Recent advances in the modeling of a drillstring inside a curved borehole
Accurate calculation of the position of the contacts between the drillstring and the borehole as well as the determination of the extent of these contacts is the true challenge of a torque-and-drag analysis. Such calculations are required to estimate the loss of torque from the rig to the bit under some assumed friction models, but also to assess the axial force on the bit (the “weight on bit”) given the known axial force at the rig (the “hook load”). Indeed, the weight on bit directly influences the rate of penetration, but also the drilling tendency of bottomhole-assemblies equipped with rotary steerable systems [3, 6]. The accurate and efficient structural analysis of such a slender body, which is constrained to deform inside a borehole, is thus of paramount importance for the modeling of drilling operations. In the late 1990’s, the state-of-the-art in torque-and-drag modeling has shifted from the “soft string model,” based on the assumption that the drillstring coincides with the borehole axis [4], to advanced numerical methods with built-in management of the contacts. Standard finite element techniques have been applied to the torque-and-drag problem, but they require a fine mesh to properly capture the response of the system in the vicinity of contacts [5]. Furthermore, the slenderness of the drillpipe, combined with the limited clearance between the drillstring and the borehole wall are two of the reasons that make the application of such standard techniques rather inefficient [2]. As an alternative, we have developed an approach that segments the drillstring between contacts and treats the series of unconstrained (or classical) elastica problems between successive contacts as a sequence of simpler auxiliary
Stability and response of closed loop directional drilling system using linear delay differential equations
A directionally controlled drilling mechanism has a lateral borehole propagation equation of state composed of spatially delayed feedback terms determined by the distribution of contact points between the borehole and drilling mechanism. A lateral borehole propagation equation is developed for a generic class of steering system. After simplification the resulting linear delay differential equation is partially validated against a Finite Element Model (FEM) of borehole propagation. The linear delay differential equations are then incorporated into various closed loop steering configurations to further demonstrate the efficacy of the approach.
Study on quantitative earthquake damage of Dujiangyan city, caused by 2008 MS=8.0 Wenchuan, China earthquake based on aerial imagery
The paper introduces quantitative earthquake damage estimation method based on remote sensing data. The quantitative remote sensing seismic damage index based on building damage extracted from the aerial image of Dujiangyan city proper, southwest China's Sichuan province, which acquired after 2008 Ms=8.0 Wenchuan earthquake. The actual seismic damage indices were determined by the ground-based investigation in the city proper. Then their quantitative relationship was established. The results indicate that it is possible to build quantitative model of remote sensing seismic damage, which will enhance significantly the practicality applying the remote sensing in emergency acquisition and analysis of seismic disaster.