How to assure vertical casing integrity?

Assuring Vertical Casing Integrity with Pipeline Inspection Technology

by B.Hostage, D. Schaper und S. Stolte

Pipelines in the oil and gas industries, as well as in the chemical and petrochemical industries, are regularly inspected regarding their integrity. The technology is called “In-Line inspection” (ILI), in which autonomous inspection tools are pumped through the pipeline usually with the product. The main objective is to detect, locate and measure defect anomalies, such as metal loss, on the inner or outer surface.

In principle, these same tools can be equally used in vertical as in horizontal installations, even though there are differences between these applications. This paper describes an application of ILI technologies used for horizontal pipelines to the area of vertical pipe inspection.

3P Services has provided ILI services for more than 25 years. Recently, ILI tools have been modified to provide a vertical inspection and a simplified set-up created for job execution. No drilling or work-over rig was required. The set-up has been used in several projects.

Like in pipelines, the adapted ILI tools work autonomously in casing applications since they are equipped with their own power supply, data processing and storage. The execution itself is effectively a wireline job without need for separate power supply or data transfer through a cable. The work site operation was planned together with the client targeting at complete sets of high resolution inspection data.

The measuring unit, equipped with different sensor types, is changeable to cover individual tasks. Ultra-sonic (UT) and Magnetic Flux Leakage (MFL) technologies are typical for wall thickness measurements. Further technologies are available for specific detection of inner corrosion, scaling and/or geometric deformations.

The development and testing of tools for a specific project is described. Operational lessons learned and potential for further application are discussed.

 

1. Introduction

 
In the last several years, 3P Services has become aware that casing integrity is becoming a sensitive matter and a consequent growing demand for integrity inspections.  There are trends indicating that inspections may be more and more required in future. The case study below comes from the UK.  Similar trends can be expected in Germany.

ILI results are now acknowledged and regularly used in pipelines as the basis for calculations of remaining life, life extension, fitness for service assessments and service up-grades. Inspection intervals can be set after analysis of the severity of defects present in a line. Necessary repairs can be planned in advance. Routine operations can be adjusted to incorporate preventive and mitigating actions.

Application of ILI technologies to casings will allow these same disciplines to be used to demonstrate and ensure casing integrity.

 

2. Technological background

 
3P Services has a profound knowledge in the area of ILI in the petroleum, natural gas, petrochemical and chemical industries through the entire energy chain from up-stream, mid-stream and down-stream. Experience ranges from field lines connecting wells to processing facilities, offshore production loops in deep water to large diameter onshore transportation lines and long distances. Inspections in downstream pipelines typically require somewhat smaller diameters. More than 100 professionals cover the spectrum of disciplines necessary to design, construct and test ILI tools as well as deliver the inspection service and reporting to operators.

The tool fleet is highly modular and under constant improvement. A wide range of standardized components, such as electronic modules, power supply, and sensor arrays, can be integrated and adapted to tool design and assemblies that are specific to the pipeline to be inspected. Diameters from 2” to 56” are readily available.

Following figure shows a variety of 3P’s ILI tools. Upper left shows a unidirectional 4” tool for measuring the internal pipe diameter. On the upper right a 18” MFL tool consisting of two modules is shown. The first module (left) is a towing module. It is equipped with sealing discs and pulls the second module through the line driven by the pressure behind the seal. A power supply and electronic to store all measurement data are integrated in the towing module. The second module is the MFL measurement unit. The tool below is a 48” bi-directional tool, which can be pumped in two directions.

Fig. 1: ILI tools of different sizes

3. Pipeline vs. casing inspection

 
In common with pipeline inspection, the objective in casing inspection is to achieve a high resolution data set about metal loss, girth weld and geometric integrity. Further interest is typically on mechanical stress in the pipelines due to dents or ovality.

There are, of course, differences between pipeline and casing inspections, some of which are summarised below:

 

  Pipeline Casing
Tool movement pumped on cable
Seal required yes no
Launch/receive traps present yes no
Typical length 5 km to 100+ km 0,5 km to 5 km
Tight bends present yes no
Temperature/pressure gradients small large
Chance to investigate results yes no

 
Some aspects of tool configuration are simpler for vertical casings compared to horizontal pipelines.  Since flow of a product is not used to propel the tool, PU sealing elements are not required. The absence of bends means that the tool bodies can be made larger.

Other aspects are more complicated. When inspection must be done under pressure, tool length may be limited by the amount of space available for a pressurized launch trap. Temperature and pressure gradients may be greater in a casing application compared to a similar pipeline, which may imply higher requirements for the tools with regard to robustness and measurement range. However, many solutions for such requirements have already been applied to different tools in the fleet. Individual tools have been pressure rated to 350 bar and temperatures to 100 °C. ATEX certification can also be achieved for EX rated zones.

 

4. Case study – LPG storage facility

 
3P Services was asked to inspect 200 m long casings of 12 ¾” and 20” diameter in an LPG facility.

 

Fig. 2: LPG Cavern schematic sketch

 
Target of the inspection was a leveling casing and a production casing. The cavern is filled with LPG through the fill line. Possible LPG vapour is vented over the vent line. Cavern pressure is controlled by pumping water over the level regulation line.

 

5. Case study – ILI equipment for vertical casing inspection

 
The objective of an inspection is to deliver detailed, high resolution information about any defects throughout the entire length and circumference of the pipe.  Sensors are arrayed to achieve a fine mesh of measurement points to detect and size pinhole and pitting features of 5 mm diameter and smaller.  Distance between measurement points in axial direction is assured by the sampling frequency and speed of tool movement. Circumferential coverage is assured by sensor spacing with overlapping measurement footprints.

Pipeline inspections use a sequence of tools to ensure that succeeding tools do not encounter a restriction that may prevent passage or lodge a tool. An un-instrumented PROFILE tool equipped with gauging discs serves to both clean the pipe and determine a minimum diameter. The next GEO tool measures pipe ID to detect and accurately locate and size geometric anomalies. These data confirm whether the pipe bore is sufficient to allow passage of the final metal loss tool.

MFL magnetizer unit is relatively massive compared to the pipe ID. This tool locates and measures internal and external metal loss.

A UT sensor array can also be used to measure wall thickness and metal loss features. A liquid coupling medium is required to provide effective transfer of ultrasonic wave energy from the tool to the pipe.

Whether MFL or UT, metal loss features are reported with sizing values for depth, length and width. A very high level of confidence in the results obtained by ILI in pipelines has been achieved by the industry through many years of development and field verification of inspection data. This level of confidence is critically important for application in casing inspections since access to any detected defect is much more difficult.

General layout of the tools used in the LPG casing inspections is shown in Fig 3. The assembly comprises 3 modules: electronic, power supply and distance measurement; metal loss measurement; and an extra weight to ensure stability during lowering and recovery.

 

 

 

 

 
Fig. 3: Schematic tool sketch

 

6. Case study – tool assembly and qualification

 
The photos below show final assemblies of the GEO and UT tools.  Dimensions and weights are indicated in the table.

Fig. 4: GEO and UT Tools

 

Dimensions of 20” tools

Length (mm) Weight (kg) Additional weight unit (kg)
GEO 2.960 190 380
MFL 3.100 340 380
UT 3.600 170 380

 
Dimensions of 12” tools

Length (mm) Weight (kg) Additional weight unit (kg)
MFL 2.300 218 110


 

The number of sensors used for each inspection tool differs depending on the type of sensor, the measurement objective (metal loss, wall thickness, etc.), minimal defect/anomaly to be detected and the size of pipeline. All sensors are distributed over the entire circumference to achieve 360 degree coverage.

Each sensor sampled at 1.000 Hz giving a distance between measurements of 0.3 mm at the maximum speed used of 0.3 m/s.

After design and assembly of the tools, each tool is qualified in accordance of ILI standards [1] [2]. Objective of the tests is to prove the mechanical passage capabilities, run behaviour for the targeted pipeline and to perform dedicated tool calibration. Defect detection and sizing ability is proven and quantified in tests using pipe joints in which representative artificial defects have been introduced.

 

Fig. 5: Pipe with reference defects

Figure 5 shows a test pipe. The defects in this test pipe are round with different depth and diameter. Performance evaluation of a UT system is done in a test structure containing liquid couplant. Together with historical data from tools of similar type, the test proves that the assembled and calibrated tool meets the performance specification.

 

7. Case study – simple set-up and inspection execution

 
The casing inspections took place in 2016. The 12 ¾” casing was inspected with an MFL tool. The 20” casing inspection was additionally supported by an Ultrasonic inspection tool and therefore benefitted from the strengths of both technologies. During the inspection the casings were partially filled with water, otherwise were empty.

In this particular case the preparation needed was limited. The production tubing and the pump were removed. Since there was no pressure at the casing head, the entire operation could be performed in the open casing without any pressure equipment and a very simple set-up. A 5 t winch with a steel cable guided by two pulleys was used. The crane was placed next to the casing head to hold one of the pulleys (Refer fig. 8 and 9). One pulley was fixed at the casing head flange with a sling. Since the winch could only able to be placed on a loose gravel surface, it was further secured to an anchor to prevent any movement.

The tool movement was controlled by the winch operator. The velocity of the inspection was between 0,1 and 0,3 m/s on both directions.

The simplified set-up for the inspection is shown in figure 7. The inspection tool is inserted vertically in the casing head. It is guided by a steel cable connected to a winch. This simple set-up was sufficient.

 

 

Fig. 6: Simplified setup

 

The winch is connected to an anchor tie point to avoid any unwanted movement. The steel cable itself is guided by two pulleys and lifted by a crane. Using this set-up, tools can be lifted and launched with an optimal angle. The tool is connected to the steel cable for the entire operation. A weight at the bottom of the inspection tool enables the measurement unit to enter the casing and provides a stable position. The tool is lowered in the casing at a speed of 0,1 to 0,3 m/s to the lowermost position and then lifted out. Measurement data are collected in both directions. The two data sets are used for analysis. The inspection process is identical for all described tools.

 

Fig. 7: Setup for the 12” casing inspection

The 20” casing was inspected with both MFL and UT tools. Each inspection run lasted several hours. The entire operation was completed in 2 days. A separate mobilisation achieved the 12 ¾” inspection.

Fig. 8: Lowering 20” UT too

 

8. Case study – inspection results

 
Immediately after the tool runs, a site report was delivered containing all relevant data about the inspection performance including data quality and completeness. A first report about the status of the casing was available a few hours after the inspection. In the final report a detailed list of any metal loss or geometric anomalies was provided showing the length, width, depth in the pipe and distance to the casing head. Several charts and other statistics were provided to summarize the features as well as charts with information of the inspection process (speed profile, rotation of tool, magnetization level or temperature). All information can be viewed and evaluated within a client software.

 

Fig. 9: Sample of MFL sensor data

 
Figure 9 shows typical raw data of a MFL casing inspection. The magnetic field measured by a single sensor is displayed by an individual line. The line on the right side represents the speed profile. On the left side the distance to the casing head can be seen.

An increase of metal results in a decrease of sensor signal. A metal loss feature results in an increase of sensor signal. The horizontal black line in the figure is caused by a girth weld. Further a mechanical centralizer, centering the casing in the borehole, can be clearly identified in the data.

Both casing inspections delivered good quality and complete data. All sensors worked as expected. Some gaps in the well documentation regarding wall thickness and installations could be closed by means of the inspection results.

 

9. Conclusion & view forward

 
Several casing inspections 12” and 20” have been concluded in shallow wells of an underground LPG storage facility. High resolution data on internal and external metal loss as well as geometric discontinuities were obtained by means of ILI equipment. No development effort was required and the need for tool modification was limited based on the existing fleet of pipeline inspection tools at 3P Services.

While pressure and temperature encountered in these shallow wells were moderate, significantly deeper wells can already be inspected. Components are available that work under temperature up to 100 °C and pressure up to 350 bar. This corresponds to wells approx. 3.000 m deep.

Inspection tools as assembled for this project can be operated from workover rigs during regular maintenance work in any type of well.

Having developed ILI tools for small diameter pipelines (2” to 4”), an in-situ tubing inspection can be done provided access to the tubing head is granted.

 

10. References

 
[1] API STANDARD 1163, “In-line Inspection Systems Qualification”, (2013)

[2] Pipeline Operators Forum, “Specifications and requirements for in-line inspection of pipelines”, (2016)

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