid line tubes.

The invention has been described with reference to various exemplary embodiments.

Modifications will suggest themselves to those skilled in the art without departing from the scope of the invention as defined by the claims.

Claims (14)

CLAIMS:

1. A component of a fluid line of a subsea power umbilical, comprising a metal tube having a semiconducting coating.

2. The component as claimed in claim 1, wherein the metal tube is formed from a ferrous metal.

3. The component as claimed in claim 2 wherein the metal is steel.

4. The component as claimed in any one of claims 1, 2, or 3 wherein the coating material is an organic semiconducting material.

5. The component as claimed in claim 4, wherein the organic semiconducting material comprises one or more of, polyethylene or polypropylene, doped with a suitable conducting material.

6. The component as claimed in claim 5, wherein the organic material is doped with a carbon material.

7. The component as claimed in any one preceding claim, wherein the coating is formed over the external surface of the metal tube.

8. The component as claimed in any one preceding claim, wherein the coating is formed by extruding onto the metal tube.

9. The component as claimed in any one preceding claim, wherein the coating material has a resistivity in the range 0.01 – 100.0 Ω.m.

10. The component as claimed in claim 9, wherein the coating material has a resistivity in the range 0.1 – 2.0 Ω.m.

11. A loaded reel for use in manufacturing an integrated power umbilical comprising a component as claimed in any one previous claim.

12. An integrated subsea power umbilical including a fluid line comprising at least one component according of any one of claims 1 to 10.

13. The integrated subsea power umbilical as claimed in claim 12, further comprising a grounding conductor to provide the semiconducting coating with a grounding path.

14. A method of manufacturing a component of a fluid line for a power umbilical, comprising providing a length of metal tube and extruding a coating onto the metal tube, the coating being semiconducting.