WO2017025099A1

WO2017025099A1 - A method of testing an unbonded flexible pipe

Info

Publication number: WO2017025099A1
Application number: PCT/DK2016/050265
Authority: WO
Inventors: Christian Ditlev CAPPELN; Anders STRAARUP; Michael Eilersen; Thorsten Holst
Original assignee: National Oilwell Varco Denmark I/S
Priority date: 2015-08-10
Filing date: 2016-08-08
Publication date: 2017-02-16

Abstract

The present invention relates to a method of testing an unbonded flexible pipe having a length and a longitudinal axis and comprising, from the inside and out, an internal armour layer, an internal pressure sheath, at least one external amour layer and an outer sheath, at least one of the armour layers is an electrically conductive armour layer. The method comprises the steps of: connecting the electrically conductive amour layer to an electrical power source; sending electrical current through the electrically conducting armour layer; obtaining electrical characteristics for the electrically conductive armour layer; and comparing the electrical characteristics with predetermined characteristics and determine if the electrically conductive armour layer is within the specification in respect of electrical properties.

Description

A METHOD OF TESTING AN UNBONDED FLEXIBLE PIPE

The present invention relates to a method of testing an unbonded flexible pipe having a length and a longitudinal axis and comprising, from the inside and out, an internal armour layer, an internal pressure sheath, at least one external amour layer and an outer sheath, at least one of the armour layers is an electrically conducting armour layer.

TECHNICAL FIELD

Flexible unbonded pipes are for example described in the standard

"Recommended Practice for Flexible Pipe", ANSI/API 17 B, fourth Edition, July 2008, and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Third edition, July 2008. Such pipes comprise an innermost sealing sheath - often referred to as an internal pressure sheath, which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or usually a plurality of armouring layers. The armouring layers may comprise an internal armour layer, referred to as the carcass on the inner side of the internal pressure sheath. On the outer side of the internal pressure sheath, one or more pressure armour layers and one or more tensile armours may be applied. Often the pipe further comprises an outer protection layer which provides mechanical protection of the armour layers. The outer protection layer may be a sealing layer sealing against ingress of sea water, and is often referred to as the outer sheath. In general flexible pipes are expected to have a lifetime of 20 years or more in operation.

Before unbonded flexible pipes are released from the manufacturer and delivered to the customer, they are tested according to "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Third edition, July 2008, part 10. The tests are denoted factory acceptance test (FAT) and include a gauge test and hydrostatic pressure test and, optionally, an electrical continuity and resistance test, in case the pipe comprises cathodical protection. Moreover, the test may include a gas-venting and flow-resistance test if the unbonded flexible pipes have gas relief valves or ports installed in the end-fittings.

International patent application WO 2011/027154 Al discloses a method for testing an unbonded flexible pipe. In this method the internal pressure sheath electrodes are placed on each side of the internal pressure sheath and a voltage is applied over the electrodes. The only capacitive coupling between the electrodes is through the internal pressure sheath. The metallic armour layers in the pipe may be used as electrodes. Recent developments in unbonded flexible pipes push towards unbonded flexible pipes comprising heating systems. In particular, a newly developed pipe enables electrical heating of the pipe by means of the armour layers, in particular the carcass and optionally a pressure armour layer or a tensile armour layer. The carcass is particularly advantageous to utilize as an electric heating element as the carcass is located in the bore of the pipe and in physical contact with the fluid to be conveyed in the in the bore of the pipe. However, it is desirable to test these heating systems before the unbonded flexible pipes are released from the manufacturer.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a test which will verify the electrical conductivity of the armour layers in an unbonded flexible pipe and thus ascertain that the heating system will work according to specifications.

The present invention also provides an option of determining the condition of the internal pressure sheath.

The present invention relates to a method of testing an unbonded flexible pipe having a length and a longitudinal axis and comprising, from the inside and out, an internal armour layer, an internal pressure sheath, at least one external amour layer and an outer sheath, at least one of the armour layers is an electrically conductive armour layer, said method comprising -connecting the electrically conductive amour layer to an electrical power source

-sending an electrical current through the electrically conductive armour layer

-obtaining an electrical characteristics of the electrically conductive armour layer comparing the electrical characteristics with predetermined electrical characteristics or values and determine if the electrically conductive armour layer is within the specification in respect of electrical properties.

By "the specification in respect of electrical properties" is meant the specifications which the unbonded flexible pipe is expected to meet in respect of electrical characteristics and defines the parameter ranges within which the unbonded flexible pipe should perform. The specifications may be determined by the manufacturer and/or by the purchaser. The test method should measure and determine that the electrical characteristics are within the defined ranges. Otherwise the unbonded flexible pipe may not meet the required specifications and may need to be discarded or optionally repaired.

It should be emphasized that the term "comprises/comprising" when used herein is to be interpreted as an open term, i.e. it should be taken to specify the presence of specifically stated feature(s), such as element(s), unit(s), integer(s), step(s) component(s) and combination(s) thereof, but does not preclude the presence or addition of one or more other stated features.

The term "measure" in measured/measuring electrical characteristics, e.g. the electrical resistance, includes both a direct measurement as well as an optional measurement of related parameters by which the electrical characteristics in question can be calculated. The terms 'measure' and

'determine' are used interchangeably.

The term "substantially" should herein be taken to mean that ordinary product variances and tolerances are comprised. Preferably the electrically conductive armour layers are connected to the power source via a first end-fitting terminating the first end of the pipe. The second end of the pipe is preferably terminated in a second end-fitting which may be adapted to establish electrical contact between the internal armour layer and the at least one external armour layer. Thus, the external armour layer may return the current to the power source and establish an electrical circuit. Alternatively, the electric current may be returned by an electric cable.

Several electrical characteristics may be used to determine if the layers in the pipe meet the set specifications such as response to high frequency signals which may reveal the dielectrical properties of the internal pressure sheath. Normally, the internal pressure sheath is not dielectrical, however, when the unbonded flexible pipe is in operation, water may migrate into the internal pressure sheath and optionally cause hydrolysis which is undesired. Water is dielectrical, and by measuring the dielectrical properties of the internal pressure sheath it is possible to determine the condition (and water content) of the internal pressure sheath.

The internal armour layer (the carcass) is placed on the inner side of the internal pressure sheath. The one or more external armour layers are placed on the outer side of the internal pressure sheath.

The term "inner side" of a layer is the side of the layer facing the axis of the pipe. The term "outer side" of a layer is the side of the layer facing away from the axis of the pipe.

In an embodiment, the predetermined electrical characteristics are obtained from an electrically conductive armour layer which is substantially free of errors.

In an embodiment, the predetermined electrical characteristics are calculated from data for the material of the electrically conductive armour layer. These calculations can be performed relatively simple and will provide numbers for the ideal characteristics of the armour layer. In an embodiment of the method, the electrical characteristics are data on the complex impedance of the pipe preferably when operated as a coaxial cable, i.e. connecting the power supply to the carcass and using one or more of the external armour layers as return path.

In an embodiment the electrical characteristic is the direct current-resistance of the electrically conductive armour layer. The resistance is measured at a specific current, and preferably at several specific currents.

In an embodiment the electrical characteristic is the alternating current- resistance of the electrically conductive armour layer. The resistance is measured at a specific current and a specific frequency, and preferably at more specific currents and specific frequencies.

In an embodiment the electrical characteristic is the impedance of the electrically conductive armour layer.

Although the electrically conductive armour layer may be manufactured from electrically conductive polymer material, the electrically conductive armour layer is preferably made from metallic material, such as duplex steel, stainless steel, carbon steel or an aluminum based alloy.

In case the selected electrical characteristic is the complex impedance of the system, one way to measure the complex impedance is to subject the pipe to a sweeping AC voltage and measure the return current as well as the phase angle of the return current. Alternatively, the pipe may be subjected to electrical pulses and the current response can be recorded and subsequently analysed using methods well known in the art.

In case only the electrical resistance of the pipe is wanted, this can be measured simply by subjecting the pipe to a voltage and record the resulting current. As the current through the carcass runs through numerous contact points, it is preferred to test the pipe at a current comparable to the envisaged operational current. Different tests for different electrical characteristics may be performed on the same unbonded flexible pipe. The tests may be performed more or less simultaneously or one after the other.

It is also possible to measure the conductivity of one or more of the armour layers. The different armour layers, e.g. a carcass and a pressure armour may be connected in series to form an electric circuit.

In an embodiment the internal armour layer, also referred to as the carcass, is a metallic and electrically conductive armour layer and the electrical current is sent through the internal armour layer. The carcass is the layer of the pipe which is placed in the bore of the pipe inside the internal pressure sheath. The bore is defined by the inner surface of the internal pressure sheath.

Thus, the carcass is closest to the longitudinal axis which also defines the center axis of the pipe, and it is advantageous to use the carcass as an electrical heating element in the pipe.

In an embodiment the at least one external armour layer is a metallic and electrically conductive armour layer and the electrical current is sent through the at least one external armour layer. The external armour layer is placed outside the internal pressure sheath. The at least one external armour layer may be a pressure armour layer or a tensile armour layer. A typical unbonded flexible pipe will comprise one or two pressure armour layers surrounding the internal pressure sheath and one or two tensile armour layers surrounding the one or two pressure armour layers.

The terms "inside" and "outside" a layer, such as e.g. the internal pressure sheath of the pipe is used to designate the relative distance to the axis of the pipe, such that by "inside a layer" is meant the area encircled by the layer i.e. with a shorter radial distance than the layer and by "outside a layer" is meant the area not encircled by the layer and not contained by the layer, i.e. with a longer radial distance to the axis of the pipe than the layer. The longitudinal axis of the pipe also defines the center axis of the pipe, i.e. "longitudinal axis", "center axis" and "axis" may be used interchangeably.

In an embodiment the internal armour layer is electrically connected to the at least one external armour layer, preferably in the second end of the pipe i.e. the end of the pipe terminated in the second end-fitting. The external armour layer is also an electrically conductive layer, and the current from the power source sent through the internal armour layer may then be returned to the power source via the external armour layer. The internal armour layer is the carcass and the external armour layer may be a pressure armour layer or a tensile armour layer.

When the internal armour layer or carcass is used as heating element and the external armour layer serves as a return path for the current, the material of the carcass may preferably have a higher electrical resistivity than the electrical resistivity of the external armour layer. Thus, more Joule heating will be generated in the carcass which is in physical contact with the fluid to be heated than in the external armour layer. In an embodiment the metallic and electrically conductive armour layer or layers are made from alloys and have specific electrical resistivity of about 10^"5 Ω-m or less.

In an embodiment the current is in the range of from 50 Ampere to about 500 Ampere. The longer the unbonded flexible pipe, the higher is the voltage required to maintain a certain current. The length of the unbonded flexible pipes to be tested in accordance with the method may vary within a rather broad range. The length may vary from about 50 m up to about 5000 m, such as from about 100 m up to about 2500 m, or from about 200 m up to 2000 m.

According to an embodiment of the method the electrical current is provided in pulses generated using a switching thyristor to the electrically conductive armour layer. As sharp pulses contain a high frequency component, this allows for probing of the electric properties of the internal pressure sheath. As an example HDPE is a non-polar material. Upon ageing of the pipe water may diffuse into the HDPE, hereby affecting the dipolar properties of the liner, which can be measured directly, and provide an indication of the condition of the internal pressure sheath.

In an embodiment the pulses have a length in the range from about 10⁵Hz to about 10Hz (PWM mode). Moreover in an embodiment the pulses have a length in the range from about 10Hz to about 10^"3Hz switch mode.

According to the method the electric current is provided as alternating current or direct current.

In an embodiment the electrically conducting armour layer may provide electric heating in the pipe and, thus, the armour layers form part of a heating system. The heating system may be formed by the carcass which may be heated by passage of the electric current (Joule-heating) and serve as the primary source of heat in the pipe. An external armour, such as a pressure armour of a tensile armour may be connected to the carcass and act as return lead to the power supply, thus forming an electric circuit.

The electrically conducting armour layer may rise the temperature in the pipe, preferably to a temperature in the range from 30°C to 130°C, such as to a temperature in the range from 40°C to 130°C. Having a temperature in the range from 30°C to 130°C is very convenient when transporting

hydrocarbonuoes fluid.

Accordingly, the pipe may be tested with a temperature in the range from 30°C to 130°C.

In an embodiment the method is performed as a factory test (FAT). Thus, the test may be used to check the electrically conductive armour layers, and possible failures in the armour layers may be discovered and repaired. In an embodiment the method is performed as an in-use test. During an in- use test where the unbonded flexible pipe is in operation, the test may check the condition of the metallic armour layers and the internal pressure sheath.

The layers of the unbonded flexible pipe are terminated in an end-fitting. Also the metallic and electrically conductive armour layer(s) is(are) terminated in the end-fitting and connected to the electrical power source via the end- fitting.

The method according to the invention provides a relatively simple and cost- effective test to determine if various electrical characteristics are in line with the predetermined electrical characteristics or values and determine if the electrically conductive armour layer is within the specification in respect of the electrical properties. The predetermined electrical characteristics or values may not be a specific value, but rather a range of values, and within this range the electrical properties of the electrically conductive armour layers in the unbonded flexible pipe are considered to be acceptable.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained in more details below in connection with an example and with reference to a drawing in which:

Fig. 1 illustrates the set-up for testing an unbonded flexible pipe according to the invention;

The drawing is schematical and only intended to illustrate the principles of the invention.

In figure 1 an unbonded flexible pipe 1 terminated in a first end-fitting 2 and a second end-fitting 3 is prepared for testing. A wire 4 connects the end- fitting to a power supply 5 which is connected to a control and measuring device 7 via a connection 6. The end-fitting 2 has connections which connect the wire to the carcass and a pressure armour in the pipe 1. The end-fitting 3 is adapted to establish electrical connection between the carcass and the pressure armour. The carcass and the pressure armour are made from stainless steel and are electrically conductive.

The power supply 5 may deliver direct current and alternating current with a voltage up to approximately 5000 V.

During operation of the unbonded flexible pipe the electrical potential difference in the electrically conductive layers should be in the range of 0.01 - 5 V/ meter pipe.

The power supply 5 comprises a Thyristor operated at 55 Hz.

The control and measuring device 7 controls the voltage in the pipe and measures and provides the desired parameters, such as electrical resistance, impedance and dielectric constant.

Example

Test of unbonded flexible pipes according to the invention.

An unbonded flexible pipe was tested according to the invention.

Data for the pipe:

Length: 125 m

Inner diameter: 300 mm

Outer diameter: 500 mm

The pipe comprises, from the inside and out, a carcass (stainless steel), an internal pressure sheath (PVDF), pressure armour layer (carbon steel), two cross wound tensile armour layers (carbon steel) and an outer sheath (HDPE) Resistance R: 0.2 ohm (direct current, carcass and tensile armour connected in series)

Temperature: approx. 45°C to approx. 51°C

Complex impedance Z: (0.28 + i*0.03)ohm (alternating current; 55Hz) Dielectric constant (measured indirectly by measuring Capacity):

C = 860 nF before FAT

C = 1830 nF after FAT (pipe filled with rape seed oil)

Claims

1. A method of testing an unbonded flexible pipe having a length and a longitudinal axis and comprising, from the inside and out, an internal armour layer, an internal pressure sheath, at least one external amour layer and an outer sheath, at least one of the armour layers is an electrically conducting armour layer, said method comprises:

-connecting the electrically conductive amour layer to an electrical power source

-sending electrical current through the electrically conductive armour layer

-obtaining electrical characteristics for the electrically conductive armour layer comparing the electrical characteristics with predetermined characteristics and determine if the electrically conductive armour layer is within the specification in respect of electrical properties.

2. A method according to claim 1, wherein the predetermined electrical characteristics are obtained from an electrically conductive armour layer which is substantially free of errors.

3. A method according to claim 1, wherein the predetermined electrical characteristics are calculated from data for the material of the electrically conductive armour layer.

4. A method according to any one of the preceding claims, wherein the electrical characteristic is the direct current-resistance of the electrically conductive armour layer.

5. A method according to any one of the preceding claims, wherein the electrical characteristic is the alternating current-resistance of the electrically conductive armour layer.

6. A method according to any one of the preceding claims, wherein the electrical characteristic is the complex impedance of the electrically

conductive armour layer.

7. A method according to any one of the preceding claims, wherein the internal armour layer is a metallic and electrically conductive armour layer and the electrical current is sent through the internal armour layer.

8. A method according to any one of the preceding claims, wherein the at least one external armour layer is a metallic and electrically conductive armour layer and the electrical current is sent through the at least one external armour layer.

9. A method according to any one of the preceding claims, wherein the internal armour layer is electrically connected to the at least one external armour layer.

10. A method according to any one of the preceding claims, wherein the electrically conductive armour layer has a specific electrical resistivity of about 10^"5 Ω-m or less.

11. A method according to any one of the preceding claims, wherein the current is in the range of from 50 Ampere to about 5000 Ampere.

12. A method according to any one of the preceding claims, wherein the electrical current is provided in pulses to the electrically conductive armour layer.

13. A method according to any one of the preceding claims, wherein the pulses have a length in the range from about 10⁵Hz to about 10Hz (PWM mode) or from about 10Hz to about 10^"3Hz switch mode.

14. A method according to any one of the preceding claims, wherein the electric current is provided as alternating current.

15. A method according to any one of the preceding claims, wherein the electric current is provided as direct current.

16. A method according to any one of the preceding claims, wherein the electrically conducting armour layer provides electric heating in the pipe.

17. A method according to any one of the preceding claims, wherein the electrically conducting armour layer may rise the temperature in the pipe, preferably to a temperature in the range from 30°C to 130°C.

18. A method according to any one of the preceding claims, wherein the method is performed as a factory test (FAT).

19. A method according to any one of the preceding claims, wherein the method is performed as an in-use test.

20. A method according to any one of the preceding claims, wherein the electrically conductive armour layer is terminated in an end-fitting.