Design for Reliability Analysis of Vibration-Induced Failures due to Equipment Assembly Bending Load and Vibration Responses
Authors: Liew Josh , Fanini Otto 
Published: November 2017

Abstract
The oil and gas industry subsurface formation evaluation instruments experience significant challenging vibrations and shock levels. Equipment development requirements for these instruments include reliability and durability under these severe operating conditions.
The engineering design for this equipment utilizes tools that enable the estimation of service lifetime, maintenance cycles, and related costs. These engineering tools model failure modes and their acceleration factors and how failures interact under certain circumstances. Laboratory test data and operations failure history are used to validate the model results. Incorporating equipment operational failure history into the reliability design after failure analysis, enables determination of failure modes and the length of stress level exposure. Before the equipment is commissioned to field operational service, it is subjected to a batch of environmental qualification tests under objective pass-fail criteria. The environmental qualification test conditions and adopted stress levels are acquired through measurements made with sensors (temperature, acceleration, and shock) in the equipment assembly during field operational conditions in targeted environments. After the equipment passes the qualification tests and final inspection, it is commissioned to field commercial service. This paper studies the development of specific equipment failures during operational field deployment after they were subjected to standard environmental qualifications tests. Various investigative actions focused on determining the cause and circumstances that led to the unexpected field failure. Results helped to introduce corrective updates to the equipment design and manufacturing, durability and reliability design models and procedures, environmental stress levels, and corresponding qualification test conditions. The equipment failures were examined, and comprehensive 3D custom vibration and stress modeling were conducted for the entire equipment assembly and each assembly module. The modeling results pinpointed and confirmed the high stress levels in the failure areas. These high stress levels exceeded the assembly construction strength thresholds, causing failures. The equipment assembly was modified and reinforced to properly support the detected stresses and provide the required lifetime reliability and durability for the operational service.
A full 3D model of the equipment assembly was used for the vibration and bending load analysis including all mechanical assembly parts, electronics modules, couplings, and attachments. The 3D model was meshed with Tet and Hex elements in ANSYS application software, failure-prone and critical regions were meshed with finer divisions. In this analysis the electronics modules assembly were considered with all parts, attachments, structural frames, linkages, carriers, and printed circuit board (PCB) modules properly attached and connected to the main chassis structural carrier. Geometries, mass, module and assembly attachments, and material properties were assigned to components in this model. External loads and boundary conditions environmentally imposed to the assembly were applied in the model. Environmental conditions, shock, and vibration (x, y, and z) recorded from similar equipment deployed in subsurface operations in equivalent wells and geological formations were used in the modeling parameters. Displacement modeling data and analysis was performed for all mechanical structural components, PCB electronics module assemblies and assembly components, and module electronics component attachments. A model harmonic analysis under static conditions was performed to detect the oscillatory modes and vibratory resonances and the extent of oscillatory displacements. A structural and main carrier chassis modal analysis was conducted for the entire model, identifying the dominant oscillatory modes and natural structural oscillatory frequencies. The displacement can be used for detection of maximum allowable plastic deformation threshold and cyclic fatigue analysis of attachments, structural support members, and linkages for equipment service lifetime durability and reliability assessment. Past field instrumented operational conditions with documented failures and lab characterization of failure modes along with failure behavior and failure triggering thresholds have provided limits for the mechanical and electronics assembly technology with maximum acceleration level of random vibration and maximum equivalent stress level tolerated by the equipment's structural assembly, standard design techniques, and materials. With these structural stress and displacement limits the 3D modeling results were inspected for the entire assembly, identifying the points in the mesh model where these limits were exceeded. The inspection determined that these recommended limits had been exceeded according to the model results, placing a reduced importance to the adjustment of tolerable maximum stresses and displacements. The mesh points with excessive stress and displacement-induced fatigue coincided with the areas where field failure had been detected in examined field failed units. Because of this modeled assembly performance result and details from the externally imposed operational shocks and vibration, the equipment mechanical and electronics assembly structural design were re-engineered to produce an updated model simulation results that did not exceed the demonstrated cumulative failure threshold stresses in lab tests and field operations. The modified equipment assembly was built and environmentally re-tested in the lab environment with more instrumentation points and scrutiny around the failure critical areas. The test results were successful. After deployment of the new and updated equipment assembly version, its field deployment has not observed similar field failures compared with the previous design version.
These modeling and engineering tools, qualification test procedures, and methods can be used to validate a new design or understand the most effective and economical approach to iterate the design before it is launched to field operations or after a field failure.