High-resolution in-line inspection of internally cement coated brine water pipelines
- DMR Inspection Technology –
by Basil Hostage and Christoph Sur
Internal corrosion is a real threat to many pipelines, whether on- or offshore. To protect carbon steel carrier pipes from the corrosive products many pipelines are internally coated.
Any internally coated pipeline presents a special challenge in terms of in-line inspection (ILI) tools”. Especially cement coated pipelines cannot be inspected using the most common ILI technologies such as MFL (Magnetic Flux Leakage) and UT (Ultrasonic Testing).
This paper describes the DMR technology and its capabilities to inspect internally cement coated pipelines. Various applications, advantages and the concept of work are outlined.
Practical experience from brine water pipeline inspections, other practical applications and potential for further development are described.
The pipelines in question are used to transport brine and having various diameters. To protect the carbon steel carrier pipes from the corrosive product (NaCl brine) the pipelines are internally cement coated.
Basic pipeline design data:
The pipelines are in service for several years and it is known that the cement liner occasionally breaks off the internal pipe wall as can be seen in figure 1. On these locations, the steel surface becomes exposed to the product. Further, cracks may occur in the cement with the same result and potential corrosion underneath the liner.
Fig. 1: View of a internally cement coated pipe spool (with outbreak of internal cement coating)
The target of the inspection is to determine presence and thickness of the cement liner over the entire length and circumference of the pipelines. Further, internal metal loss at spots where the cement is lost and underneath the cement coating shall be identified.
The ILI operations are considered a success when the following objectives are met:
- Safe pigging operations: a pipeline is inspected in a safe manner with no incidents or accidents that harm people, equipment or the environment.
- Efficient pigging operations: ILI tool runs are concluded within time and budget.
- ILI performance meets acceptance criteria: Thickness of the cement liner as well as internal metal loss is measured according to 3P specification.
To achieve this, the ILI tools for inspecting the brine pipeline system have to fulfil the following requirements:
The DMR sensor technology is a proprietary invention of 3P Services. The sensors are used on-board MFL inspection tools as a secondary sensor for internal / external feature discrimination or as stand-alone in-line inspection technology for specific high resolution measurement applications.
They are sensitive to features on the inner surface of the pipeline. These may be internal metal loss or non-ferritic internal coatings or layers like cement.
A DMR sensor delivers very accurate distance readings between it’s own location and that of a close ferritic steel surface. The sensor has a focused measurement footprint. Each measurement thus refers to a certain area of the steel surface. This sensor reference area can somewhat be compared to that of a UT sensor. The measurement does however not penetrate into the steel and has, therefore, only a single “reflector”, which is the steel surface.
DMR measurement readings are very accurate as long as the sensor is in a rectangular position facing an even steel surface (sensor position 1 and 3 in fig. 2). Transition areas (such as change in wall thickness, start of metal loss, etc.) have more than one distance value and the DMR measurement reading is subject to an averaging effect (sensor position 2 and 4 in fig. 2). At sharp transitions, certain “edge effects” may be generated.
Local metal loss is measured very well if the metal loss area is larger than the sensor measurement footprint (sensor position 3 in fig. 2). If a pit has a smaller diameter than the measurement footprint (sensor position 4 in fig. 2), it will be detected and identified, however, its depth will be undersized.
Fig. 2: DMR sensor reference areas relative to even steel surface, transmission, general corrosion and small diameter pit or pinhole
Fig. 3 shows the DMR sensor types available at present. Sensor dimensions, the measuring range, measurement footprint and the smallest detectable pit-type metal loss are outlined. When sensors are clustered together on sensor carriers, a minimum spacing between individual sensors of 10mm, 25mm and 30mm (types 1, 1.5 and 2 respectively) must be maintained in order to avoid cross sensor interference.
Fig. 3: Different DMR sensor types
Two possibilities to fix the DMR sensors are used: “stand-off” and “wall-guided”. In both cases, the DMR sensors are identical, just the sensor suspension differs.
In the “wall-guided” mode (Fig. 4-1), the sensor is integrated into a carrier that receives the force to keep it at the pipe wall and also protects it from wear. Depending on the size of this carrier, it will move over local spots of metal loss, so the sensor can measure it. If the metal loss area is large and smooth, then the sensor carrier will follow the surface and the sensor will not recognize it. The type and size of sensor carrier has an important influence on the capability to detect metal loss in the “wall-guided” mode.
In “stand-off” mode (Fig. 4-2), the sensor is fixed to the inspection tool and does not physically touch the pipe wall. The sensor is guided at a pre-set “stand-off distance” from the surface and only gives a signal when the steel surface deviates from the even shape. In this mode the ability to detect and size internal metal loss depends only on the type of sensor, its stand-off and the type and dimensions of the defect.
Fig. 4: Comparison of wall guided and stand-off sensor measurements
If an internal layer or coating is present, then the inspection properties are as follows:
The “wall-guided” (Fig. 5-1) sensor is pushed from the inspection tool by spring load against the pipe wall. It follows the physical internal surface, no matter whether it is a clean steel surface or there is any internal layer, be it an epoxy coating, a paraffin layer, or accumulations of debris. Under all circumstances it will measure its distance from the next steel surface, so it records coating or layer thicknesses as well as local internal metal loss.
“Stand-off” (Fig. 5-2) sensors ignore any non-magnetic layer and record both types of metal loss defects (be it metal loss under coating or where coating has disappeared).
Fig.5: Comparison of wall guided and stand-off sensor measurements for coated pipes
When both readings are available, “thickness of layer” readings and “metal loss” readings can be separated from each other and reported individually. This is still true if both occur at the same location.
DMR sensors may be thought of as being “short sighted” in the sense that their highest accuracy is at shortest distance. A sensor typically has a non-linear output-vs-depth characteristic. At short distance, the measurement resolution is very high and reduces as the sensor is farther away from the surface. Measurements are most accurate at small distance records, with accuracy in the range of 1/10th of a mm.
Fig. 6 shows the general limitations of the working range regarding pit type metal loss of the DMR sensor in the “wall guided” mode. On the horizontal axis the pit diameter limitations are displayed: the minimum diameter refers to the situation where the pit has a smaller diameter than the sensor reference area. The maximum limitation for wall guided derives from the sensor carrier length. If a defect is longer than the carrier, then it will follow the surface and not record metal loss. In this case, though, the defect will still be detected and measured by the stand-off sensor. On the vertical axis, the depth limitations are displayed. The darker blue area shows where the sensors give their most accurate readings. The DMR technology can even be used to measure the thickness of a paper, however, for practical reasons metal loss features with depth below 0.5mm are not sized.
Fig. 6: Working range of a “wall-guided” DMR sensor relative to pit type metal loss
The following figure shows a sketch of DMR tool combining “stand-off” and wall guided sensor array.
Fig. 7: Typical DMR tool assembly
The following data derive from a pull through test performed in a 20” cement lined pipe. The pipe was provided by the pipeline operator.
Fig. 8: “Wall-guided” data
Reporting generally follows the requirements as per the relevant paragraphs of the POF. Individual client specifications are always taken into account. Onsite operations reports are available within 24 to 48 hrs after the inspection runs and standard final reports are typically delivered within 8 weeks.
Currently DMR tools are available from 2” to 48”. Other sizes as well as dual or multi diameter tools can be built on special request. Tools are able to pass 1.5D bends (≥6”) even in back-to-back combinations. Operating temperature ranges from 0 – 70°C. High pressure capable DMR tools with up to 280bar rating have been used in deep water off-shore applications. Since the DMR tools do not require a coupling medium, inspections are possible in a liquid, gaseous or multiphase environment.
Generally all DMR tools can be set up in a uni- or bidirectional design. ATEX certification exists for tool sizes ≥6”.
Depending on the inspection target the DMR sensor technology can be combined with all other common ILI technologies like MFL or UT.
The first DMR inspection of a cement coated brine water pipeline was done in 2007. Since then several brine water pipelines have been inspected by 3P Services using DMR ILI tools. A number of pipelines have already been subject to several regular re-inspections.
Cement lined pipelines with diameters ranging from 10” to 20” and length up to 26km have been inspected. Those lines all contain 1.5D bends. Y-piece connections are present in some of the pipelines.
Already after the first DMR inspections, the pipeline operator carried out investigations in coordination with the responsible authority in order to check or cross-check the ILI results. The majority of defects detected fall either into the category of loss of cement liner or metal loss in the upper half of the line. Investigations have verified the reports and appropriate repairs have been carried out by the operator.
No metal loss underneath an undamaged coating have been found in cement lined pipelines although such defect type has been reported in other pipelines under different types of coating or internal repair.
In the last few years of operation, the presence of hard salt deposits has emerged as a challenge in terms of pipeline cleanliness. In coordination with the pipeline operator an intensive cleaning program was done prior to the DMR inspection run. Cleaning was done using different type of mechanical cleaning tools.
DMR sensor readings are hardly influenced by tool speed. They were tested and applied at speeds up to 23m/s (=83km/h). Results show practically no restriction to defect detection and sizing performance of a single measurement within the speed range up to 20m/s. Further, the same is valid for speed fluctuations. This means that acceleration and deceleration, as they occur in low pressure gas environment, do not influence accuracy of the measurement. However, as the sampling distance increasing with higher velocities, sizing performance is affected to some extent. Above 10m/s, the percentage of the internal surface covered will be below 100%.
Fig. 9: POD (Probability of Detection)
With this combination of sensors diameter restrictions due to pipe deformation (dent, ovality, etc.) can distinguished clearly from accumulations of non-magnetic debris (sand, paraffin, etc.). See Figs. 10 and 11.
Fig. 10: GEO/DMR inspection tool
Fig. 11: Correlation of GEO and DMR Sensors
The DMR technology makes an inspection of internally coated brine water pipelines possible. The data obtained are not only related to the carbon steel underneath the cement liner, but also to the cement liner itself, as the liner can be examined for thickness variations or even missing cement spots.
The accurate detection and measurement of these phenomena is an essential element in the responsible integrity management.
Subsequent key advantages of the DMR inspection technology are:
Quantitative assessment of presence of paraffin and scale