Abstract
The bottomhole assembly (BHA), the lower part of the drillstring on an oil drilling rig, extends from the drillpipe to the drill bit. High dogleg severity (DLS) curvatures during directional drilling can create fatigue issues in the drillstring because of high alternating stresses and strains around the porthole in the collar. The manufacturing tolerances affect the collar-chassis and plug-porthole clearances, as well as the porthole dimensions and can cause significant variability or uncertainty in evaluating the drill tool fatigue life.
The 3D finite element analysis (FEA) method was used for stress and strain predictions of the drill tool under the effect of pressure and DLS. The rotating bending fatigue life of the drill collar was calculated using these prediction results and the life-strain model that was created through testing the collar material. The FEA models were verified with the developed analytical calculation models for stress predictions under the effect of pressure and DLS. The relevant manufacturing tolerance ranges for the collar, chassis, porthole, and port plug were considered. The uncertainty in the fatigue life of the tool was quantified for each of these variables at the expected operating bending moment to evaluate their contributing role in the fatigue performance of the tool.
For a drill tool with a nominal diameter size of 8.25 in, it was found that the collar could show up to 76% uncertainty in fatigue life when the chassis-collar clearances were increased from the minimum to the maximum within the manufacturing tolerance bands. Similarly, the collar showed up to 38% uncertainty in fatigue life when the plug-pothole clearances were changed from the minimum to the maximum. This uncertainty quantification study provides insight into the mechanics of structural fatigue from collar assembly components and their manufacturing tolerance conditions. This helps eliminate time-consuming design iterations and expensive fatigue testing.
