NEW TECHNIQUE MONITORS PIPELINE CORROSION, CRACKING

Roe D. Strommen, Harald Horn, Kjell R. Wold
CorrOcean AS
Trondheim

Anew technique for monitoring the condition of an in-service pipeline, continuously monitors corrosion taking place at a given point on the pipeline and remaining pipe wall thickness at any time.

The "Field Signature Method" (FSM) has been field tested on a flow line on Shell Expro's Dunlin A North Sea platform.

The technique combines the advantages of corrosion probes and of non-destructive testing (NDT) inspection by being highly sensitive and responding to changes in corrosion of the actual pipe wall. It can cover relatively large areas.

FSM reduces costs, improves safety, and may reduce the required frequency of intelligent pigging. The method requires virtually no maintenance or replacement of consumables. Its service life equals that of the pipe itself.

FSM was developed and patented by the Center for Industrial Research (SI) in 1985-86. 1

CorrOcean AS acquired all rights from SI for worldwide commercial exploitation of FSM and has spent more than $3.5 million over 4 years developing it.

PROTECTING PIPELINES

Protecting pipelines from corrosion is achieved externally by use of cathodic protection (for buried or subsea pipelines) and internally by injection of inhibitors to mitigate internal corrosion.

Various inspection and monitoring techniques monitor both a pipeline's condition for early warning of failure and the efficiency of any mitigation program to reduce or arrest corrosion.

While traditional NDT techniques and in-line inspection tools (intelligent "pigs") may represent effective solutions for assessment of the condition and integrity of a pipeline, the sensitivity and accuracy of these methods may be inadequate for monitoring inhibitor performance.

In this latter case, both the sensitivity and frequency of data collection must be high regularly to produce reliable trends.

Even more important to the pipeline operator may be the economics of an inspection and monitoring program. Suitable design of an NDT-inspection and corrosion-monitoring program may help reduce the expenditures considerably.

A combination of monitoring of an actual pipeline with a certain number of FSM stations along a line combined with running smart pigs through the line at infrequent intervals may represent an optimum solution in terms of condition and integrity monitoring of the pipeline.

At the same time, such a program may be designed substantially to reduce the costs for inspection as a result of the reduced frequency required of smart pigging.

For internal corrosion monitoring, the FSM is nonintrusive: Sensing electrodes and all other equipment are placed on the outside of the pipe (or tanks or other vessels to be monitored).

Compared with traditional corrosion-monitoring methods (probes), the FSM exhibits the following operational advantages:

No components are exposed to the corrosive, abrasive, high temperature, and high-pressure environment often found in process piping or to the corrosive and often hostile environment of a subsea or buried pipeline.
There is no danger from introduction of foreign objects into the piping.
There are no consumables. The complete monitoring system can be designed for a service life comparable with that of the piping, tank, or vessel itself.
There is no danger of leaks from access fittings or unsuccessful retriever operations.
Measurements are performed on the wall of the pipe, tank, or vessel itself, not on a small probe or test piece.
Sensitivity and reliability are better than for most NDT techniques.

PRINCIPLE

The FSM method is based on feeding an electric direct current through the selected sections of the structure to be monitored and sensing the pattern of the electrical field by measuring small potential differences set up on the surface of the monitored object.

Proper interpretation of these potential differences, or rather of changes in potential differences, can lead to conclusions about, for example, general wall-thickness reduction.

Local phenomena can be monitored and located by performing a suitable number of potential measurements on a given area. Fig. 1 illustrates the principle:

Upon induction into a selected section of a structure such as a pipe, an electrical current will spread into a pattern determined by the geometry of the structure and the conductivity of the metal.

This pattern is represented by current flow lines and equi-potential lines at normal angles to the current flow (Fig. 1).

The FSM measures all electric potentials and compares them with initial values measured when monitoring of the object started. These values represent the initial geometry of the object and may be regarded as the object's fingerprint.

Hence the name of the method: field signature method.

Fig. 2 illustrates a typical arrangement for internal-corrosion monitoring of a pipe. The monitored area is located between two electrodes for feeding the excitation current.

Any potential measurement between two selected electrodes in the matrix is compared to a measurement between two reference electrodes and to the corresponding initial values when monitoring started, that is, the fingerprint.

The so-called fingerprint coefficient (FC) is calculated for each set of measurements according to the expression shown in the accompanying box.

The FC is the parameter used for analyzing corrosion rates and accumulated corrosion. It is expressed in parts per thousand (ppt) and corresponds to reduction in wall thickness in ppt during monitoring of general corrosion or erosion.

When monitoring starts, the FC values are always 0.

The reference pair of electrodes is located near the monitoring electrode matrix but in an area where corrosion will not occur. This position is necessary in order to achieve effective compensation for (small) fluctuations in temperature and excitation current.

Different practical arrangements have been developed for securing sufficiently small temperature gradients between reference and monitoring electrodes.

Fig. 2 illustrates schematically the instrument setup. By use of statistical filtering techniques, very high sensitivity is achieved.

This sensitivity facilitates high-resolution monitoring even when measurements are performed on the opposite side of the corroding wall as will be the case for internal corrosion monitoring. Resolution figures obtained in practice represent less than 0.05% (0.5 ppt) of the wall thickness.

PIN ARRANGEMENT

When designing an NDT or inspection program, the inspection engineer will select critical locations of a pipeline or a structure where the risk of corrosion, erosion, or cracking is high or serious hazards might arise in the event of failure.

Similar principles are utilized in the case of FSM. The following areas are typically chosen for monitoring:

Girth welds of pipes and pipelines
Bottom sections, for example, at 4-7 o'clock position in horizontal pipes where corrosive water may be deposited
Combinations of these and areas subject to corrosion induced by CO2, H2S, or biological activity
T-joints of pipes where there is a risk for erosion or corrosion
Pipe bends and welds
Structural node corners
Tank bottoms, inlets and outlets of tanks and vessels, and critical and stressed welds.

The selected area is fitted with the current-induction transformer and the minimum number of 24 sensing pins. This number may be increased, however, in multiples of 8 pins up to a maximum of 64 pins for one FSM spool and instrumentation module.

A typical arrangement is shown in Fig. 3.

The sensing pins may be distributed in a matrix over the critical area where the matrix spacing or pin-to-pin distance may vary typically from 2 to 3 cm (1 in.) up to 10-15 cm (4-6 in.), depending on the sensitivity required for detection of smaller pits.

With a matrix spacing of 2-3 cm, the system has proven capable of detecting and monitoring the growth of pits in welds as small as 1-2 mm in diameter and depth. A matrix spacing of 10-15 cm is used in the case of uniform corrosion or when wide and shallow pitting is expected.

A typical surface area covered per instrument module may range from 0.1 to 1.1 sq in (1-10 sq ft).

NORTH SEA TESTING

Since development of this technology was completed more than 2 years ago, as many as 20 installations have been completed or are in the process of being completed.

Two projects are for installation on subsea pipelines to be built next year.

In addition to extensive laboratory tests, some of these projects have offered opportunities for testing and evaluation of this technique, by comparison with data obtained from other inspection methods or from inspection after retrieval from plants of test sections of pipes fitted with FSM instrumentation.

One of these tests was conducted on a spool piece installation on Shell Expro's Dunlin platform in the North Sea.

The FSM si,stem was installed on a flow line of the Dunlin A platform to monitor possible corrosion in a girthweld.

The instrumented section in this case is a 10-in. OD pipe spool of 15 mm W.T. and approximately 1 in length.

A total surface area of about 0.1 sq in is covered with 48 sensing pins. The area includes the entire circumference of the pipe at the weld.

The spacing of the sensing pins (20 mm) means that the system can detect local variations in the corrosion and also small defects in, for example, the weld zone or the HAZ.

The fundamental reason for evaluating the FSM in this location was that it would enable the operator to create a baseline for the uninhibited corrosion rate and so that the FSM could be used to monitor subsequent performance of the corrosion inhibition program.

It should also be mentioned that during a previous pipework replacement program, 8 mm of additional corrosion allowance (thicker pipe) were installed to provide a luxury of living with an estimated corrosion rate of 0.5 mm/year for a period during which a credible corrosion-monitoring capability was established.

The estimated rate relates to the fact that the water cut is relatively high (75%) and the CO2 content is approximately 4.1%.

Through a fairly extensive program conducted by the operator, including inspection and NDT, the use of weight loss coupons, and inspection of actual replaced pipework, it had been possible to estimate the corrosion rate to between 0.20 to 0.55 mm/year (8-22 mils/year).

Readings obtained with the FSM system after the first 3 months of operation are shown in Fig. 4.

Utilizing an automatic procedure in the FSM Trend software package for trend analysis, the corrosion rates for the majority of the readings were in the bracket of 0.24 to 0.6 mm/year (9.5-24 mils/year). These coincide with the inspection data previously obtained by Shell.

The data obtained also indicate that this corrosion-rate data could be obtained with adequate accuracy after 2-3 weeks of monitoring.

The sensing pins were fitted around the circumference of the pipe at the girthweld (Location 1) and in an area between 6 and 8 o'clock (Location 2) downstream from the weld of Fig. 4.

The graphs show the variations in accumulated corrosion for the same two areas after 3 months.

INSTRUMENTATION

Depending on the application, the range of which is discussed presently, four different instrumentation concepts are possible for the FSM.

PORTABLE INTERROGATOR

The portable system consists of the following components (Fig. 5):

The instrumented section of the structure or pipe including the current-excitation arrangement, the sensing pins with wires and connectors, and a protective cover normally made in stainless steel.
A portable interrogator unit which can be used on any number of instrumented sections. When this instrument is connected to an instrumented section, it can run on command the measurements and store the results along with time, date, and any tag number or identification.
FSM Trend software for use with a personal computer (PC). This software accepts downloading of the readings from the portable FSM meter and takes care of historical file storage, data analysis, graphing, printing, and plotting.

CONTINUOUS LOGGING

As an alternative to the use of a portable meter, the FSM can be supplied as a standalone battery-powered logging station. The equipment consists of the following:

The instrumented FSM section with current-feeding transformer, sensing pins with wires and connectors, and the protective cover.
A self-contained, battery-powered logging station, preprogrammed for automatic interrogation at regular intervals, and storage capacity for several months of operation based on a single daily reading.
A data-retrieval unit for downloading the readings directly to a PC.
The FSM Trend software package.

ON-LINE MONITORING

Permanent instrumentation systems for on-line monitoring are based on FSM stations to be fitted locally, one at each location, and connected via a fieldbus system to a master unit in a control room.

The master unit can handle up to 15 FSM stations and is controlled by a version of the FSM Trend software for on-line monitoring installed on a standard PC.

The components of this system include the following (Fig. 6):

One or more FSM stations with field-bus interface, as described previously
Master unit installed in the control room and connected to a PC
Field-bus cable system, a cable consisting of two pairs linking in series the FSM stations to the master unit
The FSM Trend software, automatic version.

SUBSEA, REMOTE MONITORING

FSM systems are being designed as ready-made spools for subsea completions and subsea pipelines (Fig. 7).

Suitable concepts for instrumentation include the following:

Spools located close to subsea completions.
Power supply and data communication can be routed through cables run back to the subsea electronics control module and transmitted to a shore station or platform via the umbilical.
The system can be set up for remote polling via the umbilical.
Battery-powered systems with hydro-acoustic communication to the surface, either to a fixed installation within 15-20 km or via surface ship or a buoy supplied with satellite communication.
Battery-powered FSM system with an ROV or diver-retrievable datalogger. Battery is made integral with the data pack to simplify retrieval.

CRACK, CORROSION MONITORING

Because FSM technology requires virtually no maintenance nor is there need for replacement of consumables, it is particularly suitable for remote monitoring at inaccessible locations.

In combination with intelligent pigs, FSM represents the only reliable alternative to internal-corrosion monitoring subsea. Because no sensors must be replaced with FSM, the complete monitoring system can be designed for the same service life as the subsea pipework.

In addition, the FSM can monitor corrosion accurately at relevant locations subsea at actual temperatures and flow conditions,

It can monitor the performance of inhibitors as well as the influence of scale formed on the pipe wall, actual corrosion, and erosion rates in situ. This information can be transferred immediately to the surface, allowing the corrosion engineer to take immediate action.

Particularly in the case of multiphase transport of corrosive petroleum products, the FSM can be a tool for controlling inhibitor treatment of the subsea line.

ACKNOWLEDGMENTS

The authors are indebted to Brian McLoughlin and Hans van der Winden for their contribution to this work and to Shell UK Exploration & Production for permission to publish these data.