GB2554087A

GB2554087A - Umbilical fluid line and method

Info

Publication number: GB2554087A
Application number: GB1615897.4A
Authority: GB
Inventors: Ivar Skar Jan; Morten Hesjevik Sven; Roberg Andersen Tore
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2016-09-19
Filing date: 2016-09-19
Publication date: 2018-03-28
Anticipated expiration: 2036-09-19
Also published as: GB2554087B; WO2018052311A1; GB201615897D0

Abstract

A fluid line component for a subsea umbilical has two metal tubes (4) joined to form a longer component, each tube (4) being coated with a protective layer (9) along their length except at the joint (10) between the two tubes (4), where a sacrificial anode material (11) is provided. The sacrificial anode material (11) may be provided in the form of an annulus of two or separate parts around the joint. The metal tubes may be stainless, duplex or super duplex type steels.

Description

(54) Title of the Invention: Umbilical fluid line and method

Abstract Title: Sacrificial anode protection of a subsea umbilical (57) A fluid line component for a subsea umbilical has two metal tubes (4) joined to form a longer component, each tube (4) being coated with a protective layer (9) along their length except at the joint (10) between the two tubes (4), where a sacrificial anode material (11) is provided. The sacrificial anode material (11) may be provided in the form of an annulus of two or separate parts around the joint. The metal tubes may be stainless, duplex or super duplex type steels.

Fig. 2

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Fig. 2

UMBILICAL FLUID LINE AND METHOD

The invention relates to corrosion protection of tubing used in integrated power and service umbilicals.

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 lines sometimes referred to as fluid cores (generally hoses and tubes) which may transport various chemical, hydraulic and control fluids. Typically the fluid lines will be formed by steel tubes (such as carbon steel, duplex or super duplex steel). Typically the fluid lines include hydraulic control, and injection of service chemicals and hydrate inhibitors, for example. 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 lines are typically made of steel and coated with an insulator for protection against sea water corrosion. In particular, design codes may require the fluid lines to be coated with a corrosion protective sheath above about 20°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.

GB-A-2255104 discloses a flexible submarine line comprising a number of fluid/gas conducting tubes and possibly other longitudinal elements such as electrical conductors and/or optical fibre cables enclosed within a common outer cover, a number of the tubes are made of steel and each of these steel tubes is in longitudinal electrical contact with at least two sacrificial anodes, such as zinc wires. At least some of the tubes are made of stainless steel S31603 and the tubes are stranded in a tight layer on at least one layer of galvanised steel tapes arranged around a core containing the electrical conductors/optical fibres.

As explained in US-A-6012495 there are known (GB 2 255 104 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. 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. At least one of the tubes is made of carbon steel and 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(s) of at least one carbon steel tube. The line may include a sea water permeable outer cover. Thus the steel tube fluid lines are not coated with a corrosion protective layer.

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 protective insulating coating 9.

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

The first option is to reduce the induced voltage. 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. However, this is only true for a perfectly sinusoidal voltage. 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. In other words, power cores with metallic screen and an insulating sheath is not safe with respect to AC corrosion if the motor is powered by an adjustable speed drive (which is normally the case, 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 could be increased. But umbilical designs already tend to provide maximum separation between the cores 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 the induced current on the fluid line. One option is to remove the insulting coating. As noted above a coating is very often required by design codes for subsea umbilical use. Basically, corrosion preventive coating is required when operating temperatures are over 20°C. Such coatings are normally of extruded polyethylene 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 above 20°C. Thus even where the coating is not required by regulations, it is often chosen to retain a protective 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 two different umbilicals could be used (separating the power cores and optionally any optical fibres from the fluid lines) but then the advantage of an integrated umbilical is lost.

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 typically welded together. The joints are then polymer coated for example by wrapping. The coating at the joints 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 joints are particularly susceptible to cracks or crack formation. There may be quality issues of the wrapping. Defects in the wrapping may be one of the main reasons for severe AC corrosion found on the umbilical. Due to this failure the solution to date has been to split the electrical and communication cables from the umbilical.

A need arises, therefore, to mitigate corrosion of metal, particularly steel, fluid lines in a subsea power umbilical caused by cracks or blisters in the protective coating at the fabrication joints. In particular, it is desired to mitigate AC induced corrosion for subsea integrated power and service umbilicals having metal fluid lines.

SUMMARY

The invention provides a fluid line component for a subsea umbilical comprising two metal tubes joined to form a longer component, each tube being coated with a protective layer along their length except at the joint between the two tubes, wherein a sacrificial anode material is provided at the joint.

The sacrificial anode material may be made of a material that is less nobel than the metal of the two metal tubes. The sacrificial anode material may be made from an iron material, optionally a steel.

The metal tubes may be made from stainless steel. The metal tubes may be made from a duplex or super duplex steel.

The protective layer may be an insulating polymer layer.

The sacrificial anode material may be provided in the form of an annulus around the joint between the two tubes. The annulus may be formed of two or more separate parts.

The invention also provides a subsea umbilical comprising a fluid line component in accordance with the invention.

The invention further provides a method of manufacturing a fluid line component comprising joining two metal tubes to form a longer component, each tube being coated with a protective layer along their length except at the joint between the two tubes, and providing a sacrificial anode material at the joint.

The sacrificial anode material may be made of a material that is less nobel than the metal of the two metal tubes. The sacrificial anode material is made from an iron material, optionally a steel.

The protective layer may be an insulating polymer layer.

Stainless steels, in particular super duplex steels used for metal fluid lines are not normally provided with cathodic protection. The present inventors have found that in a subsea umbilical including high voltage power cores and steel fluid lines, leaving an area at the joints between tube sections (fabrication joints) means that cathodic protection can be usefully applied at the steel tube fabrication joints. The provision of an anode material at the joints provides corrosion protection (e.g. sea water corrosion protection) but the cathodic protection also protects against AC corrosion. AC corrosion may be due to induced electromagnetic fields caused either by AC power cores in the umbilical or from an external source. Providing cathodic protection at these joints gives protection against AC corrosion and enables the use of integrated power and service umbilicals. Other costly mitigating measures to protect against AC corrosion as described above may not be required.

Provides protection against AC corrosion and sea water corrosion, simplifies manufacturing (removes the separate joint wrapping step) and does not require testing.

DRAWINGS

Figure 1 is a cross section through a subsea umbilical;

Figure 2 is a cross section of a fabrication joint of fluid line for a subsea umbilical.

DESCRIPTION

As described above, the fabrication joints for fluid lines are generally wrapped in insulating material so that in addition to the extruded coating on the metal tubes the entire surface of the tube is protected. Contrary to the established procedure it has been found that leaving the joints uncoated, a large area of metal (normally duplex or superduplex stainless steel) will be available for grounding of AC currents to the surrounding sea water. This will give two interacting phenomena - namely reduced induced voltage into the steel tube as well as reduced AC density on the exposed steel surface. This will largely reduce the risk of AC corrosion. However, by leaving the joints uncoated a risk for sea water corrosion will be introduced. Stainless steels can suffer various corrosion mechanisms at elevated temperatures. The latter can be countered using cathodic protection with a sacrificial anode. A sacrificial anode will in addition result in increased robustness against AC corrosion as well.

In an embodiment, the tubing material is a stainless steel, for example of type 25 Cr duplex which is sensitive to hydrogen induced stress cracking (HISC), the anode material should not give lower potential than - 800 mV Ag/AgCl/seawater. This can be provided by using ordinary carbon steel anodes. The anodes can be clamped on the tubing locally. Cathodic protection is also efficient for protection against AC corrosion. More generally, the anode material will be less noble that the metal tube material being protected.

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

Figure 2 shows a longitudinal cross-section of a fabrication joint 10 in a fluid line component of a subsea umbilical in accordance with an embodiment. Figure 2 is not to scale. The figure shows steel tubes 4 coated each coated with a polymer corrosion protection layer 9 on their external surface. The tubes 4 and protection layer 9 form a fluid line component (4, 9). The protection layer is, for example polyethylene bonded to the tube.

The tubes 4 are connected at the fabrication joint 10 by butt welding. At the fabrication joint is an uncoated portion (i.e. with no protection layer). A reference to the length of the fabrication joint 10 is a reference to the length of the uncoated portion.

In the present invention for the fluid line of a subsea umbilical, where the chosen tube material is susceptible to corrosion, in addition to, or instead of conventional corrosion prevention measures, cathodic protection 11 is provided at joints such as fabrication joints by providing an anode material 11 at the joint. The anode material may be provided in the form of an annulus or split annulus. The anode material may be formed in any convenient manner.

The cathodic protection (11) can be installed as an annulus around the fabrication joint (10). For example, a sacrificial anode (11) can be installed as two halt shells around the joint (10) within the length of the fabrication joint (10), The anode material may be installed at the fabrication joint in any convenient manner, including by welded, clamping, heat shrinking, the use of one or more straps. An split annulus may be stretched over the tube at the joint, or two parts may be hinged and optionally bolted or otherwise secured one in position.

Typical size of the sacrificial anode can be a length of 100 mm with inner diameter corresponding to the tube (4) outer diameter. The thickness of the sacrificial anode can typically be 3-4 mm. However, length and thickness will vary with respect to needed sacrificial anode mass and thickness of the tube fabric coating. The outer diameter of the anode may conveniently be no more than the outer diameter of the fluid line component (4, 9). An example of anode mass calculation is given:

Cathodic protection does not require that the length of the annulus is the same as the length of the fabrication joint (10). The metal tubes (4) are normally provided pre-coated except for a portion at the ends so that the coating is not damaged during welding. If the length of the fabrication joint (10) - the uncoated portion - is not long enough to provide sufficient area for the desired size of sacrificial anode (11) to be contained within the length of the fabrication joint (10) then the coating may be paired back either before or after welding. Conveniently the fluid line components (4, 9) are supplied with sufficient exposed metal at their ends.

Sacrificial anode material made from iron (steel) with an anode capacity in the range of 900 Ah/Kg. Surface area to be protected is based on 1000 mm uncoated fabrication joint with an outer diameter of 25.4 mm. Dependent on the magnitude of the needed protection current approximately 0.3-0.4 Kg iron (steel) will be needed to protect the uncoated fabrication joint for 20 years.

A fluid line comprising one or more joints 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.

Claims

CLAIMS:

1. A fluid line component for a subsea umbilical comprising two metal tubes joined to form a longer component, each tube being coated with a protective layer along their length except at the joint between the two tubes, wherein a sacrificial anode material is provided at the joint.

2. The fluid line as claimed in claim 1, wherein the sacrificial anode material being made of a material that is less nobel than the metal of the two metal tubes.

3. The fluid line as claimed in claim 2 wherein the sacrificial anode material is made from an iron material, optionally a steel.

4. The fluid line component of claims 1,2 or 3, wherein the metal tubes are made from stainless steel.

5. The fluid line component of claim 4, wherein the metal tubes are made from a duplex or super duplex steel.

6. The fluid line according to any one previous claim wherein the protective layer is an insulating polymer layer.

7. The fluid line according to any one previous claim, wherein the sacrificial anode material is provided in the form of an annulus around the joint between the two tubes.

8. The fluid line according to claim 7, wherein the annulus is formed of two or more separate parts.

9. A subsea umbilical comprising a fluid line component in accordance with any one previous claim.

10. A method of manufacturing a fluid line component comprising joining two metal tubes to form a longer component, each tube being coated with a protective layer along their length except at the joint between the two tubes, and providing a sacrificial anode material at the joint.

11. The method of claim 10, wherein the sacrificial anode material is made of a material that is less nobel than the metal of the two metal tubes.

12. The method of claim 11, wherein the sacrificial anode material is made from an iron material, optionally a steel.

13. The method of claims 10, 11 or 12, wherein the metal tubes are made from stainless steel.

14. The method of claim 13, wherein the metal tubes are made from a duplex or super duplex steel.

15. The method of any one of claims 10 to 14, wherein the protective layer is an insulating polymer layer.

16. The method of any one of claims 10 to 15, wherein the sacrificial anode material is provided in the form of an annulus around the joint between the two tubes.

17. The method of claim 16, wherein the annulus is formed of two or more separate parts.

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Application No: GB1615897.4 Examiner: Mr Matthew Harle