Inspection Interval Guidelines To Reduce Drillstring Failures

Abstract

Inspection interval guidelines were developed to reduce the occurrence of costly drill string fatigue failures. The guidelines are based on a state-of-the-art fracture mechanics model and incorporate reliability data of commercial inspection services using conventional nondestructive testing techniques. On the basis of extensive full-scale testing, the model was found to predict fatigue-crack-growth behavior reliably. By establishing inspection intervals on the basis of fatigue-crack growth, inspection services are systematically and cost-effectively specified to achieve acceptable reliability (or risk). Simple equations and charts are presented in terms of practical drilling parameters to reduce typical downhole fatigue failures. IntroductionFatigue and corrosion fatigue are recognized as a major cause of downhole driIIstring failures. 1 In practice, the oil industry attempts to guard against the likelihood of these costly failures by performing periodic nondestructive inspectiops to remove damaged tubulars from service. 2 Drillstring inspections are usually conducted by service companies that perform a variety of visual, dimensional, and nondestructive inspection procedures to locate cracks and to classify the pipe for future drilling service. 3 These procedures generally consist of both automated and manual techniques to assess the condition of the connections and pipe body. Classification is normally completed in accordance with API specifications,2.4 but detailed user specifications are also frequently used to ensure the quality of inspection work performed. 5 Conventional nondestructive testing techniques are capable only of detecting fatigue damage that results in the formation of macroscopic cracks. Thus, inspection intervals are "effectively" limited by the time for undetected cracks to grow until failure. Inspections are typically performed before the start of a well but may be performed several times throughout the drilling of deep, deviated wellbores or "hard-rock" formations. Inspection intervals traditionally have been selected on the basis of experience in a given regional area, but costly failures often occur during this learning process.This paper describes the development and application of a fracture mechanics model, verified by extensive full-scale testing, to predict fatigue-crack-growth behavior in the pipe body and connections. After reliability data of commercial inspection services are discussed, simple equations and charts are presented in terms of practical drilling parameters to reduce typical downhole fatigue failures.

Fatigue of Drillstring: State of the Art

“…As per Dale [45], electromagnetic inspection method applied on drillpipe body has a probability of detection (POD) of 90-percent providing that crack depth is larger than 0.039 inch (1 mm). POD is close to 99-percent when minimum crack depth is 0.049 inch (1.25 mm).…”

“…Based on these results, Dale proposes the following example on the pipe body inspection [45]. If three inspections are planned during life duration of a drillpipe,…”

“…Formulae are existing to calculate life duration [45], e.g. (25) where fatigue crack propagation may be expressed as a function of cyclic bending stress σ b , outside diameter D, crack plane diameter d c , and fatigue constants F 0 and n, depending on steel characteristics (typically 7.25 10 5 < F 0 < 1.35 10 6 , 2.52 < n < 2.94).…”

Case Study: Drillstring Failure Analysis and New Deep-Well Guidelines Lead to Success

“…5 However, the slip/upset area of the drillpipe tube is the most common location for fatigue crack initiation sites and corresponding drillpipe tube failures. Depending on the criticality of the proposed well, it may be necessary to either pick up a new drillstring or take steps to understand the reliability of drillstring inspections.…”