Abstract:
A permanently installed monitoring system for corrosion under insulation (PIMSCUI) and a method for performing the monitoring of said system are provided. The present invention attains the objective of the invention by a fibre optic cable permanently mounted between walls of a pipeline and pipeline insulation surrounding the pipeline and placement of acoustic emitters along the length of the pipeline in mechanical contact with the optical fibre. The acoustic emitters send a pulsed acoustic signal towards the pipeline which is received by the optical fibre, the acoustic signal subsequently travels through the pipeline wall, reflecting from the inner diameter of the pipeline before the reflected pulse is received at the optical fibre.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The invention relates to corrosion monitoring in general and more specifically a permanently installed monitoring system for corrosion under insulation (PIMSCUI) and a method for performing the monitoring of said system. 
         [0003]    2. Background Art 
         [0004]    Pipeline and process vessel corrosion under insulation is an enormous and costly problem across northern European oil and gas installations, and similar weather related challenges are likely to face other growing markets around the world such as those in Canada and Brazil. At present CUI detection is a slow manual process requiring skilled inspectors with costly inspection tools. There are no permanent monitoring systems available suitable for CUI purposes. Enormous direct cost reductions could be achieved through better decision making enabled by permanent monitoring technology. Savings would be magnified through the reduction of indirect costs such as CUI related shutdowns. 
         [0005]    From prior art one should refer to US patent application US20100141281 based on WO/2009/002180 by Johnsen disclosing a fibre optic based humidity measurements. The problem is that such a measurement systems lacks a mechanism to confirm that corrosion has taken place. Furthermore the system of WO/2009/002180 refers the implementation of a grid or line of discrete and spaced humidity sensors, and as such the humidity can only be measured where the discrete sensing elements exist 
         [0006]    References should also be made to US 2012294124 A1, regarding monitoring of corrosion in a pipe. 
         [0007]    Finally from prior art one should also refer to US 2012092960 A1, regarding a system having distributed acoustic sensors wherein an optical fibre is used for distribution of signals. 
       DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0008]    Therefore, a main objective of the present invention is to provide a permanently installed monitoring system for corrosion under insulation (PIMSCUI) and a method for performing the monitoring of said system. 
         [0009]    In one aspect a permanently installed fibre optic technology for monitoring corrosion under insulation (CUI) continuously over large surface areas is provided. The technology revolves around discrete or continuous moisture under insulation monitoring using fibre optics, alone or in combination with direct measurements for CUI. The direct measurements can either be spot measurements or quantities averaged over any given length of the installation. The technology has extremely low installation overhead demands: minimal power cabling is required, and signal transmission is through the fibre optic sensing cable. 
       Means for Solving the Problems 
       [0010]    The objective is achieved according to the invention by a sensor system for corrosion monitoring as defined in the preamble of claim  1 , having the features of the characterising portion of claim  1 , and by a sensor system for corrosion monitoring as defined in the preamble of claim  1 , having the features of the characterising portion of claim  1   
         [0011]    The present invention attains the above-described objective by a fibre optic cable permanently mounted between walls of a pipeline and pipeline insulation surrounding the pipeline and placement of acoustic emitters along the length of the pipeline in mechanical contact with the optical fibre. The acoustic emitters send a pulsed acoustic signal towards the pipeline which is received by the optical fibre, the acoustic signal subsequently travels through significant depths of the pipeline wall, if desired up to the extent that said wave is reflecting from the inner diameter of the pipeline before the reflected pulse is received at the optical fibre. 
       Effects of the Invention 
       [0012]    A technical difference over WO/2009/002180 is the use of acoustic emitters. The effect of this is the ability to use acoustic means to probe corrosion conditions while the fibre is used to act as an acoustic signal receptor. A second technical difference is that the humidity sensing elements need not necessarily be discrete, and could give a spatially continuous measurement profiles over significant lengths. 
         [0013]    Furthermore the chemical sensing capability of the invention described here extends to one or more of a family of corrosion related parameters comprising of humidity, liquid water, pH, conductivity, salinity and hydrogen. 
         [0014]    These effects provide in turn several further advantageous effects:
       it makes it possible to confirm that corrosion has taken place irrespective of current or previous level of humidity   it makes it possible to confirm the likelihood that corrosion has taken place irrespective of the current or previous level of humidity   it makes it possible to probe the humidity at all points along the fibre, with a resolution limited only by the instrumentation.   it makes it possible to probe a range of corrosion related chemical parameters at all points along the fibre, with a resolution limited only by the instrumentation.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an [exemplary] embodiment of the invention given with reference to the accompanying drawings. 
           [0020]    The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings, wherein: 
           [0021]      FIG. 1  shows an embodiment for distributed monitoring of corrosion under insulation. 
           [0022]      FIG. 2  shows an embodiment for pipeline/vessel wall-thickness measurement. 
           [0023]      FIG. 3  shows an embodiment for fibre used for spatially continuous measurement of one or more of a group of corrosion related chemical parameters comprising humidity measurement, liquid water, pH, conductivity, salinity and hydrogen. The cladding is partially removed or partially tapered, and replaced with a humidity sensitive material, or a material sensitive to one of the chemical parameters in the previously mentioned group 
           [0024]      FIG. 4 . Shows an embodiment for fibre used for spatially continuous humidity measurement. The cabled optical fibre is surrounded with a humidity sensitive material. 
       
    
    
     DESCRIPTION OF THE REFERENCE SIGNS 
       [0025]    The following reference numbers and signs refer to the drawings: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 100 
                 Measurement system 
               
               
                 200 
                 Pipeline/Vessel wall 
               
               
                 210 
                 Pipeline/Vessel inner wall 
               
               
                 220 
                 Pipeline/Vessel outer wall 
               
               
                 250 
                 Pipeline/Vessel insulation 
               
               
                 260 
                 Pipeline/Vessel insulation inner wall 
               
               
                 270 
                 Pipeline/Vessel insulation outer wall 
               
               
                 300 
                 Optical fibre 
               
               
                 310 
                 Optical fibre core 
               
               
                 320 
                 Optical fibre cladding 
               
               
                 330 
                 Humidity sensitive material, or material sensitive to one or more of 
               
               
                   
                 liquid water, pH, conductivity, salinity and hydrogen 
               
               
                 340 
                 Protective jacket 
               
               
                 400 
                 Acoustic emitter 
               
               
                 410 
                 Piezo element 
               
               
                 420 
                 Controller 
               
               
                 430 
                 Power source 
               
               
                 440 
                 Clamp 
               
               
                 450 
                 Excited beam 
               
               
                 460 
                 Reflected beam 
               
               
                 500 
                 Remote signal receiver 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0026]    Various aspects of the disclosure are described more fully hereinafter with reference to the accompanying drawings. This disclosure may, however, be embodied in many different forms and should not be construed as limited to any specific structure or function presented throughout this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Based on the teachings herein one skilled in the art should appreciate that the scope of the disclosure is intended to cover any aspect of the disclosure disclosed herein, whether implemented independently of or combined with any other aspect of the disclosure. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to or other than the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim. 
         [0027]    The invention will be further described in connection with exemplary embodiments which are schematically shown in the drawings, wherein  FIG. 1  shows a typical embodiment of a measurement system  100  installed on a pipeline  200  having an inner wall  210  and an outer wall  220 . The pipeline is further provided with a pipeline insulation  250  having an outer wall  270  and an inner wall  260  preferably in close contact with the pipeline outer wall  220 . 
         [0028]    An optical fibre  300  is positioned between the outer wall  220  of a pipeline and the inner wall  260  of the pipeline insulation  250  surrounding the pipeline  200 . A fibre comprises typically a core  310  surrounded by a cladding  320 , see  FIG. 3   
         [0029]    At least one acoustic emitter  400  is positioned along the length of the pipeline in mechanical contact with the optical fibre. Each acoustic emitter comprises means for emitting an acoustic signal, typically a piezo element  410 , a means for providing operating power, typically a battery  430  and a clamp  440  for providing mechanical attachment. 
       Principles Forming the Basis of the Invention 
       [0030]    Each acoustic emitter sends an excited beam  450  of a pulsed signal towards the pipeline  200  (see  FIG. 2 ) which is received by the optical fibre  300 , the acoustic signal subsequently travels through significant depths of the pipeline wall  270 , if desired up to the extent that said wave is reflecting from the inner wall  260  of the pipeline before the reflected pulse is received at the optical fibre as the reflected beam  460 . The inward travelling and back reflected acoustic signals are converted to the optical properties of the light travelling through the fibre at the points the signals are received. These signals are then transmitted to a remote location. The time difference between the detection of the inward and outward travelling pulses is used to calculate the path of the acoustic pulse through the pipeline wall, which is a function of the pipeline wall thickness. The thickness of pipeline walls is a well-known quantity for the determination of the extent of pipeline corrosion, and forms the basis of various inspection and monitoring technologies [Clampon 2012, GE 2012]. The position and form of the acoustic emitters and optical sensing elements could be adjusted in order to excite and detect arbitrary acoustic modes, including lamb waves, controlling the sensitivity and coverage area of the node. 
         [0031]    The excited signal is emitted from the acoustic unit, typically by a piezo element. The excited and reflected signal are detected by the fibre, typically a suitable optical method for converting the acoustic signals to fibre transmitted optical signals include the use of fibre Bragg gratings. Other methods for encoding the acoustic signal include, but are not limited to, the use of Distributed Acoustic Sensing (DAS, see Optasense 2012) or elements introducing microbend losses into the fibre, detectable using e.g. optical time domain reflectometry (OTDR) methods. 
       Best Modes of Carrying Out the Invention 
       [0032]    Pipeline wall thickness measurements could be made at various points along the pipeline with the use of multiple acoustic emitters and optical multiplexing techniques. The acoustic emitters could be user controlled through wireless communication, or operate as independent units activating at predetermined time intervals. 
         [0033]    The clamps  440  should be located such that the inward travelling acoustic wave is effectively transmitted through the optical fibre and into the pipeline/vessel wall. The clamps should also ensure effective transmission of the reflected outwardly travelling acoustic wave into the optical fibre from the pipeline/vessel wall, 
         [0034]    A second property of the system is that the fibre used for transmitting the acoustic signals has optomechanical properties that are a function of the humidity in the surrounding space between the pipeline and insulation. Humidity is a known precursor to the development of corrosion. Materials such as di ureasil (Correia, 2012) and PVA/COCI2 (Cho, 2011) compounds could be used to surround the fibre to give it the desired optomechanical optical properties. Using such materials the ambient humidity could be converted to a loss parameter of the fibre, probed for example using optical time domain reflectometry (OTDR) methods. In such an embodiment, the humidity sensitive material  330  would typically replace part or all of the fibre cladding  320  which would be partially removed or tapered, see  FIG. 3 . Exposed fibre coating or humidity sensitive material could be partially or completely covered with a protective jacket  340  to increase mechanical strength. The protective jacket may or may not be permeable to water. The loss would typically result from a refractive index or absorption change of the humidity sensitive material. Alternatively the loss information could be transferred to cladded (and possibly also cabled) fibre by coating the cladding or cable with a humidity sensitive material, see  FIG. 4 . In such an embodiment the sensitive material would typically have mechanical properties that change with humidity and would be arranged in such a way to induce losses into the fibre as a function of humidity, see  FIG. 4 . Alternatively, humidity could be converted to a strain in the optical fibre, measureable using for example using non-linear optical scattering methods (distributed strain sensing, DSS, see Sensornet 2012). Typically for this type of embodiment the humidity sensitive material would have mechanical properties that change with humidity, and be located on the outside of the cladding (and possibly suitable cable also) in order to prevent loss. The use of OTDR and DSS methodologies would allow for significant lengths of optical cable to act as a single humidity sensor, with each unit length being independently probed for humidity within the limiting spatial resolution given by the light sources, transmission medium and detectors. The described modes for humidity detection also apply to the measurement of one or more of the setup of parameters indicating corrosion; presence of liquid water, pH, conductivity, salinity and hydrogen, where the humidity sensitive material is replaced with suitable material for these purposes. 
       Alternative Embodiments 
       [0035]    A number of variations on the above can be envisaged. For instance the person skilled in the art will see that the acoustic emitters can be operated in two modes. 
         [0036]    In a first mode the wall thickness is measured at a single point. Here the acoustic emitter and receiver, typically the sensitive fibre element, are positioned at substantially the same point along the pipeline, with the acoustic wave propagating substantially perpendicular to the pipeline. The wall thickness is derived from the propagation time of the acoustic wave for the forward and return pass through the pipeline wall. 
         [0037]    In a second mode an average pipeline wall thickness is measured. Here an acoustic emitter positioned at a specific point along the pipeline wall sends a guided ‘lamb wave’ through the pipeline wall to a receiver positioned further along the pipeline. The propagation time of the lamb wave is a function of the average wall thickness between the emitter and receiver. The invention also makes it possible to employ modes in the group containing longitudinal waves, transverse waves, shear wave, Rayleigh waves, Stonely Waves and Sezawa love waves, in order to assess the physical condition of the pipeline. This invention makes it also possible to measure second acoustic reflection indicating solid build-up at pipe inner wall (e.g. sand, scale, asphaltenes, waxes) cracking, pitting, density variations and other material degradation and deficiencies. The fibre can also be used for communications such as for additional instruments communicating through the fibre infrastructure including instruments for measuring salinity and temperature. The fibre can be used as part of a distributed sensing system, specifically those that measure temperature, strain, and acoustic signals. The fibre can also be used for measuring additional parameters that could act as corrosion indicators, such as the presence of liquid water, humidity, salinity, pH, conductivity and hydrogen Additional parameters could also be measured for the purpose of compensating for environmental effects on other corrosion indicator measurements. 
         [0038]    In another embodiment, the invention monitors the wear and degradation of components internal to the pipeline/process vessel. This can be achieved by measuring the evolution in space and time of (i) acoustic signals transmitted through the pipeline wall, (ii) temperature, and (iii) the previously named parameters that act as corrosion indicators. 
         [0039]    In a further embodiment a reference fibre is used, running parallel with the sensing fibre. The reference fibre could be used for example to compensate for the influence of temperature on the sensing fibre. The reference fibre could also be used to compensate for other environmental or system related effects on the sensing fibre. 
         [0040]    In another embodiment the humidity and acoustic sensing elements are on separate optical fibres. 
         [0041]    In yet another embodiment the excited and reflected acoustic signals are detected by a technology other than fibre optics, the detected signal is then encoded on the optical fibre by some means such as an amplified acoustic signal or electro optical protocols. 
         [0042]    In a further embodiment the optical fibres are single lengths, or comprised of discrete sections that are assembled into one continuous length before, during or after the installation process. A modularised embodiment would be particularly compatible with modern insulation systems that comprise many discrete insulating units. 
         [0043]    In a further embodiment, the acoustic emitter and received are replaced with other instrumentation that measures pipe vessel wall material degradation such as pitting, cracking and the presence of hydrogen typically. In such cases the fibre could be used as a medium to transmit data from these instruments to a remote location. 
       REFERENCES 
       [0044]    Clampon 2012, Clampon website; http://www.clampon.com/?page=2&amp;show=7 GE 2012, GE website; http://www.ge-mcs.com/en/ultrasound/corrosion-monitoring/rightrax-automate-lt.html 
         [0045]    Correia, S, F. H. et. al. 2012 “Optical Fiber Relative Humidity Sensor Based on a FBG with a Di-Ureasil Coating” Sensor, 12, 8847-8860. 
         [0046]    Cho, H., et. al. 2011, “Monitoring of Corrosion Under Insulations by Acoustic Emission and Humidity measurement” J. Nondestruct. Eval. 30, 59-63 Johnsen 2010, US patent US20100141281
   Sensornet 2012, Sensornet website, accessible at http://www.sensornet.co.uk/technology/distributed-strain-sensing/index.html Optasense 2012, Optasense website, accessible at http://www.optasense.com/   
 
       INDUSTRIAL APPLICABILITY 
       [0048]    The invention according to the application finds use in continuous monitoring, typically for offshore applications and other places where access is hard or limited such as in nuclear reactors