ferrx electromagnetic steel examination  
ferrx monitoring steel condition for bridge and railway structures

Game changing technology for monitoring integrity of steel structures

ferrx monitoring steel condition of offshore structures

The Ferrx method measures the steel parameters influencing the steel structures condition. Residual service life can be estimated more accurately by the combination of this condition data from critical points with predictive modelling of the whole structure. The steel’s condition parameters stress, material degradation, cracks, corrosion and erosion are measured directly on the steel.

Ferrx combines features of different potential drop techniques that makes it extremely versatile for monitoring all types of stresses and degradations that influence the structure's operational life.



Monitoring traffic loads and condition of bridges

To be able to assess a bridge’s condition and remaining safe service life, the status of the integrity of the structure is crucial information. Residual service life can be estimated more accurately by the combination of data for actual traffic loads and accurate condition data from critical points combined with predictive modelling of the whole structure. Early detection of deterioration of the structure may reduce operational costs as any required maintenance can be planned in due time and optimized with respect to the schedule for repair.

The patented FEMM (Ferrx ElectroMagnetic Method) is well suited for such applications with its unique monitoring features. FEMM measures directly the stress response in the steel structure caused by the traffic loads, and also the stress due to any resonance vibration. Furthermore, it monitors the effect these stresses have on the monitored steel, e.g. changes of residual stresses in welds and material degradation and crack initiation and growth.

Girder bridge


sensor for monitoring of traffic loads
This girder bridge is more than 60 years old. FEMM sensors are retrofitted on the lower and upper flange to monitor traffic loads. The bridge will in periods be used for the E6 traffic with a high number of heavy vehicles that may exceed the bridge classification. For monitoring the highest traffic loads the remanent stress data measured in the lower flange is used. The picture on the left shows the sensor on the lower flange.

Framework bridge


This framework bridge is more than 80 years old. Four FEMM sensors are installed on different truss elements for monitoring different types of stress. The traffic is heavy and the remanent stress is measured for monitoring the highest vehicle load in a period, and also stresses caused by 10Hz resonance vibration. The picture to the right shows one of the sensors.
bridge structural health monitoring

Railway stress monitoring

This system is installed on railway rail measuring stress in the rail for monitoring of the rail′s neutral temperature to prevent buckling-related derailments. The system is powered by solar cell panels, takes daily readings, and transfers data via the mobile network to an Internet server.   




Non-intrusive corrosion monitoring


non-intrusive corrosion monitoring

  FEMM provides non-intrusive monitoring of metal loss in pipes, including all types of corrosion and erosion. The picture shows installation of the sensor matrix on an 8-inch pipe for field application. The whole area covered by the sensor is monitored for internal localized corrosion. One autonomous system monitors three pipes, each with two sensor interfaces.

Fatigue monitoring of subsea riser


fatigue monitoring of subsea riser
  FEMM system installed on an 8" work-over riser pipe for monitoring deterioration / fatigue. It is designed for operation down to 1500m water depth and has an internal battery with capacity for approx. five years of autonomous monitoring. The system for riser inspection has been qualified by DNV-GL in accordance with DNV-RP-A203 Technology Qualification.  

When a new riser pipe is bent or loaded, the first that changes will be any residual stresses. These changes are regarded to have a significant effect on the operational life of the pipe. Furthermore, the external loads induce high stress in hot spots which can lead to crack nucleation. These surface cracks and any cracks inside the material are detected and size reported. If corrosion in the weld root it will be measured with the same sensor and give a different signal response and thus discriminated from cracks in same area.



  The FEMM method and technology were certified in 2016 by DNV GL in accordance with DNV-RP-A203. An FEMM system is used for monitoring all the different parameters just by selecting appropriate location and configuration of sensors to get the best results.

The general monitoring capabilities can be summarized as follows:

  • Stresses
    Measure elastic stress, detect plasticity when stress beyond yield, and measure changes of residual stresses

  • Remanent stress
    Measure the maximum stress since the last measurement

  • Fatigue development
    Measure the different phases of material deterioration until crack formation

  • Crack detection and crack growth monitoring
    Detect surface cracks before becoming visible and internal cracks in e.g. welds

  • Non-intrusive monitoring of internal defects
    Measure metal loss due to erosion and corrosion


Stress monitoring

Stress measurements in ferromagnetic steel are relative to a previous measurement, preferably taken with known stress or load level with sensor installed on location, or measurements on specimen with same type of steel. Calibration curves can be established based on two known loads and stress levels and quantification of measured stress level is thus possible.  

The steel ″remembers″ previous external loads, i.e. the load has changed the magnetization of the steel. This is measured by FEMM and is called remanent stress, which shows the highest stress since the last measurement. This magnetization is wiped out by the new measurement, and the permanent change can then be measured, e.g. permanent stress during the measurement. Plasticity, permanent changes in the material caused by stress exceeding yield, is detected. This is, for example, observed when monitoring areas when residual stress changes due to external loads. Changes of residual stresses related to welds are measured with sensitivity also for changes in depth location.


Fatigue monitoring

The material changes in ferromagnetic steel during a fatigue process are measured by FEMM long before cracks are visible.  

The Fatigue process refers to the irreversible changes in a material or component that is exposed to cyclic loading. These changes in steel are microstructural changes before the onset of macroscopic crack growth. Changes in microstructure are closely related to changes in the density and structure of dislocations. The fatigue process is commonly divided into three stages:

  1. Accumulation of cyclic plastic deformation, which is closely related to bulk material properties
  2. Growth of a macroscopic crack to failure
  3. Formation of slip bands and nucleation and growth of microcracks in the surface

The measured response changes according to the degree of changes in the steel impedance parameters, e.g. when dislocation density increases and later micro-crack density increases and grows to more continuous cracks, the response signals change accordingly. By analyzing the response signal, changes of magnetic permeability and electric resistance are estimated, and based on these parameters the degree of material degradation can be characterized in due time before any crack is visible. Based on many fatigue tests with different welded structures a response pattern has in agreement with theoretical material changes been verified and is used to predict fatigue development.


Crack detection and monitoring

The sensor matrix is located to cover the area where cracks are expected, most likely a weld, and it will be detected whether the crack starts in the outside surface or inner surface of a pipe or even inside the weld or HAZ area.  

The matrix sensing (sensor) direction is oriented perpendicular to the expected crack direction. If this direction is not known, a matrix sensing in both directions can be installed. When the monitoring records a significant crack, the maximum depth can be estimated. Thereafter the crack growth is monitored and reported


Corrosion monitoring

Corrosion internal in pipelines is more or less uneven and the degree of corrosion attack can vary around the pipe’s circumference e.g. be more severe in the bottom section. This makes it advantageous to monitor a continuous area of the pipe to be able to get a representative picture of the distribution.  

In general, the location expected to be most exposed to corrosion is selected for installation of the sensors. Based on the expected type and location of corrosion, e.g. localized attacks in the bottom section of a pipe, a sensing matrix is designed for best sensitivity and coverage. The whole area covered by the sensing matrix is monitored for any internal metal loss. Initially, a measurement is taken at known wall thicknesses and stored and used as a reference for the following measurements, which give the change due to corrosion attacks. When localized metal loss is detected, the actual depth of e.g. a pit can be estimated using the patented FEMM algorithm based on the transient potential drop signal.


Erosion monitoring

Erosion internal in pipes is usually localized metal loss that most frequently occurs in bends and tends to be most severe in the outer radius of the bends, however, due to turbulence also at other locations in the bend. The shape of the erosion varies related to the pipe geometry and internal flow. Both carbon steel and duplex steel can be monitored.  

The sensor matrix is designed for optimized sensitivity based on the expected shape of the eroded area. For example if the erosion is widespread and in outer radius of a bend, the matrix is distributed along the outer radius. If the erosion is expected to be a narrow groove along the outer radius of the bend the matrix can be along the pipe’s circumference which will significantly improve sensitivity. Also a combination of these matrices can be applied in case the shape of the erosion is difficult to predict. Sufficient area is covered to be sure to pick up the most severe attacks. Each Sensor Interface SI has four different sensor matrices which can be located to monitor four separate erosion locations. When metal loss is detected, the actual depth of the deepest attack within a matrix can be estimated



ferrx femm system concept

Example of installation for monitoring of four different areas for stress and cracks in bridge cover plate. One system with eight SIs can monitor 32 locations within approx. 20m. The above system is an autonomous standalone online system with several options for data transfer to the user.

A FEMM system consists of four main modules:
  • Sensor arrangement on the steel structure
  • Sensor Interface unit (SI)
  • Data Acquisition Unit
  • Dedicated SW run under MS Windows

This modularity provides for extensive flexibility to optimize the system configuration and performance for the different applications.

The system design based on many years of relevant field experience has also been optimized with respect to reliability and ease of installation, e.g. there is no steel reference needed, which significantly simplifies the preparation and installation on the steel structures. Sensors and all fixed installed components shall have the same operational lifetime as the monitored structures. The non-retrievable sensor interface is designed for at least 25 years of operation.

The sensor and sensor interface design provides for redundancy and improved reliability. One system comprising one instrument and up to eight sensor interfaces can e.g. monitor one to eight nearby pipes or on a structure up to 32 locations within 20m from the data acquisition unit.

Instrumentation modules


Sensor matrix

non-intrusive monitoring of pipes

The picture shows an example of sensor layout, this one for monitoring a pipe buttweld, and for monitoring defects both in outer and inner surfaces of the pipe wall. The very robust sensor arrangement comprises the sensing electrodes and the excitation wires, both requiring electrical contact with the steel. One or more temperature sensors monitor the structure′s surface temperature. After installation, all the connection points and wires are properly protected for the actual application.  

The sensor matrix layout design and size are based on the expected type of attacks (corrosion, erosion, or cracks), stress or fatigue and the total area to monitor. The required sensitivity for metal loss is a design factor that decides e.g. spacing between electrodes. The number of electrodes and the spacing between electrodes make up the covered area. For example, for even corrosion a few electrodes can cover several square meters of a pipe, while for pitting corrosion shorter spacing is required to obtain good sensitivity. The whole area covered by the sensor matrix is monitored. Different connection methods for making electrical contact to the steel for the sensor arrangement are available: soldering (heat controlled max 300oC), stud-welding, and spring-loaded pin arrays glued or clamped to the steel.


Sensor interface

non-intrusive monitoring of pipes

The Sensor Interface (SI) unit contains circuits for signal conditioning and digitalization and for high current excitation. Each unit handles up to four matrices with a total of 28 sensing pairs and four pairs for current excitation. Communication with the instrument is linked via a multidrop digital bus, with up to eight SI units (altogether 224 pairs) on one cable per instrument and system. The picture shows SI encapsulation for use in onshore applications.  

  • Resolution: 24 bit
  • Number of pin pairs: 28
  • Number of excitation: 4
  • Communication bus: proprietary multidrop
  • Certified for application in Ex Zone 1 when passive. (No battery in SI)
  • Ingress protection: IP68


Portable instrument

non-intrusive monitoring of pipes

This instrument is used for examination of a number of inspection points (tags) fitted with sensor matrices and Sensor Interfaces. The configuration setup for all tags to be measured are downloaded from the PC SW to the instrument before the inspection tour. Data from all measurements are intermediately stored until downloaded to the PC. In this way, a high number of inspection points can be evaluated at low cost. The instrument is powered by a rechargeable battery.  

  • Operating temperature range: -20°C to + 70°C
  • Storage temp range: -40°C to + 85°C
  • Size: width=165mm, depth=175, height=52mm
  • Ingress protection: IP67


Autonomous instrument

ferrx data data acquisition unit for autonomous monitoring

This data acquisition unit for autonomous monitoring in harsh environments is powered by an internal battery or by external power. Operation is offline or online, by cable or wireless for real-time condition data. Miscellaneous protocols are available.  

  • Operating temperature range with external power: -40°C to +85°C
  • Size: width=230mm, depth=240mm, height=110mm
  • Ingress protection: IP68



FEMM data analysis software

ferrx femm steel infrastructure data analysis software

The Windows-based PC software handles measurement data and housekeeping data from the instruments. Features and algorithms are implemented for data pre- and post-processing for the different types of defects. Results can be presented in 2D or 3D graphical plots. Software functions also include programming and configuration of monitored tags and instrument setups.  

  • Operating temperature range with external power: -40°C to +85°C
  • Size: width=230mm, depth=240mm, height=110mm
  • Ingress protection: IP68



Ferrx is an independent company located in Trondheim, Norway with the business idea to provide the FEMM technology for different onshore and offshore markets.
Ferrx patented the method now called FEMM (Ferrx ElectroMagnetic Method), and with support from several oil companies and Innovation Norway and The Research Council of Norway developed and industrialized systems for the O&G market. A system for monitoring fatigue in steel risers has been certified by DNV GL.
The FEMM technology is well suited for long term unattended autonomous monitoring of remote locations e.g. subsea or buried pipelines, or in harsh environment, e.g. for non-intrusive monitoring of high temperature pipes in refineries or nuclear power plants
This technology measures directly the actual response and condition of steel structures in selected locations and thus makes possible more accurate estimates of the condition of the whole structure and the safe operational life.


Ferrx appreciates the importance of having a team of highly qualified and motivated employees. In addition to offering interesting opportunities within development and industrialization of advanced technology products in close cooperation with clients, the employees will receive competitive compensation and be invited to share in the values they create as team members at Ferrx.
Ferrx has high ambitions for growth and is always interested in talented engineers. Students interested to do their projects or master thesis are encouraged to contact us. We can offer interesting work in the technical domains of instrumentation systems, material technology, or algorithm development in cooperation with NTNU.
Employees will have access to a modern 34-foot sailboat. The boat is used for cruising and racing and employees may participate also as racing crew.


A Method for Determination of Stress and Fatigue in Risers and Wellheads

Contact us

Ferrx as
Brøsetvn 168
N-7069 Trondheim
Phone no: +47 40001595