RIASSUNTO
Abstract
Because wells drilled by the oil and gas industry are becoming more complex, the drillstring is subjected to additional severe loading conditions, resulting in increased failure risk. It is universally known that fatigue is one of the primary causes of drillstring failure, accounting for more than 70% of the failures. Drillstring failure generally results in unexpected catastrophic twistoff of bottomhole assembly (BHA) components and costly fishing operations. For these reasons, it is important to develop a drillstring fatigue life prediction model.
Fatigue is driven by cyclic stresses and accumulates over time. These cyclic stresses can occur in a wide range of conditions, such as rotating the drillstring through a high-dogleg severity well section, and severe bending due to whirling or buckling of the drillstring. The procedure relies on the powerful and accurate drilling dynamics computation engine, which is based on a 3D finite element model and which predicts transient dynamics response and stresses along the drillstring under drilling operations loading conditions. First, the section being drilled is subdivided into multiple small intervals. For each interval, simulation is used to predict the drillstring deformation and contact force. Stresses can then be evaluated for each component in the drillstring.
Due to the cumulative nature of fatigue failure, it is necessary to track the cycle of alternative stresses of the drillstring while drilling an entire section. For stable rotary drilling, the number of stress cycles can be calculated from the number of rotation revolutions within the interval. In the real-time fatigue monitor application, the actual drillstring revolutions measured at the surface can be used directly as the stress cycle. When severe downhole vibration exists, the rain-flow counting method is used to count the stress cycles for the complex dynamics stress history. To consider mean stress effect, the Goodman law is used to compute the equivalent alternative stress. With the pre-collected fatigue S-N curves (stress level versus the cycle to failure) for different connection types and drilling tools, the fatigue life consumption in one interval is calculated. Finally, using Miner’s rule, the fatigue life results in all intervals are summed to obtain the cumulative fatigue damage to the drillstring. The fatigue model, which has been implemented as one of the key components in the drilling analysis workflow, provides engineers with the analysis capability to identify the potential factors influencing the integrity of BHA components. Two case studies validated the drillstring fatigue prediction model.
This paper presents a practical and effective procedure for calculating drillstring and BHA component life due to fatigue accumulation. This modeling tool enables engineers to employ a systematic approach for quantifying the fatigue risk during the well planning and real-time execution phase.