Loads on Drillpipe During Jarring Operations

Abstract

Summary Jarring implies heavy loads on the drillstring. The highest load on the drillpipe before jarring is at the rig floor. This paper discusses loads on drillpipe before, under, and after jarring. We show that for most situations, the shock wave from the jar impact does not imply additional load on the drillpipe compared with static load. The theoretical results are confirmed by measurements of a jarring operation with stuck point at ˜1200 m measured depth. Loads on the drillpipe can be a limiting factor in jarring operations because fear of possible additional loads from jarring dynamics may restrict the trip force (overpull) on the jar. Our main conclusion is that dynamic jar forces do not give additional loads on drillpipe. This information can be used to set an optimal trip force on the jar. Introduction Stuck pipe is an expensive event in drilling operations. Operation companies put a lot of effort into avoiding stuck pipe. Whenever a stuck-pipe situation occurs, a precise and adequate reaction from the driller will increase the probability of freeing the pipe. Publications on jarring operations are scarce if patents are not counted, with only about 15 papers published on jarring connected to drilling operations from 1975 to 1992; we cite nine of these.1–9 The main concerns in these papers are dynamics, jar performance, and effect on stuck point. Two papers7,9 discuss the acceleration phase in detail and show how speed and force change step-wise. No papers discuss the dynamic loads transmitted up the drillpipe. This paper addresses these loads. Deep, highly deviated or horizontal wells may result in operating conditions near the tensional limit of drillpipes. Stuck pipe in such wells will therefore give additional concerns about loads during jarring. Possible addition of a shock-wave amplitude on the static load may impose operational limits on the trip force of the jar. However, the jarring operation can be divided into phases in which the acceleration and postimpact phases determine the possible added load on the drillstring by the impact. Operational limits from the jar dynamics can be avoided by the use of optimized drillcollar lengths and heavy weight drillpipes (HWDP's) described in this paper. Jarring Process Our analysis will be restricted to up-jarring because it normally represents higher loads than down-jarring. The jarring cycle can be divided into the following five phases.Loading. In this phase, string is stretched to store strain energy and the jar is exposed to a (preset or operator-determined) tension force. This phase typically lasts only a few seconds, but it can take minutes when hydraulic jars with a long delay time are used.Acceleration. This phase, also called the preimpact phase, is the time from jar release until jar impact and typically lasts from 50 to 200 milliseconds. In this phase, the strain energy is convened to translation energy. The higher the speed at impact, the higher the impact force will be at the stuck point.Impact. The inside of the jar is often pictured as consisting of two parts: the hammer and the anvil. The impact phase is the short time interval, typically 10 to 50 milliseconds, when the jar hammer hits the anvil. In this phase, the bottomhole assembly (BHA) is exposed to high impact forces.Postimpact. This phase lasts until the string has come to a complete rest again.Resetting. During this phase, the string is lowered and the jar is put into a small compression force to reset the jar reset for a new jarring cycle. Fig. 1 illustrates the jarring process phases. Assumptions and Basic Equations For simplicity. the analysis will be restricted by the following assumptions:The stuck string can be divided into a few uniform sections with different cross-sectional areas but with equal material properties: the section below the jar (often called the fish section), the jarring-mass section (drill collars above the jar), the HWDP section (optional), and the drillpipe section.The drillpipe section is so long that reflections from the surface can be neglected during the acceleration and impact phases.Jar release is instantaneous, with no transition time between the loading and acceleration phases.Friction forces are neglected, both internally in the jar and externally along the drillstring.Residual vibrations in the fish section at the time of impact are neglected. Assumption 1 implies that jar accelerators are excluded. A jar accelerator placed at the top of the jarring-mass section normally increases the impact force across the jar and at the stuck point. Because it transmits shock waves poorly, it dampens the dynamic loads on the drillpipes. Consequently, the conventional jarring assemblies without an accelerator represent the worst cases for drillpipe loads. Jar release is usually not achieved instantaneously (Assumption 3). However, Assumption 3 simplifies the mathematics significantly. Because this assumption affects both relaxation of the drillstring and the impact force, the simplifications should not influence the conclusions of the paper. A further discussion of this assumption is given later. As Fig. 1 clearly shows, Assumption 4 is a simplification. If the internal jar friction force is constant, an effective jar pull equal to actual overpull minus internal friction force can be defined. The wellbore friction can be handled similarly, and the two friction forces may therefore be regarded as static forces adding to the gravitational force. Therefore, Assumption 4 is not a serious limitation.