Vibration Analysis, Model Prediction, and Avoidance: A Case History

Abstract

In recent years, there has been a greater emphasis on technical limits with respect to drilling operations. Oil and gas companies strive to be amongst the highest ranked in operational efficiency with respect to industry related benchmarking statistics. Increasingly complex drilling scenarios, both surface and down hole, coupled with extremely high operating day-rates provide a challenging environment for those partaking in such operations. The risks are high, but the rewards are as well. Companies that can operate efficiently in such scenarios will most certainly reap the rewards. Understanding technical limits and achieving operational excellence requires proper planning. Unfortunately, proper planning is only a part of the equation. The complexity of the subsurface environment generally results in deviation from planned operations. These deviations may result in significant non-productive time or in equipment failure. Reducing such incidents requires knowledge that the incident took place. Knowledge that the incident took place requires accurate data mining and retrieval as there may be significant time between operations, and knowledge transfer can be lost due to personnel changes. Additionally, an understanding of the root cause of an incident needs to be understood to prevent future incidents. This is generally more difficult to get a handle on. Applying post failure analysis utilizing engineering software applications is an essential part of determining the causes of non-productive time incidents and equipment failures. One could also surmise that it is a critical part of the planning process even though the analysis is after the fact. Reviwing lessons learned is a critical part of any planning process. In high risk, high cost environments, such as ultra-deep, ultra-deep waters, refining advanced technologies for successful completion of wells is paramount. Challenges are still very much associated with complex BHAs (bottomhole assemblies) and with the vibration of the drillstring. The more widespread use of sophisticated downhole tools --- for both directional telemetry and for logging while drilling applications (as part of the front line data acquisition system within the drilling process) --- has meant that reliability is of prime importance1–5. This paper presents and validates an existing model to predict severe, damaging vibrations. The paper also reviews analysis techniques and guidelines to successfully avoid the vibration damage to downhole tools and their associated down hole assemblies. Methodology and Calculation The dynamic analysis model is based on a forced frequency response (FFR) analysis to solve for resonant frequencies. In addition, our mathematical formulation includes viscous, axial, torsional and structural damping mechanisms. With careful consideration of input parameters and judicious analysis of results, we were able to demonstrate that drillstring vibration can be avoided by determining the three dimensional vibrational response at selected excitations that are likely to cause them. Also, the analysis provides an estimate of relative bending stresses, shear forces, and lateral displacements for the assembly used. Based on the study, severe vibrations causing potentially damaging operating conditions were avoided, a situation which was a major problem in nearby wells. The avoidance of severe vibrations resulted in documented savings in time and cost. Simple guidelines were provided to estimate the operating ranges of the drilling parameters to mitigate and avoid downhole tool failures. Extensive simulations were carried out to compare the data from the downhole vibration sensors, and the paper includes one data from a severe vibration incident where by the model estimated, predicted, and avoided severe vibration. For convenience the model2 is explained briefly in the following paragraphs. This incident took place on an offshore well. The model2 solves the linearized form of the following equation in which all the displacements and forces vary harmonically in time at the same frequency.

Modal state vectors of a free-hanging drilling riser during deployment and retrieval

“…As is well known, all-natural modes can participate in forced vibrations of marine risers, and if one of the wave frequencies is tuned to the natural frequency the riser will respond to that frequency and one natural mode will be amplified, which is called riser resonance (Dareing and Huang 1976;Samuel et al 2006;Persent et al 2009). During the deployment and retrieval, the riser structure is subjected to sizeable periodic wave loads, and resonance excited by waves may likely occur if the hang-off riser's natural frequencies are close to the frequencies of the random waves.…”