Twice in a year, on opposite sides of the world, drilling rigs collapsed after prolonged jarring operations. This paper documents the analysis performed to determine if the jars could generate enough force to overload the rig structure.
Data from the drilling logs was taken at time intervals of 10 seconds or greater. These large time intervals did not allow the peak shock loads to be recorded. The rig instrumentation or sampling rate tends to damp out these peak shock loads. Thus, during jarring operations the loading can be significantly different than that displayed by the rig instrumentation.
A dynamic finite element jarring analysis was done for both of these cases. This analysis calculates the loads along the drill string before and after the jar is released. The impact load travels up and down the string at the speed of sound in steel, causing spikes in the surface loads. To simulate these dynamic loads the model is run with a time interval of 100th of a second. This allowed the loading seen by the rig structure to be simulated. The input parameters and modeling results from one of these analyses are presented in the paper.
Jars are used in nearly every drill string in the industry and have been for some years. The invention of hydraulic jars created a tool that enhanced the use of the jars by providing an impact or "blow" level that was controllable by the driller. From the many discussions and courses taught over the past 15 years it is evident that there is still some lack of knowledge about how the jars should be used. Simple formulae are available or can be worked out by engineering sense, but some of the more sophisticated questions an engineer may want to ask are not able to be solved and answered by simple engineering tools. The dynamic response of a drill string is very complicated and is more suited to analysis by numerical finite element solutions.
Dynamic Finite-Element / Finite-Difference Model
The model used for this analysis is part of the Cerberus for Drilling™ software package. The model performs 3D a finite-element analysis (FEA) of the drill string at each specific point in time. This analysis is run repeatedly at short time steps through time using a finite-difference (FD) algorithm.
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