GB2260768A

GB2260768A - Corrosion protection of internal surfaces of J-tube by sacrificial anodes

Info

Publication number: GB2260768A
Application number: GB9221825A
Authority: GB
Inventors: Inge Jakob Vintermyr; Georg Endre Balog
Original assignee: Alcatel STK AS
Current assignee: Nexans Norway AS
Priority date: 1991-10-21
Filing date: 1992-10-16
Publication date: 1993-04-28
Also published as: GB9221825D0; NO914123L; NO914123D0

Abstract

A method for corrosion protecting the internal surfaces of a steel J-tube (1) which is secured to an offshore structure, includes the step of pulling an elongated object (3) (e.g. cable) into and through the tube, a number of sacrificial anode wires (4) being arranged on the outer surface of a certain length of the object before pulling the object into the tube, and after installing the object (3) within the tube (1), there is provided at least one low resistance electrical contact 9 between the wires and the tube. The wires (4) (e.g. zinc wires) cover a length of the object which is at least as long as the steel J-tube. <IMAGE>

Description

CORROSION PROTECTION OF INTERNAL SURFACES OF A J-TUBE The present invention relates to corrosion protection and relates more particularly to corrosion protection of so-called J-tubes which are used for accommodating elongated objects, such as electrical cables, on offshore structures. A J-tube is a tubular structure having the form of the letter J and when in use the lower part of the J-tube extends substantially horizontally near the sea bed while the upper part extends above the sea level to a desired position on the offshore structure. The tube is used for mechanical protection of submarine flow-lines, cables and umbilicals installed between the seabed and the topside of the offshore structure.

The J-tube is usually made of steel and corrosion protection thereof is usually obtained from the cathodic protection provided for the offshore structure itself.

Cathodic protection for an offshore structure can be as illustrated in Norwegian Patent No. 167559 (Shell). It is, however, well known that such cathodic protection systems do not offer corrosion protection of the internal surfaces of the steel tube.

Corrosion protection of the internal surfaces of the J-tube is usually provided by introducing corrosion inhibitors into the water contained within the J-tube. In order to prevent the corrosion inhibitors from being washed out by seawater entering the lower end of the J-tube gasket rings are usually provided at the entrance of the accommodated object(s) into the tube. However, these remedies lead to reduced heat dissipation from power cables and consequently the current carried by such cables must be reduced compared to that of an open construction.

Furthermore, the gasket rings referred to may lead to undesirable pressure against the rather sensitive surface of the elongated objects installed within the tube.

It has been proposed to install sacrificial anodes on the inside surfaces of the tube, but this proposal was abandoned as not being desirable.

An object of the present invention is to improve corrosion protection of the internal surfaces of J-tubes.

According to the present invention there is provided a method for the corrosion protection of the internal surfaces of a steel J-tube secured or securable to an offshore structure, including the steps of installing an elongate object into and through the tube after a number of sacrificial anode wires are arranged on the outer surface of a certain length of the object and providing at least one low resistance electrical contact between the wires and the tube.

Whereas cathodic protection by means of zinc wires is well known in connection with corrosion protection of cable armour, in the present invention the cable or other elongate object is now only used as a carrier of the sacrificial anode wires (e.g. zinc wires).

With the present invention the internal surfaces of the steel J-tube are efficiently corrosion protected and in the case of an accommodated power cable the current carrying capacity can be increased by a factor of 1.3 - 1.5 if this point is a bottleneck, and the risk of damaging the cable is greatly reduced.

By way of example embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a J-tube with a power cable inserted; Figure 2 is an enlarged cross-section of Figure 1; and Figure 3 shows a J-tube having an FRP or stainless steel insert.

In Figure 1 we have illustrated a J-tube 1 typically having a horizontal length L1 of some 15-100 m, a vertical length L2 of some 70-500 m below the sea level 2 and a length L3 above the sea level. The inner diameter D1 of the steel tube is some 175-300 mm, whereas an elongated object such as a power cable 3 installed within the tube has an outer diameter D2 of some 100-150mm. The power cable 3 is provided with armour and layers of corrosion protection over the armour (not shown). A number of zinc wires 4 are wound round the outer cable surface in a helical or SZ fashion, starting from a position 5 outside the lower end of the tube and ending at a position 6 outside the upper end of the tube. The zinc wires are dimensioned to give the desired protection for the expected life of the power cable.The zinc wires are bonded to the cable surface at positions 5 and 6 and possibly at other positions in between. At the upper end 9, and possibly at the lower end as well, the zinc wires 4 are electrically connected to the tube 1. There will also be electrical contact between the wires and the tube at the bend 7 of the tube. There is no electrical connection between the zinc wires and the cable armour.

With this construction the seawater 8 will fill the tube 1 up to the sea level 2.

In Figure 2 the cross-section of the tube 1 of Figure 1 shows the cable 3 and the zinc wires 4. The zinc wires are wound with a pitch of 3-20 times the diameter of the cable.

In Figure 3 is illustrated an alternative form of Jtube 10 which is suitable for accommodating the elongated object 3. The elongated object 3 has the same construction as Figure 1 with a number of zinc wires 4 wound around its surface.

In the so-called splash zone of the J-tube, extending a defined distance of some 5 m, below and above the sea level 2, a section of the steel tube is replaced by a corrosion resistance plastic (FRP) or stainless steel tube section 11. The steel tube sections 12 and 13 of the J-tube as well as the inserted tube section 11 should be provided with flanges. A cathode cable 14 interconnects the two steel tube sections 12 and 13. The tube section 11 may be extended to the top of the J-tube by making it longer and by dropping the steel tube section 13.

It should be noted that metals other than zinc or metal alloys could be used as sacrificial anode wires, as long as the material is less precious than steel, and sufficient volume of it can be provided to give protection for the desired life of the J-tube.

Claims

CLAIMS:

1. A method for the corrosion protection of the internal surfaces of a steel J-tube (1; 12,13) securable to an offshore structure, including the steps of installing an elongate object (3) into and through the tube after a number of sacrificial anode wires (4) are arranged on the outer surface of a certain length of the object and providing at least one low resistance electrical contact between the wires and the tube.

2. A method according to claim 1, in which the certain length of the object is at least as long as the Jtube.

3. A method according to claim 1 or claim 2, in which when the object (3) is installed within the tube, the wires (4) extend between defined positions (5,6) which are outside the tube ends.

4. A method according to any preceding claim, in which the wires are zinc wires which are wound helically or in an SZ fashion around the object (3) with a pitch of 3-20 times the diameter of the object.

5. A method according to any preceding claim, in which the wires are electrically connected to the upper end (9;15) of the steel J-tube (1;12).

6. A method according to any preceding claim, in which in a splash zone of the J-tube (10), which zone will extend a defined distance below and above the seal level (2) , the steel tube section is replaced by a corrosion resistant plastic (FRP) tube (11).

7. A method according to claim 6, in which steel parts (13) of the tube separated by the corrosion resistance plastic tube (11) are electrically interconnected (14).

8. A method for the corrosion protection of the internal surfaces of a steel J-tube substantially as hereinbefore described with reference to the accompanying drawing.