Experimental Study on Critical Displacement for Drill-Conductor Injection during Deepwater Drilling

Summary Drill-conductor-jetting technology is a high-efficiency, good-adaptability, and low-cost technology that has been widely applied in deepwater drilling. However, a reaming effect will be produced easily because of jet breaking and bit rotation during the jetting process, and the critical displacement would be notably affected. Also, it will experience a relatively short soaking time after installation because of the requirements of drilling timeliness, which is an important factor on the bearing safety of a conductor. Therefore, it is meaningful to study the influencing factors of construction conditions and establish a model for evaluating the value of critical displacement. In this study, field experiments on critical displacement for simulating the deepwater-drilling conditions were conducted. By analyzing the drilling hydraulic factors, the effects of soil-stress-recovery time, and the injection rate of pipe, the influence laws of different factors were obtained. The results suggest that the critical displacement increases linearly as the circulation rate of the drilling fluid increases, decreases exponentially with the increase of soil-stress-recovery time, and decreases linearly with the increase of injection rate. One model for estimating the critical displacement using experimental data and the least-squares method was proposed. The predictions showed good agreement with experimental data within suitable ranges of models. This work is expected to provide the basis for predicting conductor stability and wellhead-bearing settlement.

Response Analysis of the Free Field under Fault Movements

A quasistatic simulation of highly nonlinear problems under fault movements was carried out using the EXPLICIT module of ABAQUS. Combined with the secondary development program of the software, the application of the strain softening Mohr–Coulomb model in the simulation was realized. Free field-fault systems were simulated with two types of fault types (normal and reverse faults), four fault dip angles (45°, 60°, 75°, and 90°), and two kinds of soil (sand and clay). Moreover, the rupture laws and sensitivities of the sand and clay were studied with different soil thicknesses and different fault dip angles in the free field. The results show that the width of the ground zone with obvious deformation, which represents the point of the fault outcrop, the critical displacement of the fault, and the rupture characteristics of the overlying soil are closely related to the fault type and soil parameters. The critical displacement of the reverse fault is larger than that of the normal fault. The width of the ground zone with obvious deformation varies from 0.65 to 1.3 and does not exhibit a regular relationship with the type of soil. Compared with a normal fault, the rupture of a reverse fault is not prone to exposure at the surface.

Snap-Through of Shallow Extensible Arches Under Unilateral Displacement Control

Critical displacements are determined for snap-through of shallow, extensible, and elastic arches that are pushed downward quasi-statically at any point along the span. The initial arch is circular and unstrained, and the ends of the arch are pinned and immovable. When the vertical position at the push-down location reaches a critical value, the arch jumps into an inverted shape (unless the arch is extremely shallow). The critical displacement is given or approximated by an unstable equilibrium configuration of the unloaded arch, for which an analyical formula is derived.

Fiber Orientation Effect in the Dynamic Buckling of Debonded Regions in Laterally Loaded FRP Strengthened Walls

This paper studies the effect of lamination and fiber orientation on the geometrically nonlinear dynamic response of debonded regions in walls strengthened with FRP. The paper adopts an analytical/numerical approach and uses a specially tailored finite element formulation for the layered structure. By means of this analytical/numerical tool, two strengthening layouts for a wall segment subjected to a dynamic shear loading are compared. In the first layout, the fibers are oriented along the width and height of the segment and in the second one, they are oriented along its diagonals. The analysis reveals that the two layouts are involved with significantly different critical points and significantly different dynamic post-buckling behaviors. Specifically, it shows that the diagonal layout, which better serves the shear loading scenario, is involved with a much smaller critical displacement and a dynamic post-buckling behavior that is governed by the stiffer compressed and tensed diagonals.