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Drillpipe failures is a prevalent drilling problem. It can be put into one of the following categories: twistoff caused by excessive torque; parting because of excessive tension; burst or collapse because of excessive internal pressure or external pressure, respectively; or fatigue as a result of mechanical cyclic loads with or without corrosion. This article discusses each cause and possible preventive measures.
Pipe failure as a result of twistoff occurs when the induced shearing stress caused by high torque exceeds the pipe-material ultimate shear stress. In vertical-well drilling, excessive torques are not generally encountered under normal drilling practices. In directional and extended-reach drilling, however, torques in excess of 80,000 lbf-ft are common and easily can cause twistoff to improperly selected drillstring components.
Pipe-parting failure occurs when the induced tensile stress exceeds the pipe-material ultimate tensile stress. This condition may arise when pipe sticking occurs and an overpull is applied in addition to the effective weight of suspended pipe in the hole above the stuck point.
Collapse and burst
Pipe failure as a result of collapse or burst is rare; however, under extreme conditions of high mud weight and complete loss of circulation, pipe burst may occur.
Fatigue is a dynamic phenomenon that may be defined as the initiation of microcracks and their propagation into macrocracks as a result of repeated applications of stresses. It is a process of localized progressive structural fractures in material under the action of dynamic stresses. It is well established that a structural member that may not fail under a single application of static load may very easily fail under the same load if it is applied repeatedly. Failure under cyclic (repeated) loads is called fatigue failure.
Drillstring fatigue failure is the most common and costly type of failure in oil/gas and geothermal drilling operations. The combined action of cyclic stresses and corrosion can shorten the life expectancy of a drillpipe by thousand folds. Cyclic stresses are induced by dynamic loads caused by drillstring vibrations and bending-load reversals in curved sections of hole and doglegs caused by rotation. Pipe corrosion occurs during the presence of O2, CO2, chlorides, and/or H2S. H2S is the most severely corrosive element to steel pipe, and it is deadly to humans. Regardless of what may have caused pipe failure, the cost of fishing operations and the sometimes unsuccessful attempts to retrieve the fish out of the hole can lead to the loss of millions of dollars in rig downtime, loss of expensive tools downhole, or abandonment of the already-drilled section below the fish.
In spite of the vast amount of work that has been dedicated to pipe fatigue failure, it is still the least understood. This lack of understanding is attributed to the wide variations of statistical data in determining type of service and environment of the drillstring, magnitude of operating loads and frequency of occurrence (load history), accuracy of methods in determining the stresses, quality control during manufacturing, and the applicability of material fatigue data.
Although pipe failure cannot be eliminated totally, there are certain measures that can be taken to minimize it. Fatigue failures can be mitigated by minimizing induced cyclic stresses and insuring a noncorrosive environment during the drilling operations. Cyclic stresses can be minimized by controlling dogleg severity and drillstring vibrations. Corrosion can be mitigated by corrosive scavengers and controlling the mud pH in the presence of H2S. The proper handling and inspection of the drillstring on a routine basis are the best measures to prevent failures.