ferrx electromagnetic steel examination  
Bridge and railway Oilrig and risers

 Game-changing technology for monitoring integrity of steel structures 

The unique Ferrx technology directly measures all the steel parameters influencing the steel structures’ condition in real time, giving the best basis for safe and economic operation and more precise prediction of remaining life.
Risers and 3 templates



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 as well as crack initiation and growth.


Girder bridge

This girder bridge is more than 60 years old. FEMM sensors are retrofitted on the lower and upper flanges of the girder to monitor traffic loads. The bridge will in periods be used for the European route E6 traffic with a high number of heavy vehicles that may exceed the bridge classification. Therefore the traffic is monitored. Data is transmitted to Ferrx daily and provides for near realtime monitoring. This installation is also part of an R&D project where SINTEF Ocean has made a digital model of the bridge which, when integrating with the FEMM system, makes a digital twin of the bridge. The project is financially supported by Trøndelag’s regional research fund (RFF Trøndelag) which has been decisive for running the project.

Framework bridge

Framework bridge Framework bridge sensor and wiring
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 stress is measured for monitoring the highest vehicle load in a period, and also stresses caused by 10Hz resonance vibration. The picture above right shows one of the sensors.

Railway rail stress monitoring

This autonomous system measured the stress in railway rails for monitoring the rail′s neutral temperature to prevent buckling-related derailments. This was a demonstration installation run successfully for one year.



Non-intrusive pipe monitoring

Non-intrusive corrosion monitoring
  The picture shows installation of the sensor matrix on an 8-inch pipe for field application (before installation of protection). The whole area covered by the sensor is monitored for internal localized corrosion. One autonomous system monitors three pipes, each with two sensor interfaces.  

Other parameters that can also be monitored with the same type of sensors are longitudinal stress and hoop stress. Hoop stress can be used to calculate the pressure in the pipe. Furthermore, any vibration is monitored by measuring the dynamic stress.


Fatigue monitoring of subsea riser

fatigue monitoring of subsea riser
  FEMM system installed on an 8" work-over riser pipe for monitoring stress and 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 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. Any inside metal loss will be measured with the same sensor and give a different signal response.



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

Stress monitoring

Stress measurements in ferromagnetic steel are relative to a previous measurement. Calibration curves for elastic stress can be established based on measurements at known loads, either on the actual structure or on specimen of the same type of steel.  

Different types of stress can be monitored: elastic stress, residual stress changes and maximal stress between two measurements. Changes of stress beyond yield make permanent changes in the steel material and is detected.


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.


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 the 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 a bridge coverplate. One system with eight SI’s 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 running 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 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, for monitoring a pipe buttweld, and for monitoring defects both in outer and inner surfaces of the pipe wall. The sensor 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. 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. 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 pins. The number of pins and the spacing between pins make up the covered area. For example, for even corrosion a few pins can cover several square meters of a pipe, while for pitting corrosion shorter pin spacing is required to obtain good sensitivity. The whole area covered by the sensor matrix is monitored.


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 pin 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 pin pairs) on one cable per instrument and system. The picture shows SI encapsulation for use in onshore applications.  

  • Operating temperature range: -40°C to + 70°C
  • Size: Length=160mm, OD=55mm
  • Ingress protection: IP68
  • Certified for application in Ex Zone 1 when passive


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 tags 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.

About Ferrx and FEMM

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 called FEMM (Ferrx ElectroMagnetic Method), and with support from several oil companies and Innovation Norway and The Research Council of Norway developed and industrialized the technology. A system for monitoring fatigue in steel risers has been certified by DNV

This technology measures directly the actual response and condition of steel structures in selected locations and thus makes possible in a simple way more accurate estimates of the condition of the whole structure and the safe operational life.


We appreciate the importance of having a team of highly qualified and motivated employees. We offer interesting opportunities within advanced methods and technology 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.

Contact us

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