QUALITY INSPECTION PLANNING OF OILFIELD EQUIPMENTS USING NONDESTRUCTIVE EXAMINATION

Jude R. Boudreaux, Jim Lee, William J. Emblom and Theodore A. Kozman
Mechanical Engineering Department
University of Louisiana at Lafayette
Lafayette, Louisiana

KEYWORDS

Quality Inspection, Oilfield Equipments, ND Examination, Magnetic Particle Testing, Ultrasonic Testing

ABSTRACT

Petroleum and natural gas companies are facing many challenges in technological advancements. Due to the competitive as well as legal nature of the industry, companies are forced to have functional and reliable equipment. The costs of an equipment failure during operation can be catastrophic (i.e., rig down time, injury to personnel, or even death to personnel). In the past, pipe-handling equipment was mostly visually inspected for surface discontinuities. With the advances nondestructive examination (NDE) methods, a quality inspection procedure is developed in this research to utilize techniques such as magnetic particle and ultrasonic testing methods to improve the quality and safety of oilfield equipments.

INTRODUCTION

This research is motivated by an observation in a local oilfield equipment renting company when a piece of pipe handling equipment (i.e., elevator) was weld repaired due to a crack in the body as shown in Figure 1.

FIGURE 1. DISCONTINUITY AREA #1.

The component was weld repaired then inspected by the AC wet fluorescent magnetic particle nondestructive examination method 48 hours after the cessation of welding. The results of the examination were acceptable.

The tool was shipped to a job location where it was utilized to run casing pipe down-hole. The tool was returned to its base facility where it under-went a post-job visual examination. Visible surface cracks appeared in the area previously weld repaired as shown in Figure 2.

FIGURE 2. DISCONTINUITY AREA #2.

Another AC wet fluorescent magnetic particle examination was performed in process while grinding/gouging the cracks until there were no visible indications. The component underwent another costly weld repair. The need exists for exploration in improved equipment inspection techniques due to the fact that current industry and/or manufacturer’s requirements are not stringent enough to produce “Fit for Use” equipment consistently.

The above situation could have possibly been eliminated if a NDE method such as an ultrasonic examination was performed on the tool prior to defect removal to determine the severity and depth as well as evaluating the area of interest after defect removal to ensure that it is entirely removed. Magnetic particle testing is only sensitive to about 1/4", depending on the type of current used (AC or DC). AC will locate surface or near surface indications very well, while DC tends to travel much deeper through the component being examined. Also, field direction plays a critical role in locating defects.

This research involves quality inspection planning in the oilfield equipment renting industry with the following objectives: (1) ensure that all equipment shipped to a job location is “Fit for Use”, (2) improve customer confidence by consistently providing reliable equipment, and (3) reduce company liability by providing fully functional equipment, which would reduce the potential for rig down time, injury to personnel, or even death to personnel. In order to achieve these objectives, the following steps are necessary: (1) review and evaluate current industry accepted NDE methods, (2) perform experiments to prove the validity of selected methods, and (3) create an inspection planning procedure based on the test results.

NDE METHODS

Various NDE methods are available today. Our literature survey indicates that liquid penetrant (Lovejoy, 1991; Flaherty, 1986; Iddings, 1986) cannot detect subsurface discontinuities. Surfaces of objects to be inspected must be clean and free of organic or inorganic contamination that will prevent interaction of the penetrating media with a surface. Radiography (Halmshaw, 1995) requires expensive equipment, is hazardous to untrained individuals, and required a large or shielded inspection area. Magnetic particle is a simple yet sophisticated form of surface and subsurface examination. Ultrasonic testing is a portable and sensitive form of volumetric examination, which can locate and size internal discontinuities in thick or thin material accurately. Magnetic particle and ultrasonic testing possess the qualities of locating discontinuities in the types of material used by the oil and natural gas industry.

Magnetic Particle Examination

Magnetic particle inspection is a form of surface and sub-surface examination applicable only to ferromagnetic materials. Magnetic particle examination cannot be used with non-magnetic materials such as aluminum, copper, ceramic, plastic, magnesium, or stainless steels.

Magnetic particle is based on the principle that magnetic flux in a magnetized object is locally distorted by the presence of a discontinuity. This distortion causes some of the magnetic field to exit and reenter the test object at the discontinuity. This phenomenon is referred to as magnetic flux leakage. Flux leakage is capable of attracting finely divided particles of magnetic materials, which in turn form an outline or indication of the discontinuity on the object’s surface (Skeie and Hagemaier, 1988). Calibration requirements for magnetic yokes are relatively simple: the yoke must be able to lift a specific dead weight. For alternating current (AC) yokes, a 10 lb weight is lifted with the poles pieces spaced at the testing distance. For direct current (DC) yokes, a 40 lb weight is lifted with the poles pieces 4 to 6 inches apart. The plates for these tests must be certified for the correct weight and are considered and identified as standards (ASME, 2000).

Ultrasonic Examination

Ultrasonic inspection is a form of volumetric examination that enables the inspection of a materials internal composition determining its structural integrity through the use of sound waves without altering their capability to function as intended. Ultrasonic testing is typically performed in two ways. A beam of ultrasonic energy is directed into the test object and (1) the energy transmitted through it is measured (this technique is referred to as through transmission) or (2) energy reflected from discontinuities in the material is measured (this technique is referred to as pulse echo).

The principle applications of ultrasonic inspection consist of: (1) discontinuity detection, (2) thickness measurement, (3) determination of a materials elastic module, (4) study of a materials metallurgical structure, and (5) evaluation of the effects of processing variables on the component (Bar-Cohen and Mal, 1985).

Like many other quality control processes, ultrasonic testing compares the unknown with the known. The proper use of calibrated reference standards provides uniform test criteria independent of the testing system or location. Calibration blocks are used to check the operation of the ultrasonic instrument and transducer and to make certain adjustments to the instrument to best suit the testing condition. No single reference standard is suitable for all ultrasonic testing procedures (Graff, 1981).

EXPERIMENTS

Six experiments were performed on various components. The components were chosen at random after being cleaned from its previous job. None of the components chosen to perform NDE were previously inspected volumetrically by radiography as acceptance of the original casting pour. The experiment consists of evaluating components with conventional NDE techniques (wet fluorescent MT) then experimenting with different MT techniques including dry, wet, and different types of currents - AC, DC, and, HWDC. For the UT technique, straight and angle approach was used. The results of these experiments will either validate or invalidate conventional NDE techniques as well as yielding an inspection procedure to increase the probability of defect detection.

Experiment Design

The detailed experiment designs are shown in Tables 1 and 2. In order to quantify each experiment, the surface condition, geometry, and lighting conditions on each component were evaluated.

TABLE 1. EXPERIMENT DESIGN – MT.

|Exp |Surface |Geometry |MT Method |
|1 |Rough |Complex |AC wet |
|2 |Rough |Complex |HWDC wet |
|3 |Rough |Simple |AC wet |
|4 |Rough |Simple |HWDC dry |
|5 |Rough |Simple |HWDC dry |
|6 |Smooth |Simple |AC wet |

TABLE 2. EXPERIMENT DESIGN – UT.

|Exp |Surface |Geometry |Couplant |
|1 |Rough |Complex |Glycerine |
|2 |Rough |Complex |Ultra-Gel II |
|3 |Rough |Simple |Ultra-Gel II |
|4 |Rough |Simple |Ultra-Gel II |
|5 |Rough |Simple |Ultra-Gel II |
|6 |Smooth |Simple |Ultra-Gel II |

Rough surface conditions consisted of non-uniform surface imperfections (peaks and valleys approximately 0.020 - 0.030 of an inch in height and depth). Smooth surface conditions consisted of a uniform flat surface with minute surface imperfections.

Complex geometry components consisted of unparallel front and back surfaces greater than 15° to one another. Simple or symmetrical geometry components consisted of parallel front and back surfaces within 15° to one another. Lighting conditions were considered good at 1,000 lx for visible MT examinations and 1,000 microwatts per centimeter squared for fluorescent MT examinations at the test surface. All the experiments were performed at good lighting conditions.

Experiment Results

Results were considered valid if discontinuities were detected by both UT and MT, semi-valid if UT could not detect surface discontinuities detected by MT due to excessive surface roughness and/or non-parallel surfaces, and invalid if either inspection method detected nothing. They are summarized in Table 3.

TABLE 3. EXPERIMENT RESULTS.

|Experiment# |MT Results |UT Results |
|1 |Valid |Semi-Valid |
|2 |Valid |Invalid |
|3 |Valid |Semi-Valid |
|4 |Valid |Valid |
|5 |Valid |Valid |
|6 |Valid |Valid |

The results of these experiments validate our concerns with having only an AC wet fluorescent magnetic particle inspection as the primary gauge of component acceptance. The AC wet fluorescent magnetic particle inspection is extremely sensitive in locating surface discontinuities; however, there were several subsurface discontinuities in the test material, which it did not detect. HWDC dry particle examinations demonstrated excellent particle mobility, even to weak subsurface leakage fields. The deepest discontinuity found was 0.750 inch deep while AC wet fluorescent only located discontinuities approximately 0.015 inch deep.

There are three primary advantages to using alternating current as a magnetizing source: (1) the current reversal causes an inductive effect that concentrates the magnetizing flux at the object surface which provides enhanced indications of surface discontinuities, (2) magnetic fields produced are much easier to remove during demagnetization, and (3) the pulsing effect of the flux caused by the current reversals agitates the particles to collect at flux leakage points.

Concentration of the AC flux at the test object surface also can be a disadvantage because most subsurface discontinuities are not detected. Another disadvantage is that some specifications do not allow the use of alternating current on coated components when the thickness exceeds 0.003 inch. Also, the flux in the test object may not be at peak value, depending on where within the magnetizing cycle the current is turned off. Half-wave direct current has penetrating power approximately ten times that of alternating current. HWDC has a flux density of zero at the center of a test object and the density increases until it reaches a maximum at the object surface. The pulsing effect of the rectified wave produces maximum particle mobility for the magnetic particles; dry particles are enhanced by this effect.

The mobility of dry powder with the pulsed magnetic fields (HWDC) produced by yokes is essential for effective testing of irregular surfaces. Testing with yokes and fluorescent suspensions is increasingly required for welds that are ground smooth. While testing parts with rough surfaces, wet fluorescent particle suspensions can be trapped in depressions, creating unwanted background fluorescence.

Ultrasonic examination proved to aid in the detection of discontinuities in most cases. The main drawbacks noticed were the material surface finish and unparallel front and back surfaces. Energy loss due to casting metallurgical grain structures was not a problem area while examinations were performed.

The surface condition and geometric shape of a specimen greatly influences the transmitting and receiving of ultrasonic energy. Surface roughness complex geometric shaped specimens can cause several undesirable effects including: (1) loss of discontinuity and back surface reflections, (2) increased width of the front surface indication and consequent loss of resolving power, (3) divergence of the sound beam, and (4) spurious generation of surface waves.

QUALITY INSPECTION PROCEDURE

While testing components with unparallel surfaces (greater than 15°), the reflected energy will be directed away from the transducer at an angle, thereby receiving no response. When the front and back surfaces are parallel, the specimen is ideal for distinguishing discontinuities, which means that the sound beam is direct and we have the least amount of attenuation.

The microstructures and grain sizes of metals and alloys can be changed significantly by mechanical and thermal processes commonly used in primary mills and subsequent manufacturing operations. It is essential for the ultrasonic technician to know the actual condition of the material at the time of testing. Hot working and heat treatment of steels can produce radical changes in grain size and internal microstructure and these can in turn change ultrasonic test sensitivity and reliability.

It has also been found through this research that the method of defect excavation has the potential to cause the defect to propagate in unrealistic lengths. It is not within the scope of this paper to discuss all excavation methods; however, the most common excavation method, which is manual grinding, will be discussed.

Grinding is an operation used to remove surface or near surface discontinuities or to provide a specific surface finish. Grinding operations can also produce thermal cracks from localized overheating. The fast rate of heating and cooling encountered with grinding is often overlooked or neglected because the principle objective is complete removal of the discontinuity. Grinding cracks are typically perpendicular to the grinding wheel direction. The probability crack propagation can be greatly reduced and/or eliminated by rotating the grinding wheel 90º, and grinding in a direction that is parallel with the crack.

In addition to grinding discontinuities in the proper direction, it may prove necessary to drill through the discontinuity’s farthest edge to prevent propagation. Ultrasonic testing can easily pin point the location of a discontinuity except when the specimen’s surface is extremely rough or while testing a complex shaped specimen.

Industry Recommended Testing

According to several International Standards Organization and American Petroleum Institute Specifications and Recommended Practices (1997, 2001), the accepted form of nondestructive inspection methods to ensure quality oilfield related handling components is: ferromagnetic materials shall be examined by the magnetic particle method utilizing the wet fluorescent technique or nonferromagnetic materials shall be examined by the liquid penetrant method in regards to a surface examination and by either radiography or ultrasonic in regards to a volumetric examination. All accessible surfaces shall be examined by either magnetic particle for ferromagnetic materials or liquid penetrant for nonferromagnetic materials. Only primary load bearing castings are required to be volumetrically examined using either radiography or ultrasonic.

API documents categorize inspection requirements by category. For instance, a category I inspection involves observing the equipment during operation for indications of inadequate performance. A category II inspection involves a category I inspection plus further inspection for corrosion, deformation, loose or missing components, deterioration, proper lubrication, visible external cracks, and adjustment. A category III inspection involves a category II inspection plus further inspection, which should include NDE of critical areas and may involve some disassembly to access specific components and to identify wear that exceeds the manufacturer’s allowable tolerances. A category IV inspection involves a category III inspection plus further inspection for which the equipment is disassembled to the extent necessary to conduct NDE of all primary load carrying components as defined by the manufacturer.

These documents contain vague recommended inspection guidelines to use once a casting is purchased and put in service; for instance, these documents give no information as to which NDE method is required for a category III or category IV inspection. It is up to the manufacturer of the equipment to determine which NDE method will yield the best results, which wet fluorescent magnetic particle is recommended a majority of the time.

Current Inspection Procedure

A typical sequence of events in which equipment flows through the system once it is received from a job location to when it is shipped from stock to another job include steam cleaning, informal visual inspection, and AC wet MT. They are discussed below:

1. Steam Cleaning: As equipment is received from a job location the residue from normal job operations is removed by steam cleaning.
2. Informal Visual Inspection: Once the equipment is clean and free from dirt, grease, oil, or other foreign material an informal visual inspection is performed on visible surfaces. This informal visual inspection is a quick look for gross discontinuities. If no gross discontinuity is noticed, the equipment is put into stock.
3. AC Wet MT Testing: Should a surface discontinuity be observed during this informal visual inspection, the area of interest is then inspected using AC wet magnetic particle, repaired accordingly, re-inspected, and put into stock.

This process contains several faults: (1) lack of inspection structure, (2) lack of inspection thoroughness, i.e. the only nondestructive examination performed is AC wet magnetic particle, which can only detect surface discontinuities, and (3) unless an inspection is specified by a customer, defective equipment can be put into service leading to catastrophic results such as expensive rig down time, damage to other equipment, injury to personnel, or even death to personnel

Proposed Inspection Procedure

Based on our experiment results and evaluation of the current inspection procedure, a revised inspection procedure is developed as discussed below:

1. Steam Cleaning: As equipment is received from a job location the residue from normal job operations is removed by steam cleaning.
2. Visual Inspection: Once the equipment is clean and free from dirt, grease, oil, or other foreign material a planned visual inspection is performed on all accessible surfaces. If no discontinuity is noticed and the equipment is not scheduled to be further NDE inspected, the equipment is put into stock.
3. AC wet and HWDC dry MT: Should a surface discontinuity be observed during the visual inspection or a scheduled NDE inspection is due, the component is inspected by AC wet and HWDC dry magnetic particle when the component’s shape is complex (front and back surfaces greater 15° from one another).
4. AC Wet MT and UT Examination: AC wet magnetic particle and ultrasonic examination are used when the component’s shape is symmetrical (front and back surface within 15° - parallel to one another), repaired accordingly, re-inspected, and put into stock.

CONCLUSIONS

Engineering materials are currently being pushed to their maximum with safety factors being not as conservative when compared to 10 years ago. Metals are being alloyed to the extent that components are not nearly as massive with material reduction ratios along the range of 1.5:1. Having said this, the need for extensive inspections, both surface and subsurface is more obvious than ever before.

Each technique within the magnetic particle and ultrasonic inspection methods has advantages and limitations. The method and technique currently accepted by industry is AC wet fluorescent magnetic particle inspection. This technique is most effective in locating discontinuities that are open to the surface. This scenario gives sound reasoning why subsurface discontinuities can’t be detected; however, ultrasonic examination would compliment this situation with a volumetric examination brilliantly.

Should magnetic particle solely be utilized for defect detection, more than one technique should be deployed. As mentioned above, AC wet or dry is limited in locating discontinuities that are open to the surface. Should a discontinuity be located slightly sub-surface (approximately 0.050 inch deep), this technique will not detect it. A second technique should be utilized to ensure that any surface discontinuities found are not broken-up and don’t actually travel much deeper into the component. The HWDC dry technique proves to have the characteristics of locating discontinuities almost an inch deep into the test material.

Ultrasonic examination permits testing of a wide range of sizes and geometries. The technique detects internal, hidden discontinuities that may be deep below the surface. Transducers are available to generate waves of several types, including longitudinal, shear, and surface waves. Applications range from thickness measurements of thin steel plate to internal testing of large aircraft components.

Discontinuity depth can be indicated within millimeters, even in thick test objects. With the use of the proper frequency, a well-defined sound beam permits detection and location of the smallest critical discontinuity. Rounded, elongated, disk shaped, and cracks of almost zero thickness can be detected. There is no hazard to the operator or to nearby personnel during the use of ultrasonic testing. Access to only one surface is typically required.

The inspection costs incurred while inspecting components are minute when compared to the outcome. The benefits and costs of these nondestructive examinations will out weight the cost of rig down time, injured personnel, or even death to personnel any day. A typical ultrasonic unit will cost approximately $5,000.00 and a magnetic particle yoke is approximately $250.00. The inspection time to perform a magnetic particle examination and an ultrasonic examination is dependent on the size of component being examined, i.e. a large component is about 1½ hours while a small component would take about ½ hour.

The bottom line is that today’s economy and competitive business environment is placing more demands and emphasis on quality products and services which are backed by tested and proven procedures as well as trained and certified technicians. Utilizing the benefits of an AC surface magnetic particle exam and/or HWDC subsurface magnetic particle exam as well as complementing either of the two with ultrasonic examination while inspecting components, the probability of defect detection will increase tremendously. Utilizing these NDE methods of examination will in turn accomplish what every company should aim for: (1) ensure reliability of the end product, (2) save lives or prevent accidents, and (3) save money for the end user.

Directions for future research are: (1) assist in the implementation process and monitor the inspection results. This will provide tangible data for inspection adjustments, (2) develop detailed magnetic particle and ultrasonic inspection procedures containing essential inspection variables as well as accept and reject criteria, (3) study the time involved to train nondestructive inspectors to achieve the results yielded in this paper, (4) apply new inspection methods to the oil and natural gas industry as technology evolves, (5) apply these inspection techniques to other industries, (6) study other inspection aspects of the oil and natural gas industry, and (7) a detailed cost analysis.

REFERENCES

American Society of Mechanical Engineers (2000). Nondestructive Examination, 2000 addenda. ASME Section V. New York, NY.

American Petroleum Institute (1997). Specification for Drilling and Production Hoisting Equipment, third edition. API Specification 8C. Washington, DC.

American Petroleum Institute (2001). Specification for Drilling and Well Servicing Equipment, third edition. API Specification 7K. Washington, DC.

Bar-Cohen, Y. and A. K. Mal. (1985). “Ultrasonic Inspection.” Metals Handbook, ninth edition. Vol. 17. Metals Park, OH: American Society for Metals. pp. 231-277.

Flaherty, J.J. (1986). “History of Penetrants.” Materials Evaluation. Vol. 44, No. 12. Columbus, OH: American Society for Nondestructive Testing. pp. 1,371-1,374, 1,376,1,378, 1,380, 1,382.

Graff, K. (1981). “A History of Ultrasonics.” Physical Acoustics. Vol. XV. New York, NY: Academic Press. pp. 1-97.

Halmshaw, R., (1995). Industrial Radiography, 2nd edition. Springer.

Iddings, F.A. (1986). “The Basics of Liquid Penetrant Testing.” Materials Evaluation. Vol. 44 No. 12. Columbus, OH: American Society for Nondestructive Testing. pp. 1,364.

Lovejoy, D. (1991). Penetrant Testing: A Practical Guide. London, United Kingdom: Chapman and Hall.

Skeie, K. and D. Hagemaier (1988). “Quantifying Magnetic Particle Inspection.” Materials Evaluation. Vol. 46, No. 6. Columbus, OH: The American Society for Nondestructive Testing. pp. 779-785.