Method and device for monitoring a submarine cable

Device for monitoring a submarine cable (1) comprising at least one optical fiber (2) which is arranged in or on the submarine cable (1), at least one laser light source (3), the light of which can be coupled into the optical fiber (2), wherein portions of the light back-scattered in the optical fiber (2) can be coupled out from the optical fiber (2), detection and evaluation means (5) capable of detecting the back-scattered light and determining from the detected light spatially resolved the temperature of the optical fiber (2), detection means (6) for the electric current flowing in the submarine cable (1), evaluation means (7) capable of storing the time profile of the detected temperature and the time profile of the detected electric, wherein the evaluation means (7) are capable of calculating from the time profiles of the temperature and the electric current spatially resolved the thermal resistance of the soil surrounding the submarine cable (1) and inferring from the spatially resolved determined thermal resistance of the soil the cover height of the submarine cable (1).

This is an application based on and claiming priority to DE 10 2015 105 241.5, filed on Apr. 7, 2015 and DE 10 2015 109 493.2, filed on Jun. 15, 2015, each of which is herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a device for monitoring a submarine cable, which is used in particular to transport energy.

Definitions: In the present application, covered height of the submarine cable refers to the height of soil material arranged on top of the submarine cable, which is usually sand.

Submarine cables are increasingly being used to transport large quantities of energy of wind farms or between different countries. To secure the cable against dropped anchor and similar events, the submarine cable must typically be covered with a layer of sand having a particular statutory minimum layer thickness of sand, for example, in the order of 1.5 m. If the sand layer is completely missing, the cable can be moved by ocean currents and is thus also subjected to increased mechanical wear.

To date, no technical solutions are known for permanently monitoring the layer thickness of the sand above the submarine cable or when the sand above the submarine cables is flushed away. Therefore, inspections using diving robots and similar complex equipment must be performed in regular intervals.

The problem underlying the present invention is thus to provide a method and a device that enable reliable and cost-effective monitoring of the submarine cable.

BRIEF SUMMARY OF THE INVENTION

This is achieved according to the invention with a method having the features of claim1and with a device having the features of claim9. The dependent claims relate to preferred embodiments of the invention.

According to claim1, the method includes the following method steps:The time profile of the temperature of the submarine cable is determined spatially resolved with a fiber optic system to obtain a distributed temperature measurement,The time profile of an electric current flowing through the submarine cable is determined,The thermal resistance of the soil surrounding the submarine cable is calculated spatially resolved from the determined time profiles of the temperature and the electric current,The covered height of the submarine cable is inferred from the spatially resolved calculated thermal resistance of the soil.

In this way, the complex solutions known from the prior art, such as the regular inspections with diving robots, can be dispensed with. For this purpose, the fiber-optic system may include, for example, an optical fiber mounted on or in the submarine cable which extends, in particular, over the entire length of the submarine cable. With the aid of the optical fiber, the fiber-optic system can determine spatially resolved the temperature of submarine cable. In particular, a change of the thermal resistance can be detected at a specific location of the submarine cable by continuously determining the temperature and the electric current flowing through the submarine cable, which in turn allows conclusions about a change in the cover of the submarine cable with sand at the corresponding location.

The time profile of the temperature and/or electric current is determined over a period of at least one hour, in particular at least one day, preferably several days, for example one week. Specifically, the thermal resistance of the soil surrounding the submarine can be determined relatively accurately when the electric current during this period, in which the time profile of the temperature and/or the electric current is determined, changes.

The thermal resistance of the soil may be calculated for a plurality of locations along the length of the submarine cable, in particular for more than half of the submerged length of the submarine cable, preferably for the entire submerged length of the submarine cable. The thermal resistance of the soil can hereby be calculated simultaneously for the plurality of locations along the submarine cable, wherein in particular the thermal resistance of the soil is continuously calculated. This allows a continuous and real-time determination of the thermal resistance of the soil surrounding the submarine cable along the entire submerged length of the submarine cable, thus enabling reliable monitoring of the cover of the submarine cable.

An algorithm may be used for evaluating of the determined time profiles of the temperature and the electric current which adapts as variable parameters the thermal resistance of the soil and the temperature of the environment of the submarine cable to the measured time profiles of the temperature and the electric current. Here, the fit of these environmental parameters in the thermal model can be used to continuously determine of the sand layer thickness along the submarine cable. In particular, the outer thermal resistance is a function of the sand layer thickness and can therefore be used for its determination.

The invention thus uses the approach of determining the height of the cover from the thermal resistance around the submarine cable. It has been explicitly shown that the locations with insufficient sand layer thickness are not always found where the lowest temperatures are measured, because low temperatures can also be caused by cold water currents and are thus not necessarily a sign for an inadequate sand layer thickness. Furthermore, when only the lowest temperatures are determined, a measure for the layer thickness, which would trigger maintenance measures, would still be lacking.

The fiber optic system for distributed temperature measurement may be based on Raman or Brillouin scattering. These represent widely used, proven measurement methods, which have high accuracy and reliability.

According to claim9, the device for monitoring a submarine cable includesAt least one optical fiber for a spatially resolved temperature measurement, wherein the optical fiber is arranged or can be arranged in or on the submarine cable,At least one laser light source whose light can be coupled into the optical fiber, wherein portions of the light generated by the laser light source and back-scattered in the optical fiber can be coupled out of the optical fiber,Detection and evaluation means that detect the back-scattered light and are capable of determining from the detected back-scattered light spatially resolved the temperature of the optical fiber,Detection means for the electric current flowing in the submarine cable,Evaluation means capable of storing the time profile of the spatially resolved determined temperature and the time profile of the detected electric current, wherein the evaluation means are capable of calculating from the time profiles of the temperature and the electric current spatially resolved the thermal resistance of the soil surrounding the submarine cable and inferring from the spatially resolved determined thermal resistance of the soil to the covered height of the submarine cable.

With such a device, the cover of the submarine cable can be reliably monitored, wherein the device is particularly suited to carry out the method according to the invention.

Additional features and advantages of the present invention will become apparent from the following description of preferred exemplary embodiments with reference to the accompanying drawings, which show in:

DETAILED DESCRIPTION OF THE. INVENTION

In the figures, identical or functionally identical parts are provided with identical reference numerals.

Submarine cables can be constructed in different ways. Submarine cables for transmitting AC currents contain all three phases together. The optical fibers for linear temperature measurement along the submarine cable can have different fiber positions.

In addition to the position in the center of the submarine cable between the individual phases, laterally offset positions in direct contact with two of the three phases, as well as in the fill material of the submarine cable are also feasible. Furthermore, the optical fiber can also be located in the wire shield or in direct contact with the outer sheath of the submarine cable.

Alternatively, three individual submarine cables may also be installed, wherein each individual submarine cable has one respective electrical conductor. However, this is not implemented in practical applications due to the significantly greater complexity of the installation. In the case of DC transmission, two single-phase submarine cables are normally used. The optical fiber serving as the temperature sensor is here typically located in the wire shield, or in direct contact with the outer sheath.

In addition to a direct installation on the seabed, submarine cables may also be installed inside an additional pipe (for example, made of HDPE or concrete). This enclosure is installed in particular in areas of a transition from sea to land, and vice versa, or in areas of shallow water depth. The detailed description regarding dimensions and materials can be stored in software used by the method according to the invention, wherein the algorithm is capable of performing an analysis of the surroundings outside the enclosure.

FIG. 1shows schematically a submarine cable1which extends beyond the end shown at the right side ofFIG. 1, in particular beyond several kilometers. The submarine cable1is to be used to transport energy, so that strong electric currents will flow in the submarine cable1. An optical fiber2is arranged in the submarine cable1, which extends in particular over the entire length of the submarine cable1installed below seabed.

FIG. 2andFIG. 3show two exemplary arrangements of the optical fiber2in the submarine cable1. Several cables8, which are surrounded by a common cable shield9, are shown in the schematically depicted submarine cable1. In the first example depicted inFIG. 2, the optical fiber2runs in the cable shield9. In the second example depicted inFIG. 3, the optical fiber2runs in the gusset of the cable8.

FIG. 4shows an example of a submarine cable1in a less schematic view. This submarine cable1has three phases and is used for AC transmission. Three individual cables8are arranged in the submarine cable1, each cable8having a central conductor10for one of the three phases. The center conductor10is surrounded by an inner conductive layer11, an insulation12, an outer conductive layer13and a metallic shield14.

When using the three cables8for the three phases of AC current, the inner and the outer conductive layers11,13need not be used for the transmission of electric current, although one of the conductive layers11,13can serve as the neutral conductor. However, this is not required. Furthermore, each of the metallic shields may be connected to ground potential.

The three cables8are together surrounded by a wire screen15serving as reinforcement which is in turn surrounded by an outer sheath16. The wire screen15serving as reinforcement and the outer sheath16together form to the cable sheath9.

FIG. 4also shows various alternative arrangements of the optical fiber2in the submarine cable1. As inFIG. 3, the optical fiber2may be arranged in the gusset8the cables8. Furthermore, the optical fiber2may be disposed at other locations of the interior space of the submarine cable1. The optical fiber2may also be arranged, in analogy toFIG. 2, in the cable shield9and in particular in the wire screen15which serves as a reinforcement. The optical fiber2may also be arranged outside and in direct contact with the outer sheath16.

FIG. 5also shows an example of a submarine cable1in a less schematic view. This submarine cable1has only a single phase and is used for a DC transmission. A single cable8having a center conductor10for one phase is arranged in the submarine cable1. The center conductor10is surrounded by an inner conductive layer11an insulation12, an outer conductive layer13, a wire screen15serving as reinforcement and an outer sheath16. The wire screen15serving as reinforcement together with the outer sheath16form the cable shield9.

FIG. 5also shows various alternative arrangements of the optical fiber2in the submarine cable1. Like inFIG. 2, the optical fiber2may be arranged in the cable shield9and in particular in the wire screen15serving as reinforcement. The optical fiber2may also be arranged outside and in direct contact with the outer sheath16.

The light of a laser light source3can be coupled into the optical fiber2by using suitable coupling means4. Individual portions of the light can be back-scattered in the optical fiber2by way of temperature-dependent Raman or Brillouin scattering (seeFIG. 1). The back-scattered portions can be supplied by the coupling means4to detection and evaluation means5which capture the scattered light and determine from the detected back-scattered light spatially resolved the temperature of the optical fiber2. The fiber-optic system for distributed temperature measurement composed of the optical fiber2, the laser light source3the coupling means4and the detection and evaluation means5is known in the art.

The temperature of the optical fiber2is thus determined by the detection and evaluation means5, in particular simultaneously and continuously, at any location of the optical fiber2, and hence at each location of the submarine cable1.

FIG. 1also shows schematically detection means6for the electric current flowing in the submarine cable1. Any type of customary current measuring devices can be used as detection means6.

The device further includes evaluation means7connected to the detection means6and the detection and evaluation means5. The evaluation means7can store the time profile of the spatially resolved determined temperature and the time profile of the detected electric current.

Furthermore, the evaluation means7are capable of calculating the spatially resolved thermal resistance of the soil surrounding the submarine from the time profiles of the temperature and the electric current. This is achieved by determining with a suitable algorithm the thermal resistance and the ambient temperature of the submarine cable1as variable parameters in the adaptation of a suitable thermal model to the time profiles of temperature and electric current. The time profiles used herein may span, for example, a period of one week.

It is essential when using such a method that the electric current changes during the particular period. Optionally, a change of the electric current over time may be predefined to make the determination of the thermal resistance reliable.

The employed algorithm takes into account that external parameters, such as in particular the thermal resistance of the soil and the ambient temperature, are subject to seasonal fluctuations.

The method according to the invention enables the use of the fiber optic system for distributed temperature measurement and described calculation methods for the continuous and simultaneous determination of the thermal resistance at any location of the submarine cable.

The evaluation means7can infer from the spatially resolved determined thermal resistance of the soil the covered height at each location of the submarine cable1. To this end, data are stored in the evaluation means7or in the storage means associated with the evaluation means7which reflect the dependence of the thermal resistance around a submarine cable1on the layer thickness of a cover on the submarine cable1.

The evaluation means7can determine, in particular continuously and simultaneously, the thickness of the cover at any location of the submarine cable1

The evaluation means7can indicate to the user and/or trigger corresponding service responses when the evaluation means7determine that a part of the submarine cable1is no longer adequately covered with sand.

The temperature distribution and the time profile during cyclic load behavior in submarine cable1can be calculated with the equivalent circuit diagram shown inFIG. 6. Heat sources to be considered are here current sources Wi, thermal resistances as electric resistances Riand heat capacities as capacities (neglected here). Various thermal losses operate in a power cable, depending on whether it is operated with DC or AC current. Dielectric losses are generated in the insulator and eddy currents are generated in the metallic wire shield. Depending on the load condition and the operation of the cable, these losses contribute to different degrees to the heating of the power cable.

The temperature distribution from the electrical conductor (hot) to the environment (cold) is hence viewed like the drop of a voltage across a resistor and thus produces the temperature in the respective cable layer. The diagram was augmented with software used by the method according to the invention for the environment that is designated as soil or R4, respectively. The temperature of the optical fiber2is designated with T(DTS).

Whether the submarine is still covered and with how much protective material, is determined with an algorithm based on T(DTS)for a corresponding load behavior of the electric current. As a result, the ambient temperature T(water)and the thermal resistance of the seabed R4are obtained.

With increasing installation depth, the temperature difference at equilibrium (“steady state”) between the measured temperature of the DTS system (distributed feedback) or the temperature measured by the optical fiber2, for example, between the optical fiber2at the outer shield of the cable and the water temperature, increases due to the thermal resistance of the soil.

FIG. 7shows an exemplary dependence of the temperature difference between the temperature of the optical fiber2, on the one hand, and the water, on the other hand, on the installation depth of the submarine cable1or on the layer thickness of the cover of the submarine cable1. If the temperature of the outer shield on which, for example, the optical fiber2is arranged is approximately equal to the water temperature in the equilibrium state, then the temperature of the outer shield is already well above the ambient temperature for an installation depth of 2 m.

The employed algorithm solves the full equivalent circuit diagram schematically illustrated inFIG. 6by using an up to seven-day history for the ambient parameters ambient temperature T(water)and thermal resistance of the seabed R4for a fit that optimizes both parameters based on a least-square fit. Initial values for the fit are optionally also obtained from additional sensors. The planned installation depth of the power cable in the seabed is stored in the software. When the submarine cable1is increasingly exposed, for example, due to ocean currents, the changed environment of the submarine cable1also changes the transient temperature behavior. The algorithm attributes this to an increasingly lower thermal resistance of the soil. As seen fromFIG. 8, this effect is particularly pronounced before the submarine cable1is exposed in the ocean. A steady decrease of the installation depth can then also be observed.

This analysis can be performed in regular time- and location-dependent intervals, thus performing an analysis of the installation depth of the entire submarine cable1.

Sea beds can be composed of different materials and can thus have different thermal properties. Table 1 lists various types and describes the possible range of thermal conductivity or specific thermal resistivity. The table is taken from the publication “Cable connections within the offshore wind farm Arcadis East 1. Thermal and magnetic emissions” by Heinrich Brakelmann, Rheinberg, 2010.

Since submarine cables typically have a length of 50 km and more, the submarine cable can be surrounded by different types of sea beds. In principle, the employed algorithm is able to determine the installation depth for different types of sea beds. Additional information (geological reports, results from sand samples, and the like) and a specific association of this information with different sea beds along the cable make it possible to achieve maximum accuracy.