Summary

Rubber or steel drillpipe protectors are lateral supports placed on drillpipe between tool joints. Everyone knows placed on drillpipe between tool joints. Everyone knows that drillpipe protectors prevent drillpipe wear. An important fact not known is that drillpipe protectors will allow permissible hole curvatures (dogleg severity) greater than usual. The additional hole curvature allowed with the protectors is a result of their ability to reduce drillpipe fatigue and wear with no increase in casing wear. Conversely, for the same drilling conditions, drillpipe protectors can significantly increase drillpipe life. Range 3 drillpipe (45 ft [13.7 m] long) has the advantage of faster handling because less time is spent on connections and possibly also on round trips. Because it has the disadvantage of increased drillpipe wear and fatigue, very little Range 3 drillpipe is used today. With drillpipe protectors, the advantage of Range 3 drillpipe can be protectors, the advantage of Range 3 drillpipe can be retained, without this disadvantage. Greater permissible hole curvature (dogleg severity) when drillpipe protectors are used (on either Range 2 or Range 3 drillpipe) may permit carrying heavier bit weights. The result should be faster drilling and reduced drilling cost.

Introduction

A drillpipe protector is considered to have the same OD as the tool joint. Because of this, both the tool joint and drillpipe protector perform equivalently, and are called "supports. "Also, a steel or rubber drillpipe protector is referred to simply as a "protector."For the sake of brevity, only graphs for 4 1/2-in., 16.60-lbm/ft [11.4-cm, 24.7-kg/m] drillpipe are presented in this paper. Similar graphs for other sizes of presented in this paper. Similar graphs for other sizes of drillpipe are available from the authors. The conclusions in the paper hold true for all sizes of drillpipe.

Drillpipe Fatigue. Consider the exaggerated drawing in Fig. 1A, showing a joint of drillpipe under tension in a gradual dogleg. Bending of drillpipe is concentrated near the toot joints. At the center of a joint, the pipe is much straighter and may even contact the hole wall (Fig. 1B). Now consider Fig. 1C with a protector in the center of a joint. The pipe bending is now more evenly spread over the entire length. For all but most severe conditions, the drillpipe will not contact the wall in gradual doglegs. Following a method similar to Refs. 1 and 2, we can calculate the maximum permissible dogleg severity for no fatigue. Fig. 2 is a graph of the results for Grade E, 4-in., 16.60-lbm/ft [11.4-cm, 24.7-kg/m] drillpipe. The abscissa is hole curvature (dogleg severity) in degrees per 100 ft. The ordinate is tension* in pounds (pounds force). The curves are labelled by their 2L lengths. The 2L length is the distance between supports, either tool joints or protectors and is defined in Table 1. Points A and B indicate that for 2L= 15 ft [4.6 m] and a tension of 120,000 lbf [0.533 MN], there will be no fatigue foradogleg severity of 7 deg. /100ft, but fatigue will occur for 9 deg. /100 ft. Table 2 shows the maximum permissible dogleg severity for no fatigue at Points C, D, E, permissible dogleg severity for no fatigue at Points C, D, E, F, and G with 160,000-lbf [0.711-MN] tension. The reduction of the lateral support spacing 2L causes dramatic increases in allowable dogleg severity. Thus the use of drillpipe protectors will not only prevent visible damage (loss of drillpipe wall thickness) but also the invisible damage (fatigue). The lengths used in this paper are the result of protectors being equally spaced between tool joints. Equal protectors being equally spaced between tool joints. Equal spacing yields a constant distance between supports and the most reduction in fatigue damage. If the protectors are not equally spaced, different equations are required. For brevity, results for nonequal spacing are not included. The longest length between supports governs the fatigue life.

Casing Wear. An important question to be asked is: What effect do drillpipe protectors have on casing wear,)Casing wear is of most concern where the largest lateral force between drillpipe supports and casing occurs. This generally is in doglegs in which the drillpipe is under large tension. Casing wear at these points is a result of the force exerted by lateral supports rotated through the doglegs. A recent paper indicates that the casing wear rate is approximately proportional to the square of contact pressure (force/area). If this is used, an estimate of the pressure (force/area). If this is used, an estimate of the effect of protectors on casing wear can be made. If the number of lateral supports is doubled, the contact pressure per support is cut in half. The wear rate per pressure per support is cut in half. The wear rate per support is thus reduced by a factor of four. Since there are twice as many supports, the net effect is roughly to reduce casing wear by a factor of two.

JPT

P. 628

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