GB2521864A

GB2521864A - Pipe integrity survey

Info

Publication number: GB2521864A
Application number: GB1400184.6A
Authority: GB
Inventors: Sven Morten Hesjevik
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-01-07
Filing date: 2014-01-07
Publication date: 2015-07-08
Also published as: GB201400184D0

Abstract

A flexible pipe comprises an inner conductive pipe and an electrically insulating outer sheath in contact with an electrolyte. Damage to the electrically insulating outer sheath is detected by supplying current between an external anode and a location on the inner conductive pipe where the sheath is damaged to expose the inner conductive pipe to the electrolyte and sensing an electric field between the anode and the inner pipe. The anode may be a sacrificial anode comprising magnesium. The anode may be connected to the positive terminal of a dc power supply or power pack with the conductive inner pipe connected to the negative terminal. The pipe may comprise a cathodic protection system for reducing corrosion of a structure connected to the pipe. The invention is particularly suitable for buried flexible pipes or risers transporting oil or gas and exposed to sea water.

Description

PIPE INTEGRITY SURVEY

FIELD OF THE INVENTION

This invention generally relates to a method of detecting damage to an electrically insulating sheath or a flexible pipe, and to a system for detecting damage to an electrically insulating sheath of a flexible pipe.

BACKGROUND TO THE INVENTION

Flexible pipes for transporting fluids such as oil or natural gas may suffer damage during Installation or use. Such damage may occur when a pipe is laid down on or under the sea bed, or when pressure builds up between an Inner steel armour of a flexible pipe and an outer plastic sheath of the flexible pipe such that an outer plastic sheath cracks. The damage may result in sea water ingress and corrosion where the flexible pipe is installed offshore: the sea water may act as an electrolyte such that corrosion of the inner pipe occurs.

For buried flexible pipes, survey or Inspection of outer sheet Integrity is generally not possible without excavation in oitier to perform a visible inspection of the pipe. Such inspection, which may be carded out -for example in sea water -using a ROV (remotely operated vehicle), is costly and itself represents a risk to the integrity of the pipe.

For flexible risers and seawater exposed flexible pipes, visual Inspection can be canled out without excavation. However, marine growth may need to be removed prior to the inspection and this removal may incur time and cost and/or risk damage to the pipe.

For flexible risers, pressure testing of the outer sheet can be canted out to detect damage, using vent ports on a platform above sea level. Pressure testing of the annulus of a flexible pipe may be performed from the topside of the pipe. Such pressure testing of an annulus may detect damage between the plastic sheath and inner riser pipe of a flexible pipe.

However, pressure testing is generally not possible on a fully submerged flexible pipe with no sea surface breaking ends, or for repaired risers or risers with a damaged vent

port, for example.

Thus, the field of flexible pipe maintenance continues to provide a need for altemative and/or improved techniques hr damage detection, for example for any one or more of reduced risk of damage during damage detection, increased accuracy, sensitivity and/or rehabhty, reduced cost and/or time, etc..

SUMMARY

According to a first aspect of the present invention, there is provided a method of detecting damage to an electricafly insulating sheath of a flexible pipe, the flexible pipe having a conductive inner pipe and said sheath surrounding the inner pipe, the flexible pipe in contact with electrolyte, the method comprising: providing an anode external to the flexible pipe, to thereby provide an eiectrc field for current flow between the anode arid a location on the inner pipe when the sheath, is damaged to expose the location to the electrolyte; sensing a said electric field between the anode and the inner pipe to thereby detect damage to the sheath.

Advantageously, an embodiment may allow in situ detection of damage to the sheath so allowing measures to be taken to repair corrosion of the inner pipe, and/or to replace the flexible pipe entirely so that safe and/or reliable operating conditions are resumed. contrast to sonic flexible pipes in sea water, where it may he possible to carry out a visual inspection with a lower risk that the inspection itself will damage the pipe, embodiments may be of more advantag,e for a buried pipe that would otherwise need to be dug up for inspection.

Generally; the conductive inner pipe is metaflic, e.g, comprises a ferrous metal, such as iron or steel. The sheath may be an outermost electrically insulating layer or sheet of the flexible pipe to that in normal use the sheath is in contact with the electrolyte.

The sheath as a whole may be electrically insulating, or may have an electrically insulating coating applied preferably on the outermost surface of the sheath.

Regardless, am undamaged sheath preferably prevents electrical conduction between the electrolyte and inner pipe.

There may further be provided the method, wherein the anode is a sacrificial anode having an electrically conductive coupling to the inner pipe. Such a method niay referred to as a galvanic method. Preferably, the anode comprises magnesium to allow good damage detection sensitivity.

Additionally or alternatively, there may be provided the. method, wherein the anode comprises a positive terminal of a dc power supply, the method comprising providing an electrically conductive coupling between a negative ternmnal of the do power supply and the inner pipe. Such a method may be referred to as an impressed current method.

S

The dc (direct current) power supply may be obtained by rectification of an ac (alternating current) power source. The anode, for example where formed by the positive terminal, may comprise a material for low corrosion, e.g., an inert or noble metal.

In any embodiment, the flexible pipe may already comprise at least one cathodic protection system (CPS) for reducing corrosion of a structure connected to the flexible pipe. Such a system may be a conventional sacrificial anode CPS using a galvanic anode adjacent carefully directly connected to the metal to be protected, the anode having a higher energy level or potential relative to the metal to be protected. An electrical current to reduce corrosion may then result from a potential difference between the sacrificial anode metal and the metal to be protected. Alternatively, the CPS may be an impressed current CPS having preferably inert anodes installed in the electrolyte (e.g. soil or water) remote from the metal to be protected and connected to the positive terminal of a power supply. The metal to be protected Is connected to the negative terminal of the supply. Thus, the power supply may source a current that is impressed from the anode onto the cathode surface to be protected. In either type of CPS, current may flow from the (sacrificial or inert) anode to the metal to be protected, effectIvely to make the metal to be protected negatively charged and thus cathodic.

Either or both types of CPS may be Installed to prevent or reduce corrosion of metal elements such as metallic end fittings of a flexible pipe and/or structures connected to such end fittings.

Thus, an existing CPS on a flexible pipe to which an embodiment is to be applied may comprise its own anode and/or power supply. For such an embodiment, a sacrificial anode coupled to the inner pipe may then be provided in addition to a sacrificial anode of the CPS and/or an anode coupled to the dc power supply as mentioned above may be provided in addition to a protection system comprising a separate power supply and anode. The anode provided by the embodiment may therefore strengthen an existing electric field rather than create an initial electric field.

Optionally the flexible pipe is a buried flexible pipe preferably for transporting oil or gas e.g. from a natural oil or natural gas well. Alternatively, such a flexible pipe may not be buried and may In some embodiments be exposed to seawater, e.g. a flexible riser from a natural oil or gas well.

AccordIng to a second aspect of the present InventIon, there is provided a system for detecting damage to an electrically insulating sheath of a flexible pipe, the flexible pipe in contact with electrolyte and having a conductive inner pipe and said sheath surrounding the Inner pipe, the system comprising: an anode to provide an electric field for current flow between the anode and a location on the inner pipe when the sheath is damaged to expose the location to the electrolyte; a sensor to sense a said electnc field between the anode and the inner pipe to thereby detect damage to the sheath.

There may further be provided the system, wherein the anode is a sacrificial anode having an electrically conductive coupling to the inner pipe, the anode preferably comprising magnesium. In such a system, the anode may be coupled to an end fitting of the flexible pipe. There may further be provided the system, comprising a do power supply having a positive terminal coupled to the anode and a negative terminal coupled to the inner pipe. In such a system, the negative terminal of the power supply may be coupled to an end fitting of the flexible pipe.

There may still further be provided the system, wherein the flexible pipe comprises at least one cathodic protection system for reducing corrosion of a structure connected to the flexible pipe.

Preferably, the system comprises the flexible pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into efct, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. I shows the general principle of an embodiment applied to a buried flexible pipe.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Compared to a rigid pipe, a flexible pipe Is more generally easily handled andlor installed, for example on the sea floor. Furthermore, such pipes are able to move more easily (e.g. on the sea flooi and take up more expansion.

A flexible pipe may be used to transport a fluid such as oil, gas, produced water (produced from a well and generally to be disposed of, e.g., injected into a drainage well and then transported in rigid or flexible pipes and injected into a formation for dlspou. InjectIon water (e.g., such water being injected to Increase pressure in a well or being injected into a formation for disposal), injection methanol, production methanol, hydraulic fluid for operating valves, etc..

An aim of an embodiment of the present invention is to provide an integrity survey to detect damage to a flexible pipe, More specificafly, a preferred embodiment provides an integrity survey of an outer sheet/sheath of a flexible pipe, using electric field measurements. Damage to be detected may be to one or more ayers including an external surface surrounding ferrous material inside the pipe, e.g., steel armour inside the pipe., may give rise to corrosion of the ferrous material An electric field between an anode and an electrically conductive (e.g., steel) inner region of the pipe may arise when the region is exposed by damage to at east an outer layer of the pipe. Generally, the outer layer of a flexible pipe is at east an external surface of a sleet/sheath of the pipe, e.g., an outer coating and/or a plastic outer sheet. Detection and/or measurement of the field may allow detection and/or evaluation of the damage. Generally a flexible pipe will not have existing cathodic protection (unlike a rigid, e.g., solid steel, pipe) to protect the pipe itself. This may be because th.e outer sheet is considered sufficient protection for the inner metalic components.

However, cathodic protection may be provided with the flexible pipe in an effort to protect a metalUc structure connected to an end fitting of the flexible pipe. Generally, a flexible pipe will have an end fitting at each end. For example. such protection may be provided where end fittings or flanges assocated with a flexible pipe are exposed to electrolyte such as sea water or soil. An embodiment may increase the electric field created by such protection, to allow an integrity survey of the flexible pipe.

Alternatively, no existing electric field may be present on a flexible pipe that has no existing associated cathodic protection, so that an integrity survey embodiment creates

a new field

Where a cathodic protection system (CR9) is already installed, flexible pipe steel armour inside a damaged sheet will draw current from the CR9. The strength of the current field (current density) is generally dependent on the corrosive environment and with time the held will he reduced because of changes in the surface properties ol the steel. This means that even an improved cathodic protection survey sensor may not be able to detect coating damage unless the sensor is quite large. This is especially valid for buried pipes. However, to increase the electric current availability on potential outer sheet damages and thereby increase an electric field to be detected, an external source can be temporary installed. A combination of a cathodic protection survey and an external power source of an embodiment may he used to survey and detect damage to the outer sheet of the flexible pipe. The external source can be installed temporarily and/or connected to the pipe using for example the same ROy as used for a cathodic protection survey.

The source to provide electric current in an embodiment can be, eg,: * a sacrificial anode for example made from an alloy that provides cathodic potential representing a high driving force for galvanic current between the anode and the flexible pipe (eq., magnesium anode); and/or an electric power pack for example prepared for subsea environment (e.g. a battery with regulating possibiUties and terminals to pipe and anode respectively) Embodiments can be used for seawater exposed flexible pipes as well as flexible risers. However, embodiments are generally of more advantage for buried flexible pipes. Such flexible pipes may be buried at a minimum depth of 0.5m and thus require a relatively strong electric field for detecting damage. Even where a CR5 is n place for such a buried flexible pipe, the field would generally be to small to be detected above the ground/sea bed. Hence, the electric field of such a system is preferably added to or strengthened by use of a supplementary sacrificial anode ar.d/or impressed current anode of an embodiment, in a preferred embodiment, the integrity survey of a for example buried pipe is therefore combined with a cathodic protection survey. A combination ol a cathodic protection survey and an external power source of an embodiment may then be used to survey and detect damage to the outer sheet of the flexible pipe.

Fig. 1 shows the general principle of an embodiment applied to a buried flexibie pipe, wherein a the region of an inner pipe of the buried pipe is exposed by damage to outer layer(s) of the buried pipe. Current tiows in an electric field from a current source in the form of a sacrificial anode and/or power pack with high output current availability to the exposed region of the inner pipe. The current source is coupled to a flexible pipe end fitting, and the inner pipe completes the electrical circuit allowing current to return through the inner pipe to the end fitting. The electric field associated with such a current, or change in strength and/or distribution of such a field, is detectable by a sensor for example operated by a remotely operated vehicle.

An embodiment utilizes a sensor to detect damage to the outer sheet of a flexible pipe or riser, the embodiment creating or strengthening an electric field using an external source connected to the end fitting of the pipe. By creating or/increasing the electric field, the detection sensitivity of outer sheet damage will be increased, and this is especially useful for buried flexible pipes. Preferably the sensor is has a sensitivrty of 0.2 -0.3 microvoks/cm (tN/cm) or better, preferably 0.luV!cm or less. The sensor may be a cathodic protection survey sensor, for example composing two or more reference cefls separated by a short thstance across which an electric field can be measured.

More preferably, the sensor has an array of multiple electrodes (e.g., 10, 20; 30; 100).

The sensor may he mountable on a fixture for fitting on a remote operating vehicle.

The creation or strengthening of the held by an embothment may be achieved with an initial or additional sacrificial anode to give a (higher) current driving force, for example a sacrificial anode comprising magnesium. Such an anode may be connected to an end fitting of the pipe that is exposed to &ectrolyte, e.g., sea water. This end fitting may auow connection to, e.g., steel, and thus allow good electrical contact to the inner pipe of the flexible pipe. For practical purposes the sacrificial anode is preferably arranged closest to the end fitting. However, if necessary the sacrificial anode could be moved closer to the flexible pipe.

The sacrificial anode may comprise a single anode, one at each end of the flexible pipe, and/or a plurahty of two or more anodes spaced along the length oF the flexible pipe. The size of the or each anode, determining the current output of the anode, is preferably determined on a case to case basis. For example, where the flexible pipe is relatively long andlor connected to a large metallic structure, larger anode(s) may be required. The or each anode may for example be a 100kg 200kg anode with corresponding surface area for providing adequate current for the damage detection, and may be comprised of a plurality of smaller anode units. Preferably the anode comprises magnesium (e.g., comprises a magnesium alloy). However, additionally or alternatively the anode comprises zinc and/or aluminium, which may provide a driving voltage of 200 -. 250mV. Generally. magnesium will give a higher driving voltage, e.g., 600 -700niV.

An embodiment comprising an impressed current arrangement having an anode and an electric power supply, e.g., battery or derived from a power generator can he used to provide higher power than a sacrificial anode arrangement and thus potentially greater damage detection sensitivity. Ann impressed current arrangement generally composes a voltage regulator. Where the original power source is an alternating current (ac) source, a rectifier may be employed to provide a direct current (dc) output.

The anode, which may comprise an inert material, e.g., coated in a metallic oxide, is preferably placed near the flexible pipe to supply the impressed current. Similarly as for the sacrificial anode arrangement, the anode for the impressed current may comprise a single anode, or one at each end of the flexible pipe, or a plurality of two or more anodes spaced along the length of the flexible pipe.

B

Whether the above sacrificial anode and/or impressed current techniques are implemented, preferably the electric field achievable and detectable when damage has occurred Is at least 0.1 uV/cm, more preferably 0.2 -0.3uV/cm or hIgher. In thIs regani, it is noted that an end fitting of a pipe may be connected to an adjacent metallic structure, e.g., a manifold structure or a further pipeline continuing from the flexible pipe under inspection. Such a structure may distort the electric field from the anode of an embodiment. Therefore in an embodiment some of the sacrificial anode current and/or impressed current may leak to such a structure rather than to the damaged region of the flexible pipe. This may reduce damage detection sensitivity. In such a situation, it may be more desirable to implement an impressed current embodiment with hIgher power availabilIty.

For safety reasons, current surge protection may be provided in an impressed current embodiment for example where the flexible pipeline carries oil or gas.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.