WO2008033028A1

WO2008033028A1 - Mechanical fastening device for cathodic protection of an insulated pipeline

Info

Publication number: WO2008033028A1
Application number: PCT/NO2007/000315
Authority: WO
Inventors: Geir Endal; Per Egil Kvaale; Rolf Morten Nes
Original assignee: Statoilhydro Asa
Priority date: 2006-09-11
Filing date: 2007-09-06
Publication date: 2008-03-20
Also published as: NO328083B1; NO20064080L

Abstract

Mechanical fastening device for cathodic protection of an insulated pipeline (1), comprising: a fastening ring (6) of diameter to fit around the insulated pipeline, means (7) for electrical contact, and means (8) for fastening of, or connection to, a sacrificial anode, distinguished in that the means (7) for electrical contact are in the form of bolts that can be screwed through the fastening ring and the insulating coating of the pipeline, to good electrical contact with the pipeline (1), as the length of the bolts and the fastening ring are adapted such that bolts are kept pre-tensioned in good electrical contact with the pipeline in the full range of pressure and temperature in the pipeline and the surroundings.

Description

Mechanical fastening device for cathodic protection of an insulated pipeline

Field of the invention

The present invention relates to corrosion protection of structures, in particular pipelines. More specifically, the invention relates to a mechanical fastening device for cathodic protection of a pipeline, to which mechanical fastening device a sacrificial anode is fastened or can be connected.

Background of the invention and prior art

Almost all metallic materials are thermodynamically unstable, which results in a natural tendency of corrosion. Corrosion is an electrochemical reaction that result in oxidation and thereby deterioration of a metallic structure, such as a pipeline. Corrosion is enhanced by the presence of an electrolyte such as seawater.

By cathodic protection of a pipeline by use of sacrificial anodes, the pipeline is galvanically connected to the sacrificial anodes, whereby the pipeline constitutes the cathode and is protected, while the sacrificial anodes corrode and thereby decompose instead of the pipeline. In patent publication US 3,623,968 a sacrificial anode and a pipe protected thereby are described. More specifically, a sacrificial anode and a protective coating for application on joints between welded sections of a pipeline are described, for which the surface coating of the pipeline has been removed before welding. The sacrificial anode is shaped as a ring or tubular shell that preferably is equipped with nibs for electrical contact with the pipeline. The ring or shell is tightened with a dedicated bolt, whereby the nibs are pressed to electrical contact with the pipeline. The space between the pipeline and the ring or shell is filled with mastics, before or after applying the ring or shell, and a protective, polymeric coating. In one embodiment the nibs are replaced with a setscrew 62, threaded in a bore of the shell, which setscrew is used to provide electrical contact with the weld 16. The sacrificial anode of US 3,623,968 is relatively laborious to use, it is adapted for use on welded joints having metal exposed and use of mastics, and it is not adapted for use on thermically insulated pipelines that are operated in a wide range of pressure and temperature, without previous removal of a thick insulating layer. In patent publication US 3,553,094 a device for cathodic protection of a metal pipe is described. The device is in the form of a sacrificial anode arranged with or having electrical connection to, a contact member arranged tightly inside a band that is tightened around the pipe. The contact member has legs that by tightening of the band can be pressed through a surface coating. For some embodiments the legs may be deformed after penetrating the outer coating, whereby a larger contact area for better electrical contact is provided. The fastening takes place by tightening a threaded bolt 54, which tightens the band.

For pipelines at large water depths, and in particular thermo-insulated pipelines having a thick outer insulating layer, significant deformation takes place both of the insulating layer and the metal pipelines, because of large variations in pressure and temperature during operation. The above-mentioned devices are not used for such pipelines, because of insufficient ability to maintain good electrical contact over time at large variations of pressure and temperature. The pressure variations can be several hundred bars, and the temperature variations can be above 200 ⁰C. At present at so- called dobler plate is welded to such pipelines, to which dobler plate a bracket is welded, for cathodic protection with sacrificial anodes of thermo-insulated pipelines at large water depths. The fastening device of the sacrificial anode is fastened to said bracket. Said method has proved to cause problems, because the welding results in welding stresses and transformed micro structure, which result in susceptibility of hydrogen induced cracking under the prevailing conditions with an electrochemical reaction with respect to the cathodic protection, in particular for high-strength pipe materials, such as 13 % Cr-steel and duplex steel. Such pipelines are usually equipped with sections of so-called clad steel with anodes arranged, which is complicated and expensive. There is a demand for providing a mechanical fastening device for cathodic protection of an insulated pipeline, in particular a thermo-insulated pipeline, without requirement for removal of insulating coating and without the above-mentioned problems or deficiencies.

Summary of the invention

The above-mentioned demand is met by providing a mechanical fastening device for a cathodic protection of an insulated pipeline, according to claim 1.

Figures Figure 1 illustrates the present technology for cathodic protection of a thermo- insulated pipeline of 13 % Cr-steel, and

Figure 2 illustrates a mechanical fastening device according to the present invention.

Detailed description

Figure 1 illustrates the steel wall 1 of a pipeline, with a dobler plate 2, a fastening bracket 3 and an insulating coating 4, according to prior art. The insulating coating 4 is arranged around the fastening bracket 3 after it has been welded to the pipe wall, before or after the anode 5 has been welded to the fastening bracket. Over time the insulating coating has tendency to loosen from the bracket under the prevailing conditions. Access for seawater and hydrogen into the welded area is thereby provided. The welding results in susceptibility of hydrogen embrittlement, with resulting cracking. This has been observed on pipelines of 13 % Cr-steel (ferritic stainless steel) and duplex stainless steel, and is particularly dangerous if the welding results in significant formation of martensite and other hard phases that are particularly susceptible to hydrogen induced cracking.

Figure 2 illustrates a mechanical fastening device according to the invention, for cathodic protection of a thermo-insulated pipeline, where a fastening ring 6 of diameter to fit around the thermo-insulated pipeline is illustrated, and four devices 7 for electrical contact, and a device 8 for fastening of, or connection to, a sacrificial anode. The devices 7 for electrical contact are bolts that can be screwed through the fastening ring and the insulating coating of the pipeline, to good electrical contact with the pipeline, as the length of the bolts and the fastening ring are adapted such that the bolts are pre-tensioned in good electrical contact with the pipeline in the full range of pressure and temperature in the pipeline and the surroundings. At least two, preferably three, four or more devices for electrical contact 7 are used, more preferably three or four, most preferably four, in the form of four bolts 7 having softer tips that are deformable against the pipeline metal. Three or most preferred four bolts 7 for electrical contact give a strong and stable fastening with good current distribution, while the fastening is simple and quick to carry out. The fastening ring has holes with threads. The bolts 7 have a softer tip 7a, with good electrical conductivity, and are fully threaded or without threads in an area near the tip. The insulating coating on the pipeline comprises an inner FBE epoxy layer 9 (FBE-fusion bonded epoxy), a PP- coating layer 10 (PP-polypropylene), a layer of PP-foam 11, and a PP-covering cap 12. The insulating per se can be differently constructed and comprise other materials, but in general has a significant total thickness. Further, an open gap 13 is illustrated between the fastening ring and the insulation. The fastening ring 6 consists of bolted half-parts, bolted together with bolts 14. A fastening ring means any ring, sleeve, cover or other element that can be arranged around the pipeline, outside the thermo-insulating coating or -layer. The fastening ring is functioning as a spring with adapted stiffness/elasticity, and it may comprise specific spring zones, for example with incorporated compound springs, leaf springs or plate springs. The fastening ring is preferably splitted into two or more parts that are bolted and/or hinged together, for simple fastening. The expression a device for fastening of , or connection to, a sacrificial anode, means that one or more sacrificial anodes are fastened directly on the fastening ring, or an electrical connection is arranged, preferably in the form of conductors between the sacrificial anodes and either the devices for electric contact or the fastening ring. For the embodiment illustrated on figure 2 this is in the form of space for clamp connections that can be fastened under the bolt heads of the bolts 7. With the area for pressure and temperature of the pipeline and surroundings, is meant pressure from 0 bars to 800 bars in the pipeline, temperature from -5 to +200 °C in the pipeline, pressure outside the pipeline from 1 bar to about 200 bars, and temperature outside the pipeline from -5 °C to +60 °C. The pre- tensioning is up to 0,9 times the yield strength of bolts and fastening ring at the prevailing conditions.

The bolts for electrical contact have preferably a softer tip, deformable against the pipeline, and of high conductivity, for example made of copper, aluminum, silver, brass or other metals or alloys having high electrical conductivity and sufficiently lower strength than the pipeline material. Thereby, the yield strength of the softer material of the bolt, optionally the whole bolt, is at least 20 MPa lower than the yield strength of the pipeline material, preferably at least 50 MPa lower, such that stress concentrations into and deformation of the pipeline material are avoided. Most preferably the correct strength or hardness of the bolt tips 7a and correct tightening torque for the bolts 7 are verified by tests for each use.

In a preferable embodiment the anode is connected directly to the bolts, in order to reduce the transition resistance. Alternatively, the anode is fastened directly to the fastening ring. The fastening ring is preferably adjustable. The fastening ring can be electrically conductive and is thereby preferably prepared of metal. Optionally, the fastening ring can be electrically non-conductive, and be made of a composite material or polymer material. In a preferred embodiment the fastening ring is made of a surface treated metal, and is not in electrical contact with neither the bolts for electrical contact nor the sacrificial anode, by use of an electrically isolating surface treatment and optionally isolating bolt sleeves, which reduce disturbance of the electrochemical reaction because of the fastening ring per se. In a preferable embodiment the bolts are massive bolts of a softer material than the pipeline material, and with high electrical conductivity, for example copper, silver, brass or bronze.

In a preferable embodiment the fastening ring comprises a current bus bar that is fastened tightly by bolt tightening, as the sacrificial anodes are cast around the bus bar and are in electrical contact with said bus bar.

In a further preferred embodiment the bolts are equipped with a pre-tensioning device, preferably in the form of a spring that is arranged around each bolt between the pipeline surface and fastening ring, between a flange on the bolts and the fastening ring. Thereby a more even pre-tensioning or biasing is achieved over a larger strain with respect to deformations.

With the mechanical fastening device according to the invention welding on pipelines is avoided, which results in that stress concentrations, transformation of micro structure and danger of hydrogen-induced cracking are avoided. Further, use of so-called clad-pipes with anodes, which presently is used for 13 Cr-pipes, duplex pipe and other relatively stainless, high-strength pipeline materials are avoided. It is not necessary to remove the insulation before installation. The deficiencies related to dobler plate and bracket, as mentioned above, are avoided. Fabrication and laying of a pipeline become less expensive, because it is not required with any particular preparations, and the pipeline can be uniformly thermo-insulated over its full length. The device according to the invention can be retrofitted at demand and it can easily be retrofitted sub-sea by special adaptation, by use of remotely operated vehicles, optionally by divers in shallow waters. Said particular adaptation comprises bolts with a head shape such that the bolts can be tightened by use of the manipulator of the remotely operated vehicle, with controlled torque applied. Further, the devices for electrical contact with the pipeline material are arranged symmetrically around the circumference, and not in one point at one side of the pipeline, which must be assumed to result in a more even current distribution. Fastening of the bolts is done with torque tool, with adjusted torque large enough to penetrate the insulation and deform the tip of the bolt, but low enough not to damage the pipe metal.

Tests have proved that the fastening with bolts that are screwed through the insulating coating and simultaneously ensures good electrical contact, resists forces during laying and optional burying of pipelines, as the forces of up to 50 kN from gravel fillings have proved not to result in displacement of the device.

The device will preferably be installed such that a gap is achieved between the fastening ring and surface of the insulation, to ensure free flow of seawater and space to take up possible marine growth or scales, making any preparations on the pipeline in the form of cleaning and clarifying of the surface unnecessary. Further, the bolts 7 and 14 can be point- welded to the fastening ring when correct installation has been made, before deployment from the laying vessel to sea. For the embodiment with an electrically non-conductive fastening ring, including an electrically isolated fastening ring of metal, correct and high conductivity as installed can simply be checked by measuring the resistance between bolts that provide electrical contact, and it can easily be verified that there is no electrical contact between said bolts and fastening ring if the fastening ring includes a measuring terminal or similar.

Claims

177635-EHClaims

1. Mechanical fastening device for cathodic protection of an insulated pipeline (1), comprising: a fastening ring (6) of diameter to fit around the insulated pipeline, means (7) for electrical contact, and a means (8) for fastening of, or connection to, a sacrificial anode, characterized in that the means (7) for electrical contact are in the form of bolts that can be screwed through the fastening ring and insulating coating of the pipeline, to good electrical contact with the pipeline (1), as the length of the bolts and the fastening ring are adapted such that the bolts are kept pre-tensioned in good electrical contact with the pipeline in the full range of pressure and temperature in the pipeline and the surroundings.

2. Device according to claims 1, characterized in that the bolts have a softer tip (7a) that is deformable against the pipeline and has high electric conductivity.

3. Device according to claim 1, characterized in that the anode is electrically connected directly to the bolts.

4. Device according to claim 1, characterized in that the anode is fastened to the fastening ring.

5. Device according to claim 1, characterized in that the fastening ring is split into two parts that are bolted and/or hinged together.

6. Device according to claim 1, characterized in that the means (7) for electrical contact comprises four bolts with soft tip that is deformable against the pipeline metal.

7. Device according to claim 1, characterized in that a gap is present between the insulation of the pipeline and the fastening ring.

8. Device according to claim 1, characterized in that an inner part of the bolt is made of copper and an outer part of steel.

9. Device according to claim 1, characterized in that the bolts are massive bolts of copper, brass, silver or other softer material than the pipeline, and with high electrical conductivity.

10. Device according to claim 1, characterized in that the fastening ring comprises a current bus bar that-is fastened by tightening the bolts, as the sacrificial anode is cast around the current bus bar.

11. Device according to claim 1 , characterized in that springs for pre-tensioning are arranged around each bolt, between a flange on the bolt and the fastening ring.

12. Device according to claim 1, characterized in that the fastening ring is surface treated and electrically insulated from the anode and the devices for electrical contact.

13. Device according to claim 1, characterized in that the pipeline is a thermically insulated (thermo insulated) pipeline.