GB2180073A

GB2180073A - Detecting wear in insulating sheaths of submerged lines

Info

Publication number: GB2180073A
Application number: GB08619821A
Authority: GB
Inventors: Christian Babin; Francois Marie Robert
Original assignee: Societe National Elf Aquitaine
Current assignee: Societe National Elf Aquitaine
Priority date: 1985-08-30
Filing date: 1986-08-14
Publication date: 1987-03-18
Also published as: GB8619821D0; FR2586818B1; NO863232D0; NO863232L; IT8667680A0; BR8604127A; FR2586818A1; GB2180073B; IT1215186B

Abstract

To detect wear in an insulating sheath on the outer surface of a submerged line having metal armouring (1), the line constants R, C, L and G of a line in good condition are used to calculate the apparent impedance Za in accordance with the equations: <IMAGE> R being the resistance, C the capacity, L the inductance, G the conductance, Zc the characteristic impedance, Zu the load impedance,w the pulsation, f the frequency of the current, I the length of the line, n the propagation constant according to n = 2ROOT (R+jLw) (G+jCw> the line being considered to be formed between the armouring (1) and the conducting medium surrounding the outer insulating sheath; then during use thereof on the service site in connecting its metal armouring (1) to a measuring apparatus the surrounding liquid providing the return path to a connection (2), in periodically injecting a low frequency current and in measuring the value of its apparent impedance Za whose deviation with respect to the initial apparent impedance signifies wear of the insulating sheath. <IMAGE>

Description

SPECIFICATION A method of detecting wear in a submerged line in a conducting medium and containing a duct or an electric cable BACKGROUND OF THE INVENTION 1. Field of thelnvention The present invention relates to a method of detecting wear in a line provided with an external insulating sheath containing a hydraulic duct, electric cable or a combined electro-hydraulic bundle, said duct and said cables or combined bundles being provided with an inner metal sheathing, immersed in a conducting medium such as sea water.

2. Description ofthePriorArt The oil fields situated on the sea bed comprise underwaterwells which are connected to platforms by umbilical hydraulic and electric control lines. Such umbilicals are protected by external plastic material casing, covering a metal sheathing, inside which hydraulic lines and/or electric cables are placed in a plastic material sheath. Each umbilical is suspended by its connection head above the platform and leads along a column of the platform through glass fiber and steel protective tubes as far as the foot of the platform, then it travels over the underwater ground to reach the well heads. Oscillations ofthe platform cause wear of the external plastic material sheath during the downward travel along the column ofthe platform.Such wear results in a reduction of the thickness ofthe sheath and the appearance of cracks. The insulating qualities of the sheath regress, so that the electric resistance between the metal sheathing and the sea water decreases in the vicinity ofthewear point.

The control line situated in the center of the umbilical and the sea water are in electric contact, whereas the metal sheathing is isolated by its two sheaths from the sea environment, except if there is an insulating defect ofthe external sheath.

Methods are known for detecting wear ofthe insulating sheath which consists in measuring the resistance between the inner metal sheathing or armouring and an additional metal casing with which the external plastic sheath is surrounded. Thus, variations of the real impedance due to the defect ofthe insulating sheath may be measured but it requires the addition of an external metal sheath, which contrary to a plastic material sheath does not slide well and is therefore subjected to rapid wear and in addition results in a considerable extra cost.

SUMMARYOFTHE INVENTION The present invention is based on the idea that an umbilical, as well as any electric or hydraulic line comprising an external insulating sheath and a sheathed metal armouring, immersed in a conducting medium behaves like an electric line with two conductors whose surrounding conducting medium serves as return conductor and that by injecting a low frequency current into the metal armour and by using the conducting medium as return conductor, measurement of the apparent impedance of the circuit allows the defects ofthe insulating sheath to be detected.

When an electric line is of great length, the current cannot be considered as constant along the line and in such a line of length 1 (see Figure 1) each elementary length may be represented by a series impedance dZs = dr + jdA corresponding to the longitudinal resistive losses of conductors and to the inductive effects between the conductors, and buy a shunt admittance dYd = dg + jdc corresponding to the losses of conductance through the insulator and to the capacitive effects between the conductors.

The line constants being foran homogeneous line: dr line resistance: R= dx linecapacitance: - = dx dh line inductance: L = dx dg lineconductance: G= 9 line conductance: G dx Figure 1 gives the representation of a dx element.

It isfurther known thatthe characteristic impedance of the line is:

whereas the propagationn constant of a line which characterizes the attenuation of awave during its travel and its propagation speed along the line is:

The method of the invention is characterized in that fist of all the line constants R, C, Land G of a line in good condition are determined so as to calculate its apparent impedanceZa from the equation

Zc being

R being the resistance, C the capacitance, L the inductance, G the conductance, Zu the low impedance of infinite value for a line in good condition, Wthe period 2Pf, fthe frequency of the current, 1 the length ofthe line, n the propagation constant according to

Then during its use in the field, its metal armour is connected to a load and measuring circuit, a low frequency current is injected periodically and the value of the instantaneous apparent impedanceZa is measured whose differencewith respectto the initial apparent impedance meanswear ordamage ofthe insulating sheath.

Is should be noted thatthe apparent impedance Za varies in the following way. In the case of an open circuit attheend of dwell insulated line in good condition, the value ofthe return current 12 is O, Zu = and Za = Zc.

cth n 1. It is then easy to verify that in the case of an ideal lines is equivalent to a pure capacity. The line is predominantlycapacative in the real case. In the case of a short circuit due to piercing of the external insulating sheath, the voltage at this position isV2 = 0,Zu = 0 and Za = Zc.thn 1.Za is equivalent to a pure inductance. The circuit is predominantly inductive in the real case.

When the line is connected to a load impedance Zu, V2 = Zu-12 and the apparent impedance Za may be calculated from the above mentioned formula.

In practice, for detecting a defect in the insulating external sheath of the umbilical, the behavior of the electric line formed bythe metal armouring (conductor 1), the external insulating sheath and the conducting liquid (conductor2) is examined. The metal sheath being normally insulated, damage to the external insulating sheath will result in a lowering ofthe resistance between the metal sheath and the conducting liquid, representing more particularly the case of a line connected to a load impedance,whereas breakage ofthe external insulating sheath will result in the metal sheath coming into contact with the conducting liquid, representing more particularly the above mentioned case of the short circuit.

Often the medium in which the umbilical is immersed in its service side is not a homogeneous medium and the line constants R, C, 1 and G are difficult to estimate for they vary, because of the presence of metal structures along the umbilical and particularly the line resistance R of the return conductor, the inductance L and capacity C as a function of the forms and relative positions ofthe conductors.

To take these variations into account, it is preferable to operate as follows.

Aftercalculating the apparent impedance of the line in good condition as mentioned above, the line is immersed at its service site and its apparent impedance is measured so as to checkwhetherthe result ofthe measurement conforms to the result of the theoretical calculation effected previously and, in the case of non conformity, the values ofthe line constants are rectified, which depend more particularly on the environment formed by the immersion medium.

The method ofthe invention allows not only the presence of a defect in the insulating sheath to be detected but this defect can also be located with respect to the distance which separates it from the line head.

For this, the value of the load impedance Zu and ofthe length 1 are varied by successive approximations until the apparentimpedance calculated is equal to the apparent impedance measured, the value of 1 thus found corresponding to this distance from the line head.

The invention also provides forthe detection and location or of wear in insulating sheath of a line passing through several environmental media in its service site, each medium being electrically homogeneous but different from the others.

Forthis, the line is virtually broken up into a succession of sections, each corresponding to a homogeneous service site, for each section its line constants R, C, L, G; R1, C1, L1, G;Rn ,Cn 1,Ln ,Gn 1nRn,Cn,L,Gnare previously determined for calculating the apparent impedance of each section Zar, Zan , Zan. A start is made with the last section by connecting the whole line to a load impedance Zu and the apparent impedanceZan seen from the head of section n is calculated from constants Rn,Cn,Lcand Gnat the length Ln. Forthe last but one section n-l,the apparent impedance Zan 1 at the head of the section n-l is calculated from Run 1, Con 1, On 1, Gn "1n using as load impedance valueforthis section the value ofZa.Thus, progressivelythe initial apparent impedance ofthe line is calculated by assigning to each section a load impedance equal to the calculated apparent impedance ofthe preceding section, that isto say having a higherindex(n for n-l, n-l for n-2 etc).

During immersion ofthe line,forZu equal to ocfora line in good condition, it is checked thatthecalculated apparent impedance is equal to the measured apparent impedance and in the case of non conformitythe values are adjusted in accordance with the above mentioned method.

Then, a lowfrequencycurrent is injected at the head ofthe line in the metal armouring andtheapparent impedance measured at the head ofthe line, the difference between the measured apparent impedance and the initial calculated apparent impedance signifying wear of the external sheath.

Depending on the differences between the values of calculated Za and measured Za at the head of the line, the defective part of the line is located. The calculation istightened up by considering the section situated in this part and the part where the defect is located is searched for by successive approximations, by approximating the calculated apparent impedance as close as possible to the measured apparent impedance and the position ofthe defect is approximately derived therefrom.

BRIEF DESCRIPTION OFTHE DRAWINGS The method of the invention will be better understood from the example illustrated by the accompanying drawings, in which: Figure lisa graphic representation ofthe elementary line model, Figure2showsthe load circuit connected to the metal armouring of a line plunged in sea water and intended to be connected to a measuring apparatus, Figure 3shows a diagram of the apparent impedances for a line of several sections with defects appearing at the end of sections n, n-l, n-2, etc and Figure 4gives a representation of the apparent impedance seen from the line head formed of two sections for different values and positions of a defect located in the second section.

DESCRIPTION OF THE PREFERRED EMBODIMENT The line constants are determined in the following way. The line resistance R is placed in series with the line resistance ofthe metal armouring, measured possibly before installation ofthe line, and with the resistance of the return conductor, that is to say of the conducting liquid medium.

The line capacity C is a capacity between the metal armouring and the return conductor, which may be estimated from measurements made on an insulating sheath sample, at several current frequencies, which measurements are reduced by calculation to the real dimensions of the umbilical by means of known formulae.

The line inductance L is the sum of the inductances proper to the conductors and mutual inductances between the outgoing and return conductors. They may be calculated from the geometry of the conductors and their nature.

The line conductance G is the conductance between the metal armouring and the return conductor, which may be estimated from measurements made on an insulating sheath sample, at several currentfrequencies, which measurements are reduced by calculation to the real dimensions of the umbilical by means of known formulae.

Once the values or R, C, Land G have been determined as mentioned above, these results are confirmed by measurements on an immersed umbilical in a known environment. The apparent impedance of an umbilical section is measured underthese conditions and the values of the parameters are adjusted on which uncertainties exist, particularly those influenced by the environment such as the inductance L. It will be considered that the new values of R, C, Land G represent the umbilical in the service environment.

For carrying outthese measurements, the metal armouring 1 of line 1 (Figure 1) is bared atthe line head, itis connected electrically to a circuit comprising a low frequency current generator whose opposite pole is connected to the conducting liquid medium 2. Conductors attheterminals of a resistor Rand a conductor connected to said opposite pole ofthe generator allow the voltage V and the current I to be measured and the modulus and the phase shiftofthe apparentimpedanceZa to be worked outfrom these measurements.

Thus, by using the above mentioned circuit the apparent impedance (modulus) corresponding to the case Zu = X may be measured. This measurement serves in the step of checking the line constants for comparing the values of the calculated and measured impedances and rectifying if need be the values ofthese constants.

The measuring circuit and apparatus remaining installed in the service site, a low frequency current is injected into the metal armouring and the apparent impedance is measured. When a variation of apparent impedance is observed corresponding to the appearance of a defect in the insulating sheath, the apparent impedance is calculated by varying the position (parameter 1 in the formula of Za) ofthe defect by successive approximations, until a value of 1 is found again corresponding to the equality ofthe value ofthe measured impedance with the calculated one.

When it is a question of a line of which several (four in the diagram of Figure 3) sections passthrough homogeneous media different from each other from the electric point of view, the line constants are determined previously on sections n, n-l,n-2 and n-3 whose lengths are known, as described above.

During immersion of the line in the service site and connection of the armouring to the load and measuring circuit indicated above, the setting ofthe line constants is checked.

Should a defect appear, the apparent impedance Za is calculated atthe head ofthe lineforshortcircuits appearing at the ends of each section; the calculated values are plotted on a diagram (Figure 3) representing as ordinates the imaginary component of the impedance (the inductive component Lw and the capacitive component 1/Cw) and as abscissa the real component R ofthe impedance. Then the impedance Za measured at the head ofthe line is plotted on the diagram.Depending on the position where this measured Za point is situated,the identity ofthe section having this defect is deduced, in the case shown, in the diagram, the defect is located in the last but one section n-l . Location ofthe defect may be further refined by varying by successive approximation calculations the position and the value of the defect in the incriminated section until the measured value of Za is found again. Figure 4 is a graphic representation example ofthe results ofcalculations carried out in the case of a line formed oftwo sections, with section number 2 defective.

The calculations thus made allow tables to be plotted which are curves plotted respectively for distances separating the defects from the head of the line by 0, 40, 80, 120, 160 and 200 m (iso-position curves). Then,the points of these curves corresponding to a same value of the defect in ohms, are connected together, respectively ofO (short circuit), 2,5,10,20 and x ohms (iso-importance curves). As polar coordinates (modulus and argument) the measured impedance is plotted on such a diagram and the defect is located. Thus, in the case represented,the measured impedance Za, 9.6 ohms and 44.5 , point indicated by arrow D corresponds to a defect of 1 ohm situated at 100 m in the second section.

Claims

1. A method of detecting wear in a line having an external insulating sheath containing a hydraulic duct, an electriccableora combined electric-hydraulic bundle, said ductand said cables or combined bundles being provided with an inner metal armouring immersed in a conducting medium, wherein the line constants R. C. L and G fora line in good condition are determined forcalculating its apparent impedance Za according to the equations:

R being the resistance, Cthe capacity, Lthe inductance.G the conductance, Zcthe characteristic impedance, Zu the load impedance,wthe pulsation 2irf, fthefrequency of the current, Lthe length of the line, n the propagation constant according to

then during its use on the service site, its metal armouring is connected to a measuring apparatus, a low frequency current is periodically injected and the value of its apparent impedance Za is measured, whose differencewith respectto the initial apparentimpedance means wearor damage tothe insulating sheath.

2. The method asclaime in claim 1,wherein, after calculating the apparent impedance ofthe line in good condition, it is immersed in its service site and its apparent impedance is measured for checking whetherthe result ofthe measurement conforms to the result of the theoretical calculation effected previously and, in the case of non conformity, the line constantvalues are rectified which depend more particularly on the environment formed by the immersion medium.

3. The method for detecting and locating wear in a line as claimed in claim 1, wherein after detecting a variatiorZthe apparent impedance which means wear or damage to the insulating sheath of a line immersed on its service site, the value ofthe load impedance Zu and ofthe length 1 ofthe line are varied by successive approximations until the calculated apparent impedance is equal to the measured apparent impedance, the value of 1 thus found corresponding to the location of the position of wear of the sheath.

4. A method of detecting and locating wear in a line running in its service site through several environmental media, each medium being homogeneous electrically but different from the others, acclaimed in one or more of claims 1 to 3, wherein the line is virtually broken up into a succession of sections, each corresponding to an homogeneous environment, for each section the line constants R, C, L, G;R1, C1, L1, G1 are previously determined and soon then, when the line is immersed in its service site, its line head armouring is connected to a measuring apparatus and its apparent impedance is measured so asto checkwhetherthe result of the measurement conforms to the result of the theoretical calculation made previously and, in the case of non conformity, the line constant values are rectified which depend more particularly on the environment, a low frequency current is injected periodically during its use on the service site at the head of the line, the apparent impedance ofthe line is measured whose difference with respect to the initial apparent impedance means the wear of the sheath, then the value of the load impedance Zu is varied by successive approximations for variable line lengths L, until the calculated apparent impedance atthe head of the line is as close as possible to the measured apparent impedance, calculated impedance at the head of the line being obtained by calculating the apparent impedance ofthe section n loaded bythe load impedanceZu and using thevaluethus calculated for determining the apparent impedance of the preceding section n-1 and soon progressively up to the head ofthe line, the value of 1 thus found corresponding to the location of the defect of the sheath.