Game-changing technology for integrity monitoring of steel structures

Ferrx and FEMM

Ferrx was started in 2005 based on the transient potential drop technique developed by SINTEF in Oslo. Ferrx patented the method now called FEMM (Ferrx ElectroMagnetic Method), and with support from several oil companies continued the development and industrialization mainly focused on monitoring fatigue in steel risers, and a system for riser inspection has been certified by DNV GL.

FEMM measures the factors influencing safe operational lifetime of steel structures, e.g. by monitoring welds at critical points for changes of residual stresses, material deterioration, crack initiation and growth, and also weld root corrosion, all with the same type of sensors.

Numerous stress and fatigue tests of the method's different features and with different types of specimens and structures have validated the monitoring capabilities.

FEMM 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.

The 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.

The features for early warning of any damage that can cause fatigue in vital structure will improve operational safety and can reduce the cost and time for required maintenance. It will, in addition, reduce the risk of unwanted incidents.

How it works


Fatigue 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: accumulation of cyclic plastic deformation which is closely related to bulk material properties, formation of slip bands and nucleation and growth of microcracks in the surface, and finally growth of a macroscopic crack to failure.

Measure fatigue

The material changes during a fatigue process cause typically reduced permeability and an increase of electrical resistance in the surface. These changes in the outer surface and beneath are monitored by the FEMM's transient voltage response. This response changes according to the degree of changes in these parameters, e.g. when micro-crack density increases and grows to more continuous cracks, the response signal changes accordingly. By analyzing the response signal, changes of permeability and 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

Measure stresses

Stress measurements in ferromagnetic steel are relative to a previous measurement, preferably taken with known stress levels. Calibration curves can be established and quantification of actual stress level is thus possible.

Plasticity, permanent changes in the material caused by stress exceeding yield, is detected. This is, for example , observed when monitoring areas with residual stress. Changes of residual stresses related to welds are measured with sensitivity also for changes in depth location. 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 measurement, and the permanent change can then be measured.

Differentiation between these changes are related to the analyses of changes of permeability and resistance and the depth of these changes. This has been verified by several high cycle fatigue tests and miscellaneous measurements of elastic and plastic stresses.

Measure cracks

The pin 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. The matrix sensing 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 internal in pipelines is more or less uneven and the degree of corrosion attack can vary 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 and degree of attacks.

Measure corrosion

The location expected to be most exposed to corrosion is selected for installation of the FEMM 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 and installed. The whole area covered by the sensing matrix is monitored for any internal metal loss. Initially, a measurement is taken and stored and used as a reference for the following measurements, which give the change due to corrosion attacks. When significant 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 signal.


Erosion internal in pipes 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 bend geometry and flow. Both carbon steel and duplex steel can be monitored.

Measure erosion

The sensing pin matrix is designed for optimized sensitivity based on expected shape of erosion, e.g. if erosion is wide and in outer radius, the matrix is distributed along the outer radius. If the erosion is expected to be a narrow groove along the outer radius the matrix can be along the bend'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 4 different matrices which can be located to monitor 4 separate erosion locations. When significant metal loss is detected, the actual depth of the deepest attack within an matrix can be estimated based on the patented FEMM algorithm.


System components overview

A FEMM system consists of four main modules: the sensor arrangement on the steel structure, the sensor interface unit, the data acquisition unit, and the dedicated PC SW. This modularity provides for extensive flexibility to optimize the system configuration and performance for many applications. This 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 structure.

Sensors and all fixed installed components shall have the same operational lifetime as the monitored structures and 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 bridge up to 32 cracks within 20m from the data acquisition unit.

System Components

The FEMM technology has been designed with flexibility for different applications onshore, topside, and subsea. Electronics and software designs are similar for all applications. The sensor pin layout, instrument encapsulations and software configuration are selected for the actual application.

Monitoring fatigue in pipe butt weld

The sensor

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 pins and the excitation current, both requiring electrical contact with the steel. One or more temperature sensors monitor the structure's surface temperature. The Sensor Interface (SI) cylinder shown above on the right comprises the interfacing and digitizing circuits and the current excitation circuits.

The pin 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 pin matrix is monitored.

Different connection methods for making electrical contact to the steel for the sensor arrangement are available: soldering (heat controlled max 300degC), stud-welding, and spring loaded pins. After installation, all connection points are properly protected.

Component sensor interface.jpg

Sensor interface

The Sensor Interface (SI) unit contains signal conditioning and digitalization circuits and a high current excitation circuit. 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. The picture shows SI encapsulation for use in onshore applications. (IP68).

Portable instrument

Portable instrument

This instrument is used for examination of a number of inspection points (tags) fitted with pin 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 SW. 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: -20degC to + 70degC. Storage temp range: -40degC to + 85degC. Size: width=165mm, depth=175, height=52mm. IP67.

Component Autonomous Instrument

Autonomous instrument

This data acquisition unit for autonomous monitoring in harsh environment 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: -40degC to + 85degC (depending on type of battery). Size: width=230mm, depth=240mm, height=110mm. IP68.

Subsea sensor and SI protection

Subsea integrated sensor and SI protection

The left sketch shows sensor installation on an 8-inch riser pipe with a subsea SI unit included. The encapsulation is adapted to the size of the monitored area and available space. A robust composite shell filled with soft silicone rubber protects the instrumentation fixed to the pipe. The shell gives sufficient impact protection during handling and deployment.

It is designed for maximum 1500m water depth. The internal silicone is pressure compensated and has an operational lifetime of minimum 25 years. This system for riser inspection has been qualified by DNV GL according to DNV-RP-A203.

FEMM Data analysis software

FEMM Data analysis software

The FEMM 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. Data results for export are available in SQLite 3 and CSV file formats. Software functions also include programming and configuration of monitored tags and instrument setups.


Different field and full-scale tests and monitoring installations have demonstrated the versatility of the Ferrx monitoring technology.

OG onshore-offshore

O&G onshore and offshore

FEMM technology is well suited for long term unattended monitoring of remote steel structures with sensors having a lifetime equal to that of the structure itself. The system for riser inspection has been qualified by DNV-GL in accordance with DNV-RP-A203 Technology Qualification. The FEMM sensor technology is also well suited for harsh environments like high-temperature pipes in refineries.

 Other infrastructure

Wind turbine structures

About a quarter of the costs of offshore wind energy are related to the operation and maintenance of wind farms. More cost efficient O&M can be obtained by condition monitoring which will predict wear of the components in order to perform planned maintenance, while avoiding unplanned production interruption and unplanned maintenance as much as possible. The limitations inherent in current simulation technology pose a major challenge for the optimization of wind turbine structures. Significantly more accurate estimates of such structure's condition can be achieved by measurement data giving actual condition for certain points of the structure. In this way FEMM monitoring of e.g. critical points can be used to calibrate the model and thus achieve more accurate condition data for the whole structure and potentially allow for reduced safety margins.

Highway infrastructure

Highway infrastructure (i.e bridges, ...)

To be able to assess a bridge's condition and the remaining safe service life, the actual integrity of the structure is crucial information. Residual service life can be estimated by the combination of accurate condition data from critical points and predictive modelling of the whole structure. The Ferrx technology has unique capabilities for early detection of deterioration in steel material which can lead to fatigue. Typical applications can be monitoring critical areas that are assumed subjected to the highest fatigue loads. FEMM measures the actual effect stress loads have on the monitored steel, e.g. changes of residual stresses in welds and material degradation that can lead to crack initiation. Eventually this may lead to unacceptable cracks and less structural reliability than required. Crack initiation is detected with high sensitivity before the cracks become visible and crack growth is then monitored and quantified.

Monitoring railway neutral temperature

Monitoring railway neutral temperature

System 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 mobil network to an Internet server

Monitoring corrosion in pipe

Monitoring corrosion in pipe

The picture to the left was taken during installation of pin matrix on an 8-inch pipe spool for the customer. The whole area covered by the pin matrix is monitored for internal localized corrosion. Several spools were instrumented for monitoring general and localized corrosion. One autonomous system monitored three spools, each with two sensor interfaces

Monitoring fatigue in pipe butt weld

Monitoring fatigue in pipe butt weld

When a new riser pipe is bent or loaded, the first that changes will be 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.

FEMM monitors these changes of residual stresses and any permanent changes of the material caused by load variations. These changes may be defects internal in the pipewall e.g. creep or changes in the outer surface i.e. microcracks and macrocrack initiation. Internal defects (cracks and metal loss) are also monitored, and all these will influence the safe service life.

With FEMM the depth position of defects is measured, either if it is within the pipewall or inside the pipe. The picture shows a typical pin layout for monitoring a pipe's buttweld and Heat Affected Zone. In this case the wires are soldered to the pipe surface and the connection points are protected with a robust ceramic epoxy.

About Ferrx

Ferrx is an independent company located in Trondheim, Norway with the business idea to provide the FEMM technology for different onshore and offshore markets. This technology monitors the actual condition of steel structures and thus can prevent loss of assets and pollution of the environment. The Ferrx FEMM has been developed and tested for two decades, first by SINTEF and since 2005 by Ferrx with the support from several oil companies, Innovation Norway and The Research Council of Norway.

During the last years the instrumentation technology has been industrialized and the industrialization, marinization, and production of the instrumentation systems are being done in cooperation with specialized suppliers . The first commercial systems for onshore have been delivered and installed. Different types of instrumentation systems are available both for onshore and offshore/subsea applications.

The FEMM is protected by two patents and all rights are vested in Ferrx. The system for riser inspection has been qualified by DNV-GL in accordance with DNV-RP-A203 Technology Qualification.

Participants in development of the Ferrx Technology Ferrx has close cooperation with
BP p.l.c.

Det norske oljeselskap ASA

DONG Energy A/S

Innovation Norway

Lundin Norway AS

Norwegian National Rail Administration

Shell Technology Norway AS

The Research Council of Norway

Total E&P Norge AS
Ironhaven BV

SINTEF Ocean Marine Structure Laboratory


Achilles JQS certified logo

Referenced article in the text

Fatigue: irreversible changes in a material or component that is exposed to cyclic loading.


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.

Leisure activities

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.


Ferrx has high ambitions for growth and is always interested in talented engineers. We can offer interesting work in the technical domains of instrumentation systems, material technology, electromagnetic modeling and simulation.

Sailboat Ferrx

Contact page

For more information, please contact us at:

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

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