Effects of Drilling Vessel Pitch or Roll on Kelly and Drill Pipe Fatigue

Abstract

Abstract Roll or pitch of a drilling vessel results in bending of the kelly and the first joint of drill pipe. Cumulative fatigue damage of these members due to cyclic stresses is calculated, and means for prevention of fatigue failure are devised. The effects of variation of the cyclic stress magnitude due to weather, kelly motion, changes in hookload and other factors are fully taken into account. SUMMARY AND CONCLUSIONS Fatigue damage of the kelly and the first joint of drill pipe, induced by vessel roll or pitch, is greatly minimized through use of either a gimbal kelly bushing or a one plane roller bushing. The fatigue damage, which occurs when each joint of drill pipe is the first below the kelly, should be kept reasonably small, such as 10 per cent of the drill pipe life. It is probable that some past drill pipe failures were caused by excessive fatigue damage due to roll or pitch, without having been recognized as such, because the failure occurred at a much later time when the particular point was far down the hole. Means for calculating the cumulative fatigue damage of the first joint of drill pipe are given in this paper. The rate of this damage decreases as the kelly goes down and as the drilling rate becomes greater. Also, the rate of this damage decreases as the hookload increases because most of the bending then occurs in the kelly. The rate of fatigue damage increases with the roll or pitch angle and the severity of the corrosion environment. Drill pipe fatigue damage may be made smaller by increasing the length of kelly or kelly subs. Generally, fatigue damage of the hexagonal portion of the kelly is negligible due to high-quality steel and frequent cold-working of the surfaces in the kelly bushing. Only in extremely unfavorable conditions (small vessel, frequent heavy seas) will fatigue failures of the hex portion become probable. The round section of the kelly is not cold-worked and may suffer appreciable fatigue damage, particularly at the point just below the hex section. This damage may be controlled by using a kelly whose hex section is sufficiently long, since the bending stress decreases fast with distance from the kelly bushings. Introduction When drilling from a floating vessel, the drilling string is subjected to stresses which arise from heave, roll, pitch and horizontal movement of the ship. Stresses generated by these conditions were investigated by Lind. However, Lind used the strength of pipe as the criterion of failure, while actually in almost all cases, failures are fatigue failures due to cyclic stress reversal at lower stress levels. The first purpose of this paper is to determine the fatigue life of that part of the drill string which is affected by roll or pitch motion, i.e., the kelly and the first joint of drill pipe below the kelly. The second purpose of this paper is to adapt the known concept of cumulative fatigue damage to drill string problems. It is hoped that this concept will be used extensively in the future in calculations of cumulative damage of drill pipe due to vibrations and crooked-hole conditions. The latter investigation has already been initiated within the AAODC and the API. A method, which the authors believe to be original, for calculation of cumulative fatigue damage through use of so-called pseudo curves (stress vs cycles to fatigue failure) is developed and applied in this paper. Complete results of this investigation are presented in the text preceding the section entitled "Calculations of Cumulative Fatigue Damage". Reading this last section is necessary only for performing similar calculations. Derivation of formulas is presented in the Appendix. IDEALIZED SYSTEM Consider Fig. 1, a diagram which shows a derrick in an inclined position due to the roll of the ship. The figure shows, in an exaggerated way, the resulting bending of the kelly, kelly subs and drill pipe. This bending is somewhat relieved by the swivel bail and by the use of either a one plane kelly bushing or a gimbal-mounted bushing. It is assumed throughout this investigation that one of these bushings is used. The calculations and examples which follow are based on the experience and conditions prevailing on the catamaran "C.P. Baker" of the Reading and Bates Offshore Drilling Co. This vessel has been used in 1962–63 by Pan American Petroleum Corp. in drilling operations off the Louisiana coast. The numerical dimensions needed in this study are shown in Fig. 2, except the length of the swivel bail (DR in Fig. 1), which was assumed to be 6 ft. A one plane kelly bushing was used on the catamaran. JPT P. 77ˆ