Dale B.A., and Moyer, M.C., Exxon Production Research Co. (USA)
Drill-string failures continue to be a costly problem for the oil industry. To reduce the occurrence of these failures, operators and drilling contractors typically employ service companies to perform periodic nondestructive inspections to remove damaged tubulars from service. Nonetheless, drill-string failures continue to occur, most often as a result of some form of metal fatigue. This paper presents the results of several field evaluation programs which were conducted to assess the performance of commercial drill-string inspection services employing conventional non-destructive testing techniques. A practical estimate of the effectiveness of these commercial inspection services is made using modern fracture mechanics concepts and fatigue crack growth data obtained from full-scale testing. Based on the data presented in this paper, commercial drill-string inspection services are found to be statistically quantifiable, however their effectiveness is substantially less than optimum. Improved flaw detection technology is needed to most effectively reduce typical downhole fatigue failures.
Drill-string failures are a serious problem that have troubled the petroleum industry for many years, often resulting in lost rig time, damaged tubular goods, and occasionally abandoned or side-tracked wells. Operators and drilling contractors actively try to prevent these costly failures by performing periodic nondestructive inspections to remove damaged tubulars from service. Depending on the severity of drilling, these inspections are often performed several times for key components of the drill string during the course of a well.
Drill-string inspections are usually performed by service companies. These commercial inspection service companies typically perform a variety of visual, dimensional, and nondestructive flaw inspection procedures to classify the pipe for future drilling service. These procedures usually consist of both automated and manual techniques to assess the condition of the connections, drill-pipe body, and drill-pipe transition regions. Classification is normally completed In accordance with API specifications, but detailed user-specifications are also frequently used to ensure the quality of inspection work performed.
Despite taking these preventative steps, drill-string failures continue to occur, usually due to some form of metal fatigue. Fatigue is a progressive mechanism that primarily results from the synergistic actions of both cyclic stress and corrosion over time. Fatigue damage is initially manifested in the form of microscopic cracks (i.e. fatigue crack initiation). Fatigue cracks usually originate in the thread roots of the connections or in the drill-pipe body and transition regions as shown in Figure 1. These microscopic cracks develop until macroscopic cracks appear which then propagate through the wall thickness until failure, resulting in what is commonly known as a "washout" or "twist-off". Conventional nondestructive testing techniques are only capable of detecting macroscopic cracks. Thus, for inspections to be of any practical value, inspection intervals are "effectively" limited by the time for undetected cracks to grow until failure. The sensitivity and reliability of these commercial inspection services strongly influence their effectiveness in reducing drill-string failures.
Within the last twenty years, a great deal of effort has been directed toward the better understanding of nondestructive inspection performance. This largely occurred as a result of a new philosophy that evolved into engineering practice known as "defect tolerant" design — the principal concept that all structures or components possess defects, either from manufacture or from service. Consequently, the fatigue life or service life between inspections can be established based on fatigue crack growth. The role of nondestructive inspection is important to this design basis since critical flaws (i.e. defects) must be identified and either removed or repaired to avoid failure. Performance testing of nondestructive inspections has been widely accepted and practiced by other industries, but to a lesser extent within the oil field.
This paper presents the results of several field evaluation programs that were conducted to determine the effectiveness of commercial nondestructive inspection services. The results of this investigation are valuable for establishing practical inspection intervals based on modern fracture mechanics concepts and conventional nondestructive testing techniques. More importantly, though, the results of this investigation support the development and use of improved law detection technology to cost-effectively reduce downhole fatigue failures.
A brief review of the terminology associated with performance testing of nondestructive inspections is warranted.