WO2014128439A1

WO2014128439A1 - Rejuvenation of subsea electrical distribution systems

Info

Publication number: WO2014128439A1
Application number: PCT/GB2014/050430
Authority: WO
Inventors: Neil Irwin Douglas; Paul Robert OVERTON; Alistair John WRIGHT
Original assignee: Viper Subsea Technology Limited
Priority date: 2013-02-20
Filing date: 2014-02-14
Publication date: 2014-08-28
Also published as: GB2522351A; GB201402587D0; GB201302938D0; NO20151062A1; NO344030B1; GB2522351B; GB201504747D0

Abstract

A rejuvenation method for a cable used in a subsea environment, the method comprising applying a bias signal to a conducting element (2a, 2b) of the cable (2), the bias signal being selected to improve the insulation properties of the cable (2).

Description

Rejuvenation of Subsea Electrical Distribution Systems

This invention relates to a method of rejuvenating parts of an electrical subsea distribution system, and to an associated apparatus. The invention relates, in particular, to a method and apparatus for improving the electrical insulating properties of a cable of such a system, for example a subsea umbilical cable, a jumper, a flying lead or the like.

It is frequently necessary to sleeve an electrically conducting wire with an electrically insulating material, for example to prevent electrical coupling between adjacent wires within a cable, or to a conducting medium within which the wire is disposed. It is important that the insulation material performs adequately in the task, and one important figure of merit is the insulation resistance of the insulating material.

In subsea applications, insulated wires are used in the conductive medium of seawater, and the insulation material prevents electrical losses thereto, as well as fulfilling a number of other functions. Subsea cable insulation may degrade over time, with the insulation resistance eventually becoming unacceptably low. One cause of failure is seawater ingress into and through the insulation due to, for example, long term degradation of the insulating material, manufacturing faults or other means. Such a failure may result in short circuits between conductors and/or current flows from live conductors to earth.

These types of failure can eventually lead to total loss of subsea electrical control and hence to an unplanned shutdown of production from one or more wells. Present solutions to the problem rely heavily on intervention to disconnect subsea equipment, cables and connectors and to replace with new items. Such intervention is very inefficient, time consuming and expensive.

WO2010/136284 describes an arrangement for monitoring the insulation resistance of an ungrounded electrical network such as that found in electric and hybrid vehicles. It involves injecting a test voltage including an AC component and a DC component into the network. Appropriate monitoring allows measurements of the insulation resistance to be made. A need exists for a method and apparatus that is capable of improving the insulation properties of cables, particularly subsea cables as used in a subsea electrical distribution system. Whilst as mentioned above WO2010/136284 describes an arrangement for monitoring insulation resistance (albeit in an application far removed from the present invention), it does not provide teaching regarding enhancement of or rejuvenation of the electrical insulating properties of a network or parts thereof.

US2010/0122453 and US2009/0133799 both describe arrangements intended for use in the rejuvenation of cables.

According to the present invention, there is provided a rejuvenation method for a cable used in a subsea environment, the method comprising applying a bias signal to a conducting element of the cable, the bias signal being dynamically controlled and selected to improve the insulation properties of the cable.

The bias signal may comprise a bias voltage, preferably applied between ground and the conducting element. It includes a DC component. It may additionally have a time varying component. Where a time varying component is present, it may be of sinusoidal, square, or triangular waveform, for example.

The bias signal may alternatively comprise a bias current.

The bias signal is preferably selected such that, in the event of an electrical current flowing between the conducting element and a salt containing liquid of the subsea environment, an electrochemical reaction is promoted resulting in the generation of a barrier material restricting further leakage current flow and so enhancing the insulation resistance of the cable.

The invention also relates to an apparatus configured to perform the method described hereinbefore.

The invention will further be described, by way of example, with reference to the following drawings, in which:

Figure 1 is a simplified schematic diagram of an apparatus according to an embodiment of the invention; Figure 2 is a diagram illustrating the apparatus of Figure 1 in greater detail;

Figure 3 is a graph showing insulation resistance plotted against time, showing the improvement in the insulation properties of a cable over time as a method according to an embodiment of the invention is used thereon;

Figure 4 is a diagram illustrating the operation of the invention; and

Figure 5 is a diagram illustrating the relationship between the voltage and current with varying insulation resistance levels.

Figures 1 and 2 show a cable rejuvenation apparatus comprising a voltage source 1 (including a part 1 a arranged to output a DC signal and a part 1 b arranged to output a time varying component upon which the DC signal is superimposed) and a current limiting resistor 3. The voltage source 1 is connected to the conducting elements 2a, 2b of a subsea cable, umbilical or other part 2 of a subsea electrical distribution system. The system is illustrated as including two conducting elements 2a, 2b coupled to one another and coupled to other parts of the system by way of transformers. Resistances Ri_i and R_L2 denote the insulation resistance associated with the insulation of each conducting element 2a, 2b, and R_L3 denotes the resistance between the conducting elements 2a, 2b.

The cable rejuvenation apparatus may be a suitably programmed Viper Line Insulation Monitor device known as a V-LI FE device and available from Viper Subsea. However, this need not always be the case.

The present applicant has identified and proven that the insulation properties of a subsea cable can be markedly improved by applying a cable healing or rejuvenation method according to an embodiment of the invention. Figure 3 shows the results of a trial in which a suitably programmed V-LI FE device was connected to a subsea cable that was known to have poor insulation properties. The measured insulation resistance was approximately 160 kQ before the cable rejuvenation method was applied. During the period of the trial, the V-LIFE device was configured as a cable rejuvenation apparatus according to an embodiment of the invention, arranged to apply a bias voltage to the conducting elements 2a, 2b of the cable selected to improve the electrical insulation properties of the cable. The V-LIFE was configured to repeatedly apply a bias voltage to the cable and, whilst the bias signal was being applied, to conduct a sequence of measurements upon the cable. As described below, whilst the bias signal was applied, an electrochemical reaction was promoted serving to improve or enhance the insulation resistance of the cable. Over a majority of the time the V- LIFE was configured to operate in a rejuvenation and measurement phase in which whilst the bias signal was applied to promote the electrochemical reaction and so serve to rejuvenate the cable, the V-LIFE device further operated to monitor the insulation resistance. During this phase, the bias voltage was connected via the current limiting resistor 3. The presence of the current limiting resistor ensures that the applied current resulting from the application of the bias voltage will be maintained at an acceptably low level even if the insulation resistance of the cable is low.

The graph of Figure 3 shows insulation resistance measurements taken from the V- LIFE device over a period from the 2 January 2014 to 21 January 2014 during which tests were conducted to confirm the effectiveness of the invention. Figure 3 shows the insulation resistance markedly improving over this period with a substantially linear trend, up to a maximum of 100 ΜΩ. As mentioned above, the rejuvenation effect stems from an electrochemical reaction occurring between the material of the conducting elements 2a, 2b of the cable and the salts, primarily NaCI, present within the seawater in which the system including the cable 2, or at least part thereof, is located, the reaction being promoted by the application of the bias voltage. In the trial mentioned above, the conducting elements 2a, 2b were of copper form.

As illustrated in Figure 4, in the event of a failure in the insulation 7 surrounding one of the conducting elements 2a, 2b, say element 2a, one or more pores 5 may be formed in the insulation 7. The pore 5 may extend completely through the insulation 7 as illustrated or may extend only part way through the insulation 7. The application of the bias signal between ground and the element 2a will result in the production of Cu⁺ ions which will tend to migrate towards the seawater and in the production of CI^" ions which will tend to migrate towards the element 2a, these migrations resulting in the formation of an electrical leakage current. In addition, it will promote the formation of CuCI salt which will accumulate, initially, primarily as a solid within the pore 5 forming a barrier impeding the aforementioned migrations and so reducing the leakage current. By reducing the leakage current, it will be appreciated that the insulating properties of the cable 2 have been rejuvenated. In addition to collecting within the pore 5, some of the salt will tend to accumulate on the surface of the cable 2 around the pore 5.

It will be appreciated that, over time, some of the formed salt will tend to dissolve or otherwise be carried from the pore 5. Such removal will tend to result in a slight increase in the leakage current, resulting in the generation of fresh salt replacing that which has been removed. Accordingly, the rejuvenation of the cable is self- maintaining.

In some circumstances the electrochemical reaction also promotes the formation of Cu₂0 which, again, will serve as a barrier material. The formation of Cu₂0 occurs if the seawater present within the pore 5 becomes increasingly alkaline. The pore 5 will typically be of very small dimensions and so the flow of seawater into and from the pore 5 will be restricted. Accordingly, the make-up of the seawater within the pore 5 will change over time as the electrochemical reactions take place. The formation of Cu₂0 may thus be related to the spacing of the conductor 2a from, for example, a steel shielding provided around the cable 2.

The magnitude of the applied bias signal, and any time varying component thereof, are conveniently controlled by monitoring the insulation resistance of the cable and adjusting one or other or both of these parameters to optimise the insulation resistance or maintain the insulation resistance within an acceptable range. By way of example, the applied bias voltage signal may be selected so as to control the rate of the electrochemical reaction and thereby avoid or reduce to acceptable levels the generation of gases as part of the electrochemical reaction or as a result of electrolysis, and also to ensure that the barrier material is maintained at a level sufficient to maintain the insulation resistance at an acceptable level, whilst also minimising the loss of conductor material from the conductors. By measuring the insulation resistance whilst the rejuvenation method is in use, it will be appreciated that the applied bias signal can be actively and dynamically controlled to achieve optimisation of the effects mentioned above. The applied bias signal will thus vary depending upon the measured insulation resistance, and as a result damage to the conductors arising from the application of too large or too small a bias signal can be avoided or limited to an acceptable level.

The nature of the failure of insulation may take several forms. For example the insulation may degrade substantially uniformly over large lengths of the cable, may suffer from a single point failure or may be subject to a distributed failure such as water treeing. The insulation resistance measurements allow information relating to the nature of the failure to be derived, and it is possible to control the applied bias signal depending upon the nature of the fault to optimise rejuvenation of the cable.

As mentioned above, the primary purpose of the current limiting resistor 3 is to ensure that in the event that the insulation resistance falls to a low level, the applied bias voltage signal does not result in the generation of an excessively high current. In practise, the current limiting resistor 3 and the insulation resistance form a potential divider. For half of the applied bias voltage to be dropped across the insulation, the insulation resistance would need to fall to a level substantially equal to the resistance of the current limiting resistor 3.

Figure 5 illustrates the relationship between the applied bias voltage and current signals and the insulation resistance, showing that whilst the insulation resistance is high, the application of a large bias voltage signal will only result in the supply of a small current. As the insulation resistance drops, for example as a result of degradation thereof, to avoid the supply of an excessive current, the applied bias voltage signal will need to be reduced.

It will be appreciated that an operator will be able to predict, based upon the design of the system and insulation resistance measurements taken over time, an expected lifetime for the cable, and by how much the lifetime can be increased by the use of the invention.

The specific embodiment describes the use of an appropriately configured V-LIFE as a cable healing or rejuvenation apparatus, but as mentioned hereinbefore any other suitable equipment may be used. The use of the cable rejuvenation process described hereinbefore has a deleterious effect on the cable conductor due to the electrochemical reaction resulting in the loss of conductor material therefrom. The applicant has considered a number of modifications to the process by which the cable healing or rejuvenation method may be optimised to achieve the appropriate balance between improving insulation properties while minimising damage to the conductor. For example, the cable healing method may be optimised by appropriate configuration of the duration of the bias voltage application, but also the polarity, amplitude and waveform shape of the bias voltage. Suitable waveforms may include sinusoidal, triangular and square waves, for example. The amount of conductor material loss, and the impact of this loss upon the predicted lifetime of the cable, can be determined by the operator.

Although the description hereinbefore relates to the rejuvenation of cables with copper conductors, the invention is not restricted in this regard and may be applied to other forms of cable. By way of example, it may be applied to cables having aluminium conductors. In the case of cables with aluminium conductors, the electrochemical reaction may promote the formation of an Al₂0₃ barrier material layer. A different magnitude of bias signal may be required to promote the occurrence of this reaction.

Whilst specific embodiments of the invention are described hereinbefore, it will be appreciated that a number of modifications and alterations may be made thereto without departing from the scope of the invention, as defined by the appended claims.

Claims

A rejuvenation method for a cable used in a subsea environment, the method comprising applying a bias signal to a conducting element of the cable, the bias signal being selected to improve the insulation properties of the cable.

The method according to Claim 1 , wherein the bias signal comprises a bias voltage.

The method according to Claim 2, wherein the bias voltage is applied between the conducting element and ground.

The method according to Claim 2 or Claim 3, wherein a current limiting resistor is used to limit the magnitude of a leakage current arising from the application of the bias voltage.

The method according to any of Claims 2 to 4, wherein the bias voltage has a DC component.

The method according to any of Claims 2 to 5, wherein the bias voltage has a time varying component.

The method according to Claim 6, wherein the time varying component is one of: sinusoidal, square, triangular in waveform.

The method according to any of Claims 2 to 7, wherein a period of time during which the bias voltage is applied is optimised to enhance the insulation resistance properties of the cable.

The method according to any of Claims 2 to 8, wherein an amplitude of the applied bias voltage is optimised to enhance the insulation resistance properties of the cable.

10. The method of Claim 1 , wherein the bias signal comprises a bias current signal.

1 1. The method according to any of the preceding claims, wherein the bias signal is controlled to result in the promotion of an electrochemical reaction resulting in the formation of a barrier material tending to reduce electrical current flow between the conducting element and seawater.

12. The method according to Claim 1 1 , wherein the bias signal is controlled using information relating to the measured insulation resistance and variations in resistance over time.

13. The method of Claim 1 1 or Claim 12, wherein the barrier material comprises at least one of CuCI, Cu₂0 and Al₂0₃.

14. The method according to any of the preceding claims, wherein the bias signal is chosen depending upon the material of the conducting element.

15. An apparatus configured to perform the method of any preceding claim.