System Concepts

Component Sensor Interface

A FEMM system consists of 4 main modules: the sensor arrangement on the steel structure, the sensor interfaces, the data acquisition unit, and the dedicated PC SW. This modularity provides for extensive flexibility to optimize the system configuration and performance for any application. One system comprising one instrument and up to 8 sensor interfaces can monitor one to eight nearby pipes or locations.

This system design based on many years of relevant field experience has been optimized with respect to reliability and ease of installation. The same electronics and software designs are applied for all applications, and adaptation to different applications comprises mainly the encapsulation and the configuration made in software.

Sensors 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.

Sensor Configuration

Sensor configuration

The figure shows the principle of sensor layout for monitoring internal corrosion in a pipe. The sensor comprises the sensing pins and the excitation, 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 sensor interfacing and digitizing circuits and the current excitation circuits.

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

Each SI unit has 28 sensing pin pair inputs and 4 pairs of current excitation outputs. Up to 8 SI units can be run from one instrument which makes a system of up to 224 pin pairs. The SI units can be installed on the same pipe or several nearby pipes and thus one system can monitor several pipes.

The pin matrix layout design is mainly based on expected type of attacks (corrosion, erosion, or cracks), stress or fatigue and the total area to monitor. For metal loss required sensitivity is a design factor that decides e.g. spacing between pins. Number of pins and spacing between pins make up the covered area. For example, for even internal or external corrosion a few pins can cover several square meters of a pipe, while for pitting corrosion short pin spacing is required to obtain good sensitivity.

System Components

The FEMM technology has been developed for different applications onshore, topside and subsea. Electronics and software designs are similar for all applications. Sensor design, encapsulations and software configurations are adapted to the actual application.

The main system components are: sensor with sensor interface normally permanently attached to monitored structure, a data acquisition unit, and a PC program for data handling and reporting.

Component Sensor Interface
Sensor Interface

The Sensor Interface (SI) unit contains signal conditioning and digitalization circuits and a high current excitation circuit. Each unit handles 28 sensing pin pairs and 4 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 left shows encapsulation for use in onshore applications.

Component Handheld Instrument
Handheld Instrument for inspection of locations (tags)

The Handheld Instrument is used for inspection of locations (tags) with FEMM sensors. Configuration setups for all tags to be inspected during one inspection tour are downloaded to the instrument prior to inspection. Measurements are taken automatically when the instrument is connected to a tag and the tag has been identified. 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.

Component Autonomous Instrument
Autonomous Instrument

Data acquisition unit for autonomous monitoring at preprogrammed intervals. It is powered by an internal rechargeable battery or by external power for offline or online operation, by cable or wireless. Miscellaneous protocols are available.

Subsea Instrument Cylinder Encapsulation
Subsea autonomous instrument

Picture showing FEMM instrument cylinder installed on an 8-inch WOR pipe. It is designed operation down to 1500m water depth. Internal battery capacity is 5 years of monitoring. The instrument operates autonomously, either offline storing data locally or online transferring data to the user.

Subsea sensor for pipes
Subsea integrated sensor and SI protection

Left figure shows a scaled sketch of sensor protection on an 8-inch riser pipe with a subsea SI unit included. This design is made for drilling and work over risers. 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. The 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 all 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 set-ups.

The patented Transient Potential Drop method

General layout of four-point potential drop sensor.

A and B in the above figure are connections for excitation current, and C and D make a sensing pin pair for measuring the potential drop response signal. Normally, many potential drop pin pairs are applied per current excitation pair.

Transient Drop method Waveforms

Conceptual waveforms. Excitation current pulse, Response signals a(t) and n(t), which are two transient curves measured at the pin pairs for two different steel conditions, and Deviation d(t) between the two Response signals.

The electric Excitation current, Response signal (Transient Potential Drop) and finally the Deviation are shown in the above figure. The Excitation current pulse has a typical duration of 0.5 s. Two Response signal transient curves measured at two different states of the steel are shown. Here is shown the transient voltage response, a(t), of the monitored object and the transient voltage response, n(t), measured at the evaluated material condition, in this case when being exposed to higher stress. The curve to the right in the figure shows the Deviation d(t) between the two response transients: d(t) =[n(t)/a(t) - 1]*1000 [ppt] (parts per thousand).

The transient shape of the response signal is due to the skin effect when injecting a current step in ferromagnetic steel. The transients are characterized by their time constants τ (tau) which are related to the steel parameters presented in the following equation: τ ≈ ¼ a2 σ µ0 µr, where a is the wall thickness, σ is the electrical conductivity, µ0 is the magnetic permeability of free space and µr is the relative magnetic permeability.