Summary

Drillstrings made before 1982 were not designed to meet the rigorous demands of today's drilling. Deeper wells and higher-angle holes are requiring higher pump pressure, and more tensile and torque capacity. Back-reaming, with the use of a power swivel, is agitating these conditions even further.

Recent work has been done to improve the fatigue life of drillpipe by making drillpipe with longer internal-taper upsets and stress-relief grooves.

Even though improvements have been made, slip damage still takes its toll on drillpipe. This paper addresses the failures caused by slip damage and offers a solution for solving this problem. Finite element work and fatigue testing has been done to prove the effectiveness of this new drillpipe design.

Introduction

Premature failures in drillpipe can be very costly. At a time when oil and gas prices are low, the prevention of failures could well be the difference in showing a profit or a loss in new drilling ventures.

Drillpipe slips cause damage to the drillpipe tubes, which in turn creates stress risers in an area that is already subjected to high stresses. With sufficient bending, a fatigue crack will develop causing a premature failure.

Fatigue

Most premature failures in drillpipe are caused by fatigue damage. Fatigue in drillpipe is the result of rotational bending which produces cyclic stress reversals, below the tensile strength but above the endurance limit of the drillpipe. Theoretically, the endurance limit of steel is a stress level below which the drillpipe would run forever without fatigue damage. However, drilling environments are usually corrosive so the endurance limit of steel will be reduced or eliminated, depending on the degree of corrosiveness. Fatigue is cumulative and doesn't go away; it just continues to build up until the drillpipe finally fails.

Most fatigue failures occur within 3 to 4 ft from each end of a joint of drillpipe. This is a high-stress area where the drillpipe upsets fade out into the thin-wall tube section (Fig. 1). We have learned through the years that several things can cause an acceleration of fatigue damage, which reduces the life of the drillpipe. Some of these conditions are as follows.

Running Drillpipe in Compression

When rotary drilling first started, drillpipe weight was applied to the bit to make it drill. As wells got deeper and formations got harder, more weight was required to make the bit drill, and the drillpipe started failing. It was learned that rotating drillpipe in compression caused cyclic stress reversals which resulted in rapid fatigue failures. To solve this problem, heavy bars of steel, called drill collars, were run between the bit and the drillpipe to supply weight for the bit. The problem was that sometimes an insufficient amount of drill-collar weight was run to offset the buoyant force of the drilling fluid and the bounce of the bit. Also in directional wells, not all of the drill-collar weight was available for the bit. The buoyed drill-collar weight had to be multiplied by the cosine of the angle of inclination to determine the amount of usable bit weight.

In horizontal wells, drillpipe is sometimes deliberately run in compression between the drill collars and/or the heavy-weight drillpipe (HWDP) and the bit. The reason for this is that the drill collars and/or HWDP must be placed in the vertical or near-vertical section of the hole to provide a component of weight for the bit. By using light-weight drillpipe in the horizontal section of the hole, there is less torque-and-drag, which permits drilling a longer horizontal section of hole. The only problem is that, when too much weight is applied, the drillpipe will buckle and become fatigued. At the present time, this limits the length that a horizontal lateral can be drilled.

Too Much Change in Cross Section

A large difference in cross-sectional area between the drill collars and the relatively light-weight drillpipe can cause rapid accumulation of fatigue damage in the drillpipe directly above the drill collars. W.L. Kirk¹ suggested that the polar section modulus (PSM) ratio should be used to control this change in cross section. He said in normal drilling conditions, the PSM ratio should not exceed 5.5 and, in a corrosive environment, 3.5 should be the maximum ratio. The equation for PSM is

• Equation 1

where D is the outside diameter (in inches) and d is the inside diameter (ID) (in inches).

In most cases, application of this principle will show that smaller drill collars and/or HWDP will be required in the transition zone between large drill collars and drillpipe.

Drilling Through Doglegs

Many years ago, Arthur Lubinski² illustrated in his work that drilling through doglegs can cause severe fatigue damage to the drillpipe. He developed curves for estimating cumulative fatigue damage that can be found in the American Petroleum Inst. (API) RP-7G. These curves are for the drillpipe weight hanging below the dogleg, without the weight of the bottomhole assembly (BHA). His idea was that the BHA weight would be on the bit while drilling and therefore would not be added to the drillpipe weight below the dogleg. A worst-case situation would be to rotate the pipe with the bit off-bottom, when a dogleg is near the surface. When this happens, the BHA weight and drag would have to be added to the drillpipe hang-down weight, below the dogleg. This can cause rapid fatigue damage and is a common occurrence in today's drilling with power swivels, as the well is back-reamed during trips. Reduction in rev/min, and slower upward motion of the drillpipe, will help reduce fatigue damage when these conditions exist. It seems that doglegs are becoming more and more prevalent today, while drilling with navigational-type steering tools, because of the ease in making fast directional changes. It appears that heat checking of drillpipe is also on the rise, resulting from key seats being formed in these doglegs. Directional changes should be gradual to prevent these problems.

Whipping Action

Fatigue damage is caused in drillpipe by whipping action that is created when unstabilized drill collars are rotated at a high rev/min. An excessive number of drill collars and/or a relatively large hole size will cause fatigue damage to grow at a rapid rate. Critical rotary speeds can cause even greater fatigue damage to drillpipe and in some instances will buckle the pipe. These conditions can also cause eccentric wear.